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Quantitative studies of hydrogen bonding in ortho-substituted phenols using proton magnetic resonance. 1963

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QUANTITATIVE STUDIES OP HYDROGEN BONDING IN ORTHO-SUBSTITUTED PHENOLS USING PROTON MAGNETIC RESONANCE by ERNEST ALBERT ALLAN B.Sc., The 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 the Department of Chemistry We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY; OF BRITISH COLUMBIA, .APRIL,. 1963. In presenting t h i s ' t h e s i s ' i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e for reference and study. I further agree that per- mission for extensive copying of t h i s t h e s i s f o r . s c h o l a r l y purposes may be granted by the Head of my Department or by h i s representatives. I t i s understood that copying, or p u b l i - c a t i o n of t h i s t h e s i s for f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission. Department of The U n i v e r s i t y of B r i t i s h Columbia, Vancouver 8, Canada. i A B S T R A C T The chemical s h i f t of protons i n i n t r a m o l e c u l a r hydrogen bonds has been measured i n ij.1 ortho s u b s t i t u t e d phenol type compounds. The change i n chemical s h i f t " ^ ^OH " on f o r m a t i o n of these hydrogen bonds i s taken as the d i f f e r e n c e between the i n f i n i t e d i l u t i o n chemical s h i f t of the parent phenol compound i n CClj^ s o l u t i o n and the chemical s h i f t measured f o r the p r o t o n i n the i n t r a - m olecular hydrogen bond. This change i n chemical s h i f t i s c o r r e l a t e d with the corresponding frequency s h i f t " £\~VOH " i n the -OH s t r e t c h i n g r e g i o n of the i n f r a - r e d spectrum. The d i l u t i o n chemical s h i f t f o r the -OH p r o t o n i n the o-halophenols has been i n v e s t i g a t e d over a concen- t r a t i o n range 1 - 5 mole % i n CS2 and a temperature r e g i o n -53 to 1 0 7 ° C Using the i n f i n i t e d i l u t i o n s h i f t v a l u e s , the e q u i l i b r i u m constants of the c i s - t r a n s c o n v e r s i o n were obt a i n e d . Values f o r £ C I S , the chemical s h i f t of the completely hydrogen bonded form; & TRANS' t h e cilem^c&l s h i f t of the unbonded form, and AR , the enthalpy of formation of the hydrogen bond, were a l s o c a l c u l a t e d . Prom these r e s u l t s a value f o r AH, the enthalpy of f o r m a t i o n f o r the dimer was c a l c u l a t e d , assuming t h a t the major dimer species i n s o l u t i o n was formed from the combination of a c i s and t r a n s bonded form. i i Temperature s t u d i e s of the change i n chemical s h i f t of the -OH p r o t o n f o r 2 , 1 ^ , 6 - t r i h a l o s u b s t i t u t e d phenols i s a l s o r e p o r t e d . The temperature range i n t h i s case was 0°G to 111°0. ABSTRACT APPROVED v i i ACKNOWLEDGMENT I wish to express my thanks to Dr. L. W. Reeves f o r h i s a s s i s t a n c e and s u p e r v i s i o n throughout the course of t h i s work. i i i TABLE OP CONTENTS Page ABSTRACT i ACKNOWLEDGMENT ' v i i LIST OP TABLES v LIST OP FIGURES v i CHAPTER I - INTRODUCTION 1 I) Gen e r a l C o n s i d e r a t i o n s 1 i i ) I n f r a r e d and D i p o l e Moment Studie s o f In t r a m o l e c u l a r Hydrogen Bonds i n Phenols $ CHAPTER I I - EXPERIMENTAL PROCEDURE 8 i ) P r e p a r a t i o n of Samples 8 i i ) S y n t h e s i s o f 2,i}.,6-trisubstituted halophenols 10 i i i ) Measurement of Chemical S h i f t s 12 CHAPTER I I I - EXPERIMENTAL RESULTS l l j . i ) C o r r e l a t i o n of Chemical S h i f t w i t h i n f r a r e d s t r e t c h i n g frequency l lj. i i ) Temperature S t u d i e s of o-Halophenols 20 i i i ) Temperature S t u d i e s of 2,lj.,6-Trisubstituted Phenols 25 CHAPTER IV - DISCUSSION 27 i ) C o r r e l a t i o n o f Chemical S h i f t s with I n f r a r e d S t r e t c h i n g Frequency 27 i i ) C a l c u l a t i o n o f E n t h a l p i e s of Formation f o r the I n t r a m o l e c u l a r Hydrogen Bond i n o-Halophenols 30 TABLE OP CONTENTS i i i ) C a l c u l a t i o n o f E n t h a l p i e s o f Formation o f the C i s - T r a n s Dimer f o r o-Halophenols i v ) V a r i a b l e Temperature S t u d i e s of 2,I|.,6-trisubstituted Phenols BIBLIOGRAPHY V LIST OP TABLES Page TABLE 1 TABLE 2 TABLE 3 TABLE k TABLE 5 TABLE 6 TABLE 7 TABLE 8 TABLE 9 I n f i n i t e d i l u t i o n s h i f t s o f parent phenol compounds 1I4. ^ 0 ^ , M a n ^ A V O H f o r phenol type compounds c o n t a i n i n g an i n t r a m o l e c u l a r hydrogen bond 1 7 Values of the chemical s h i f t c£,in c y c l e s / s e c from cyclohexane f o r o - c h l o r o - phenol i n the range s t u d i e d 2 1 Values of the chemical s h i f t i n c y c l e s / s e c from cyclohexane f o r o-bromo- phenol 2 2 Values of the chemical s h i f t cCi i n c y c l e s / s e c from cyclohexane f o r o-iodo- phenol 2 3 Values of the chemical s h i f t r\ i n c y c l e s / s e c from cyclohexane f o r o - f l u o r o - phenol 2l|. Values of the chemical s h i f t cSM i n c y c l e s / s e c from cyclohexane f o r some 2 , l j . , 6 - t r i h a l o s u b s t i t u t e d phenols 26 Values of K ) f T ) and AH f o r o-chloro, o-bromo and o-iodophenol 3 5 Values of K^,^ f o r o-bromo, o - c h l o r o and o-iodophenol l\2 v i LIST OP FIGURES To f o l l o w page: FIGURE 1 - R e p r e s e n t a t i v e pmr spectrum of o-halophenols showing s i d e bonds a p p l i e d b e f o r e and a f t e r -OH resonance peak. FIGURE 2 - P l o t of A C f M v s f o r the compounds g i v e n i n Table 2. FIGURE 3 - D i l u t i o n chemical s h i f t s f o r the -OH p r o t o n i n o-chlorophenol as a f u n c t i o n of temper- a t u r e . FIGURE k - D i l u t i o n chemical s h i f t s f o r the -OH p r o t o n i n o-bromophenol as a f u n c t i o n of temper- a t u r e . FIGURE 5 - D i l u t i o n chemical s h i f t s f o r the -OH pr o t o n i n o-iodo and o - f l u o r o p h e n o l as a f u n c t i o n o f temperature. F I G U R E 6 - The i n f i n i t e d i l u t i o n s h i f t " A " of o-bromophenol p l o t t e d a g a i n s t temperature. F I G U R E 7 - Chemical s h i f t s of some 2,lj.,6-trihalo- s u b s t i t u t e d phenols p l o t t e d a g a i n s t temperature. F I G U R E 8 - P l o t of l o g K x C t > a g a i n s t J<£- f o r o-bromo, o-chlor o and o-iodophenol. 13 19 21 22 2k 25 26 kl 1 CHAPTER I INTRODUCTION. i ) General C o n s i d e r a t i o n s The study and i n t e r p r e t a t i o n o f f a c t o r s which account f o r the form a t i o n o f a hydrogen bond has been o f c o n s i d e r a b l e i n t e r e s t to chemists f o r some y e a r s . Many experimental techniques, i n c l u d i n g d i e l e c t r i c constant measurements, vapour p r e s s u r e s t u d i e s and s p e c t r o s c o p i c methods, p a r t i c u l a r l y i n the i n f r a r e d r e g i o n , have been adopted f o r the study of such bonding (1) (2) ( 3 ) . Since 1953* the use of n u c l e a r magnetic resonance spectroscopy as a means of s t u d y i n g the hydrogen bond has i n d i c a t e d t h a t t h i s r e l a t i v e l y new method w i l l equal, I f not surpass, o t h e r means f o r o b t a i n i n g i n f o r m a t i o n (Ij.). The r e c e n t l y p u b l i s h e d work of Pimentel and M c C l e l l a n (3) i s p a r t i c u l a r l y u s e f u l , s i n c e a g e n e r a l survey o f a l l p e r t i n e n t l i t e r a t u r e on hydrogen bonding to 1956, as w e l l as some to 1959* i s i n c l u d e d as an appendix. Pimentel and M c C l e l l a n d e f i n e a hydrogen bond i n the f o l l o w i n g manner: "A hydrogen bond e x i s t s between a f u n c t i o n a l group -X-H and an atom or group of atoms Y i n the same or d i f f e r e n t molecule when (a) there i s evidence o f bond f o r m a t i o n ( a s s o c i a t i o n or c h e l a t i o n ) (b) there i s evidence t h a t t h i s new bond l i n k i n g 2 -X-H and Y s p e c i f i c a l l y i n v o l v e s the hydrogen a l r e a d y bonded to X . " # U s i n g t h i s d e f i n i t i o n , one must d i s t i n g u i s h between the two types of hydrogen bonding l i k e l y to occur; the i n t e r m o l e c u l a r , where the f u n c t i o n a l groups -Xr-H and Y are i n d i f f e r e n t molecules, and the i n t r a m o l e c u l a r , where both -X-H and Y are i n the same molecule, with the d i s t a n c e between them f a v o u r a b l e to the f o r m a t i o n o f a bond. In 1953 A r n o l d and Packard (5) (6) d i s c o v e r e d t h a t the chemical s h i f t o f the -OH pr o t o n s i g n a l o f e t h y l a l c o h o l was both c o n c e n t r a t i o n and temperature dependent. Upon d i l u t i o n i n non-polar s o l v e n t s or upon an Increase In temperature the s i g n a l was observed to s h i f t to h i g h e r f i e l d . No s i m i l a r s h i f t was observed f o r the methyl or methylene p r o t o n s . The l i f e t i m e o f the hydrogen bond i s very s h o r t and the h i g h f i e l d s h i f t on d i s s o c i a t i o n of these bonds i s a time average of the v a r i o u s environments i n the s o l u t i o n (7) ( 8 ) . The average s h i e l d i n g parameter thus i n c r e a s e s as hydrogen bonds are broken i n the system, with consequent resonance at h i g h e r f i e l d f o r the p r o t o n i n v o l v e d . F u r t h e r s t u d i e s on e t h a n o l (9)> v a r i o u s phenols ( 1 0 ) , and a c e t i c a c i d (11) seem to co n f i r m t h i s i n t e r - p r e t a t i o n . Pople and M a r s h a l l ( 1 2 ) , i n an attempt to e x p l a i n •K-S.O.Plmentel and A.L.McLellan, The Hydrogen Bond. W.H. ., Freeman & Co., San F r a n c i s c o (I96OJ, p.fe> 3 the low f i e l d s h i f t of a p r o t o n on f o r m a t i o n of a hydrogen bond, c o n s i d e r e d the c o n t r i b u t i o n to the chemical s h i f t o f a hydrogen atom by an e l e c t r i c f i e l d E. The r e s u l t o b t a i n e d cf = - 881 a* E Z (1) 2l5 m c*- where ^ = the chemical s h i f t m s mass of the p r o t o n c «... v e l o c i t y of l i g h t a = the Bohr r a d i u s g i v e s a s h i f t of l\. p.p.m. f o r an e l e c t r i c f i e l d of O.llj. atomic u n i t s , which i s the f i e l d t h a t would a r i s e from a s i n g l e e l e c t r o n a t a d i s t a n c e of l . l f A. Pople, Schneider and B e r n s t e i n (13) a l s o c a l c u l a t e d the c o n t r i b u t i o n to the chemical s h i f t produced by the magnetic f i e l d induced by the Y atom on bonding, but obtained a r e s u l t which p r e d i c t e d a s h i f t o p p o s i t e to that observed. Another term g i v i n g a s i m i l a r r e s u l t would be the amount of c o v a l e n t c h a r a c t e r of the bond X-...H -Y, but t h i s would be d i f f i c u l t to e v a l u a t e q u a n t i t a t i v e l y . Pople (12) concluded that e l e c t r o s t a t i c e f f e c t s were of c o n s i d e r a b l e importance, s i n c e observed chemical s h i f t s are always to lower f i e l d when a hydrogen bond i s known to be formed. The major purpose of t h i s work was to evaluate the chemical s h i f t of the -OH group ( a t low c o n c e n t r a t i o n s , to reduce i n t e r m o l e c u l a r e f f e c t s ) f o r compounds known to c o n t a i n an i n t r a m o l e c u l a r hydrogen bond. The change i n chemical s h i f t A . C £ M from the i n f i n i t e d i l u t i o n chemical s h i f t of the parent compound was then c o r r e l a t e d with the change i n frequency A > ^ H of the -OH s t r e t c h i n g frequency i n the i n f r a r e d r e g i o n . Por some weakly bonded s p e c i e s , a study of the chemical s h i f t as a f u n c t i o n of temperature and c o n c e n t r a t i o n allowed the c a l c u l a t i o n of a heat o f f o r m a t i o n A E f o r the hydrogen bond. An ortho s u b s t i t u e n t on a phenol or n a p t h o l i s f a v o u r a b l y s i t u a t e d f o r the f o r m a t i o n of an i n t r a m o l e c u l a r bond with the -OH group o f the compound. The study was c o n f i n e d to compounds of t h i s type. 5 ( i i ) I n f r a r e d and D i p o l e Moment Studie s o f I n t r a m o l e c u l a r Hydrogen Bonds i n Phenols, . v Evans (1I4.) has r e p o r t e d an estimated b a r r i e r to r o t a t i o n f o r the -OH group i n phenol o f 3 . 7 kcal./mole. Upon p l a c i n g a s u b s t i t u e n t capable of hydrogen bond form a t i o n In the ortho p o s i t i o n the two s i t e s would no 1 l o n g e r be e q u i v a l e n t and the r e l a t i v e p o p u l a t i o n o f the unbonded to the bonded form ( t r a n s / c i s ) - w o u l d depend on the s t r e n g t h o f the hydrogen bond formed, n e g l e c t i n g i n t e r m o l e c u l a r e f f e c t s . In the -OH s t r e t c h i n g r e g i o n two bands are observed due to the trans and c i s forms. I f i n t e n s i t y measurements are used, an estimate o f the e q u i l i b r i u m c o n s t a n t may be made. One o f the f i r s t such s t u d i e s was made by Wulf and L i d d e l i n 1 9 3 5 ( 1 5 ) ( l 6 ) . Using d i l u t e s o l u t i o n s o f o-chlorophenol i n carbon t e t r a c h l o r i d e they observed two bands (7050 cm""' and 69IO cm" 1) i n the I n f r a r e d which were a t t r i b u t e d by P a u l i n g (17) to the frequency f o r a f r e e -OH (trans) and a bonded -OH ( c i s ) . S e v e r a l other workers s t u d i e d s i m i l a r compounds, but most of the work p r i o r to 1 9 5 3 was d i s c o u n t e d by Rossmy, L i i t t k e and Mecke ( 1 8 ) , who made new measurements on c a r e f u l l y p u r i f i e d o-chloro, o-bromo and o-iodophenol and 3tated that a l l e a r l i e r q u a n t i t a t i v e measurements were i n e r r o r due to the f a c t t h a t no s p e c i a l care had been taken by e a r l i e r workers to remove the phenol which was i n v a r i a b l y p r e s e n t . T h e i r 6 values f o r the t r a n s / c i s r a t i o (25° C) i n o - c h l o r o , o-bromo and o-iodophenol i n carbon t e t r a c h l o r i d e s o l u t i o n were' 1/37, and 1/12 r e s p e c t i v e l y . C a l c u l a t e d v a l u e s of were l . i j . kcal/mole f o r o-iodophenol and 2.1 kcal/mole f o r o-bromophenol. I t was s t a t e d t h a t the value f o r o-chlorophenol would be h i g h e r . Probably the most r e l i a b l e , and the most complete, i n f r a r e d data on the -OH s t r e t c h i n g r e g i o n of ©-substi- t u t e d phenols i s t h a t o b t a i n e d by Baker and co-workers (19-22). Fo r the o-halophenols, a f t e r p u r i f i c a t i o n by s e v e r a l passes through a gas chromatograph, Baker o b t a i n e d a c c u r a t e -OH s t r e t c h i n g f r e q u e n c i e s and from the band i n t e n s i t i e s c a l c u l a t e d t r a n s / c i s r a t i o s of 1/56, 1/38 and 1/13.5 f o r o - c h l o r o , o-bromo and o-iodophenol r e s p e c t i v e l y at 25° C. Subsequently, he i n v e s t i g a t e d the change i n frequency ( ) r e l a t i v e to phenol f o r a s e r i e s o f o - s u b s t i t u t e d phenols and concluded t h a t A HT« i s rough l y a measure of the s t r e n g t h o f the hydrogen bond formed. He f u r t h e r s t u d i e d 2 , 1 ^ , 6 - t r i h a l o s u b s t i t u t e d phenols where two bands are observed I f the halogens at the 2,6 p o s i t i o n s are d i f f e r e n t . Jones and Watkinson (23), i n t h e i r s t u d i e s of o-halophenols, obtained r e s u l t s markedly d i f f e r e n t from Baker's. Ri c h a r d s and Walker (2i|.-27) have i n v e s t i g a t e d a s e r i e s of s i m i l a r o r t h o - s u b s t i t u t e d compounds i n v a r i o u s s o l v e n t s and, where p o s s i b l e , have attempted to c o r r e l a t e 7 t h e i r d i p o l e moment measurements with the i n f r a r e d data of Baker. They have measured the t r a n s / c i s r a t i o s f o r the o-halophenols (26) In carbon t e t r a c h l o r i d e , c y c l o - hexane, benzene and dioxan. The order i n cyclohexane and carbon t e t r a c h l o r i d e was found to be I > F > B r and C l ; i n benzene I ? B r > C l and F; and i n dioxan I > B r > C l > P . A p r e v i o u s d i p o l e moment study i n carbon t e t r a c h l o r i d e by A n g i l o t t l and Gurran (28) gave the order F > C l > B r f o r the three o-halophenols s t u d i e d . 8 CHAPTER I I . EXPERIMENTAL PROCEDURE. i ) P r e p a r a t i o n of Samples. Commercially a v a i l a b l e compounds ( i . e . Eastman Kodak or A l d r i c h Chemicals) known to c o n t a i n an i n t r a - m o l e c u l a r hydrogen bond of the type -OH—Y were p u r i f i e d by f r a c t i o n a l d i s t i l l a t i o n o r r e c r y s t a l l i z a t i o n from s u i t a b l e s o l v e n t s such as cyclohexane, chloroform, ether, a l c o h o l or acetone. The s y n t h e s i z e d 2 , 1 ^ , 6-trihalo- s u b s t i t u t e d phenols were p u r i f i e d by r e c r y s t a l l i z a t i o n from n-heptane or pentane. F o r the temperature s t u d i e s , o - c h l o r o , o-bromo and o - f l u o r o p h e n o l were p u r i f i e d by two passes through a Beckman Megachrom gas chromatograph. Reagent grade carbon t e t r a c h l o r i d e and carbon d i s u l p h i d e which had been d r i e d over phosphorus pentoxide and d i s t i l l e d were used as s o l v e n t s i n the p r e p a r a t i o n of samples. In each case approximately one mole % o f i n t e r n a l standard (cyclohexane or t e t r a m e t h y l s i l a n e ) was added to the f r e s h l y d i s t i l l e d s o l v e n t , which was s t o r e d i n g l a s s e t h e r b o t t l e s . A l l s o l u t i o n s were made up by weight u s i n g a standard M e t t i e r a n a l y t i c a l balance and the mole f r a c t i o n o f phenol c a l c u l a t e d . In most cases s o l u t i o n s of approximately 5 ml. volume were made up and a p o r t i o n of the s o l u t i o n t r a n s f e r r e d to 5 mm. pyrex tubes which had been d r i e d and s t o r e d i n a d e s s i c a t o r over phosphorus 9 pentoxide. The tube was tranferred to a vacuum system, the solution was frozen i n l i q u i d nitrogen, and the tube sealed af t e r a i r over the sample had been pumped o f f . Two tubes of each solution were made. For the work on c o r r e l a t i o n of chemical s h i f t with infrared stretching frequency, carbon tetrachloride was used as solvent, but since i t s melting point i s too high for low temperature work (-23° G), carbon disulphide was used for the subsequent temperature studies (m.p. -108°C). Benzene, acetone, chloroform, or sim i l a r solvents are unsuitable f o r this type of study, since hydrogen bonding with solvent molecules would occur. 10 i i ) S y nthesis o f 2,1|.,6 t r i s u b s t i t u t e d h a lophenols. (a) 2-Fluoro-lj.,6-diiodophenol and 2-Ghloro- I4., 6 - d i i o dophenol The procedure f o l l o w e d was e s s e n t i a l l y that o u t l i n e d by Baker (21). E i g h t grams of potassium hydroxide and 0.10 mole o f the 6-halophenol were d i s s o l v e d i n 150 ml. of water. In a second beaker, $i\ gm. of p u l v e r i z e d i o d i n e were d i s s o l v e d i n a c o l d s o l u t i o n of 26 gm. of potassium hydroxide i n lj.00 ml. of water and immediately added to the phenol s o l u t i o n . Cold, 2$% s u l p h u r i c a c i d was added dropwise u n t i l a pH of 7- was reached. The excess of Iodine was removed by the a d d i t i o n o f s o l i d sodium t h i o s u l p h a t e . The o i l which formed s o l i d i f i e d on c o o l i n g i n i c e and was r e c r y s t a l l i z e d twice from n-heptane. m e l t i n g pt.(obs.) r e p o r t e d (21) ( c o r r . ) 2-fluoro-l J.,6-diiodophenol 56-56.5° C; 56-57° C 2-chloro -I|.,6-diiodophenol 93-95° 0 96 0 C (b) 2-Pluoro-ij.,6-dibromophenol and 2-Chloro-lj.,6- dibromophenol Prepared by the method o f R a i f o r d and Le Rosen(29). Twenty ml. of g l a c i a l a c e t i c a c i d and 0.10 mole of the ortho halophenol were cooled i n i c e and 12 ml. o f bromine were slowly added. The s o l u t i o n was l e f t 11 f o r one hour and was then poured i n t o i c e water. A 20%> s o l u t i o n of sodium b i s u l p h i t e was added to remove excess bromine. The crude product was d i s s o l v e d i n a $% s o l u t i o n o f sodium hydroxide, d e c o l o r i z e d w i t h c h a r c o a l , and f i l t e r e d . The r e s u l t i n g s o l u t i o n was a c i d i f i e d w i t h d i l u t e s u l p h u r i c a c i d to p r e c i p i t a t e the product, which was r e c r y s t a l l i z e d twice from n-heptane. m.p. (obs.) m.p.(reported) ( c o r r . ) 2-Pluoro-if,6-dibromophenol 33-3k-5°c 3k~35°G 2-Chloro-lf,6-dibromophenol 70-71°G; 12 i i i ) Measurement o f Chemical S h i f t s . A standard V a r i a n I4.O Mc/sec Vij.300 High R e s o l u t i o n Spectrometer was used f o r a l l measurements, except where noted. In order to achieve the base l i n e s t a b i l i t y - r e q u i r e d f o r measurements a t c o n c e n t r a t i o n s o f one mole %, a twelve v o l t b a t t e r y was p l a c e d i n p a r a l l e l with the f i l a m e n t supply to the V - ^ I O V a r i a n RF u n i t . Chemical s h i f t s were measured u s i n g the s i d e band technique, and sid e band f r e q u e n c i e s were measured si m u l t a n e o u s l y by adopting the c i r c u i t recommended by V a r i a n NMR Instrument Owner B u l l e t i n , February 1, i 9 6 0 , which allowed the H e w l i t t Packard model 522B E l e c t r o n i c Frequency Counter to co n t i n u o u s l y monitor the s i d e bands a p p l i e d . A H e w l i t t Packard 200 CD O s c i l l a t o r was used to supply s i d e bands. The v a r i a b l e temperature apparatus was t h a t d e s c r i b e d by R e i d and Connor (30). For the work on c o r r e l a t i o n o f chemical s h i f t s with -OH s t r e t c h i n g frequency, the measurements were made by superimposing a sideband f o r the i n t e r n a l cyclohexane standard upon the phenol -OH band u s i n g a Dumont type 30I4- AR O s c i l l o s c o p e . Two measurements on each o f the two samples were c o n s i d e r e d s u f f i c i e n t . The estimated accuracy i s 1.5 c y c l e s / s e c , or O.Ol}. p.p.m. Si n c e chemical s h i f t s were r e q u i r e d to £ 0.1 c y c l e s / s e c f o r the temperature s t u d i e s , another technique was adopted. The phenol -OH band 1 3 was recorded on c h a r t paper with s i d e bands before and a f t e r i t , u s i n g a Speedomax model H r e c o r d e r (see f i g u r e 1 ) . The s i d e band was switched o f f and changed while sweeping through the -OH band. The d i s t a n c e between s i d e bands was from 1 0 - 1 2 c y c l e s / s e c and sweep r a t e s were such that the l e n g t h on the c h a r t paper was approximately 1 cm./cycle. F i v e to e i g h t measurements were taken f o r each sample a t a l l temperatures. The i n t e r n a l standard f o r the o-halophenols was cyclohexane, while f o r the 2,1].,6 t r i s u b s t i t u t e d h a l o - phenols t e t r a m e t h y l s i l a n e was used and the valu e s reduced to that f o r cyclohexane. As measured i n t h i s l a b o r a t o r y , the cyclohexane s i g n a l i s 57.6 c y c l e s / s e c (ij.0 Mc/sec RF) or l.i|i). p.p.m. to low f i e l d from t e t r a m e t h y l s i l a n e . The average standard d e v i a t i o n f o r o-chloro and o-bromophenol measurements was i 0 . 1 c y c l e s / s e c ; f o r o - f l u o r o p h e n o l 0 . 2 c y c l e s / s e c . and f o r the t r i s u b s t i t u t e d phenols i 0 . 5 c y c l e s / s e c . / I \( Figure 1, Representative pmr spectrum ofo-halophenols showing sidebands applied before and after the phenol -OH resonance peak. The sample i n t h i s case was o-chloro- phenol (2 mole%) at 27 C. The sweep rate was .86 cm./cycle, . / 111. CHAPTER I I I EXPERIMENTAL RESULTS. I) C o r r e l a t i o n of Chemical S h i f t w i t h i n f r a r e d s t r e t c h i n g frequency. The v a l u e s f o r the chemical s h i f t at i n f i n i t e d i l u t i o n and the -OH s t r e t c h i n g frequency f o r the parent compounds phenol, 1-naphthol and 2-naphthol are giv e n i n Table 1 below: TABLE 1 Compound ( r e f e r r e d to i n f i n i t e d i l u t i o n i n CCl^) ( i ) (Observed) p.p.m. (2) (Corrected) p.p.m. (3) cm"1 phenol 3.12 2.80 3604 1-naphthol 3.78 2.86 3594 2-naphthol 3.k9 2.66 3601 Column 1 g i v e s the observed i n f i n i t e d i l u t i o n s h i f t of the compound, while column 2 g i v e s the chemical s h i f t c o r r e c t e d f o r r i n g c u r r e n t u s i n g the model of Pople (31). The c i r c u l a t i n g c trons of the benzene r i n g l e a d to an a d d i t i o n a l magnetic f i e l d a t the protons near the r i n g which r e i n f o r c e s the main f i e l d H Q, l e a d i n g to resonance at lower v a l u e s of H D than would normally be the case. This l e a d s to an approximate c o r r e c t i o n f o r the chemical s h i f t , g i v e n by: 15 Ao-= - e2- a 2 - 2 m c * R where: e = charges on the e l e c t r o n m - mass of an e l e c t r o n a - r a d i u s o f the benzene r i n g ( 1 . 1 ) . A) c a v e l o c i t y o f l i g h t R = d i s t a n c e from the pro t o n under c o n s i d e r a t i o n to the centre of the r i n g A C T S the change i n chemical s h i f t due to the i n f l u e n c e o f the ~fT e l e c t r o n s . These c o r r e c t i o n s were a l s o made f o r the compounds c o n t a i n i n g i n t r a m o l e c u l a r hydrogen bonds. The e x p e r i - m e n t a l l y observed v a l u e s , along with c o r r e c t e d v a l u e s f o r CT are g i v e n i n Table 2 . Column 3 gives the value o f A O ~ H ( c o r r e c t e d ) from the parent compound w h i l e column Ij. g i v e s the change i n frequency A V o H (cm" 1). A graph of AV£H a g a i n s t & 0 ^ H i s giv e n i n f i g u r e 2 , the numbers on the graph r e f e r r i n g to the compounds i n Table 2 ( 3 2 ) . I n f r a r e d data were ob t a i n e d on a P e r k i n Elmer model 2 1 Double Beam i n f r a r e d spectrometer u s i n g yearn., or 1 m m . sodium c h l o r i d e c e l l s . A l l measurements were checked, where p o s s i b l e , w i t h those o f Baker ( 2 0 ) ( 2 1 ) . With the e x c e p t i o n o f A\Crt f o r o - n i t r o p h e n o l , where Baker's value of 3k& cm"' di s a g r e e s with aur value of 361). cm"', there was good agreement. The 2 , ^ , 6 t r i h a l o s u b s t i t u t e d phenols, where the 2 , 6 p o s i t i o n s c o n t a i n d i f f e r e n t halogens, are not i n c l u d e d i n f i g u r e 2 , s i n c e two bands are observed i n the i n f r a r e d , corresponding 16 to the two d i f f e r e n t "bonds formed. As stated previously, the magnetic resonance signal observed i s an average of these two forms (8). 17 TABLE 2. Compound ACT AV O M c o r r e c t e d c o r r e c t e d p.p.m. p.p.m. p.p.m. cm" 1. S a l i c y l a l d e n y d e -9i5i -8.83 6.71 471 2. 5-N i t r o s a l i c y l a l deny de -10.05 -9.37 7.25 500 3. 5-Br omo s a l 1 c y l a l deny de -9.38 -8.70 6.58 454 4. o-Nitrophenol -9.14 -8.1+6 6.34 l o t 5. 2,4 D i n i t r o p h e n o l -9.64 -8.96 6.84 388 6. Methyl S a l i c y l a t e -9.12 -8.44 6.32 395 7. o-Bromo Benzoic A c i d -11.25 -10.70 8.58 570, 96 8. o-Chlorophenol -3.97 -3.2? 1.17 61 9. 2,4 D i c h l o r o p h e n o l -3.99 -3.31 1.19 63 10. 2,4,6 T r i c h l o r o p h e n o l -4.20 -3.52 1.40 75 11. o-Bromophenol -3.95 -3.27 1.15 92 12. 2,4-Dibromophenol -3.98 -3.30 1.18 83 13. o-Iodophenol -3.68 -3.00 O.98 105 l4. o-Me thoxyphenol -3*98 -3.30 1.18 60 15. 2,6 DImethoxyphenol -3.82 -3.14 1.02 56 16. o - A l l y l p h e n o l -3.87 -3.19 1.07 63 17. o - C r e s o l -3.12 -2.44 0.32 -8 -» c f . r e f . 20. 18 TABLE 2 (cont'd) Compound p.p.m. c o r r e c t e d c o r r e c t e d p.p.m. p.p.m. cm -1 18. -12.^9 -11.54 942 (Overlaps (-C-H (stret<ch ( 3000cm-« 19. -11.27 -10.59 847 20. -11.13 HC=N -1045 8.33 5kk 21. -11.36 -10.68 8.56 532 22. -11.03 -10.08 7.96 531 19 TABLE 2 (cont'd) Compound ( V - <Fj < <T - 0"OH) ^ p.p.m. c o r r e c t e d p.p.m. c o r r e c t e d p.p .m. cm" 23. l - N i t r o - 2 - n a p h t h o l -10.53 -9.70 7.04 538 24. 2,1+ D i n i t r o - l - n a p h t h o l -11.20 -10.28 7.42 543 25. 2,5 D i c h l o r o p h e n o l -I+.02 -3.34 4- 1.22 65 26. o-Fluorophenol -3.62 -2.94 0.82 18 27. o-Phenylphenol -3.67 -2.99 0.87 45 28. lj>-Chloro-2-nitrophenol -9.06 -8.38 6.26 354 29. 2-Bromo-l+-phenylphenol -3.93 -3,25 1.13 75 30. 1-Bromo-2-naphthol -4.45 -3.62 1.79 82 31. 2,4-Dibromo-l-naphthol -4.4o -3.48 1.54 89 32? o ( M e t h y l t h i o ) p h e n o l -4.67 -3.99 1.87 194 33 a 2 Methyl-6(Methylthio) phenol -5.20 -4.52 2.40 208 31^ 0 - ( i s o p r o p y l t h i o ) p h e n o l -5.18 -4.50 2.30 205 35. 2,3,4,6- Te t r a c h l o r o p h e n o l - 4 . 1+3 -3.75 I.63 80 36. 2-Chloro-l+,6-dibromophenol -4.27 -3.59 1.47 - • 37. 2-Chloro-l+,6-diiodophenol-l+.32 -3.64 1.52 - 38. 2,]+, dichloro-6-iodophenol-4* 28 -3.60 1.48 - 39. 2-Fluoro-4>6-dibromo- phenol -3.85 -3.17 1.05 - 1+0. 2-Fluoro-4> 6-di iodophenol-4* C-6 -3.38 1.26 - 1+1. 2,1+ dibromo-6-iodophenol -4.27 -3.59 1.47 - (a) I wish to express my thanks to Dr. A.W. Baker f o r s u p p l y i n g these compounds. 500- 3001- 100 Z3» 22* •? 2* I* 3* •28 32 • 33,34 13* 10 ,25 •26 I I 1 • I I I I I I I I I I I I I I I I 8 9 10 ACT PP-M- OH F i g u r e 2. P l o t o f A C£ wvs f o r the compounds given i n T a b l e 2. Numbered p o i n t s r e f e r t o compounds as l i s t e d . 20 i i ) Temperature S t u d i e s o f o-Halophenols. The r e s u l t s o f the chemical s h i f t measurements ( r e l a t i v e to cyclohexane) a t v a r i o u s temperatures are gi v e n i n t a b l e s 3 to 6, along with the values o f C^,^ , the e x t r a p o l a t e d i n f i n i t e d i l u t i o n s h i f t . The g r a p h i c a l r e s u l t s f o r o b t a i n i n g C^,^ a r e g i v e n i n f i g u r e 3 f o r o - c h l or ophenol., f i g u r e 4 f o r o-bromophenol, and f i g u r e 5 f o r o-iodophenol and o - f l u o r o p h e n o l . F i g u r e 6 shows a p l o t of C^oo a g a i n s t temperature f o r o-bromophenol, which was used to o b t a i n a t r i a l v a lue f o r <C , the chemical s h i f t of the phenol -0-H when completely hydrogen bonded. S i m i l a r p l o t s were made f o r o-chlo r o and o-iodophenol (see d i s c u s s i o n ) . The s o l i d p o i n t s on the graph f o r o - f l u o r o p h e n o l ( f i g . 5>) r e p r e s e n t data o b t a i n e d by L . W . Reeves a t 60 Mc/s d u r i n g a v i s i t to the N a t i o n a l Research C o u n c i l i n the summer o f 1961. The measurements were reduced to I4.0 Mc/s f o r i n c l u s i o n i n the same f i g u r e . 2 1 Table 3. Values o f chemical s h i f t i n c y c l e s / s e c from cyclohexane obtained a t v a r i o u s c o n c e n t r a t i o n s and temperatures f o r o-chl o r o p h e n o l , a l o n g w i t h the e x t r a p o l a t e d v a l u e s f o r &^ obt a i n e d front F i g u r e 3» C O N C E N T R A T I O N ( M O L E % ) TEMPERATURE 0 1.22 2.02 3.Id 4.03 4.99 107°C 151.2 151.5 152.3 152.6 153.1 153.4 82°C 152.6 153.2 153.5 154.0 154.6 155.3 6S°c 153.3 - 154.7 155.5 155.9 27°C 156.1 153.6 159.6 160.0 161.1 162.3 . -1°C 157.4 159.7 161.5 163.5 165.5 166.9 -53°C 159.2 - 176.6 16*5.9 - 196.9 CONCENTRATION (MOLE %) Figure 3» D i l u t i o n chemical s h i f t s at s i x temperatures f o r the -0-H proton i n o-chlorophenol at 1-5 mole fo of the phenol i n carbon disulphide. The numbers r e f e r to cyclohexane as an i n t e r n a l standard. 22 Table 4. Values o f chemical s h i f t Sn i n c y c l e s / s e c from cyclohexane obtained a t v a r i o u s c o n c e n t r a t i o n s and temperatures f o r o-bromophenol, a l o n g w i t h the e x t r a p o l a t e d v a l u e s f o r c$*iv»̂ » o b t a i n e d from F i g u r e 4. C O N C E N T R A T I O N ( M O L E % ) TEMPERATURE 0 1.16 2.02 3.13 3.74 4.61 107°G 150.4 - 151.5 - 152.0 152.8 82°C - 152.4 153.1 153.3 154.1 154.3 63o c 152.7 153.2 153.7 154.0 154.7 155.3 27°C 154.5 155.5 156.2 157.2 157.9 159.0 -1°C 155.4 156.8 153.1 159.4 160.7 161.9 -18°C 156.0 157.8 159.0 161.6 - 164.1 -53°C 156.6 162.5 167.9 173.0 177.4 133.5 1 4 6 P_ - BROMOPHENOL I S O - 1 5 2 ———2-— -o-J°7 ° C I S 4 _ ~ ° — ~ —__2___ 68 °C 1 5 6 I S S 27 °C "UJ 1 1 6 0 -1 °C 8 1 6 2 F R O M  1 6 4 -18 °C o i 'S E C  1 6 6 - CO - J jj 1 6 8 c\ u I— I T O £ C O i 172 IC AI  1 7 4 1 • 1 L U • X o 1 7 6 1 7 8 1 6 0 -53 °C 1 8 2 , 1 . 1 , 1 . 1 . 1 , 1 . 1 . X . I , 1.0 20 3.0 4.0 5.0 CONCENTRATION ( M O L E % ) F i g u r e 4. D i l u t i o n chemical s h i f t s a t s i x temperatures f o r the -0-H proton i n o-bromophenol. 23 Table 5. Values of chemical s h i f t c£ M i n cycles/sec from cyclohexane at various concentrations and temperatures f o r o-iodophenol, along with the extrapolated values f o r C^^^ obtained from Figure 5. C O N C E N T R A T I O N ( H O L E % ) TEMPERATURE 0 1.37 2.11 3.00 4.21 4.73 107 GC 141.5 142.2 - 142.6 143.5 143.6. 82°C 142.3 143.4 143.6 143.9 144.5 145.0 63°C - 144.3 145.7 146.6 27°C 145.3 147.0 147.6 143.2 149.9 150.7 -1°G 146.5 143.4 149.3 151.2 153.2 154.7 -22°C 152.4 152.3 154.6 160.0 161.6 -53°C - 157.3 164.9 163;6 179.5 132.7 2k T a b l e 6. Values o f chemical s h i f t ^ i n c y c l e s / s e c from cyclohexane o b t a i n e d a t v a r i o u s c o n c e n t r a t i o n s and temperatures f o r o - f l u o r o p h e n o l . C O N C E N T R A T I O N ( M O L E % ) TEMPERATURE O.966 1.91 2.85 3.62 5.35 50°C - 136.4 140.1 143.1 149.0 a27°C 133.5 144.6 149.5 152.6 160.9 23°C - 144.7 150.7 153.7 162.1 -1°C - 156.8 166.1 171.7 182.6 -l3©c - 165.1 173.3 133.2 136.9 a Data o b t a i n e d by L.W. Reeves a t 60 Mc/sec w h i l e a t the N a t i o n a l Research C o u n c i l , Ottawa, i n August 1961. Values are reduced t o those f o r 40 Mc/sec f o r comparison. CONCENTRATION ( K O L E ^ ) I Figure 5. D i l u t i o n chemical s h i f t s f o r the -G-H proton i n o-iodo and o-fluorophenol. C-BROMOPHENOL The values of O k . — o b t a i n e d from F i g u r e I4. p l o t t e d a g a i n s t temperature 25 i i i ) Temperature S t u d i e s o f 2 , 1 + , 6 - T r i s u b s t i t u t e d Phenols. Chemical s h i f t measurements over the temperature range 0 to 110° C f o r 2-fluorb-i | . , 6-dIbromophenol, 2 - f l u o r o - l | . , 6 - d i i o d o p h e n o l , 2-chloro-l | . , 6-dibromophenol, and 2-chloro-i | . , 6-diiodophenol are g i v e n i n t a b l e 7. In these cases the chemical s h i f t s were independent of c o n c e n t r a t i o n i n the range s t u d i e d (to 1+ mole % ) . F i g u r e 7 i s a graph of these r e s u l t s p l o t t e d a g a i n s t temperature. 26 TABLE 7 Values of chemical shift SH in cycles/sec from cyclohexane for some' 2,4»6-trisubstituted phenols at various temperatures. The chemical shift was independent of concentration in the range studied. COMPOUND TEMPERATURE o°c 25°c 5l°C 73°G 111GC 2-Chloro-l|.,6-diiodophenol 174*6 173.6 171.6 169.1 168.9 2-Chloro-4»6-dibromophenol 174»3 172.3 167.O 167.O - 2-Fluoro-4,6-diiodophenol l68.3 161.5 155.0 152.6 149.9 2-Fluoro-4»6-dibromophenol 165.2 154'0 150.0 45.7 147.6 TEMPERATURE °C 27 CHAPTER IV. DISCUSSION. i ) C o r r e l a t i o n o f Chemical S h i f t s w i t h I n f r a r e d S t r e t c h i n g Frequency. An o r t h o - s u b s t i t u t e d phenol or naphthol i s f a v o u r a b l y s i t u a t e d f o r the f o r m a t i o n of an i n t r a - m o l ecular hydrogen bond wi t h the -OH group of the phenol. I f the p o s s i b i l i t y o f a s t r o n g hydrogen bond e x i s t s , the measured chemical s h i f t w i l l be t h a t of the hydrogen bonded s p e c i e s (33) (3k)» D u t f o r the weakly bonded compounds, an e q u i l i b r i u m o f the type Trans C i s w i l l e x i s t , and the chemical s h i f t w i l l be a weighted average of the two forms (8). I f the chemical s h i f t i s measured i n a non p o l a r s o l v e n t at low c o n c e n t r a t i o n s ( ^ 1 mole %) the value obtained w i l l be v e r y n e a r l y t h a t of the completely hydrogen bonded form. P l o t t i n g the v a l u e s o b t a i n e d f o r A O £ w , the change i n chemical s h i f t , a g a i n s t A\£ H , the change i n i n f r a r e d s t r e t c h i n g frequency, shows t h a t there i s a degree of c o r r e l a t i o n ( f i g u r e 2), i n s p i t e of s c a t t e r . One may say t h a t the g r e a t e r the change i n chemical s h i f t , the g r e a t e r the change i n s t r e t c h i n g frequency, 28 and the s t r o n g e r the bond formed. Upon examination of f i g u r e 2, one can see t h a t the compounds measured f a l l i n t o three main c l a s s e s . The f i r s t c l a s s c o n s i s t s p r i m a r i l y of f i v e membered hydrogen bonded r i n g s of the type: where X can be 0, G l , Br, I or P. The second group c o n s i s t s of f i v e membered r i n g s c o n t a i n i n g an -0-H...S bond, which;appears to be s t r o n g e r than the p r e v i o u s s e r i e s . I t i s p o s s i b l e t h a t the l a r g e r sulphur atom makes the d i s t a n c e s f o r bonding more f a v o u r a b l e . The t h i r d c l a s s c o n s i s t s of s i x membered r i n g s w i t h the hydrogen bonded to an oxygen or n i t r o g e n atom, e.g. o - n i t r o p h e n o l . The l a r g e r s i z e of the r i n g and the d i r e c t i o n a l nature of the donor o r b i t a l s favours a s t r o n g e r hydrogen bond p r o v i d i n g one assumes that the predominant term i s e l e c t r o s t a t i c (35) (36). J . R. M e r r i l l (38) has r e c e n t l y p u b l i s h e d a c o r r e l a t i o n of A C f O H w i t h A V £ H f o r s e v e r a l s u b s t i - t u t e d o-hydroxybenzophenones, and Gutowsky (39) n& s e s t a b l i s h e d a s i m i l a r c o r r e l a t i o n of A C f O H w i t h A ^ 0 « the change i n c a r b o n y l s t r e t c h i n g frequency, f o r a s e r i e s of compounds, w i t h r e s u l t s s i m i l a r to those i n f i g u r e 2. 29 Two exceptions to the c l a s s e s g i v e n above must be noted. o - A l l y l p h e n o l has a 6-§- membered r i n g w i t h the hydrogen bond formed with the Tf e l e c t r o n s of the a l l y l group and i s found i n the weakly bonded f i r s t c l a s s . The l a r g e chemical s h i f t o b t a i n e d f o r o-bromobenzoic a c i d i s b e l i e v e d to be due to the f o r m a t i o n of a hydrogen bonded dimer r a t h e r than an i n t r a m o l e c u l a r bond w i t h the bromine. The chemical s h i f t i s approximately t h a t which would be expected f o r a dimer (37) (11) and the i n f r a r e d spectrum shows frequency s h i f t s of 9° and 570 cm"' r e s p e c t i v e l y . The f i r s t , a very weak bond, i s that which would be expected f o r an -0-H...Br bond (21) ( t a b l e 2 ) , while the second i s t h a t expected f o r a dimer ( 11) . One may conclude t h a t even a t c o n c e n t r a t i o n s of one mole % there i s l i t t l e f o r m a t i o n of an i n t r a m o l e c u l a r bond. 30 i i ) C a l c u l a t i o n o f E n t h a l p i e s of Formation f o r the I n t r a m o l e c u l a r Hydrogen Bond i n o-Halophenols. The observable chemical s h i f t f o r a compound c o n t a i n i n g a s t r o n g I n t r a m o l e c u l a r hydrogen bond i s t h a t of the hydrogen bonded s p e c i e s (33) (3^) (38), and the chemical s h i f t i s independent of c o n c e n t r a t i o n as l o n g as there are no i n t e r m o l e c u l a r c o n t r i b u t i o n s . However, f o r a weakly bonded s p e c i e s (the f i r s t c l a s s d i s c u s s e d i n the previous s e c t i o n ) a t any c o n c e n t r a t i o n there w i l l e x i s t an e q u i l i b r i u m Trans C i s which w i l l c o n t r i b u t e to an average chemical s h i f t . A f u r t h e r c o n t r i b u t i o n w i l l be o b t a i n e d from any i n t e r m o l e c u l a r bonding. By measuring the chemical s h i f t ( <£M ) over a range of c o n c e n t r a t i o n s one i s able to o b t a i n an e x t r a p o l a t e d I n f i n i t e d i l u t i o n s h i f t ( C $ n o 0 ) which may be a t t r i b u t e d to the e q u i l i b r i u m as i n (1), s i n c e the e x t r a p o l a t i o n w i l l e l i m i n a t e i n t e r m o l e c u l a r e f f e c t s . U s i n g the a n a l y s i s of Gutowsky and S a i k a (8), one may express the i n f i n i t e d i l u t i o n s h i f t c £ M a 0 a s : X - f C ^ -V " X c c f t (2 ) I 31 where ")/ - mole f r a c t i o n of the phenol i n the trans form. Y. s mole f r a c t i o n of the phenol i n the c i s form. = chemical s h i f t of the trans form. <£c = chemical s h i f t of the c i s form. Sc and d .̂ are assumed to be invariant. The equilibrium expression f o r (1) at any temper- ature may be written TTRANSI W T I - V c l (3) K . C T ) = [CIS] " t X c ] • [ X c ] since XT )^<.~ -̂ • From (2), one obtains (4) Substituting for X C I N (3)» an expression for the equilibrium constant K , ̂  at any temperature (T) i s obtained i n terms of , and £ : ^ineo ^ n o w n ^ r o m the extrapolations of figures 3» k- and f> for the compounds o-chloro, o-bromo and o-iodophenol, If one observes the slopes of the l i n e s for o-fluorophenol 32 ( f i g u r e 5), i t i s e v i d e n t t h a t they d i f f e r from those of the p r e v i o u s three compounds and i n f a c t resemble the curve one obtains f o r phenol (33), where there i s no p o s s i b i l i t y of an i n t r a m o l e c u l a r hydrogen bond being formed. This was taken as evidence that the i n t r a - m o l e c u l a r bond formed, i f any, i s so weak t h a t the p r e v i o u s a n a l y s i s would not apply, s i n c e i n t e r m o l e c u l a r e f f e c t s are s t i l l c o n s i d e r a b l e a t 1 mole % t which i s the l i m i t of s e n s i t i v i t y f o r the spectrometer used. F o r the o t h e r three phenols, lower temperatures w i l l s h i f t the e q u i l i b r i u m so t h a t the c o n c e n t r a t i o n of the c i s form i s i n c r e a s e d . At s u f f i c i e n t l y low temper- a t u r e s the phenol w i l l be almost 100$ c i s form, which w i l l g ive a value f o r <£ c . In order to o b t a i n a value f o r , C^ijvioo f°r each phenol was p l o t t e d a g a i n s t temperature as i n f i g u r e 6, and an e x t r a p o l a t i o n to the maximum val u e made. The r e s u l t s were a l s o f i t t e d to an e q u a t i o n of the type X - A -V- BT CT 2" (6) where T = temperature °K A, B, G are constants to be determined. I t was p o s s i b l e to f i t experimental values of c T n ^ o t o equation 6 w i t h i n i .1 c y c l e s / s e c f o r o-bromo and o-chlorophenol and to w i t h i n "± .2 c y c l e s / s e c f o r o-iodophenol. The equations obtained were: 33 f o r o-bromophenol, c £ , _ = 151.6 -M5.75 * I O " X ) T _ (1.60 x i o _ , Q ) T L f o r o - chlorophenol, £ = 153.6 -*-(6.83 x 10" 2)T - (2.00 x 1 0 " * ) T X and f o r o-iodophenol, X = Ikl.k 4-(2.i|l x 10" l)T - (9.86 x 1 0 - S ) T X . Using these equations, one may o b t a i n an estimated value f o r t£ , s i n c e i t i s p o s s i b l e t o determine the value f o r which the eq u a t i o n i s a maximum; that i s , when ACS*.) ~ICrf = ° Having found the temperature a t which Q ^ ^ i s a maximum, "TOO one can s u b s t i t u t e back i n t o the equ a t i o n to o b t a i n C^rvieo (max.)^ W h i c h i s approximately equal to . The r e s u l t s of t h i s method are giv e n below: o-bromophenol: cTM4o(max.) * ^ m 156.8 c y c l e s / s e c a t 172°K b-chlorophenol: cf M «o^ m a x «)~ <$*c • 159.5 c y c l e s / s e c at 171°K o-iodophenol: (max.) 2 B ii+8.9 c y c l e s / s e c a t 121°K Thus, i n order to determine values f o r the e q u i l i b r i u m constant K«^ T) , i t remains to evaluate C ^ - » t n e chemical s h i f t o f the trans form. In order to do t h i s , one may make use of the f o l l o w i n g r e l a t i o n s h i p : A F = A H - T A S = - R T JL K K T ) (7) 34 According to J a f f e (38), entropy terms f o r these systems are very c l o s e to zero, so one may w r i t e ^ • (8) A reasonable value f o r cTT w i l l be t hat which makes (8) approximately constant f o r a l l measured temperatures. In a d d i t i o n , one then o b t a i n s a value f o r A H, the enthalpy of f o r m a t i o n o f the hydrogen bond, which should be r e a s o n a b l y good. The r e s u l t s of t h i s a n a l y s i s f o r the three o-halophenols s t u d i e d are g i v e n i n Table 8. Values of K , ( T ) a t 2$°C agree r e a s o n a b l y w e l l w i t h those of Baker (21), and a l s o w i t h those of Rossmy, Lilittke and Mecke, but not w i t h those of Jones and Watkinson (23). However, n.m.r. data do not support Baker's c o n t e n t i o n t h a t the -0-H..P bond i s stronger than the -0-H...I bond i n these phenols. J 35 TABLE 8. Values of K { ^ and A H f o r o- c h l o r o , o-bromo and o-iodophenol. o-Chiorophenol Temp., ° K K , ( T > A H ( c a l mole" 1) 380 1/23 2334 355 1/27 2320 3lp. 1/30 2308 300 1/56 2392 272 1/91 2^32 mean value A H - 2356 c a l mole" t£c = -l59»5 c y c l e s / s e c from cyclohexane <£ T = -V- 3k- c y c l e s / s e c from cyclohexane o-Bromophenol Temp., ° K K U T > AH ( c a l mole -' ) 380 l/lk 2011 3kl 1/22 2095 300 1/38 2l6l 272 1/58 2182 253 1/87 2254 mean value AH a 2llp. c a l mole""* -«- £ = -157.2 c y c l e s / s e c from cyclohexane £ ^ = - 51.9 c y c l e s / s e c from cyclohexane 36 TABLE 8 (cont'd) o-Iodophenol Temp., °K K , ^ AH ( c a l mole" 1 ) 380 1/7.6 1529 355 1/9.5 1581 300 1/19 17^8 272 1/26 17I46 mean value A H = 1650 c a l mole" Q £ = -lij.8.9 c y c l e s / s e c from cyclohexane _ 84.9 c y c l e s / s e c from cyclohexane -«- In order to o b t a i n more reasonable v a l u e s f o r o-bromophenol, i t was found necessary to change the value of c£ by O.lj. c y c l e s / s e c to -157.2 c y c l e s / s e c . This i s w i t h i n the e r r o r i n e x t r a p o l a t i o n . The value i n i s extremely s e n s i t i v e to s m a l l changes i n , s i n c e K | ( T) i s so s m a l l . However, A H i s not a p p r e c i a b l y a l t e r e d by t h i s , and t h e r e f o r e the values f o r AH are more r e l i a b l e than those f o r £ 37 i i i ) C a l c u l a t i o n of E n t h a l p i e s of Formation of the C i s - t r a n s Dimer f o r o-Halophenols. I f one assumes t h a t the change i n chemical s h i f t from the e x t r a p o l a t e d i n f i n i t e d i l u t i o n s h i f t a t low c o n c e n t r a t i o n s i s due p r i n c i p a l l y to the f o r m a t i o n of dimers of the type CIS TRANS CIS TRANS a f u r t h e r a n a l y s i s may be made to o b t a i n e n t h a l p i e s of f o r m a t i o n f o r these dimers. I t Is c o n s i d e r e d t h a t the c o n c e n t r a t i o n of the t r a n s - t r a n s dimer i s always very much l e s s than t h a t of the c i s - t r a n s dimer, s i n c e the concen- t r a t i o n of the t r a n s form i s low, as shown by the values f o r K t ( T ) i n t a b l e 8. One f u r t h e r assumes that t h i s value of K ^ ( T) Is a p p l i c a b l e a t c o n c e n t r a t i o n s o f one mole %. I f there are i n i t i a l l y 'a' moles of phenol and 'ms' moles of CS^, and one l e t s 'm̂ ' - number of moles o f c i s form and 'm̂ ' 8 number of moles of dimer, one may w r i t e as the number of moles p r e s e n t a t e q u i l i b r i u m f o r each form: 3 8 Trans (a-m c-mj) C i s ( m c - uij) Moles Mole F r a c t i o n (a - m c - nij) (a - m^) Hr m s (n^ - m^) (1) (a - mj ) + m s C i s . . . t r a n s m, dimer (a - mj) + m s One can then d e f i n e an e q u i l i b r i u m constant K ^ ^ j i n mole f r a c t i o n u n i t s : [ c i s . . . t r a n s ] - m̂  (a - mj-mi^) (2) K ZCT) ~ C c i s l { t r a n s ] £ a - m c - mj ̂  £ m c - mjy The f o l l o w i n g chemical s h i f t s are needed: Q = measured chemical s h i f t at f i n i t e concen- M c t r a t i o n 'c' ^ = chemical s h i f t s of the trans and c i s forms, chemical s h i f t s of the trans and c i s forms i n the dimer. r e s p e c t i v e l y , as determined i n s e c t i o n ( i i ) ° T « J , 0 C a One may then w r i t e (8): where )Cr} X c ; a r e r e s p e c t i v e l y the mole f r a c t i o n s ( 3 ) of t r a n s , c i s and dimer forms. S u b s t i t u t i n g , one obtains: 39 4 Assuming that the chemical s h i f t of the c i s proton i n the dimer i s approximately equal to t h a t of the f r e e c i s proton; t h a t i s , r j £ ^ 2t (£ • , i t i s p o s s i b l e to r e w r i t e (1+) as: Prom the p r e v i o u s s e c t i o n , A l s o , r \ > A as w % , > O Since m^ i s s m a l l , i f one assumes that a - m^&a equation (5) may be r e w r i t t e n as: <£wT" ( a - m ^ C ^ . + ( I^SnJ) C ^ T A S o l v i n g f o r ny, d L e t A _ ko S u b s t i t u t i n g i n the e x p r e s s i o n f o r KjjfT) ( 2 ) > K 2 ( T ) = aA (a - a A t a < ) (1) (a - m c - aA) (mc * aA) Prom the p r e v i o u s s e c t i o n ( i i ) , m c = a (10) S u b s t i t u t i n g t h i s i n ( ), and n e g l e c t i n g terms i n 1 x. - - (K t ( T j ) and (A) , which are small, the f o l l o w i n g e x p r e s s i o n i s obt a i n e d f o r KJ»(T) : A ( (11) With the e x c e p t i o n of the chemical s h i f t f o r , a l l value s are known. Prom the work done i n s e c t i o n ( i ) , a reasonable value f o r a pro t o n i n an -0-H..0- hydrogen bond would be approximately 8 p.p.m. or 320 c y c l e s / s e c . Therefore, t h i s value was used i n c a l c u l a t i n g K j, ^ . Values f o r <^^ c were taken from f i g u r e s , 3, k a n d 5 at one mole %. The a n a l y s i s w i l l not apply f o r the h i g h e r temperatures (108°G), s i n c e the c o n c e n t r a t i o n o f the trans form i s a p p r e c i a b l e , which w i l l make the assumptions made i n d e r i v i n g K ^ ) i n v a l i d . By p l o t t i n g l o g K x ^ a g a i n s t l / T , one obt a i n s A H from the slope of the s t r a i g h t l i n e formed. kl The graph of these r e s u l t s i s given i n figure 8. As would be expected, A H f o r the dimer i s more or less independent of a p a r t i c u l a r phenol, since the X substituent would play l i t t l e part i n dimer formation. The estimated accuracy i s £ .5 k a l mole. The values of 5.8, 5.6, S.k kcal-mole" 1 f o r the o-bromo, o-chloro, and o-iodophenol dimer are what would be expected for an - 0 - H . . . 0 hydrogen bond. S 9 i k i k j j %~k 6 ~i i i i ~ § i i i ~*>f©6n 3 IO 3 3 4 2 TABLE 9. Compound Temperature K i ^ - r } °K ' o-bromophenol 380 2.57 3lA 3.78 » 300 11.22 '< 272 24.10 » 253 46.89 o-chlorophenol 355 4*54 » 34L 7.04 » 300 23.48 » 272 55.08 o-iodophenol 380 1.5 355 1.52 tt 300 5.24 272 12.9 1 1+3 i v ) V a r i a b l e Temperature S t u d i e s of 2 , 4 , 6 - t r i s u b s t i t u t e d phenols. As s t a t e d p r e v i o u s l y , the chemical s h i f t o f the -OH pro t o n o f the 2 , 4 , 6 - t r i s u b s t i t u t e d phenols was found to be independent of c o n c e n t r a t i o n i n the range s t u d i e d (2-ij. mole This may be a t t r i b u t e d to the f a c t t h a t the bulky e f f e c t o f the halogens prevents i n t e r m o l e c u l a r a s s o c i a t i o n . I t was thought that such compounds might a l s o have a chemical s h i f t which would be independent o f s o l v e n t , but a study of 2 , 4 , 6 - t r i e h l o r o p h e n o l showed t h a t t h i s was not the case, s i n c e a s h i f t was observed. Although the chemical s h i f t of the phenol p r o t o n Is the same f o r i n e r t s o l v e n t s such as C S 2 , 0 0 1 ^ and cyclohexane ( -4 .32 p.p.m.), there i s a s h i f t i n benzene to - 3 . $ 3 p.p.m., i n chl o r o f o r m to -4 .2i j . p.p.m., and i n acetone, to 8.94 p.p.m., i n d i c a t i n g that there i s s t i l l s o l v e n t i n t e r a c t i o n . In phenol i t s e l f there i s a very strong c o n c e n t r a t i o n dependence ( 1 0 ) but the i n f i n i t e d i l u t i o n s h i f t should be independent of temperature, s i n c e there i s a symmetrical b a r r i e r f o r the r o t a t i o n o f the hydroxy group about the C-0 bond (lij.). T h i s should a l s o be tr u e f o r any phenol symme t r i c a l l y s u b s t i t u t e d a t the 2 , 6 p o s i t i o n s . However, where the s u b s t i t u e n t s a t the 2 , 6 p o s i t i o n s are d i f f e r e n t , and there i s a p o s s i b i l i t y o f hydrogen bonding or s t e r i c h indrance, the b a r r i e r to r o t a t i o n w i l l be unsyrametrical. This should r e s u l t i n a temperature dependent chemical s h i f t f o r the phenol proton, s i n c e the p o p u l a t i o n s a t each ortho p o s i t i o n w i l l vary with temperature. I f the d i f f e r e n c e i n energy between the two forms i s l a r g e r , one should expect a l a r g e temperature dependence; while a s m a l l e r dependence would be expected i f the energy d i f f e r e n c e i s s m a l l . At h i g h e r temperatures the chemical s h i f t w i l l tend to become temperature independent. An examination of the r e s u l t s f o r the f o u r compounds s t u d i e d ( f i g u r e 7) w i l l show t h a t t h i s i s the r e s u l t obtained. For 2-chloro-4 , 6-dibromphenol and 2 - c h l o r o - l+ , 6 - d i i o d o p h e n o l , where the d i f f e r e n c e i n energy between the two hydrogen bonded forms i s s m a l l , the chemical s h i f t becomes independent of temperature a t about 5 0 ° C. f o r 2-chloro-J+ , 6-dibrompphenol and a t about 70°C. f o r the o t h e r . Since the energy d i f f e r - ence i s g r e a t e r f o r the c h l o r o - i o d o compound, one would expect a constant chemical s h i f t to occur at a h i g h e r temper- ature, as i s the case. For 2-fluoro-ij . , 6-dibromophenol and 2 - f l u o r o - i j . , 6 - d i i o d o p h e n o l , where the d i f f e r e n c e i n energy i s much g r e a t e r , there i s no i n d i c a t i o n t h a t the measured chemical s h i f t i s tending to a constant value i n the range s t u d i e d . 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