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Stereoelectronic effects on the basicities of carbonyl compounds Granger, Maurice Roy 1961

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(i)  STEREOELECTRONIC EFFECTS ON THE BASICITIES OF CARBONYL COMPOUNDS by MAURICE ROY GRANGER B.Sc,  U n i v e r s i t y o f B r i t i s h Columbia, 1959  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n t h e Department of Chemistry We a c c e p t t h i s t h e s i s as conforming t o t h e required  standard  THE UNIVERSITY OF BRITISH COLUMBIA April,  1961  In presenting the requirements  this thesis  in partial fulfilment  f o r an a d v a n c e d d e g r e e a t  the  of B r i t i s h Columbia, I agree that  the  it  and s t u d y .  freely  agree that for  for reference  permission for  s c h o l a r l y purposes  Department that  available  extensive  of  University  L i b r a r y s h a l l make I  copying of  further this  thesis  may be g r a n t e d by t h e Head o f my  o r by h i s r e p r e s e n t a t i v e s .  It  copying or p u b l i c a t i o n of t h i s t h e s i s  is  understood  for  financial  g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n .  Department o f  Chemistry  The U n i v e r s i t y o f B r i t i s h V a n c o u v e r #, C a n a d a . Date  May U, 1961  Columbia,  (ii) ABSTRACT The b a s i c i t i e s o f f i f t e e n o r t h o - s u b s t i t u t e d benzoic a c i d s and f i v e benzophenone analogues  have been measured  w i t h the aim o f s t u d y i n g the e f f e c t s o f s t e r e o e l e c t r o n i c f a c t o r s on the s t r e n g t h s o f very weak bases. The ortho e f f e c t s o f the s u b s t i t u e n t s on the b a s i c i t i e s o f the b e n z o i c a c i d s have been e s t i m a t e d by two methods. The f i r s t method compares the p % u  +  values of the corresponding  o r t h o - and p a r a - s u b s t i t u t e d a c i d s and uses t h e d i f f e r e n c e between t h e n as a measure o f the o r t h o e f f e c t on assumption  the  t h a t the e l e c t r i c a l e f f e c t s o f s u b s t i t u e n t s  a r e equal i n the ortho and para p o s i t i o n s . method i s t h a t o f McDaniel  The  second  and Brown (10).  The ortho e f f e c t s o f a l k y l s u b s t i t u e n t s a r e base weakening and i n c r e a s e i n the o r d e r methyl i-propyl  t-butyl.  <  ethyl <  These e f f e c t s a r e a t t r i b u t e d to the  b u l k e f f e c t o f the s u b s t i t u e n t s , and i n p a r t i c u l a r , s t e r i c i n h i b i t i o n o f resonance The halogen  i n the conjugate  acid.  s u b s t i t u e n t s , on the o t h e r hand, appear to have  base s t r e n g t h e n i n g o r t h o e f f e c t s i n the o r d e r F I  ^  to  >  Cl >  Br >  0, which a r e a t t r i b u t e d t o s t a b i l i z a t i o n o f the protonated  a c i d by i n t r a - m o l e c u l a r hydrogen bonding.  In a  similar  manner, oxygen c o n t a i n i n g s u b s t i t u e n t s such as methoxyl and c a r b o x y l a r e base s t r e n g t h e n i n g . The b a s i c i t i e s o f these benzoic a c i d s do not follow the l i n e a r r e l a t i o n s h i p s of e i t h e r Taft  (21) o r  (iii) Farthing, and Nam  (22)  which have been developed to c o r r e l a t e  the r e a c t i o n s o f o r t h o - s u b s t i t u t e d benzene d e r i v a t i v e s ,  a  f a c t which s e r v e s t o emphasize the h i g h dependence o f ortho e f f e c t s upon the nature o f the r e a c t i o n . The b a s i c i t i e s of benzophenone analogues  i n which t h e  phenyl r i n g s a r e r i g i d l y h e l d have a l s o been s t u d i e d .  The  b a s i c i t i e s were found to be i n the o r d e r 9-anthrone> 2,3;6,7-dibenzotropone > 2,3;6,7-dibenzosuberone > benzophenone > 9-fluorenone.  The h i g h b a s i c i t y o f anthrone  i s a t t r i b u t e d to the p l a n a r i t y o f the molecule which a l l o w s a h i g h degree o f resonance i n both the unprotonated protonated species.  and  The low b a s i c i t y o f f l u o r e n o n e i s  a t t r i b u t e d t o i t s s i m i l a r i t y t o cyclopentadienone i n which the tendency o f the c a r b o n y l group to p o l a r i z e i n the u s u a l d i r e c t i o n , and so i n c r e a s e the e l e c t r o n d e n s i t y on oxygen, i s g r e a t l y reduced through an e f f o r t to aromatize the r i n g .  (iv) TABLE OF CONTENTS Page INTRODUCTION 1  The Ortho E f f e c t The Hamrrtett E q u a t i o n and i t s  Failure for  O r t h o - S u b s t i t u t e d Benzene D e r i v a t i v e s The B a s i c i t i e s . o f  Very Weak Bases  - - - - -  -6  - - - - - - - - - 1 2  Mode, o f P r o t o n a t i o n o f the C a r b o x y l Group - - - - - 15 OBJECTS OF THE PRESENT RESEARCH  - -  lg 19  METHODS OF APPROACH EXPERIMENTAL U l t r a v i o l e t S p e c t r a and Determination o f 1.  Basicities  Materials - - - - - - - - - - - - - 2 2  (a)  Benzoic A c i d s  (b)  Benzophenone Analogues - - - - - - - -  (c)  Sulphuric Acids - - - - - - - - - -  -22  - - 2 7  2.  Measurement o f U l t r a v i o l e t Spectra - - - - - 27  3.  Treatment o f Data  4.  (a)  G r a p h i c a l Method  (b)  L i n e a r Method  _ _ _  32 3#  Estimated E r r o r i n B a s i c i t y Determinations  - 42  RESULTS AND DISCUSSION P a r t I - Benzoic A c i d s - - - - - - - - - - - - - - 1.  Alkyl  2.  Halogen S u b s t i t u e n t s  3.  Oxygen C o n t a i n i n g S u b s t i t u e n t s  Substituents  -43  - - - - - - - - - - - - - 4 9 - - - - - - - - - - - - 5 5 - - - - - - - 5 9  (v) Page Part I  (cont.)  4.  Phthalic Acids  - - - - - - - - - - - - - - 6 5  5.  Search f o r L i n e a r R e l a t i o n s h i p s - - - - - -71  P a r t I I - Benzophenones - - - - - - - - - - - - - -  #2  SUGGESTIONS FOR FUTURE WORK  89  BIBLIOGRAPHY  90  (vi) LIST OF TABLES Page I  P h y s i c a l Constants o f Benzoic A c i d s - - - - - - -  -25  II  P h y s i c a l Constants o f Benzophenones - - - - - - -  -26  III  U l t r a v i o l e t S p e c t r a l Data on t h e B a s i c i t y o f S a l i c y l i c A c i d - - - 30  IV  U l t r a v i o l e t S p e c t r a l Data on t h e B a s i c i t y o f Benzophenone - - - 31  V  B a s i c i t y Constants, A b s o r p t i o n Maxima and Slopes o f Linear B a s i c i t y P l o t s o f S u b s t i t u t e d Benzoic A c i d s - - - - - - - - - - - -  VI  -44  Comparison o f t h e Base S t r e n g t h s o f Ortho-, Meta- and P a r a - S u b s t i t u t e d Benzoic A c i d s - - 45  VII  B a s i c i t i e s o f Mono-Substituted  Pyridines (pK ) a  and Benzoic A c i d s (pKBpf ) - - -4S +  VIII  Conformational E q u i l i b r i a o f Halocyclohexanes  IX  E x p e r i m e n t a l and C a l c u l a t e d B a s i c i t y  - - -58"  Constants  o f Isomeric P h t h a l i c A c i d s - - 66 X  A c i d I o n i z a t i o n Constants o f t h e Isomeric P h t h a l i c A c i d s - - - - -66  XI  P o l a r and S t e r i c S u b s t i t u e n t s Constants f o r O r t h o - S u b s t i t u e n t s Using T a f t ' s Method  XII  78"  E l e c t r o n i c and S t e r i c S u b s t i t u e n t Constants f o r Ortho S u b s t i t u e n t s U s i n g F a r t h i n g ' s and Nam's Method  XIII  79  B a s i c i t y Constants, A b s o r p t i o n Maxima and Slopes o f L i n e a r B a s i c i t y P l o t s o f S u b s t i t u t e d Benzophenones -S3  (vii) LIST OF FIGURES Page 1.  Curve o f  AD versus H  0  f o r S a l i c y l i c A c i d - - - - - -35  2.  Curve o f  A D versus H  Q  f o r Benzophenone  3.  Anomalous Curve o f  AD v e r s u s H  0  f °  r  -------36  Ortho-Nitro  Benzoic Acid - -  4.  ADBH  Linear Plot of log  AD  -  +  -  AD  -37  v e r s u s Ho f o r  ADB  S a l i c y l i c A c i d - - - 40  ADBH  5.  -  +  A  D  Linear Plot of log  v e r s u s Ho f o r AD  -  A D  B  Benzophenone - - - 41 6.  R e l a t i o n s h i p between p K o f P y r i d i n e s and pKBH  7.  Substituted  a  +  ° f S u b s t i t u t e d Benzoic A c i d s - - 47  R e l a t i o n s h i p between E a c t f o r R e a c t i o n  RC5H4N  C H 3 I — > RC5H4N+CH3l~  R e l a t i o n s h i p between A pKgjj  f  and for  A  B  and  ApK i  +  B  - -52  Halo-Substituted  Benzoic A c i d s and Van d e r Waals R a d i i - - - - - - 9.  U l t r a v i o l e t A b s o r p t i o n Curves o f O r t h o - P h t h a l i c - - - 70  Acid i n Sulphuric Acid Solutions 10. R e l a t i o n s h i p between p K and p K H a  B  +  of  Ortho-Substituted Benzoic Acids 11.  R e l a t i o n s h i p between pKjgij; + o f  - - - - - - - - 7 2  Ortho-Substituted - - - 74  B e n z o i c A c i d s and 0~ 12.  -61  R e l a t i o n s h i p between p K H B  +  of  Ortho-Substituted  B e n z o i c A c i d s and E  s  - - -  -75  (viii) Page log 13. R e l a t i o n s h i p between  K/K  0  ^JJE  f o r Ortho-  S u b s t i t u t e d Benzoic A c i d s and  ^£ Using  T a f t ' s Method log 14. R e l a t i o n s h i p between  K/K ?  S u b s t i t u t e d Benzoic A c i d s and  80  0  f o r Ortho+  Using  CPs F a r t h i n g ' s and Nam's Method - - - - - - - S l  o  (ix)  ACKNOWLEDGEMENTS  I wish to express my  s i n c e r e thanks to Dr. R.  Stewart  f o r h i s s t i m u l a t i n g and p a t i e n t guidance throughout the course o f t h i s work. I am a l s o i n d e b t e d t o Dr. H a r o l d Schechter o f Ohio S t a t e U n i v e r s i t y f o r the supply o f c h e m i c a l s . F i n a n c i a l a s s i s t a n c e from the N a t i o n a l Research C o u n c i l o f Canada i n the form o f a s t u d e n t s h i p i s a l s o gratefully  acknowledged.  (1) INTRODUCTION The Ortho. E f f e c t . The anomalous e f f e c t s of ortho substituents on the rates of reaction of substituted benzene compounds were f i r s t noted by Meyer (1) i n 1394.  He observed appreciable  reductions  i n the rates of the acid-catalyzed e s t e r i f i c a t i o n s of ortho substituted benzoic acids.  S i m i l a r l y , the r e l a t i v e l y high  strengths of the ortho substituted benzoic acids have long been known and were put on a quantitative basis by Dippy and coworkers i n the 1930 s (2). r  E f f e c t s of ortho  substituents,  such as these, on reaction rates and e q u i l i b r i a have been termed "ortho or proximity e f f e c t s " . Several explanations  of ortho e f f e c t s have been advanced.  Meyer suggested that they were due primarily to the bulk of the ortho substituent, which would hinder the approach of an attacking group to the reaction centre, and that they depended l i t t l e on the chemical character of the substituent. and Callow (3)  Sidgwick  l a t e r postulated the formation of a six-membered  "chelate" r i n g involving i n t e r n a l hydrogen bonding to account f o r the high v o l a t i l i t y of certain ortho phenols as i n I .  0  I  (2) T h i s type o f hydrogen bonding was l a t e r shown t o account f o r the  enhanced a c i d i t y o f s a l i c y l i c a c i d  bonded form o f s a l i c y l i c a c i d ( I I ) ,  (4).  I n t h e hydrogen  the p o s i t i v e end o f the  p h e n o l i c h y d r o x y l group l i e s v e r y near the c a r b o x y l group, p u l l i n g e l e c t r o n d e n s i t y away from the c a r b o x y l hydrogen atom and e a s i n g i t s d e p a r t u r e as a p r o t o n .  Similar  hydrogen  bonding o c c u r s t o s t a b i l i z e the a n i o n ( I I I ) and decrease the ease o f r e a s s o c i a t i o n .  n Evans  m  (5>) has p o s t u l a t e d s i m i l a r i n t r a m o l e c u l a r  hydrogen  bonding t o occur even i n the case o f ortho-methyl groups. However, t h i s s u g g e s t i o n i s u s u a l l y looked upon w i t h s k e p t i c ism  today. Baddeley (6) has suggested t h a t s t e r i c i n h i b i t i o n o f  resonance i s the cause o f the i n c r e a s e d s t r e n g t h s o f the ortho s u b s t i t u t e d b e n z o i c a c i d s .  To understand t h i s phenomenon  we must c o n s i d e r the resonance s t r u c t u r e s f o r the u n d i s s o c i a t e d  (3) and d i s s o c i a t e d forms o f benzoic a c i d . f o r the b e n z o i c a c i d molecule may  Resonance s t r u c t u r e s  be w r i t t e n as f o l l o w s .  S t r u c t u r e s f o r the a n i o n such as  are unimportant s i n c e two n e g a t i v e charges must be brought  (4) close together.  S i n c e resonance  i s more important i n the  u n d i s s o c i a t e d form, i t w i l l s t a b i l i z e the molecule to  relative  the a n i o n and so w i l l c o n s t i t u t e an a c i d weakening e f f e c t  r e l a t i v e to the i n d u c t i v e e f f e c t alone o f the phenyl  group.  A b u l k y ortho s u b s t i t u e n t w i l l f o r c e the c a r b o x y l group out o f the plane o f the benzene r i n g and so i n h i b i t  resonance,  thus r a i s i n g the f r e e energy o f the u n d i s s o c i a t e d form ive  relat-  t o t h a t o f the a n i o n and so r e s u l t i n g i n an a c i d s t r e n g t h -  ening e f f e c t .  T h i s phenomenon has been d i s c u s s e d from a  more thermodynamic viewpoint by Zawidzki (7).  Zawidzki  observes t h a t the i n h i b i t i o n o f resonance i n the u n d i s s o c i a t e d form w i l l g i v e r i s e to an i n c r e a s e i n enthalpy, H, o f t h i s form and t h e r e f o r e decrease the A H to  of ionization.  Owing  the decrease i n r i g i d i t y o f the u n d i s s o c i a t e d form, as a  r e s u l t o f the l o s s o f resonance, t h e r e w i l l be an i n c r e a s e i n i t s entropy, S, and t h e r e f o r e a l o w e r i n g (more n e g a t i v e ) of the  AS  of i o n i z a t i o n .  AF  we  see t h a t these two  - AH  R e c a l l i n g the f a m i l i a r equation,  - T AS  (1)  e f f e c t s oppose one another.  The  effect  on the e n t h a l p y change i s , however, u s u a l l y l a r g e r than the e f f e c t on the entropy change and so the o v e r a l l e f f e c t i s a decrease i n the in  A ? o f i o n i z a t i o n and t h e r e f o r e an i n c r e a s e  acidity. The i d e a s o f Watson (8)  from those o f Meyer.  were probably the most extreme  Watson a t t r i b u t e s ortho e f f e c t s i n  ortho benzoates to be due wholly to hydrogen  bonding  either  (5)  i n the ground or t r a n s i t i o n states of the reactants.  The  fact that no evidence of hydrogen bonding has been obtained f o r many substances which exhibit ortho effects i s explained, he says, by the f a c t that such hydrogen bonding need only occur during the passage of the reaction system over the "energy  pass".  More recently i t has become accepted that ortho effects must be attributed to a combination of the factors so f a r discussed.  From a comparison of the r e l a t i v e b a s i c i t i e s  of pyridine and the p i c o l i n e s with the d i s s o c i a t i o n energies of the addition compounds of trimethylboron and these compounds, Brown ( 9 ) has shown that the effect of an ortho methyl group i s primarily a bulk e f f e c t and does not involve hydrogen bonding.  This conclusion may be reasonably extended to  other a l k y l groups.  I t i s now accepted that the so-called  bulk e f f e c t i s due to a combination of front s t r a i n ("F s t r a i n " ) , s t e r i c i n h i b i t i o n o f resonance, s t e r i c hindrance to solvation ( 1 0 ) and k i n e t i c energy e f f e c t s ( 1 1 ) .  F s t r a i n i s a term  originated by Brown and r e f e r s to s t e r i c interaction a r i s i n g between groups on two d i f f e r e n t atoms during the formation of a bond between these atoms.  Any s t e r i c factor which  causes changes i n the motions of atoms i n a molecule i s termed a k i n e t i c energy s t e r i c e f f e c t .  I t i s p l a i n to see that  such a f a c t o r w i l l a f f e c t the entropy and thus the free energy change of the reaction.  I t i s well known that i n any i o n i z -  ation process, solvation of the ions so formed i s an important s t a b i l i z a t i o n process.  The presence of bulky groups adjacent  to the reaction centre w i l l hinder the approach of solvent  (6) molecules and therefore r e s u l t i n d e s t a b i l i z a t i o n of the ions and hence a decrease i n the degree of i o n i z a t i o n . McDaniel and Brown (10) have demonstrated that the ortho effects of halo substituents  are due both to bulk e f f e c t s  and to i n t e r n a l hydrogen bonding of the halo group with the adjacent reaction centre i n the order  F>Gl>Br>I.  As  already mentioned, the a b i l i t y of n i t r o and hydroxyl groups to p a r t i c i p a t e i n i n t e r n a l hydrogen bonding with a functional group i s well known.  The factor or factors of those mention-  ed which contribute most to the observed ortho e f f e c t i n any one  case w i l l depend upon the nature of the p a r t i c u l a r  functional group and thus i t i s often d i f f i c u l t to predict the effect of a given ortho substituent  i n a particular  reaction even though i t s behavior i n other reaction  series  may be known.  The Hammett Equation and I t s Failure f o r Ortho-Substituted Benzene Derivatives. Observations that the effects o f substituents  i n many  reaction series involving meta- or para-substituted benzene derivatives could be correlated with the acid i o n i z a t i o n constants o f the corresponding benzoic acids (12, 13, 14, 15>) led Hammett to propose the existence o f a l i n e a r free energy relationship between the e f f e c t s of meta and para and the r e a c t i v i t y of benzene derivatives  substituents  (16, 17,).  This  relationship i s embodied i n the well-known Hammett equation. (2)  (7)  Here k and k  Q  a r e the r a t e o r e q u i l i b r i u m constants  r e a c t i o n s o f s u b s t i t u t e d and u n s u b s t i t u t e d t i v e l y , 0" i s a constant  f o r the  compounds,  c h a r a c t e r i s t i c o f a given  respec-  substituent  and measures the a b i l i t y o f the s u b s t i t u e n t t o withdraw o r donate e l e c t r o n s , and p i s the r e a c t i o n constant  and measures  the s e n s i t i v i t y o f a g i v e n r e a c t i o n t o changes i n s u b s t i t u e n t s . The  Hammett equation,  however, i s l i m i t e d t o t h e r e a c t i o n s  o f meta- and p a r a - s u b s t i t u t e d  compounds and i s i n v a l i d f o r  a l i p h a t i c and o r t h o - s u b s t i t u t e d  compounds,  Hammett s a i d  t h i s was l i k e l y an i n d i c a t i o n t h a t t h e f r e e energy r e l a t i o n s h i p i s e s s e n t i a l l y a r e l a t i o n between t h e p o t e n t i a l energy changes d u r i n g a r e a c t i o n and n o t between the k i n e t i c energy changes (1&).  T a f t has demonstrated t h a t t h i s i s indeed  t h e case and t h a t when the Hammett e q u a t i o n i s f o l l o w e d , the k i n e t i c energy e f f e c t s , i f any, a r e embodied i n t h e suscepp  t i b i l i t y constant  (19),  Reasons f o r t h i s f a i l u r e o f t h e Hammett equation can b e s t be understood i f we c o n s i d e r the t h e o r e t i c a l b a s i s o f t h e equation. K  Q  An e q u i l i b r i u m process w i t h e q u i l i b r i u m  has a standard  f r e e energy change A F  Q  constant  associated with i t  as g i v e n by the equation AF  Q  = -RT I n K  Q  A s i m i l a r p r o c e s s i n t h e same r e a c t i o n s e r i e s w i t h e q u i l i b r i u m constant  K has. a f r e e energy change  AF  = -RT I n K  (8) Thus the Hammett e q u a t i o n may  be r e w r i t t e n i n the form  l o g K = CTp +  - AF =  or The  log  RTCrp  K  0  -AF  0  f r e e energy changes a s s o c i a t e d w i t h r e a c t i o n s w i t h i n a  s e r i e s are thus l i n e a r l y r e l a t e d to the 0" v a l u e s , which are by d e f i n i t i o n measures o f the e l e c t r o n i c e f f e c t s o f subs t i t u e n t s on the f r e e energy o f the system.  These e f f e c t s  a r e embodied mainly i n the enthalpy term o f the f r e e energy expression  (equation  1) and u s u a l l y to a s m a l l e r extent  the e n t r o p y term and a r e the major c o n t r i b u t i o n to p o t e n t i a l energy changes i n the system.  in  the  Since o r t h o  sub-  s t i t u e n t s i n t e r f e r e w i t h the r e a c t i n g s i d e c h a i n i n a number o f ways and  i n a v a r y i n g manner depending on the n a t u r e o f  the s i d e c h a i n , and  so produce v a r i a b l e entropy and k i n e t i c  energy e f f e c t s , and  s i n c e i t i s not p o s s i b l e to make a  separation o f ortho  o f the e l e c t r o n i c and  s t e r i c e f f e c t s , the e f f e c t s  s u b s t i t u e n t s on the f r e e energy o f r e a c t i o n cannot  be a c c u r a t e l y p r e d i c t e d . t h a t no  simple and  I t i s t h e r e f o r e not s u r p r i s i n g  g e n e r a l l y a p p l i c a b l e l i n e a r f r e e energy  relationship exists f o r ortho-substituted r e a c t i o n s In which the  compounds.  entropy changes are e i t h e r  r e l a t e d by the Hammett equation The most e x t e n s i v e  Only  constant  o r p r o p o r t i o n a l to the enthalpy changes appear to be  cor-  (20).  attempts to develop a more  i z e d Hammett type o f r e l a t i o n s h i p a p p l i c a b l e to and  general  general-  aliphatic  o r t h o - s u b s t i t u t e d benzene d e r i v a t i v e s have been made by  (9) T a f t (21).  T a f t s work has been concerned w i t h a T  separation  o f the p o l a r , resonance and s t e r i c e f f e c t s o p e r a t i n g f r e e energy changes d u r i n g a r e a c t i o n .  on the  He has d e f i n e d a  rr* p o l a r s u b s t i t u e n t constant  <->  i n terras o f the r a t e s o f  normal e s t e r h y d r o l y s i s by the equation 0 " %  J _  log(k/k ) Q  B  -  logtk/kjj  2.48 L where t h e k s a r e the r a t e constants f  o f b a s i c and a c i d  h y d r o l y s i s o f s u b s t i t u t e d and u n s u b s t i t u t e d ortho  esters.  (3)  J  a l i p h a t i c and  Assuming t h a t t h e p o l a r , resonance and s t e r i c  e f f e c t s c o n t r i b u t e i n d e p e n d e n t l y t o t h e f r e e energy o f a c t i v a t i o n f o r the r e a c t i o n s t u d i e d , t h a t s t e r i c and resonance e f f e c t s a r e the same i n c o r r e s p o n d i n g a c i d i c and a l k a l i n e r e a c t i o n s , and t h a t t h e p o l a r e f f e c t s o f s u b s t i t u e n t s a r e much g r e a t e r i n a l k a l i n e than i n a c i d h y d r o l y s i s , 0~  should  be a measure o n l y o f t h e p o l a r e f f e c t o f the s u b s t i t u e n t . T a f t has found t h a t c e r t a i n r e a c t i o n s o f o r t h o - s u b s t i t u t e d compounds i n which s t e r i c f a c t o r s a r e constant  follow a linear  p o l a r r e l a t i o n s h i p o f t h e form log(k/k ) = Q  p  i s a constant  series to polar  °~ f  (4)  g i v i n g the s u s c e p t i b i l i t y o f a given  reaction  substituents.  On the b a s i s t h a t the s u s c e p t i b i l i t y o f a c i d h y d r o l y s i s to p o l a r e f f e c t s i s v i r t u a l l y zero and t h a t the t r a n s i t i o n s t a t e i n such a h y d r o l y s i s i s s a t u r a t e d w i t h r e s p e c t t o  (10)  conjugation, constant  E E  The  S  T a f t has  further defined a s t e r i c  substituent  by  s  =  log(k/k ) 0  order of decreasing  a l i p h a t i c and  ortho  E  (5)  A  s  values  so measured, both f o r  s u b s t i t u e n t s , conforms to the o r d e r  i n c r e a s i n g approach between the s u b s t i t u e n t and centre.  T a f t has  of  reaction  shown t h a t the s t e r i c s u b s t i t u e n t  constants  are determined by both p o t e n t i a l energy ( s t r a i n ) and k i n e t i c energy (hindrances  t o motion) s t e r i c e f f e c t s .  Certain reactions of ortho-substituted  compounds i n  which p o l a r e f f e c t s are s m a l l o r n e g l i g i b l e have been found to f i t a l i n e a r s t e r i c energy r e l a t i o n s h i p o f the log(k/k )  =  Q  &  i s a measure o f the  6E  (6)  S  s u s c e p t i b i l i t y of a given  s e r i e s to the s t e r i c requirements o f the By  combining equations (4)  more g e n e r a l  (6),  and  reaction  substituents. T a f t has  obtained  a  Hammett-type r e l a t i o n s h i p which should be a p p l i c -  a b l e to r e a c t i o n s i n which both p o l a r and vary  form  simultaneously  and  independently w i t h structure.,  CT*p* +  iog(k/k ) = G  steric effects  £E  S  The main f a u l t o f t h i s equation i s t h a t resonance and other than p o l a r and  s t e r i c are  (7) effects  excepted.  A s i m i l a r type o f r e l a t i o n s h i p has been used w i t h success by F a r t h i n g and Nam  (22)  to c o r r e l a t e r e a c t i o n r a t e and  equil-  (11) i b r i a o f a number o f o r t h o - s u b s t i t u t e d effect  o f an ortho s u b s t i t u e n t  reaction i s  compounds.  on the f r e e energy change o f a  taken as the sum o f the e l e c t r o n i c and  e f f e c t s o f the  The  steric  substituents.  Thus the Hammett e q u a t i o n may be w r i t t e n log (K/K ) 0  =  CT  E  p  +  E  <T  S  p  (8)  s  where the s u b s c r i p t s S and E r e f e r to s t e r i c and e l e c t r o n i c effects,  respectively.  CT^ i s assumed to be e q u a l to  the c o n v e n t i o n a l Hammett s u b s t i t u e n t uent i n the para p o s i t i o n . subject  constant f o r a  T h i s assumption,  substit-  although  to c r i t i c i s m , has o f t e n been made, and w i t h some  support.  Firstly,  the s i m i l a r i t y o f the  ortho s u b s t i t u e n t s to the Hammett assumption.  Secondly,  0~" v a l u e s  for  0~p l e n d support to  Charton ( 2 3 )  has found t h a t  this  the  e l e c t r i c a l e f f e c t s o f s u b s t i t u e n t s a r e s i m i l a r i n the ortho and para p o s i t i o n s .  He has s t u d i e d the e f f e c t  of  ortho s u b s t i t u e n t s X on the r e a c t i o n c e n t r e Y separated from the aromatic r i n g by a group Z.  (12) Assuming s t e r i c e f f e c t s to be absent i n r e a c t i o n s o f t h i s type,  Gharton has  found the e l e c t r i c a l e f f e c t s o f  ortho  s u b s t i t u e n t s to be o f the same order o f magnitude as those f o r para s u b s t i t u e n t s . by o b s e r v i n g the  These r e s u l t s may  be r a t i o n a l i z e d  t h a t the resonance e f f e c t o f a s u b s t i t u e n t i s  same i n the ortho and  para p o s i t i o n s and by assuming  the i n d u c t i v e e f f e c t a c t s p r i n c i p a l l y through the and  i s therefore equivalent  s i n c e i t may  i n the ortho  and  7T -bonds  para p o s i t i o n s (24).  a c t i n e i t h e r d i r e c t i o n around the r i n g  In view o f such behavior, the assumption o f F a r t h i n g Nam  does not  constant  seem too unreasonable.  <J~ i n equation (8)  ortho  substituent  i s d e f i n e d analogously  0  u s u a l Hammett meta and  The  and  para s u b s t i t u e n t constants  to  as  the  the  e f f e c t o f the s u b s t i t u e n t on the a c i d i o n i z a t i o n constant of benzoic a c i d .  The  s t e r i c s u b s t i t u e n t constant  Cfg i s  then d e f i n e d as the d i f f e r e n c e  The B a s i c i t i e s o f Very Weak Bases. Amongst those atoms commonly o c c u r r i n g i n  organic  compounds the most b a s i c a r e n i t r o g e n and oxygen. b a s i c i t i e s o f these atoms are due  The  to the unshared p a i r s o f  e l e c t r o n s which they possess i n t h e i r o u t e r s h e l l i n t h e i r normal c o v a l e n t  state.  Nitrogen,  as the more b a s i c o f the two  has  atoms.  as r e l a t i v e l y s t r o n g bases and  long been  recognized  Amines are w e l l known  t h e i r b a s i c i t i e s have been  r e a d i l y measureable i n d i l u t e aqueous s o l u t i o n w i t h i n  the  (13) common pH range. ethers,  Oxygen c o n t a i n i n g bases such as a l c o h o l s ,  aldehydes, ketones, c a r b o x y l i c a c i d s , amides, a c i d  h a l i d e s and e s t e r s , on t h e o t h e r hand a r e extremely weak bases.  S i n c e t h e r e had been no q u a n t i t a t i v e , measure o f the  a c i d i t y o f the v e r y a c i d i c media r e q u i r e d t o i o n i z e these bases u n t i l r e l a t i v e l y r e c e n t times, t h e b a s i c i t i e s o f these weak bases went unmeasured. of the H  Q  Only a f t e r t h e i n t r o d u c t i o n  a c i d i t y f u n c t i o n by Hammett (25) d i d such measure-  ments become f e a s i b l e . The  H  Q  a c i d i t y f u n c t i o n i s e s s e n t i a l l y an  extension  o f the pH s c a l e t o h i g h l y a c i d i c media and i s based on the i o n i z a t i o n o f uncharged bases i n such media.  The e q u i l i b r i a  so measured a r e t h e r e f o r e o f t h e form B: + E H  Q  f  i  > BH  +  i s d e f i n e d by the equation H  c  = log f B C  BH  +  pK +  (9)  BH  +  Cgand c + a r e the d i r e c t l y measureable c o n c e n t r a t i o n s B H  of  the i n d i c a t o r base and i t s conjugate a c i d , r e s p e c t i v e l y , and  Kgpj+  i s the thermodynamic i o n i z a t i o n constant o f t h e  conjugate a c i d o f the i n d i c a t o r i n terms o f and  r e f e r r e d t o d i l u t e aqueous s o l u t i o n .  concentrations  By measuring  the b a s i c i t i e s o f a s e r i e s o f i n d i c a t o r bases Hammett was a b l e , by a stepwise procedure t o d e r i v e t h e H over a wide range o f a c i d i t i e s .  Q  function  I t i s therefore  possible  (14) to measure the b a s i c i t i e s o f other i n d i c a t o r s o f  similar  s t r u c t u r e to those used to e s t a b l i s h the s c a l e simply  by  a r e p e t i t i o n o f Hammett s procedure. 1  One  disadvantage i n the H  Q  f u n c t i o n as i t stands,  a r i s e s from the c h o i c e o f the r e f e r e n c e I n d i c a t o r s . N i t r o g e n type bases were used as i n d i c a t o r s i n the more weakly a c i d r e g i o n s ; (corresponding were used;  i n the more s t r o n g l y a c i d r e g i o n  to about 70%  H2S0 [ ) oxygen c o n t a i n i n g bases i  f  f i n a l l y , a t n e a r l y 100$  i n g bases were again employed.  H S0^> n i t r o g e n c o n t a i n 2  Strictly  speaking,  only  the  b a s i c i t i e s o f compounds which a r e s t r u c t u r a l l y s i m i l a r t o and i o n i z e i n the same r e g i o n as the r e f e r e n c e i n d i c a t o r s can be c o n f i d e n t l y measured w i t h a c c u r a c y . o f the H  Q  Remeasurements  f u n c t i o n once u s i n g o n l y oxygen c o n t a i n i n g bases  and once u s i n g o n l y n i t r o g e n c o n t a i n i n g bases would appear very d e s i r a b l e . Using the concepts o u t l i n e d above, Hammett and have measured the b a s i c i t i e s o f a few n i t r o g e n and c o n t a i n i n g bases (25,  26,  27).  The  ratio  determined by s p e c t r o p h o t o m e t r y means.  c  B  coworkers oxygen  /cgjr+has been  Similar indicator  methods-have been used by D a v i s and Geissman (23) to measure the b a s i c i t i e s o f s e v e r a l f l a v a n o n e s ,  by Stewart and  Yates  and Edward to determine the b a s i c i t i e s o f acetophenones, benzaldehydes, benzoic a c i d s and benzamides (29,  30,  31>  32)  and by A r n e t t to measure the base s t r e n g t h s o f p h e n o l i c ethers  (33,  34).  P r a t t and Matsuda (35)  have measured the  relative  b a s i c i t i e s o f a number o f ketones, e s t e r s , ethers and  alcohols  (15) by measuring t h e e f f e c t s o f these bases on t h e r a t e s o f acid catalyzed reaction.  A method based on t h e d i s t r i b u t i o n  o f t h e base and i t s conjugate  a c i d between o r g a n i c and  aqueous l a y e r s has been used by P l a t t n e r t o measure t h e b a s i c i t y o f azulene  (36) and by A r n e t t  base s t r e n g t h s o f s a t u r a t e d e t h e r s . have a p p l i e d t h e techniques  (37)  t o measure t h e (38)  Gordy and.Stanford  o f i n f r a - r e d spectroscopy  to the  measurement o f weak bases by s t u d y i n g t h e e f f e c t s o f weak bases on t h e OD s t r e t c h i n g f r e q u e n c i e s o f Mode o f P r o t o n a t i o n o f the Carboxyl  deuterio-methanol.  Group.  Although t h e r e i s o n l y one l i k e l y mode o f p r o t o n a t i o n o f aldehydes and ketones, s i n c e t h e c a r b o n y l oxygen i s the o n l y b a s i c atom  present,  0 « R  l  -  +  H ^ " %  G  OH || Ha. - C - R  2  >  4  R  X  OH | - C - R  2  c a r b o x y l a c i d s can undergo p r o t o n a t i o n e i t h e r a t t h e c a r b o n y l oxygen t o g i v e s t r u c t u r e VI o r a t t h e ether oxygen t o g i v e structure VII. +  0H  0  II  R - C - OH  <  >  .OH R-C+ 0H X  vi  a  VI  b  . '  || t R _ c - OHo 2  VII  (16) In t h e p a s t , t h e r e has been l i t t l e evidence t o f a v o u r one s t r u c t u r e o r t h e o t h e r and both s t r u c t u r e s have  appeared  equally often i n the l i t e r a t u r e . R e c e n t l y , t h e b a s i c i t i e s o f a number o f meta- and p a r a - s u b s t i t u t e d b e n z o i c a c i d s have been measured by Yates (39).  T h i s worker has found t h a t t h e b a s i c i t i e s o f these  a c i d s c o r r e l a t e b e s t w i t h the Hammett equation i f the  0~*  v a l u e s o f Brown and Okamoto ( 4 0 ) a r e used i n s t e a d o f t h e o r i g i n a l Hammett  CT v a l u e s .  The  C T s u b s t i t u e n t constants  a r e a new s e t o f constants suggested by Brown f o r use when a s u b s t i t u e n t can e n t e r i n t o d i r e c t c o n j u g a t i o n w i t h a p o s i t i v e charge a t t h e r e a c t i o n c e n t r e .  The c o r r e l a t i o n o f  the b a s i c i t i e s o f t h e a c i d s w i t h t h e  values i s therefore  (J  evidence i n f a v o u r o f s t r u c t u r e VT as the predominating p r o t o n a t e d s p e c i e s s i n c e a s u b s t i t u e n t i n t h e ortho o r para p o s i t i o n can conjugate d i r e c t l y w i t h the p o s i t i v e charge i n VI but cannot do so i n V I I . A d d i t i o n a l evidence f o r s t r u c t u r e VI was found i n a l i n e a r r e l a t i o n s h i p between the P % u i n g acetophenones.  +  v a l u e s o f the correspond-  T h i s l i n e a r r e l a t i o n s h i p i s an i n d i c a t i o n  o f t h e s i m i l a r i t y i n t h e modes o f p r o t o n a t i o n o f t h e two c l a s s e s o f compounds and s i n c e acetophenones can p r o t o n a t e o n l y on the c a r b o n y l oxygen, i t i s an i n d i c a t i o n o f c a r b o n y l oxygen p r o t o n a t i o n i n the b e n z o i c a c i d s .  I t should be  emphasized, however, t h a t although VT i s t h e more important form, both forms p r o b a b l y e x i s t i n e q u i l i b r i u m i n s o l u t i o n . I t was found a l s o t h a t the  p  v a l u e f o r a Hammett  plot  (17) o f the b a s i c i t i e s o f the b e n z o i c a c i d s was much lower than the  p  v a l u e s f o r s i m i l a r p l o t s f o r the c o r r e s p o n d i n g  benzaldehydes and acetophenones.  T h i s has been i n t e r p r e t e d  to i n d i c a t e the g r e a t e r importance o f resonance w i t h i n the p r o t o n a t e d c a r b o x y l group and thus the g r e a t e r importance o f s t r u c t u r e Yla  than  VTb.  (13) OBJECTS OF THE PRESENT RESEARCH The o b j e c t o f t h i s r e s e a r c h has been to e l u c i d a t e the e f f e c t s o f s t e r e o e l e c t r o n i c f a c t o r s on the s t r e n g t h s o f v e r y weak oxygen c o n t a i n i n g o r g a n i c b a s e s .  With t h i s aim i n mind,  two major groups o f o r g a n i c compounds have been s t u d i e d . The b a s i c i t i e s o f a s e r i e s o f mono-ortho-substituted benzoic a c i d s and a s e r i e s o f benzophenone d e r i v a t i v e s and analogues have been measured.  An attempt w i l l be made t o determine  which, i f any, f r e e energy r e l a t i o n s h i p s the p r o t o n a t i o n o f the o r t h o - s u b s t i t u t e d a c i d s and ketones f o l l o w and what f a c t o r s account f o r the anomalous e f f e c t s o f ortho s u b s t i t u e n t s i n this series.  A j u s t i f i c a t i o n o f the d i r e c t i o n and magnitude  o f the ortho e f f e c t s may then be p o s s i b l e .  Through a study  o f the b a s i c i t i e s o f the benzophenone analogues such as f l u o r e n o n e and anthrone, the e f f e c t s o f m o l e c u l a r shape on b a s i c i t i e s w i l l be examined.  (19)  METHODS OF APPROACH.  Two g e n e r a l methods have been used t o e v a l u a t e the magnitude and d i r e c t i o n o f t h e e f f e c t s o f ortho s u b s t i t u e n t s on t h e b a s i c i t i e s o f t h e b e n z o i c a c i d s .  The f i r s t method i s  based on t h e assumption o f e q u a l i t y o f e l e c t r i c a l e f f e c t s o f s u b s t i t u e n t s i n t h e o r t h o and para p o s i t i o n s .  The second  oi - s u b s t i t u t e d  method which uses t h e a c i d i t y c o n s t a n t s o f  p y r i d i n e s as a standard o f r e f e r e n c e has been used by McDaniel and Brown t o q u a n t i t a t i v e l y e v a l u a t e t h e e f f e c t s o f ortho s u b s t i t u e n t s i n a number o f aromatic  e q u i l i b r i u m systems (10).  S i n c e t h e e f f e c t o f any s u b s t i t u e n t s on a r a t e o r e q u i l i b r i u m process should be made up o f c o n t r i b u t i o n s from s t e r i c and e l e c t r o n i c e f f e c t s , and i n view o f t h e known s i m i l a r i t y o f t h e e l e c t r i c a l e f f e c t s o f s u b s t i t u e n t s i n t h e ortho and para p o s i t i o n s , as mentioned e a r l i e r , the d i f f e r e n c e i n the b a s i c i t i e s o f o r t h o - and p a r a - s u b s t i t u t e d b e n z o i c a c i d s should serve as a t l e a s t a q u a l i t a t i v e i n d i c a t i o n o f t h e magnitude and d i r e c t i o n o f t h e s t e r i c o r ortho e f f e c t s o f the s u b s t i t u e n t s . This difference  A  g  i s d e f i n e d as  Ag P o s i t i v e values o f A  = p K + (Ortho) - p K + B H  B  B H  (Para)  i n d i c a t e a base s t r e n g t h e n i n g ortho  e f f e c t whereas n e g a t i v e v a l u e s i n d i c a t e a base weakening effect.  The v a l u e s o f A g. f o r t h e s u b s t i t u e n t s s t u d i e d  i n t h i s work have been determined  using the b a s i c i t i e s of  (20) the o r t h o - s u b s t i t u t e d benzoic a c i d s measured In t h i s work and  the b a s i c i t i e s o f the p a r a - s u b s t i t u t e d  by Stewart and Yates  a c i d s measured  (31)»  A procedure s i m i l a r to t h i s has been used by T a f t i n comparing the r a t e s o f m e t h a n o l y s i s and h y d r o l y s i s o f and  para-substituted  ortho-  1-menthyl benzoates and  e t h y l benzoates  r e s p e c t i v e l y (41).  T a f t assumed the r a t i o k  p  measure o f the ortho  e f f e c t and  order  CH3O < C l < CH3  < Br < N0  /k  to be  Q  found i t to i n c r e a s e i n the 2  i n c r e a s i n g s t e r i c requirements.  which i s the o r d e r  of  An assumption which i s  i m p l i c i t i n both these procedures i s t h a t the v a l u e s o f p  r e a c t i o n constant para-substituted  are i d e n t i c a l f o r both the o r t h o -  compounds.  That t h i s may  i s i n d i c a t e d by the f a c t t h a t the i o n i z a t i o n o f meta- and water i s 1.000  ^  p  the and  not always be  so  value f o r the a c i d  para-substituted  whereas the  a  benzoic acids i n  v a l u e f o r the i o n i z a t i o n o f  o r t h o - s u b s t i t u t e d b e n z o i c a c i d s i s 1.787. The ortho  other method which has been used to e v a l u a t e  the  e f f e c t s o f s u b s t i t u e n t s on the b a s i c i t i e s o f benzoic  a c i d s i s t h a t used by McDanlel and Brown (10) who the a c i d i o n i z a t i o n constants  have used  o f the conjugate a c i d s o f  s u b s t i t u t e d p y r i d i n e s as a r e f e r e n c e  system.  f o l l o w i n g evidence i t i s assumed t h a t the  On  the  0< - s u b s t i t u t e d  p y r i d i n e system i s r e l a t i v e l y f r e e o f ortho e f f e c t s . Firstly,  s t e r i c i n h i b i t i o n o f resonance i s i m p o s s i b l e  p y r i d i n e system and be  i n the  secondly, hydrogen bonding e f f e c t s should  s l i g h t except f o r a few  substituents.  Measurements o f  (21) the p K s o f mono-substituted methyl, e t h y l , i - p r o p y l , and T  a  t - b u t y l p y r i d i n e s i n d i c a t e t h a t F - s t r a i n i n the P< - s u b s t i t u t e d p y r i d i n e s i s n e g l i g i b l e except f o r p o s s i b l y v e r y b u l k y s u b s t i t u e n t s such as t - b u t y l ( 4 2 ) .  S o l v a t i o n e f f e c t s and  s t e r i c hindrance t o s o l v a t i o n a l s o appear t o be unimportant except i n t h e cases o f the most b u l k y s u b s t i t u e n t s .  Accord-  i n g t o McDaniel and Brown, i f the s u b s t i t u t e d p y r i d i n e s do form a system  e s s e n t i a l l y f r e e o f ortho e f f e c t s , then  d e v i a t i o n s from l i n e a r i t y on a p l o t between the p K  &  values  o f s u b s t i t u t e d p y r i d i n e s and those o f the c o r r e s p o n d i n g s u b s t i t u t e d aromatic a c i d o r base should p r o v i d e a q u a n t i t a t i v e measure o f the ortho e f f e c t o f the s u b s t i t u e n t i n the p a r t i c u l a r o r g a n i c a c i d o r base under c o n s i d e r a t i o n . The r e l a t i v e v i r t u e s o f both these methods a r e d i s c u s s e d at a l a t e r  point.  (22)  EXPERIMENTAL Ultraviolet 1.  Spectra and Determination o f B a s i c i t i e s .  Materials (a)  Benzoic A c i d s Ortho-substituted benzoic a c i d s a v a i l a b l e  commercially were p u r i f i e d e i t h e r by s e v e r a l o r by vacuum d i s t i l l a t i o n .  recrystallizations  Samples o f o - e t h y l , o - i s o p r o p y l ,  o - t e r t i a r y b u t y l b e n z o i c a c i d s were k i n d l y s u p p l i e d by Dr. H a r o l d Schechter o f Ohio S t a t e U n i v e r s i t y . constants o f the p u r i f i e d acids together with  The p h y s i c a l literature  v a l u e s a r e l i s t e d i n Table I . (b)  Benzophenone Analogues Benzophenones a v a i l a b l e commercially were p u r i f i e d  by s e v e r a l r e c r y s t a l l i z a t i o n s  and t h e i r m e l t i n g p o i n t s  compared w i t h l i t e r a t u r e v a l u e s .  2,3;6,7-dibenz-2,6-  c y c l o h e p t a d l e n e - l - o n e (2,3r6,7-dibenzosuberone), ( X I ) , was prepared from b e n z a l p h t h a l i d e , ( V I I I ) , method o f Cope and Fenton  (48).  (47)  by the  2,3;6,7-dibenzotropone,  ( X I I I ) , was prepared from the suberone by bromination w i t h N-bromo-succinimide and subsequent t r i e t h y l a m i n e (4#). below.  dehydrobromination  with  The s y n t h e t i c schemes a r e o u t l i n e d  The p h y s i c a l c o n s t a n t s o f the ketones a r e l i s t e d  i n Table I I .  (23)  (25)  Table I PHYSICAL CONSTANTS OF BENZOIC ACIDS  Benzoic A c i d  M.p, o r B.p. and R e f r a c t i v e Index  L i t e r a t u r e Value o f P h y s i c a l Constants  H  M.p. 121-122  M.p. 122 (43)  0-CH3  M.p. 100-101  M.p. 104  (43)  M.p. 65.0-65.5  M.p. 65.6  (44)  O-i-C^Hy  M.p. 57.5-58.5  M.p. 59.4-59.9 1 (44)  o-t-C H  M.p. 68.5-69.0  M.p. 68.5  (45)  o-F  M.p. 121-122  M.p. 122  (43)  o-Cl  M.p. 143-144  M.p. 142  (43)  o-Br  M.p. 147.5-149  M.p. 147-150  (43)  o-I  M.p. 159-160  M.p. 162  (43)  o-OH  M.p. 157-158  M.p. 159  (43)  0-CH3O  M.p. 97-98  M.p. 98  (43)  0-C2H5O  B.p. 150-155 (1mm.) = 1.5410  B.p. 216-229 (90mm.)  (46)  o-N0  M.p. 147-148  M.p. 147.5  (43)  o-C0 H  M.p. 219-219.5  M.p. 206-208  (43)  m-C0 H  M.p. 346-348  M.p. 330  (43)  p-C0 H  Subl . c a . 300  Subl . c a . 300(43)  M.p. 161.5-162.5  M.p. 160  o-C H 2  5  4  2  2  2  2  a  CX-C4H4  9  a  <* -naphthyl  (43)  (26)  Table I I PHYSICAL.CONSTANTS OF BENZOPHENONES  Ketone  M.p. o r B.p. and R e f r a c t i v e Index  Benzophenone  M.p.  49-49.5  M.p.  47-48  4,4 -dimethoxy benzophenone  M.p.  142-143  M.p.  142-143  9-Fluorenone  M.p.  80-81.5  M.p.  83.0-83.5  (51)  9-Anthrone  M.p.  153-156  M.p.  154-155  (52)  2,3;6,7-dibenzosuberone  B.p. 149-151 (0.3mm.) B.p. 148 (0.3mm.) (48) n** = 1.6330 n j = 1.6332  2,3r6,7-dibenzo-  M.p. 88-89  ,  tropone  L i t e r a t u r e Value o f P h y s i c a l Constants  (49) (50)  r  M.p. 87.6-88.5 (48)  (27) (c)  Sulphuric Acids  Standard  aqueous s u l p h u r i c a c i d s o l u t i o n s r a n g i n g from  45$ t o 96$ by weight H^SO/,. were prepared by s u i t a b l e o f F i s h e r C.p. Reagent grade s u l p h u r i c a c i d  (96$ m i n . ) .  The c o n c e n t r a t i o n s o f each s o l u t i o n were determined o f weighed samples w i t h 0 . 1 N standard sodium ( s t a n d a r d i z e d w i t h potassium  Long (53).  by t i t r a t i o n  hydroxide  a c i d p h t h a l a t e , A n a l a r Reagent)  using phenolphthalein i n d i c a t o r . s o l u t i o n were determined  dilution  The H  Q  v a l u e s o f each  from t h e standard curves o f P a u l and  The, a c i d s o l u t i o n s were s t o r e d i n t h e dark i n  ground g l a s s - s t o p p e r e d b o t t l e s and t h e i r c o n c e n t r a t i o n s were checked p e r i o d i c a l l y . 2.  Measurement o f U l t r a v i o l e t  Spectra.  A sample o f each b e n z o i c a c i d and benzophenone was weighed on a microbalance  i n a 50 m l . v o l u m e t r i c f l a s k and  -3 d i s s o l v e d i n s p e c t r a l grade acetone t o prepare a 10  M stock  s o l u t i o n from which 0 . 1 ml. a l i q u o t s were p i p e t t e d i n t o 5 o r 10 ml. v o l u m e t r i c f l a s k s .  The acetone was removed under  vacuum and t h e f l a s k s made up t o volume w i t h s u l p h u r i c a c i d o f the d e s i r e d c o n c e n t r a t i o n , r e s u l t i n g i n a s u b s t r a t e c o n c e n t r a t i o n o f the o r d e r o f 10 ^ M, s u i t a b l e f o r t h e measurement o f u l t r a v i o l e t spectra."  A number o f such s o l u t i o n s  o f a c i d s t r e n g t h s necessary t o produce from 0 t o 100$ p r o t o n a t i o n o f t h e s u b s t r a t e were p r e p a r e d . In cases o f i n s u f f i c i e n t water s o l u b i l i t y o f the u n i o n i z e d form o f the s u b s t r a t e , s t o c k s o l u t i o n s were made up i n e t h a n o l and 0 . 0 5 m l . a l i q u o t s were p i p e t t e d i n t o 10 ml. v o l u m e t r i c f l a s k s u s i n g a 0 . 1 0 0 ml. Hamilton  s y r i n g e w i t h a Chaney a d a p t i o n .  (28) In these cases t h e e t h a n o l was not evaporated but l e f t i n t h e f l a s k a f t e r the addition o f the a c i d .  A suitable  correction  was made f o r t h e d i l u t i o n o f the a c i d by t h e e t h a n o l .  I t was  found t h a t t h e presence o f t h e e t h a n o l i n t h e a c i d i n no way a f f e c t e d the u l t r a v i o l e t s p e c t r a o f the s u b s t r a t e and t h a t identical P % H +  v a l u e s ( w i t h i n experimental e r r o r ) c o u l d be  o b t a i n e d by both methods.  Since t h e l a t t e r method minimizes  e r r o r s due t o s o l u b i l i t y d i f f i c u l t i e s , i t i s recommended  that  t h i s method be used e x c l u s i v e l y i n t h e f u t u r e whenever s o l u b i l i t y problems a r e suspected.  The o n l y compound  encountered i n t h i s work whose s o l u b i l i t y was too s m a l l t o enable i t s d i s s o l u t i o n i n the more d i l u t e a c i d s o l u t i o n s i n q u a n t i t i e s s u f f i c i e n t f o r s p e c t r a l measurement, even u s i n g c e l l s o f 10 cm. path l e n g t h , was benzanthrone.  For t h i s  reason, i t s b a s i c i t y c o u l d not be measured. The u l t r a v i o l e t  s p e c t r a o f the unprotonated and protonated  b e n z o i c a c i d s and ketones i n 45$ and 96$ H^SO^  respectively  were measured on a Cary model 14 r e c o r d i n g spectrophotometer u s i n g 1 cm. quartz c e l l s and the wavelengths t i o n o f both the unprotonated s p e c i e s , thereby l o c a t e d . c o r r e s p o n d i n g t o the TT  o f maximal absorp-  ( ?^ ) and p r o t o n a t e d u  The wavelengths  (?i^)  chosen were those  > IT e l e c t r o n i c t r a n s i t i o n s r a t h e r  than those c o r r e s p o n d i n g t o the n  > 7T t r a n s i t i o n s  since  the former peaks a r e u s u a l l y b e t t e r d e f i n e d and l e s s s u b j e c t to medium e f f e c t s .  The u l t r a v i o l e t s p e c t r a o f the u n i o n i z e d  and i o n i z e d forms o f 9-fluorenone each showed two w e l l d e f i n e d a b s o r p t i o n maxima.  Two pKj3g+ v a l u e s were thus o b t a i n e d  (29) depending on the wavelengths  chosen f o r 7\  u  and  7\^„  absence o f medium e f f e c t s these v a l u e s should be the d i f f e r e n c e between them may p r e c i s i o n o f the r e s u l t s (32).  In the  identical;  be taken as a measure o f the A s o l v e n t b l a n k o f the same  a c i d c o n c e n t r a t i o n as t h a t o f the sample s o l u t i o n was used i n each c a s e .  The o p t i c a l d e n s i t i e s o f each i n d i v i d u a l  solution,  r e p r e s e n t i n g v a r i o u s stages o f i o n i z a t i o n o f the s u b s t r a t e , were measured i n 1 cm.  quartz c e l l s a t ^  u  and  Beckman model DU spectrophotometer, whose c e l l was m a i n t a i n e d a t 25°C by  7^  with a  compartment  thermospacers.  To make sure t h a t the s u b s t r a t e was undergoing no i r r e v e r s i b l e changes such as s u l p h o n a t i o n i n the s t r o n g a c i d s , the spectrum o f a 96%  a c i d s o l u t i o n o f the s u b s t r a t e was  measured on the Cary immediately a f t e r mixing and then again about one hour l a t e r . e s s e n t i a l l y unchanged. to 45$  In a l l cases, the spectrum The 96% a c i d s o l u t i o n was  was then d i l u t e d  and the spectrum o f the r e s u l t i n g s o l u t i o n measured.  In a l l cases, the spectrum o f t h i s s o l u t i o n compared c l o s e l y with t h a t o f a 45$ c o n t r o l s o l u t i o n and a l s o with t h a t o f the s u b s t r a t e i n water o r e t h a n o l . R e p r e s e n t a t i v e samples o f u l t r a v i o l e t s p e c t r a l data are shown i n T a b l e s I I I and  IV.  (30)  Table I I I  ULTRAVIOLET SPECTRAL DATA ON THE BASICITY OF SALICYLIC ACID.  %H S0^  -H  45.1  2.86  0.702  0.083  -0.619  64.5  5.00  0.750  0.148-  -0.602  70.0  5.67  0.763  0.222  -0.541  73.9  6.15  0.679  0.388  -0.291  76.9  6.55  0.608  0.626  0.018  6.93  0.505  0.894  O.389  84.0  7.48  0.365  1.220  0.855  88.3  8.03  0.285  1.240  0.955  95.6  8.97  0.237  1.270  1.033  2  79.9  D  G  D  u  Substrate concentration  /V  u  -  239  y  AD  ±  « 7.91 x 10~-> M  *± ~  260  (31)  Table IV  ULTRAVIOLET SPECTRAL DATA ON THE BASICITY OF BENZOPHENONE  $H S0 2  -H  4  D  0  Di  u  AD  45.4  2.90  0.730  0.008"  -0.722  59.3  4.38  0.690  0.020  -0.670  64.3  4.98  0.619  0.051  -O.567  69.6  5.62  0.475  0.196  -0.279  73.6  6.13  0.300  0.452  0.152  75.8  6.42  0.17S  0.590  0.412  79.2  6.8"7  0.110  0.340  0.730  81.7  7.19  0.063  0.939  0.376  36.2  7.77  0.055  1.000  . 0.945  . 94.I  3.73  0.053  1.065  1.012  Substrate C o n c e n t r a t i o n  ^  u  - 262  vyA  B  4.45 x 10~5 M  ^ i ~ 344 xyj  (32) 3.  Treatment o f Data. The b a s i c i t i e s o f the b e n z o i c a c i d s and benzophenones  were determined from t h e u l t r a v i o l e t s p e c t r a l data by two methods. (a)  G r a p h i c a l Method T h i s method o f d e t e r m i n a t i o n o f p % ^  +  v a l u e s from  u l t r a v i o l e t s p e c t r a l data i s e s s e n t i a l l y t h a t o f Davis and Geissman ( 2 8 ) .  In the f o l l o w i n g c a l c u l a t i o n s ,  optical  d e n s i t i e s a r e used i n p l a c e o f m o l e c u l a r e x t i n c t i o n  coefficients.  T h i s procedure i s j u s t i f i e d s i n c e the c o n c e n t r a t i o n o f t h e stock s o l u t i o n f o r a g i v e n compound i s t h e same throughout. I - f r a c t i o n o f t h e s u b s t r a t e i n the i o n i z e d form.  If ^  u  - wavelength o f maximal a b s o r p t i o n o f the u n i o n i z e d form. « wavelength o f maximal a b s o r p t i o n o f the i o n i z e d form.  D = o p t i c a l d e n s i t y o f a m i x t u r e o f the i o n i z e d and u n i o n i z e d forms. D  u  * o p t i e a l d e n s i t y o f the u n i o n i z e d form.  D^ - o p t i c a l d e n s i t y o f the i o n i z e d form, then assuming B e e r s Law t o h o l d under the experimental T  conditions,  S u b t r a c t i n g ( i i ) from ( i ) and AD  »  (D)>-  letting - (0)-^  (33) Since values constant  such as (D )>^ may be assumed t o be n e a r l y u  over a s m a l l range o f a c i d c o n c e n t r a t i o n i n t h e  neighbourhood o f 5 0 $ i o n i z a t i o n , t h e f r a c t i o n i o n i z e d i s l i n e a r l y r e l a t e d t o AD. Thus ( i i i ) may be rearranged I  and w r i t t e n a s  « K + c AD  1  where  and  K  Since  H  Q  then  H  Q  *. l o g  =  [unionized! [ ionized"]  b In [  « b In  i I  fl  Q  + pK  PK H  +  B  + B H  - K - cADjf K + cAD  A p l o t o f A D against H slope dH  i7  +"  Q  w i l l be a sigmoid-type curve o f  /dAD, and a t the i n f l e c t i o n  point  (34) Therefore,  at the i n f l e c t i o n  1 -  A D + KJ  fraction unionized «  or and  [c  point  thus The  H inflection  i n each  of the  sigmoid curve.  sigmoid-type  may  due  be  AD  curve  of "straight"  gave a v e r y poor  on t h e p K g f j  to the  +  tangent  point.  was  portion  symmetrical. 3)  (Figure  For t h i s reason,  shift  (11  little  T h i s anomaly  mjm ) on p r o t o n a t i o n .  curves, the i n f l e c t i o n p o i n t  determined  t o t h e u p p e r and  by  lower  t a k i n g the I n f l e c t i o n  between t h e s e t a n g e n t s  curve  v a l u e so d e t e r m i n e d .  small s p e c t r a l  c o u l d a l t e r n a t i v e l y be  c u r v e and  versus  c u r v e s , a l t h o u g h some c u r v e s were n o t  In the cases o f symmetrical  lines  Q  A l l t h e compounds s t u d i e d gave  w i t h an i n d e f i n i t e i n f l e c t i o n i s put  +  case as the m i d - p o i n t  Ortho-nitro benzoic acid  weight  ionized  B  point o f the H  chosen  AD + K  fraction  PK H  s 0  C.  =  along the  drawing "flat"  two  straight  p o r t i o n s o f the  p o i n t a s t h a t p o i n t midway " v e r t i c a l " portion of  the  curve. R e p r e s e n t a t i v e p l o t s o f the d a t a t r e a t e d by are  shown i n F i g u r e s 1 a n d The  t o be  determined  species.  e r r o r s due weighing  2.  method w h i c h has b e e n d e s c r i b e d e n a b l e s p % j j + w i t h good a c c u r a c y  e r a b l e o v e r l a p of the ated  t h i s method  I t has  values  e v e n when t h e r e i s c o n s i d -  s p e c t r a o f t h e p r o t o n a t e d and  unproton-  the a d d i t i o n a l advantages o f m i n i m i z i n g  t o c h a n g e s i n medium e f f e c t s and  e r r o r s due  to  o f s a m p l e s and measurement o f s o l u t i o n v o l u m e s s i n c e  (35)  CONCN.=7.91x10"S>M.  0.8  a*  AD  pK  B H  +=-6.80  -H,  -Oiff 3  +r. 0  +  *.0  » —  5.0  H  6.0  H  7.0  FIG.1-CURVE O F AD VERSUS F O R S A L I C Y L I C ACID.  1-  8.0  H  Q  to  (36)  I.3L  +  CONCN.=  0.8  +  O.f  +  4.45x10" M 5  AD o  +  pK  -o.»  a  u  +  = -6.20  +  - H ,  -o.g  H  3.0  -i  V.O  FIG.2-CURVE FOR  1—  S.O  OF  1-  I  (,.0  AD  7.0  VERSUS  BENZOPHENONE  I  10  8.0  Ho  01)  CONCN.=1.58x10" M. 4  0.0+  +  o  +  -0.0H  +  AD  pKou =-718 +  -o.o 8  4  i 30  i i*.0  -H 1  5.0  t6.0  o  H  7.0  1-  1.0  $.0  FIG.3-ANOMALOUS C U R V E O F AD VS. H O R T H O - N I T R O BENZOIC ACID.  Q  FOR  (3d)  the d i f f e r e n c e o f two o p t i c a l d e n s i t i e s i s used. disadvantage  The major  i s t h e a r b i t r a r y method which must be used to  locate the i n f l e c t i o n point. (6)  L i n e a r Method T h i s method, which makes use o f the same data as t h e  g r a p h i c a l method, has been d e s c r i b e d by Yates ( 3 9 ) . Let  AD  (DL.  = B  - (Dk  A<  i n an a c i d c o n c e n t r a t i o n where  Alt  the f r a c t i o n o f t h e base i o n i z e d is effectively AD g  - (D)^. - (^jVu  +  B  "*"  na  n  ac  zero,  ^-^ c o n c e n t r a t i o n where  the f r a c t i o n o f t h e base i o n i z e d is effectively and  AD  =  ( D L . - (Dk  zero,  i n a sample s o l u t i o n c o n t a i n i n g a m i x t u r e o f both forms.  Then  AD  = (D )^ - (D )  B  u  u  A D H = (Di)^. - ( D i )  X u  +  B  and  Thus,  AD = [ ( D ^ . - ( D ) J + [ ( D ^ . AD  + B H  (1-1) [ ( D i ) ^ - ( D ) . t  -AD -AD  U  =  u H  0  B  H  +  -  r  +  u  (D ) u  (Di) J A  - (Di)^]  k  r  K  i o n i 2 e  A  D  against H -  (D )^-  [unionized] , , -_ t * d] P BH  a plot o f log AD  u  f r a c t i o n unionized fraction ionized  - i«„ - log A D  A  I f ( D i ) ^ - (Du)^ +  B  (D )^ - (  - (D^. +  U  AD  GSince  A u  A D  0  should g i v e a s t r a i g h t  B  T i n e o f u n i t s l o p e w i t h the H  Q  i n t e r c e p t equal t o P % H  +  D i  ]Jl  (39) T y p i c a l p l o t s u s i n g t h i s procedure are shown In F i g u r e s 4 and  5.  The advantage o f t h i s method o v e r the g r a p h i c a l method i s t h a t the need f o r a r b i t r a r i l y choosing t h e i n f l e c t i o n p o i n t o f a curve i s e l i m i n a t e d s i n c e the b e s t s t r a i g h t may  be found by the method o f l e a s t squares.  The  line  linear  method, however, i s not a p p l i c a b l e when data i s used which, when p l o t t e d by the g r a p h i c a l method, g i v e s unsymmetrical sigmoid c u r v e s , s i n c e i n these cases s t r a i g h t l i n e s a r e not obtained.  In some cases i t was  found t h a t the p o i n t s on a  p l o t u s i n g the l i n e a r method c o u l d b e s t be j o i n e d by  two  i n t e r s e c t i n g s t r a i g h t l i n e s r a t h e r than by one s t r a i g h t In these cases the H s l o p e was  Q  line.  i n t e r c e p t o f t h a t l i n e o f the two whose  n e a r e r u n i t y was  taken as the pKgH'*" v a l u e .  In a l l  such cases, t h i s l i n e corresponded to the more symmetrical h a l f o f the sigmoid c u r v e .  I t was  found too, t h a t even  when good s t r a i g h t l i n e s were o b t a i n e d by the l i n e a r method, the s l o p e s o f these l i n e s o f t e n d i f f e r e d c o n s i d e r a b l y from unity.  T h i s phenomenon i s most l i k e l y due to a  combination  o f medium e f f e c t s and a d e v i a t i o n o f the i o n i z a t i o n process from t h e o r e t i c a l b e h a v i o r . To o b t a i n the r e s u l t s which f o l l o w , both o f the above methods have been used and the r e s u l t s averaged u n l e s s s t a t e d otherwise.  (W>)  H  Q  INTERCERT = -6.75  S L O P E = 1.11 C O N C N . = 7>91x10~ M. 5  13  <3 i  Of  CD  B  4  i  18  o  -04  4  -o/?  4  FIG.4-LINEAR VERSUS  H  Q  BASICITY  FOR  PLOT O F  L Q G ^ f e f f i  SALICYLIC ACID.  A  D  _  A  D  b  H INTERCEPT=-6.12 SLOPE = 0.87 CONCN. = 4.45x10~ M. 0  5  1.6  |.2  0.8  «>  "  ex 0  i  a 3  0.4- . •  cr-> O  -0.8  •  S.O  6.0  7.0  AP)  _AT Q L  FIG.5-LINEAR BASICITY PLOT OF L O G ^ AD-AD, B VERSUS H FOR BENZOPHENONE. Q  (42) 4.  Estimated E r r o r i n B a s i c i t y Determinations. D a v i s and  Geissman (28)  suggest t h a t pK g+ B  determined by t h e i r method are accurate unit.  Yates (39)  f e e l s t h a t the v a l u e s  method are l i k e l y accurate and  to 0.05  good p l o t s are o b t a i n e d .  t h a t most o f t h e p K f f  +  B  v a  values  t o a t l e a s t 0.1 obtained  by  u n i t where simple  H  Q  either spectra  However, i n view o f the f a c t  l © s measured were i n the  region  u  70 to iQffo s u l p h u r i c a c i d , which i s the r e g i o n i n which the H  G  and  s c a l e i s considered  to be not f i r m l y e s t a b l i s h e d  i n view o f p o s s i b l e e r r o r s i n s t a n d a r d i z i n g  acids i n t h i s high concentration  B  +  values  between d u p l i c a t e runs may  sulphuric  range, i t does not seem  unreasonable to estimate an a b s o l u t e i n the measured p K f i  (53)  e r r o r o f 0.1H  even though t h e  0  unit  deviations  be much l e s s than t h i s .  (43)  RESULTS AND  DISCUSSION  P a r t I - Benzoic A c i d s . The b a s i c i t y constants and a b s o r p t i o n maxima o f the o r t h o - s u b s t i t u t e d b e n z o i c a c i d s s t u d i e d i n t h i s work are l i s t e d i n T a b l e V.  I t i s emphasized t h a t although t h e  v a l u e s are g i v e n to 0 . 0 1  H  c  to be a c c u r a t e o n l y to - 0 . 1  P%g  u n i t , the w r i t e r c o n s i d e r s them unit.  In o r d e r t o compare the  e f f e c t s o f o r t h o s u b s t i t u e n t s w i t h those o f meta and  para  s u b s t i t u e n t s on the base s t r e n g t h s o f b e n z o i c a c i d s ,  the  b a s i c i t i e s o f the meta- and p a r a - s u b s t i t u t e d a c i d s by Stewart and Yates  +  ( 3 1 ) are l i s t e d i n T a b l e VT.  determined I t should  be noted t h a t the pKgrj+ v a l u e o f b e n z o i c a c i d measured by Stewart and Yates d i f f e r s by approximately 0 . 1 t h a t found by t h i s worker.  unit  from  T h i s presumably-is due to e r r o r s  i n the s t a n d a r d i z a t i o n o f one o f the s e t s o f standard s u l p h u r i c acids.  In o r d e r to e l i m i n a t e t h i s d i s c r e p a n c y and to  simplify  the comparison o f the b a s i c i t i e s o f a l l the a c i d s , the v a l u e s f o r the meta- and p a r a - s u b s t i t u t e d a c i d s have been brought i n t o l i n e w i t h those f o r t h e o r t h o - s u b s t i t u t e d a c i d s by 0 . 0 9 to the v a l u e f o r each meta- and p a r a - s u b s t i t u t e d  adding acid.  These " c o r r e c t e d " v a l u e s a r e l i s t e d i n T a b l e VI under the headings  ' -pKgjj+ Meta ( C o r r . ) " and "-pKgjj Para r  +  The f a c t t h a t one  (Corr.)".  set o f v a l u e s r a t h e r than the o t h e r  was  " c o r r e c t e d " should not be taken as an I n d i c a t i o n o f any preference.  The v a l u e s o f Stewart and Yates are i n f a c t  supported by the f a c t t h a t the base s t r e n g t h o f benzoic  acid  (44) Table V B a s i c i t y Constants,  A b s o r p t i o n Maxima and Slopes o f L i n e a r  B a s i c i t y P l o t s o f S u b s t i t u t e d Benzoic  Acids. I n 4 5 $ H 2 4 I n 96$H S04  -pK +  S 0  2  BH  Substituent  Gra.  Lin.  Ave.  Slope ''max  log max  Hydrogen  7.19  7.14  7.17  1.00  233  4.00  261  4.14  o-Methyl  7.15  7.08  7.12  1.15  234  3.33  265  4.13  o-Ethyl  7.15  7.10  7.13  1.03  233  3.33  266  4.14  o-i-Propyl  7.25  7.21  7.23  1.09  233  3.71  265  4.06  o-t-Butyl  7.57  7.64  7.61  0.95  232(s )3.36  265  3.73  o-Fluoro  7.54  7.65  7.60  1.12  229  3.97  257  4.13  o-Chloro  7.64  7.70  7.67  0.99  233  3.79  263  4.11  o-Bromo  7.75  7.70  7.73  1.20  236°  3.30  265  4.04  o-Iodo  7.72  7.80  7.76  1.53  232(s ) 3 . 3 4 266  3.37  o-Hydroxyl  6.80  6.75  6.73  1.11  237  3.95  260  4.20  o-Methoxyl  6.04  6.18  6.11  0.32  240  3.92  262  4.20  o-Ethoxyl  6.05  6.18  6.11  0.33  240  3.91  262  4.15  o-Nitro  7.18  6.85  7.02  0.93  266  3.69  277  3.74  o-Carboxyl  6.00  5.92  5.96  0.39  231  3.33  264  m-Carboxyl  7.10  7.10  7.10  0.61  232  2.97  293  3.23  p-Carboxyl  7.56  7.30  7.43  0.93  245 .  4.23  260  4.43  o<-G4H4  7.20  -  7.20  -  235(s )4.29  253  4.11  e  log max  11  e  c  d  3.39  d  a - a good l i n e a r p l o t was not obtained ; pKgH v a l u e chosen from Ho i n t e r c e p t o f t h a t p o r t i o n o f p l o t whose s l o p e was c l o s e s t to u n i t y . ^ b - s p e c t r a l data o b t a i n e d from r\ > TT a b s o r p t i o n maxima c  - In 6 5 $ H 2 S 0 4  d  - In  37$H2S04  e - c<-Naphthyl Gra. - G r a p h i c a l  L i n . - Linear  Ave. - Average  (45) Table VI Comparison of the Base Strengths of Ortho-, Meta-, and Para-Substituted Benzoic Acids*  Substituent  -pKBH Ortho  -pKBH -PKBH Meta Meta (Corr.)  -pKBH -pK H Para Para (Corr.)  Hydrogen  7.17  7.26  7.17  7.26  7.17  0  Methyl  7.12  7.19  7.10  6.92  6.83  -0.29  Ethyl  7.13  6.92  6.83  -0.30  i-Propyl  7.23  t-Butyl  7.61  -  Fluoro  7.60  7.62  Chloro  7.67  Bromo  +  +  +  B  -  6.91  a  6.82  a  -0.41  6.90  6.81  -0.80  7.53  7.30  7.21  -0.39  7.73  7.64  7.48  7.39  -0.28  7.73  7.65  7.56  7.51  Iodo  7.76  7.64  7.55  7.50  7.41  -0.35  Hydroxyl  6.78  7.47  7.38  6.67  6.58  -0.20  Methoxyl  6.11  7.45  7.36  6.68  6.59  0.48  Ethoxyl  6.11  7.56  7.47  6.61  6.52  0.41  Carboxyl  5.96  7.43  1.47  Nitro  7.02  C4H  7.20  b 4  A  7.97  -  7.10  -  7.88  8.10  -  a  7.08  = pKBH Ortho - pK H +  B  a  B  +  8.01 6.99  Para(Corr.)  a - Calculated or estimated value b - o< - or fl -naphthyl  7.42  a  a  -0.31  0.99 -0.21  (46) determined by t h e m . i s i n exact agreement w i t h t h a t o r i g i n a l l y found by Hammett  value  (26).  As mentioned e a r l i e r , two methods have been used to e v a l u a t e the e f f e c t s o f the ortho s u b s t i t u e n t s basicities  o f the benzoic a c i d s .  on the  One method assumes the  e l e c t r i c a l e f f e c t s , o f ortho and para s u b s t i t u e n t s and uses the d i f f e r e n c e s  i n the b a s i c i t i e s  to be equal  o f the o r t h o - and  p a r a - s u b s t i t u t e d a c i d s to be a measure o f the ortho e f f e c t the s u b s t i t u e n t .  This, d i f f e r e n c e  v a l u e s are l i s t e d i n Table V I ;  is  called  of  Ag and i t s  negative values i n d i c a t i n g  base weakening e f f e c t s and p o s i t i v e v a l u e s i n d i c a t i n g base strengthening  effects.  The second method, which was f i r s t used by McDaniel and Brown (10), uses the i o n i z a t i o n constants o f c < - s u b s t i t u t e d p y r i d i n e s as a r e f e r e n c e system.  U s i n g t h i s method,  the  pKgjj+ v a l u e s f o r the o r t h o - , meta- and p a r a - s u b s t i t u t e d benzoic a c i d s have been p l o t t e d a g a i n s t the p K v a l u e s  for  a  the c o r r e s p o n d i n g p y r i d i n e s and the b e s t s t r a i g h t l i n e drawn through the p o i n t s c o r r e s p o n d i n g to the meta- and p a r a s u b s t i t u t e d compounds.  This plot i s  the data l i s t e d i n Table V I I .  shown i n F i g u r e 6 and  The d e v i a t i o n o f the  points  f o r the o r t h o - s u b s t i t u t e d compounds from t h i s l i n e has been called  ApK  B H  +  and has been taken as a measure o f the ortho  e f f e c t o f the s u b s t i t u e n t  on the b a s i c i t y o f the benzoic  acid.  Some o f the o r t h o - s u b s t i t u t e d benzoic a c i d s s t u d i e d have not been t r e a t e d i n t h i s manner because o f the l a c k o f p K f o r the c o r r e s p o n d i n g p y r i d i n e s .  a  I t has been noted by  values  I  1  1  -  1  S.0 7.S -pKD|_| FIG.6-RELATIONSHIP B E T W E E N pK p K O F SUBST. B E N Z O I C ACIDS. a  B  H  +  +  1  7.0 L.S O F SUBST. PYRIDINES  ,  AND  (48) T a b l e VII B a s i c i t i e s of Monosubstituted P y r i d i n e s ( p K ) and a  Benzoic A c i d s (PKRH**). Substituent  -pK +(Acid)(Corr.)  pK (Pyridine)  H  7.17  5.17  0-CH3  7.12  5.97  JH-CH3  7.10  5.68  p-CH  6.83  6.02  0-C2H5  7.13  5.97  p-C H  6.83  6.02  o-i-C3Hy  7.23  5.83  p-i-C H  6.82  6.02  2  3  5  3  7  BH  a  APKBH  -0.08  -0.09 -0.17  o-t-C H  9  7.61  5.76  p-t-C H  9  6.81  5.99  o-F  7.60  -0.44  ra-F  7.53  2.97  o-Cl  7.67  0.72  m-Cl  7.64  2.84  o-Br  7.73  0.90  m-Br  7.56  2.84  o-I  7.76  1.82  m-I  7.55  3.25  o-OH  6.78  4.47  0.52  0-CO2H  5.96(6.26)  1.08  1.65  4  4  a  1  -0.54 0.59 0.31 0.22 0.02  a - C o r r e c t e d f o r a s t a t i s t i c a l f a c t o r o f 2 i n Kgfj+  (49) McDaniel and Brown t h a t c e r t a i n s u b s t i t u e n t s  should not be  i n c l u d e d i n an e v a l u a t i o n by t h i s method s i n c e t h e y may e x h i b i t unusual e f f e c t s .  In p a r t i c u l a r , n i t r o , c a r b o x y l ,  and  hydroxyl  groups may i n v o l v e e x t r a resonance i n e i t h e r the p y r i d i n e molecule o r the p y r i d i n i u m  i o n , a l s o , c< - p i c o l i n i c a c i d may  i n v o l v e hydrogen bonding and <X -hydroxy p y r i d i n e e x i s t s as a keto-enol  tautomeric mixture.  The v a l u e s f o r these s u b s t i t -  uents, although p l o t t e d f o r the sake o f i n t e r e s t , a r e given little  significance.  a l k y l and ortho-halo  F o r these reasons, o n l y t h e o r t h o benzoic acids are s e r i o u s l y  considered  by t h i s method. A d i s c u s s i o n o f the ortho  e f f e c t s evaluated  methods which have been d e s c r i b e d  by t h e two  i s s i m p l i f i e d by d i v i d i n g  the s u b s t i t u e n t s i n t o groups and d i s c u s s i n g each group separately.  For t h i s purpose, we s h a l l f i r s t  a l k y l substituents  consider the  (with which the oi -naphthyl group  shall  be i n c l u d e d ) , secondly the halo s u b s t i t u e n t s and f i n a l l y t h e oxygen c o n t a i n i n g s u b s t i t u e n t s .  I t i s seen t h a t the ortho  e f f e c t s , as determined by the f i r s t method, assuming s i m i l a r e l e c t r i c a l e f f e c t s i n t h e ortho  and para p o s i t i o n s , a r e base  weakening f o r a l l except methoxyl, e t h o x y l , n i t r o and substituents.  carboxyl  On t h e other hand, t h e method o f McDaniel and  Brown i n d i c a t e s o n l y the a l k y l s u b s t i t u e n t s t o be base weakening  1.  and a l l others t o be base  Alkyl  strengthening.  Substituents.  The t r e n d o f the ortho  e f f e c t s o f the a l k y l  substituents  (50) on the b a s i c i t i e s of benzoic a c i d s i s p a r t i c u l a r l y c l e a r . Methyl and  e t h y l groups e x e r t s i m i l a r e f f e c t s , the  group a somewhat l a r g e r  e f f e c t and  considerably larger e f f e c t . observed by Brown w i t h the addition the  compounds o f  i-propyl are but the  the t - b u t y l group a  S i m i l a r b e h a v i o r has s t r a i n e n e r g i e s i n the  o(. - a l k y l p y r i d i n e s  geometries of the  groups can  rotate  group.  so t h a t  t-butyl  boron He  attributes  i-propyl  ethyl  and  t h e i r s t e r i c requirements  group, however, r o t a t i o n  r e d u c t i o n i n s t r a i n and  results.  An  I t i s i n t e r e s t i n g that  that  t h i s may  a p l o t of e i t h e r  A g  w e l l be  activation  o f methyl i o d i d e w i t h the  s u b s t i t u e n t s are and  case. for  energy f o r  cK - a l k y l  pyridines alkyl  e x e r t i n g s i m i l a r e f f e c t s i n both t h i s  i n the  p r o t o n a t i o n o f the  T h i s p l o t i s shown i n F i g u r e The  the  ApKgji+  or  i s approximately l i n e a r i n d i c a t i n g that the  reaction  cannot r e s u l t  a sharp i n c r e a s e i n s t r a i n  these a l k y l s u b s t i t u e n t s a g a i n s t the reaction  In  examination o f Leybold models o f these o r t h o -  a l k y l benzoic acids indicates  (54)  The  been  l i t t l e g r e a t e r than those o f the methyl group.  case o f the  i n any  the  (54).  slow r i s e i n s t r a i n from methyl to e t h y l to  groups to the  i-propyl  ortho b e n z o i c  acids.  7.  ortho e f f e c t s o f the a l k y l groups may  be most  r e a d i l y e x p l a i n e d i n terms o f s t e r i c i n h i b i t i o n o f resonance. Whereas In the a c i d i o n i z a t i o n o f b e n z o i c a c i d s ,  resonance i s  o n l y o f importance i n the u n d i s s o c i a t e d molecule and importance i n the  a n i o n , i n the b a s i c  of  no  i o n i z a t i o n o f benzoic  a c i d , resonance i s important i n both the molecule and  ion  (51) but p a r t i c u l a r l y  so i n the l a t t e r .  Resonance forms o f the  molecule and c a r b o n y l oxygen p r o t o n a t e d s p e c i e s a r e as shown.  <  (  >  *  XIV  A  H V  HO.  0^  ON  XV I t i s reasonable t o assume t h a t resonance w i l l be more important i n the c a t i o n , XV, than i n the molecule, XIV, s i n c e  (52)  I  FIG.7-E  I a c t  1  I  1  t  I  FOR REACTION ^ - R C ^ H ^ N + M e l  —*c<-RC5H4N MeI +  VS.A (a) AND ApK B  B H  +  (b).  (53) charge s e p a r a t i o n s t r u c t u r e s  a r e not i n v o l v e d i n the former.  Resonance e f f e c t s would t h e r e f o r e tend t o lower the energy o f the  cation  r e l a t i v e to that  o f the molecule and so tend t o  decrease the enthalpy o f i o n i z a t i o n .  An i n h i b i t i o n o f t h i s  resonance, would i n c r e a s e the e n t h a l p y o f i o n i z a t i o n and, ignoring and  entropy e f f e c t s , i n c r e a s e the f r e e energy o f i o n i z a t i o n  t h e r e b y decrease the e q u i l i b r i u m  reaction The  constant o f the p r o t o n a t i o n  as has been observed. r e s u l t s o f entropy e f f e c t s a r e more d i f f i c u l t t o  predict.  I n h i b i t i o n o f resonance w i l l cause a l o s s o f r i g i d i t y  i n the protonated form and t h e r e f o r e an i n c r e a s e i n the entropy o f t h i s form and thus an i n c r e a s e i n the entropy o f i o n i z a t i o n ( i . e . i t w i l l become l e s s n e g a t i v e ) .  Similarly,  since  solvation  o f the c a t i o n i s presumably more important than  solvation  o f the molecule, hindrance t o s o l v a t i o n  will  r e s u l t i n an i n c r e a s e i n the entropy o f i o n i z a t i o n .  also These  e f f e c t s w i l l thus a c t i n the o p p o s i t e d i r e c t i o n t o the e n t h a l p y changes i n determining the o v e r a l l f r e e energy o f ionization.  S i n c e t h e b a s i c i t y constants o f the o r t h o - a l k y l  b e n z o i c a c i d s are o b v i o u s l y d e c r e a s i n g i n the o r d e r o f expected increasing  s t e r i c e f f e c t s i t appears t h a t ,  i f other  are n e g l i g i b l e , the enthalpy change i s i n c r e a s i n g  effects  faster  than t h e entropy change. I t appears reasonable t o assume t h a t o t h e r phenomena t o which ortho e f f e c t s a r e commonly a t t r i b u t e d The  are n e g l i g i b l e .  e f f e c t s o f b u l k y groups on r o t a t i o n a l and v i b r a t i o n a l  motions w i t h i n m o l e c u l e s should be s i m i l a r i n both the  (54) unprotonated and p r o t o n a t e d b e n z o i c a c i d s . should t h e r e f o r e c a n c e l .  These e f f e c t s  For s i m i l a r reasons, F s t r a i n  i n t e r a c t i o n s should be unimportant  i n these compounds.  F i n a l l y , as mentioned e a r l i e r , i t i s now  g e n e r a l l y accepted  t h a t i n t e r n a l hydrogen bonding i s unimportant groups.  with a l k y l  T h i s w i l l c e r t a i n l y be t r u e i n the s t r o n g l y a c i d  s o l u t i o n s i n which the b a s i c i t y c o n s t a n t s were measured, s i n c e even i f i n t e r n a l hydrogen bonds i n v o l v i n g a l k y l groups d i d form, they would be v e r y weak and i t i s i n c o n c e i v a b l e t h a t they c o u l d compete w i t h the s t r o n g e x t e r n a l hydrogen bonds i n v o l v i n g the c a r b o x y l group and the s o l v e n t . The ortho e f f e c t o f the c< -naphthyl group appears to be s i m i l a r t o t h a t o f t h e methyl and e t h y l groups i f i t i s assumed t h a t the e l e c t r i c a l e f f e c t s o f o< - and |S -naphthyl groups a r e the same.  T h i s , however, i s u n l i k e l y s i n c e t h e r e  i s evidence to i n d i c a t e t h a t the e l e c t r i c a l e f f e c t o f the a r y l group i s d i f f e r e n t i n the two p o s i t i o n s .  Schenkel  t55)  has c a l c u l a t e d the T T - e l e c t r o n d e n s i t i e s f o r the d i f f e r e n t i s o m e r i c p o s i t i o n s i n naphthalene  and found i t t o be  over  twice as g r e a t i n the t X - p o s i t i o n than i n the j$ - p o s i t i o n . From t h i s f a c t by i t s e l f one would p r e d i c t oi -naphthoic to be a s t r o n g e r base than |3 -naphthoic a c i d . however, t h a t the o p p o s i t e i s the case. due to s t e r i c i n h i b i t i o n o f resonance  I t appears,  This i s probably  and s t e r i c  hindrance  to s o l v a t i o n by the p e r i hydrogen which would c o u n t e r a c t and p o s s i b l y overcome the e l e c t r i c a l base s t r e n g t h e n i n g effects.  acid  (55;):  XVI In summary, i t appears t h a t the ortho e f f e c t s o f substituents  on the b a s i c i t i e s  r e c o n c i l e d i n terms o f s t e r i c s t e r i c hindrance to s o l v a t i o n .  alkyl  o f benzoic a c i d s may be i n h i b i t i o n o f resonance and The enthalpy c o n t r i b u t i o n s  to these e f f e c t s appear to outweigh the entropy f a c t o r s .  2.  Halogen S u b s t i t u e n t s . The ortho e f f e c t s o f the halogen s u b s t i t u e n t s  easy to r e c o n c i l e .  are l e s s  Although both methods o f e v a l u a t i n g  ortho e f f e c t s  seem to apply e q u a l l y w e l l i n the case o f  substituents,  the r e s u l t s o f the two methods appear to be  c o n t r a d i c t o r y i n the case o f the halogens.  alkyl  The method o f  McDaniel and Brown i n d i c a t e s the e f f e c t s o f these  substituents  on benzoic a c i d s to be base s t r e n g t h e n i n g , w h i l e the o t h e r method i n d i c a t e s them to be base weakening.  Since the  (56) assumption o f e q u a l i t y o f e l e c t r i c a l e f f e c t s i n the ortho and para p o s i t i o n s  i s a d m i t t e d l y open to a c e r t a i n amount o f  c r i t i c i s m and s i n c e  e l e c t r i c a l e f f e c t s are l i k e l y to be more  important i n the case o f halo s u b s t i t u e n t s than i n the of a l k y l substituents,  case  t h i s method o f e v a l u a t i n g ortho. e f f e c t s may  be i n v a l i d f o r halogen s u b s t i t u e n t s .  For t h i s  reason,  the  ortho e f f e c t s o f the halogens e v a l u a t e d by McDaniel'.s and Brown's method are c o n s i d e r e d more a c c u r a t e and o n l y they w i l l be d i s c u s s e d  here.  I t should be noted a t t h i s time t h a t the d i f f e r e n c e s the p K s o f the T  a  c<-halo p y r i d i n e s are much l a r g e r than  whereas the d i f f e r e n c e s  between the pKgjf*'s o f the  a c i d s are q u i t e s m a l l .  (56).  I t t h e r e f o r e appears t h a t the d e v i a t i o n s  expected  benzoic  Brown p o i n t s out t h a t the p K  cx-fluoro p y r i d i n e may be i n e r r o r by as much as  in  a  - 0.3  of pK  o f the  a  halogen  p o i n t s from l i n e a r i t y i n F i g u r e 6 may be due more so to  the  pK  the  a  v a l u e s o f the p y r i d i n e s than to the pKgjj+ v a l u e s o f  acids.  I f this i s true,  then the measured ortho e f f e c t s w i l l  be l a r g e r than they a c t u a l l y a r e ; ortho e f f e c t s It i s  unit  should however be the  the r e l a t i v e o r d e r of same.  seen t h a t the o r d e r o f the ortho e f f e c t s o f  halo s u b s t i t u e n t s i s I < B r ( C l < F , which i s i n c r e a s i n g hydrogen bonding power.  the  the  the o r d e r o f  A s i m i l a r b e h a v i o r to  t h i s has been observed by McDaniel and Brown with the p K o r t h o - h a l o phenols and o r t h o - h a l o a n i l i n e s  (10).  ortho e f f e c t o f f l u o r i n e may be e x p l a i n e d ,  therefore,  o f an i n t e r n a l l y hydrogen bonded s t r u c t u r e o f the  T a  s of  The l a r g e i n terms  conjugate  (57) acid,  XVII.  XVII T h i s would s t a b i l i z e  the conjugate a c i d r e l a t i v e to  unprotonated form and so i n c r e a s e the b a s i c i t y Because o f i t s  small size,  should be n e g l i g i b l e . of  lessening  and i o d i n e .  constant.  any b u l k e f f e c t s o f the  S i m i l a r hydrogen bonded  the  fluorine  structures  importance may be drawn f o r c h l o r i n e , bromine The n e g l i g i b l e  as would have been expected, unimportant h e r e .  ortho e f f e c t of iodine  indicates,  t h a t hydrogen bonding i s  It i s possible,  however,  t h a t a weak  hydrogen bond may e x i s t but t h a t the b u l k e f f e c t o f i o d i n e a c t i n g i n the same manner as the b u l k e f f e c t o f the groups,  is  alkyl  t e n d i n g to weaken the b a s i c i t y o f the benzoic a c i d .  These two e f f e c t s a c t i n g t o g e t h e r c o u l d r e s u l t net ortho  i n only a s m a l l  effect.  The work o f B e r l i n and Jensen  (57) on the  conformational  (53)  e q u i l i b r i a o f halocyclohexanes suggest t h a t the s t e r i c of  halogen groups, with the e x c e p t i o n o f f l u o r i n e , are v e r y  similar.  and  effects  These workers have measured the  equilibrium  determined the f r e e energy change A F f o r the p r o c e s s .  T h e i r r e s u l t s are l i s t e d i n T a b l e V I I I .  •TABLE V I I I CONFORMATIONAL EQUILIBRIA OF HALOCYCLOHEXANES. •'C6 11-X H  The  AF (cal./mole)  F  250  Cl  513  Br  430  I  431  s m a l l f l u o r i n e has the s m a l l e r s t e r i c e f f e c t and a l t h o u g h  the o t h e r t h r e e halogens have s i m i l a r e f f e c t s ,  c h l o r i n e appears  to- have the l a r g e s t and i o d i n e the s m a l l e s t .  B e r l i n and  (59) Jensen e x p l a i n t h i s apparent d i s c r e p a n c y by n o t i n g t h a t both the c o v a l e n t r a d i i and the p o l a r i z a b i l i t i e s o f the atoms are important i n d e t e r m i n i n g the s t e r i c r a d i i have l a r g e p o l a r i z a b i l i t i e s , clouds and t h e r e f o r e  effect.  Atoms with l a r g e  hence e a s i l y deformed  r e l a t i v e l y small conformational  electron  preferences,  whereas atoms w i t h s m a l l e r r a d i i have lower p o l a r i z a b i l i t i e s and thus c o n f o r m a t i o n a l p r e f e r e n c e s more i n l i n e w i t h t h e i r van der Waals r a d i i . o f the halogens  therefore vary i n opposite  resulting in l i t t l e Although the basicities  The p o l a r i z a b i l i t i e s and c o v a l e n t directions  net v a r i a t i o n i n s t e r i c  steric  thereby  effect;;  e f f e c t s o f the halogens  on the  o f b e n z o i c a c i d s appear to be n e g l i g i b l e ,  i n t e r e s t i n g to note t h a t the ortho e f f e c t ,  radii  it  ApKg^of  is  these  atoms i s i n v e r s e l y p r o p o r t i o n a l to t h e i r van der Waals r a d i i (57a) and t h a t a p l o t o f these q u a n t i t i e s line  (Figure 8).  results  in a  T h i s b e h a v i o r appears almost too  to be c o i n c i d e n t a l .  straight  fortuitous  In the l i g h t o f what has been s a i d ,  the  most ready e x p l a n a t i o n appears to be t h a t the hydrogen bonding power o f the halogens  i s i n v e r s e l y p r o p o r t i o n a l to t h e i r van  der Waals r a d i i and thus F i g u r e 8 i s r e a l l y a p l o t o f against  ApKgjjf  hydrogen bonding power r a t h e r than a g a i n s t van der  Waals r a d i i .  3.  Oxygen-Containing S u b s t i t u e n t s . A l l the oxygen c o n t a i n i n g o r t h o - s u b s t i t u e n t s  w i t h the p o s s i b l e strengthening  exception o f h y d r o x y l ,  effects.  studied,  e x h i b i t base  Because o f the l a c k o f data on the  (60) p K s o f s u b s t i t u t e d p y r i d i n e s , the o r t h o e f f e c t s o f o n l y f  a  h y d r o x y l and c a r b o x y l groups c o u l d be estimated by M c D a n i e l ' s and Brown's method.  These authors p o i n t out t h a t  0<-hydroxy  and <?C-carboxy p y r i d i n e should not be i n c l u d e d i n the p y r i d i n e reference  system because these groups may i n v o l v e e x t r a resonance  i n e i t h e r the p y r i d i n e molecule o r p y r i d i n i u m i o n and because p i c o l i n i c a c i d may i n v o l v e hydrogen bonding and  ©(-hydroxy  p y r i d i n e e x i s t s as a k e t o - e n o l t a u t o m e r i c m i x t u r e . reasons, effects  the v a l u e s o f  For these  A g o b t a i n e d by assuming e q u a l e l e c t r i c a l  i n the ortho and para p o s i t i o n s have been used as  measures o f the ortho e f f e c t s o f t h i s T h i s method, however, d i d f o r the a l k y l electrical  group o f  substituents.  may not prove as s a t i s f a c t o r y as  substituents  since  i n the p r e s e n t  it  case  e f f e c t s w i l l p r o b a b l y be o f g r e a t e r importance.  The h y d r o x y l and a l k o x y l groups are i n many ways s i m i l a r and f o r t h i s reason w i l l be d i s c u s s e d t o g e t h e r . effect  o f the h y d r o x y l group estimated u s i n g the p y r i d i n e  system as a r e f e r e n c e e f f e c t whereas the effect.  The ortho  standard i s  a base  strengthening  A g value i n d i c a t e s a s m a l l base weakening  The ortho e f f e c t s o f the a l k o x y l groups, on the  o t h e r hand, are d e f i n i t e l y base  strengthening.  The a c i d s t r e n g t h e n i n g e f f e c t o f the o r t h o - h y d r o x y l group i n s a l i c y l i c a c i d i s w e l l known and i s a t t r i b u t e d to a s t a b i l i s a t i o n o f the benzoate a n i o n by i n t e r n a l hydrogen bonding w i t h the h y d r o x y l group (4).  In view o f t h i s and  the apparent importance o f hydrogen bonding i n the protonated halo-benzoic acids i t  seems reasonable to expect  similar  (61)  FIG. 8 - R E L A T I O N S H I P B E T W E E N ApK FOR HALO-SUBSTITUTED BENZOIC ACIDS A N D V A N D E R W A A L S RADII. B  H  +  (62)  phenomena to be important i n the conjugate a c i d (XX, X X I ) .  a c i d of  In a s i m i l a r manner, i n t e r n a l hydrogen  bonding may be important i n d e t e r m i n i n g the base o f the o r t h o - a l k o x y l benzoic a c i d s . s t u d i e s on b e n z o i c a c i d s ,  Brooks and coworkers ( 5 3 ) as  c h e l a t e d s t r u c t u r e XVIII i s  well  internally  the predominant form o f o r t h o -  methoxy b e n z o i c a c i d i n carbon t e t r a c h l o r i d e .  constant  strengths  From t h e i r i n f r a - r e d  as Forbes and h i s group ( 5 9 ) conclude t h a t the  necessarily  salicylic  be t r u e , however,  i n a solvent  o f high d i e l e c t r i c  such as c o n c e n t r a t e d s u l p h u r i c a c i d .  form o f t h i s a c i d may l i k e w i s e  T h i s may not  The protonated  be hydrogen bonded as i n XX.  The observed base s t r e n g t h e n i n g e f f e c t o f the ortho methoxyl group i s t h e r e f o r e  accounted f o r i f one makes the  reasonable  assumption t h a t hydrogen bonding i s more important i n XX than i n XVIII. The a p p a r e n t l y s m a l l o r t h o e f f e c t o f the h y d r o y l group indicates  that i f  s t r u c t u r e s XX (R = ff) and XXI are i m p o r t a n t ,  t h e i r e f f e c t must be counteracted by a base weakening It  i s possible  that a p a r t i a l explanation of t h i s  entropy e f f e c t s .  effect.  lies in  The hydrogen bonding i n XXI would tend to  h o l d the h y d r o x y l group r i g i d l y and so lower the entropy o f the c a t i o n . (i.e.  T h i s would r e s u l t  i t w i l l become more n e g a t i v e ) o f i o n i z a t i o n and t h e r e f o r e  a greater free  energy o f i o n i z a t i o n and hence a s m a l l e r  e q u i l i b r i u m constant hand,  i n a decrease i n entropy  and a lower b a s i c i t y .  On the  other  entropy e f f e c t s may be s m a l l e r i n the case o f o r t h o -  methoxy benzoic a c i d since i n s t r u c t u r e XX (R^CR^) the methyl  (63)  R= H, CHj C J H J (  XX  R =H  XXI  ( 6 4 )  group would s t i l l be f r e e entropy o f i o n i z a t i o n ,  to move.  Thus the decrease i n  and thereby the base weakening  effect,  would be s m a l l e r than i n s a l i c y l i c a c i d .  It i s also  t h a t the hydrogen bonding i n the conjugate  a c i d s o f the  a l k o x y l a c i d s w i l l be s t r o n g e r than t h a t a c i d of s a l i c y l i c a c i d , a l k y l groups i s in  salicylic  i n the  s i n c e the i n d u c t i v e  the  possible ortho-  conjugate  effect of  the  g r e a t e r than t h a t o f the h y d r o x y l hydrogen  a c i d and may t h e r e f o r e  result  i n a greater  electron  d e n s i t y on the a l k o x y l oxygen than on the h y d r o x y l oxygen. The ortho e f f e c t o f the n i t r o group appears to be v e r y large.  I t must be emphasized,  however,  that t h i s value  is  i n doubt because o r t h o - n i t r o benzoic a c i d gave a poor and unusual i o n i z a t i o n c u r v e , hence the pKgjj"'"  value i s  uncertain,  and because the pKgj|+ o f p a r a - n i t r o benzoic a c i d was and not measured. ipate  calculated  The a b i l i t y o f the n i t r o group to p a r t i c -  i n i n t r a m o l e c u l a r hydrogen bonds i s  i n d i c a t e d by a  comparison o f the m e l t i n g p o i n t s o f o r t h o - and p a r a - n i t r o benzoic a c i d s ,  1 4 7  and  2 4 2 ° C ,  respectively  i n t r a m o l e c u l a r bonds have l o n g been thought confirmed ( 6 0 ) , If,  therefore,  ( 4 3 ) .  (3),  Similar and r e c e n t l y  to e x i s t i n o r t h o - n i t r o phenol ( s t r u c t u r e the ortho e f f e c t  o f the n i t r o group i s  I).  base  strengthening,  i t may be a t t r i b u t e d to i n t r a m o l e c u l a r hydrogen  bonding i n the  conjugate  a c i d as shown i n X X I I .  (65)  XXII  4.  Phthalic Acids The b a s i c i t y constants o f the three i s o m e r i c p h t h a l i c  a c i d s have been measured. values  The experimental v a l u e s and the  c a l c u l a t e d by the Hammett equation u s i n g the  Hammett 0" v a l u e s and the  p  original  v a l u e o f 1.03 determined by  Stewart and Yates (31) f o r the p r o t o n a t i o n o f meta- and p a r a s u b s t i t u t e d b e n z o i c a c i d s are l i s t e d i n Table IX.  Table IX. (see  page 66)  (66) Table IX EXPERIMENTAL AND CALCULATED BASICITY CONSTANTS OF ISOMERIC PHTHALIC ACIDS.  (Experimental) Measured Corrected  Acid  a  (Calculated)  u  b  Ortho-phthalic  5.96  6.26  Isophthalic  7.10  7.40  7.55  0.36  Terephthalic  7.43  7.73  7.96  0.73  a - Corrected f o r a s t a t i s t i c a l  factor of 2 i n K g+. B  b - L . P . Hammett, P h y s i c a l Organic Chemistry, M c G r a w - H i l l Book C o . , I n c . , New York, M . Y . , 1940, p . 1$$.  The most n o t i c a b l e the ortho isomer.  of  I t appears t h a t the ortho e f f e c t s o f  c a r b o x y l group are great electronic  f a c t o r i s the enhanced b a s i c i t y  enough to more than overcome  base weakening e f f e c t s o f t h i s group.  the  the  A similar  behavior has l o n g been r e c o g n i z e d w i t h regard to the a c i d i t y o f o r t h o - p h t h a l i c a c i d as compared to i t s  enhanced  isomers  (61).  The f i r s t and second i o n i z a t i o n c o n s t a n t s o f the p h t h a l i c  acids  are l i s t e d  i n Table X . Table X .  ACID IONIZATION CONSTANTS OF THE ISOMERIC PHTHALIC ACIDS Acid  p^  1  PKa  II  Ortho-phthalic  2.98  5.28  Isophthalic  3.46  4.46  Terephthalic  3.51  4.82  (61)  (67) Both the high f i r s t and  ionization  constant o f the ortho isomer  t h e l a r g e d i f f e r e n c e between i t s f i r s t  ionization  bonding  (62)  As i n s a l i c y l i c  acid,  XXIV the f i r s t  e x p e c t e d t o be a b n o r m a l l y h i g h ; hydrogen  a negatively  ionization but s i n c e  constant i s the remaining  atom o f t h e m o n o - a n i o n i s i n c o r p o r a t e d  charged c y c l i c  c o n s t a n t i s e x p e c t e d t o be  system,  the second  a b n o r m a l l y low.  manner, i n t r a - m o l e c u l a r h y d r o g e n satisfactory  of  a s shown b e l o w .  XXIII  acid.  second  c o n s t a n t s have b e e n a c c o u n t e d f o r i n t e r m s  i n t r a - m o l e c u l a r hydrogen  acidic  and  ionization  In a  similar  b o n d i n g a p p e a r s t o be  e x p l a n a t i o n o f t h e enhanced  into  basicity of  a  this  (68)  l~0  \  N  ©H  XXV  XXVI  Attempts were made to measure the second constants, pKgHH > ++  o  f  phthalic acids.  t n e  protonation  However, no  f u r t h e r u l t r a v i o l e t s p e c t r a l changes appeared t o take place a f t e r the f i r s t i o n i z a t i o n with the meta and para isomers even i n a c i d concentration as high as 30$ fuming sulphuric a c i d (H ^12.8).  A pKg ++ value o f -11.4 was,  />  0  obtained  HH  f o r ortho-phthalic acid.  however,  This value should be  assumed t o be only approximate f o r several reasons. the H  Q  First,  scale changes very r a p i d l y i n t h i s region and small  e r r o r s i n s t a n d a r d i z a t i o n o f the sulphuric a c i d could r e s u l t i n large errors i n H .  Secondly, since the e q u i l i b r i u m  Q  i n v o l v e d i s o f the type BH  +  +  H  +  BHH  (69) and not o f the  the H  Q  type  s c a l e does not r e a l l y a p p l y .  The a c i d i t y s c a l e used  f o r s t u d y i n g such e q u i l i b r i a should be a H u s i n g s i n g l y charged c a t i o n s molecules.  Finally,  +  s c a l e b u i l t up  as i n d i c a t o r s r a t h e r than n e u t r a l  the u l t r a v i o l e t  s p e c t r a o f the  phthalic  a c i d s p e c i e s i n d i c a t e t h a t the f i r s t p r o t o n a t i o n i s not b e f o r e the sedond p r o t o n a t i o n b e g i n s . shown i n F i g u r e 9 .  It is  These s p e c t r a are  seen t h a t the peak a t 264 r y x ,  assumed to correspond to the mono-protonated s p e c i e s , t o grow a f t e r  Although these  throw some doubt on the v a l i d i t y o f the  is  it  appears t h a t the P % H H ' ' "  +  factors  second p r o t o n a t i o n ° ^ ortho-phthalic acid  h i g h e r than t h a t f o r the meta and para i s o m e r s .  unexpected  continues  the peak a t 305 n y u , c o r r e s p o n d i n g to the d i -  p r o t o n a t e d s p e c i e s begins t o form.  constant,  complete  i n view o f the l a r g e d i f f e r e n c e  This  is  between the  first  and second a c i d i o n i z a t i o n c o n s t a n t s o f o r t h o - p h t h a l i c  acid.  On the o t h e r hand, i t may be t h a t the f i r s t  and sedond  p r o t o n a t i o n s o f the meta and para isomers both o c c u r over a narrow range o f a c i d c o n c e n t r a t i o n s one d e t e c t a b l e s p e c t r a l  and thereby produce o n l y  shift.  In summary, i t may be s a i d t h a t the ortho e f f e c t s the  substituents studied,  on the b a s i c i t i e s  may be a t t r i b u t e d to two major causes. uents appear to weaken the b a s i c i t i e s  o f benzoic  Ortho-alkyl o f the a c i d s  of acids substit-  largely  (TO)  220  a*0  300  340  380  X ( m u ) F I G . 9 - U . V .A B S O R P T I O N C U R V E S O F o - P H T H A L I C A C I D I N S U L P H U R I C A C I D S O L U T I O N S  (71) through b u l k e f f e c t s whereas halogen and o x y g e n - c o n t a i n i n g substituents  appear to enhance the b a s i c i t i e s by i n t r a -  m o l e c u l a r hydrogen bonding which tends to s t a b i l i z e  the  p r o t o n a t e d s p e c i e s r e l a t i v e to the n e u t r a l m o l e c u l e .  5.  Search f o r L i n e a r R e l a t i o n s h i p s . An attempt has been made to determine whether the pKg^ " 1  v a l u e s o f the o r t h o - s u b s t i t u t e d benzoic a c i d s f o l l o w a l i n e a r r e l a t i o n s h i p o f any s o r t , whether i t be a l i n e a r f r e e  energy  r e l a t i o n s h i p o f the Hammett type o r whether i t be some form o f a l i n e a r s t e r i c r e l a t i o n s h i p o f the type found by T a f t I t has been found t h a t the b a s i c i t i e s  o f the  acids  q u a l i t a t i v e l y f o l l o w a l i n e a r r e l a t i o n s h i p w i t h the o f the c o r r e s p o n d i n g a c i d s .  (21).  acidities  A p l o t o f these q u a n t i t i e s  is  shown i n F i g u r e 10.  T h i s i s o f course e q u i v a l e n t to a p l o t  of  where  0~ a g a i n s t pI^H* Q  G~ i s d e f i n e d as the 0  constant f o r ortho s u b s t i t u e n t s the u s u a l Hammett s u b s t i t u e n t substituents.  substituents  i n an analogous manner to  constants f o r meta and para  The p o i n t s i n F i g u r e 10 appear to f a l l r o u g h l y  on two s t r a i g h t l i n e s ;  the oxygen c o n t a i n i n g  on one l i n e and the o t h e r s u b s t i t u e n t s  substituents  on the o t h e r .  This  b e h a v i o r i n d i c a t e s t h a t s i m i l a r f a c t o r s are a f f e c t i n g both the a c i d i t i e s and b a s i c i t i e s  o f the a c i d s s u b s t i t u t e d with  oxygen c o n t a i n i n g groups and t h a t s i m i l a r f a c t o r s are the a c i d i t i e s and b a s i c i t i e s acids.  This i s  o f the a l k y l - and h a l o -  somewhat i n c o n t r a s t to the  affecting substituted  conclusions  drawn i n the f o r e g o i n g d i s c u s s i o n where i t was concluded t h a t  (12)  FIG.10-RELATIONSHIP BETWEEN pK AND pK OF ORTHO-SUBST. BENZOIC ACIDS. a  +  B H  C73) s t e r i c f a c t o r s were most important w i t h the a l k y l s u b s t i t u t e d a c i d s whereas i n t r a - m o l e c u l a r hydrogen bonding, was most important with the h a l o - and o x y - s u b s t i t u t e d a c i d s . The agreements o f the b a s i c i t i e s with the l i n e a r p o l a r and s t e r i c r e l a t i o n s h i p s (equations 4 and 6) investigated.  has a l s o been  The r e s u l t s f i t n e i t h e r o f these  relationships  well.  (4) log The PKBH  +  K / K  =  0  6"E  (6)  S  v a l u e s o f those a c i d s f o r which  0"~ v a l u e s are  a v a i l a b l e (Table X I ) show o n l y a v e r y approximate agreement w i t h equation 4 and the p o i n t f o r o r t h o - n i t r o b e n z o i c a c i d (Figure 11).  i s f a r o f f the l i n e  Likewise, a poor agree-  ment w i t h equation 6 i s shown i n F i g u r e 12,  a l t h o u g h the  p o i n t s f o r the h a l o - s u b s t i t u t e d a c i d s appear to l i e on a l i n e by themselves.  T h i s b e h a v i o r i s not too  surprising  i n view o f the s p e c i f i c nature o f s t e r i c e f f e c t s . v a l u e s o f 0"  de-  and E  s  were determined  from e s t e r h y d r o l y s e s  and o n l y r e a c t i o n s which have s i m i l a r p o l a r and requirements 4 and 6.  The  steric  to these can be expected to f o l l o w equations  S i n c e the p r o t o n a t i o n o f benzoic a c i d s may  q u i t e d i f f e r e n t p o l a r and s t e r i c requirements  from  the  h y d r o l y s i s o f benzoate e s t e r s a f i t to equations 4 and should not n e c e s s a r i l y be expected.  have  6  I t i s i n t e r e s t i n g to  note, however, t h a t the a c i d i o n i z a t i o n c o n s t a n t s o f o r t h o -  (Ik)  k  o-EtO -MeO  -o.'a.  o  c r *  w  o.'a  o.V o/j ww  FIG. 11 - RELATIONSHIP B E T W E E N p K ORTHO-SUBSTITUTED BENZOIC ACIDS AND c r *  B H  +  ua  OF  (75)  0.4-  0.S  1.0  FIG. 12-RELATIONSHIP BETWEEN p K OF ORTHO-SUBSTITUTED BENZOIC ACIDS AND E . +  B H  s  (76) s u b s t i t u t e d benzoic a c i d s do f o l l o w equation 4 q u i t e w e l l . More g e n e r a l equations which take i n t o account simultaneous and independent v a r i a t i o n s i n p o l a r and s t e r i c e f f e c t s might be expected to c o r r e l a t e the b a s i c i t y better.  results  As mentioned e a r l i e r , equations 7 and 8 which are  o f t h i s type have been used s u c c e s s f u l l y by T a f t and F a r t h i n g and Nam to c o r r e l a t e the r a t e s and e q u i l i b r i a o f a number o f r e a c t i o n s o f o r t h o - s u b s t i t u t e d compounds.  log K/K  - <r*p* + 6~E  0  l o g K/K  (7)  S  =  0  + <J"S f>S  CRpE  (3)  Whereas T a f t ' s e q u a t i o n 7 i g n o r e s e l e c t r i c a l e f f e c t s than p o l a r e f f e c t s , all  a  other  F a r t h i n g ' s and Nam's equation 8 i n c l u d e s  e l e c t r o n i c e f f e c t s on the assumption t h a t they a r e equal  i n the ortho and para p o s i t i o n s .  Taft's  0~  and E  s  are  d i r e c t l y measureable q u a n t i t i e s , whereas 0~g and (Ts are d e r i v e d from measureable  quantities.  The b a s i c i t i e s o f the b e n z o i c a c i d s show a q u a l i t a t i v e agreement w i t h equations 7 and 8. i s improved i f a m o d i f i e d form, l o g K/K  0  ^  0"EfE  C o r r e l a t i o n by e q u a t i o n 8  equation 8a, i s +  °"sPs  used. (8a)  a - The v a l u e s o f CT and E used a r e r e l a t i v e t o the methyl group; Ko i s the e q u i l i b r i u m constant o f the m e t h y l - s u b s t i t u t e d compound. s  (77) +  The s u b s t i t u e n t s  constants  u s i n g Brown's G"p v a l u e s  +  CTg and  a r e o b t a i n e d by  (40) i n p l a c e o f Hammett's 0~p  v a l u e s as a measure o f the e l e c t r o n i c e f f e c t s .  The 0~p  v a l u e s have been used by Brown f o r r e a c t i o n s i n which d i r e c t c o n j u g a t i o n can o c c u r between the s u b s t i t u e n t centre. pKgu and  +  and the r e a c t i o n  Yates (39) found b e t t e r c o r r e l a t i o n between the  v a l u e s o f meta- and p a r a - s u b s t i t u t e d b e n z o i c  O  than w i t h 0 .  t h a t the present r e s u l t s than equation 3.  acids  I t i s t h e r e f o r e not s u r p r i s i n g should f i t  equation 3a b e t t e r  P l o t s o f the b a s i c i t y c o n s t a n t s  using  equations 7 and 3a are shown i n F i g u r e s 13 and 14 and the data l i s t e d i n T a b l e s XI and X I I .  In both c a s e s ,  the  p o i n t s c o r r e s p o n d i n g to o r t h o - n i t r o benzoic a c i d l i e off  the  well  line.  The poor agreement o f the data w i t h a l l the  equations  mentioned serves to emphasize the s p e c i f i c nature o f e f f e c t s and t h e i r h i g h dependence on the n a t u r e o f reaction centre.  Since s t e r i c  from one r e a c t i o n to another,  it  f a c t o r s v a r y so  steric  the  greatly  i s d o u b t f u l whether a  s a t i s f a c t o r y g e n e r a l equation r e l a t i n g them w i l l ever be obtained.  (78) Table XI Polar and Steric Substituent Constants f o r Ortho-Substituents ' Using Taft's Method. (21) Ortho-Substituent  0"  E  s  Log  (%H / OBH ^ +  K  Methoxyl  -0.22  0.99  -1.01  Ethoxyl  -0.18  0.90  -1.01  Fluoro  0.41  0.49  0.48  Chloro  0.37  0.18  0.55  Bromo  0.38  0.00  0.61  Methyl  0.00  0.00  Iodo  0.38  -0.20  0.64  Nitro  0.97  -0.75  -0.10  0  +  (79) Table XII E l e c t r o n i c and S t e r i c S u b s t i t u e n t Constants f o r OrthoSub s t i t u e n t s u s i n g F a r t h i n g ' s and Nam's Method. (22)  Log Ortho-Substituent  + K  OBH"  +  °~"E  +  +  =  0""p  ^"o  ^~S  4.  " G"o~ ^~E  Methyl  -0.05  -0.31  0.29  0.60  Ethyl  -0.04  -0.29  0.43  0.72  i-Propyl  0.06  -0.28  0.56  0.34  t-Butyl  0.44  -0.25  0.67  0.92  Fluoro  0.43  -0.07  0.93  1.00  Chloro  0.50  0.11  1.26  1.15  Bromo  0.56  0.15  1.35  1.20  Iodo  0.59  0.13  1.34  1.21  Hydroxyl  -0.39  -0.92  1.22  2.14  Methoxyl  -1.06  -0.76  0.11  0.37  Nitro  -0.15  0.78  2.03  1.25  -0.13  0.51  0.64  (X -Naphthyl  0.03  O-EtO  «f o +  o-Me(J  a * 3.0 4  o 4  o-N0 -3.0  Z  10  2  10  FIG.I3-RELATIONSHIP B E T W E E N L O G % . FOR ORTHO-SUBST BENZOIC ACIDS AND  7Q* USING T A F T ' S M E T H O D .  FIG. 1 4 - R E L A T I O N S H I P ACIDS  AND  / t T  s  B E T W E E N  USING  LOG XK  FARTHING'S  l  0  AND  F O R  ORTHO-SUBST.  NAM'S  METHOD.  (32) Part II -  Benzophenones  The a b s o r p t i o n maxima and b a s i c i t i e s analogues  o f the  s t u d i e d are l i s t e d i n Table X I I I .  benzophenone  It i s  again  emphasized t h a t although the b a s i c i t y constants are given to 0.01 H  0  unit,  they are l i k e l y a c c u r a t e to no more than  ±0.1 u n i t . The b a s i c i t i e s  o f the o r t h o - s u b s t i t u t e d b e n z o i c  acids  were s t u d i e d because these compounds p r o v i d e an o p p o r t u n i t y f o r d e t e r m i n i n g the r e s u l t s e f f e c t s on the s t r e n g t h s  of s t e r i c  and e l e c t r o n i c p r o x i m i t y  o f weak b a s e s .  With the  exception  o f 4j4 -dimethoxybenzophenone, the the compounds l i s t e d  in  T  Table X I I I ,  on the otherhand, p r o v i d e a s e r i e s i n which  ordinary proximity factors basicities.  should have l i t t l e  e f f e c t on the  The major f a c t o r a f f e c t i n g the b a s i c i t i e s  these compounds should be the s t e r i c  i n h i b i t i o n of  of  resonance  r e s u l t i n g from the l a c k o f c o p l a n a r i t y o f the aromatic r i n g s and the c a r b o n y l group.  This stereoelectronic  effect  is  c o n t r o l l e d by the s e m i - r i g i d s t r u c t u r e o f the molecules and thus i t s pKgjj+  e f f e c t s on the b a s i c i t i e s  can be s t u d i e d .  o f 4j 4'-dimethoxybenzophenone  measured by Stewart  (63);  its  has been p r e v i o u s l y  d e t e r m i n a t i o n was  i n t h i s work as a check o f the method. r e l a t i v e to benzophenone  The  Its  repeated  enhanced b a s i c i t y  i s undoubtedly due to the  electron-  d o n a t i n g e f f e c t s o f the para-methoxyl groups. The o r d e r o f the b a s i c i t i e s  o f anthrone,  and benzophenone are e a s i e s t to r e c o n c i l e .  dibenzosuberone The v a l u e s may  be compared with those p r e v i o u s l y measured f o r anthrone and  (83) Table XIII B a s i c i t y C o n s t a n t s , A b s o r p t i o n Maxima and Slopes o f L i n e a r B a s i c i t y P l o t s o f S u b s t i t u t e d Benzophenones.  In 4 5 f e S 0 4 In 96$H S04  -pK H*  2  B  Compound  Gra.  Lin.  Ave.  Slope ^jnax  Benzophenone  6.20  6.12  6.16  0.87  262  4,4 dimethoxy benzophenone  4.39  4.38  4.39  0.86  300  9-Fluorenone  6.81  6.48  6.65  1.00  257  9-Anthrone  4.99  5.05  5.02  0.93  2,3;6,7dibenzo 5.64 suberone  5.73  5.69  2,3;6,7dibenzo 5.23 tropone  5.27  5.25  f  b  max  *max  ^max  4.22  344  4.37  403  4.63  4.92  266  4.72  275  4.18  351  4.38  0.94  276  4.13  358  4.36  1.07  262  4.60  293  4.96  b  a  t  4.33  a  a - In 23fcS04 b - A good l i n e a r plot was not obtained; PKBH value chosen from Ho intercept of that portion of p l o t whose slope was closest to unity. +  (84) benzophenone o f -5.5  and -6.0,  respectively  anthrone i s a near p l a n a r molecule  (65),  (64).  Since  the 7T - e l e c t r o n  o v e r l a p between the benzene n u c l e i and the c a r b o n y l group w i l l be near a maximum.  T h e r e f o r e , the s t a b i l i z a t i o n o f  the conjugate a c i d o f anthrone by resonance ening effect)  (a base  should a l s o be a t a maximum, and the  o f the molecule  strength basicity  high.  XXVII  The r e l a t i v e l y h i g h d i p o l e moment o f anthrone (66)  indicates  (3.46  debye)  t h a t s i m i l a r resonance may a l s o be important  i n the unprotonated form.  Such resonance would tend to  i n c r e a s e the e l e c t r o n d e n s i t y on the oxygen and so would a l s o be a base s t r e n g t h e n i n g  effect.  Jones has c a l c u l a t e d the minimum angle between the planes o f the benzene n u c l e i i n benzophenone  to be 30°  (65).  Assuming t h a t both phenyl r i n g s are e q u i v a l e n t and s t r i v i n g f o r a maximum degree o f c o n j u g a t i o n with the c a r b o n y l group,  (85) one might d i s t r i b u t e t h i s angle e q u a l l y between the two h a l v e s o f the molecule and so a r r i v e a t an angle o f 15° between each r i n g and the c a r b o n y l group (67).  T h e r e f o r e , s i n c e the  o f TT - o r b i t a l o v e r l a p and the degree o f resonance i n v e r s e l y as c o s 0 2  (0  i s the i n t e r p l a n a r angle  resonance s t a b i l i z a t i o n i n benzophenone and i t s a c i d w i l l be lower than t h a t i n anthrone.  degree  varies ),  the  conjugate  T h i s f a c t should  account f o r the lower b a s i c i t y o f benzophenone compared to anthrone. Models i n d i c a t e the maximum degree o f TT - o v e r l a p between the aromatic r i n g s and the c a r b o n y l group to be about the same i n dibenzosuberone as i n benzophenone. a l o n e , one might p r e d i c t equal b a s i c i t i e s  From t h i s  f o r the two m o l e c u l e s ,  whereas T a b l e X I I I i n d i c a t e s the suberone to be the base.  It  fact  stronger  should be remembered, however, t h a t the r i n g s i n  benzophenone are f r e e to r o t a t e - and due to thermal v i b r a t i o n s v e r y p r o b a b l y do - whereas the r i n g s i n dibenzosuberone are held r i g i d l y .  T h i s p r o b a b l y r e s u l t s i n the average  degree  o f c o n j u g a t i o n i n benzophenone b e i n g l e s s than t h a t i n the suberone.  The chances f o r resonance s t a b i l i z a t i o n o f the  conjugate a c i d w i l l t h e r e f o r e be g r e a t e r i n the l a t t e r and the b a s i c i t y t h e r e f o r e h i g h e r .  The i n d u c t i v e e f f e c t o f the  methylene g r o u p s - i n dibenzosuberone which i s absent  in  benzophenone may a l s o c o n t r i b u t e to the enhanced b a s i c i t y o f the former. At f i r s t g l a n c e , anomalously low.  the b a s i c i t y o f fluorenone appears  Since fluorenone i s a p l a n a r molecule  (86) one might expect i t If,  however,  pentadienone,  to have a s i m i l a r b a s i c i t y to anthrone.  we c o n s i d e r fluorenone as a s u b s t i t u t e d a reason f o r i t s  cyclo-  low b a s i c i t y may be seen.  Cyclopentadienone has o n l y f i v e  TT - e l e c t r o n s  in its  ring  system but would p r e f e r s i x i n o r d e r to form a pseudo-aromatic system w i t h An+2 "FT-electrons.  T h i s tendency should l e a d  to a p o l a r i z a t i o n o f the carbon-oxygen bond i n an  opposite  d i r e c t i o n to u s u a l w i t h a decreased e l e c t r o n d e n s i t y on the oxygen atom.  A similar effect,  although to a l e s s e r e x t e n t ,  should o c c u r i n fluorenone as shown i n s t r u c t u r e X X V I I I .  +  XXVIII Evidence f o r p o l a r i z a t i o n o f t h i s  s o r t i s found i n the  c a r b o n y l s t r e t c h i n g frequency o f f l u o r e n o n e ,  which i s 1720 cm."  (63) as compared to t h a t i n anthrone o f 1667 e m . - l (69). The decreased e l e c t r o n d e n s i t y on oxygen w i l l lower  its  (87) b a s i c i t y as evidenced i n the pK pj+ v a l u e . B  difference  i n the b a s i c i t i e s  The 1.6  units  o f anthrone and f l u o r e n o n e  i n d i c a t e s t h a t t h i s tendency f o r a r o m a t i z a t i o n i s  sufficient  to overcome much o f the base strengthening, e f f e c t o f due to the p l a n a r i t y o f the m o l e c u l e , responsible It  which appears to be  f o r the enhanced b a s i c i t y o f a n t h r o n e .  i s noted t h a t the b a s i c i t y o f the  i s o n l y s l i g h t l y h i g h e r than t h a t o f  dibenzotropone  dibenzosuberone.  Tropone, the parent compound, i s known to be q u i t e i n that i t  conjugation  forms h y d r o c h l o r i d e and p i c r a t e s a l t s  basic  (70).  In a d d i t i o n , i t undergoes none o f the u s u a l c a r b o n y l r e a c t i o n s , such as the formation o f 2 , 4 - d i n i t r o p h e n y l h y d r a z o n e s , and has an anomalously low i n f r a - r e d c a r b o n y l frequency (71)  and a h i g h d i p o l e moment (4.3 debye)  (72).  (163$ cm. "'") -  The low  c a r b o n y l frequency and h i g h d i p o l e moment are i n d i c a t i v e o f a h i g h e l e c t r o n d e n s i t y on the oxygen and are i n harmony with i t s  r e l a t i v e l y high b a s i c i t y .  On t h i s b a s i s one would  expect dibenzotropone to have a b a s i c i t y as high o r even higher.  The f a c t t h a t i t  ment w i t h i t s  does not i s ,  however,  in  agree-  known p h y s i c a l and chemical p r o p e r t i e s :  has a normal d i p o l e moment, (1660 cm.-1) (71)  is  its  carbonyl stretching  s i m i l a r to t h a t o f benzophenone,  it  frequency it  forms no s a l t s with a c i d s and r e a c t s w i t h 2 , 4 - d i n i t r o p h e n y l hydrazine.  From t h i s  the molecule i s f o l d e d  evidence  i t has been concluded t h a t  (73).  An attempt was made to measure the b a s i c i t y o f benzanthrone  (XXIX) w i t h the thought t h a t i t may be  quite  (88) basic since i t  i s r e a l l y a derivative of  perinaphthenone  (XXX) which i s known to be a r e l a t i v e l y s t r o n g base w i t h a p K + o f -3.9 BH  (74)  XXIX  XXX  As mentioned e a r l i e r ,  the b a s i c i t y o f t h i s  compound could  not be measured by the methods used i n t h i s work due to h i g h i n s o l u b i l i t y i n aqueous a c i d s . f o r i t o f -3.2 which i n d i c a t e s  has,  however,  that i t  A basicity  constant  appeared i n the l i t e r a t u r e  i s indeed a c o n s i d e r a b l y  base than the o t h e r benzophenone analogues work.  its  (64),  stronger  studied i n  this  (89) SUGGESTIONS FOR FUTURE WORK. An i n v e s t i g a t i o n o f the b a s i c i t i e s benzamides seems o f i n t e r e s t  of  ortho-substituted  i n t h a t i t may t e n d to throw  some l i g h t on the q u e s t i o n o f N - p r o t o n a t i o n versus O - p r o t o n a t i o n o f amides.  I f the b a s i c i t i e s  o f the amides demonstrate a  s i m i l a r dependence upon the ortho e f f e c t s o f as the b a s i c i t i e s If,  o f the a c i d s ,  O - p r o t o n a t i o n w i l l be i n d i c a t e d .  on the o t h e r hand, the b a s i c i t i e s  dependence upon ortho e f f e c t s , Further studies i n t h i s  substituents  show a d i f f e r e n t  N - p r o t o n a t i o n w i l l be i n d i c a t e d .  l a b o r a t o r y on the  basicities  o f the benzophenone analogues are a n t i c i p a t e d .  In p a r t i c u l a r ,  a s y n t h e s i s and d e t e r m i n a t i o n o f the pKgpj o f 2,3;7>8-dibenzo+  cyclo&ctanone i s i n t e n d e d . most s t a b l e  Models i n d i c a t e t h a t the  sterically  conformation o f t h i s molecule i s one i n which  the p r o p y l e n e b r i d g e h o l d s the p h e n y l r i n g s v i r t u a l l y p e r p e n d i c u l a r to the c a r b o n y l group. c o n s i d e r a b l e decrease  T h i s should r e s u l t i n a  i n b a s i c i t y from t h a t o f  benzosuberone.  The b a s i c i t y o f 2 , 2 ' -dimethylbenzophenone may a l s o interesting.  prove  A model o f t h i s molecule shows t h a t i t s  rings  may assume a conformation s i m i l a r to t h a t i n benzophenone. 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