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The mechanism of permanganate oxidations : pivalaldehyde, benzaldehyde and p-nitro-phenyltrifluoromethylcarbinol. Fleming, Donald George 1963

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THE MECHANISM OP PERMANGANATE OXIDATIONS: PIVALALDEHYDE, BENZALDEHYDE AND p-NITROPHENYLTRIFLUOROMETHYLCARBINOL by DONALD.GEORGE FLEMING B.Sc., U n i v e r s i t y o f B r i t i s h Columbia May, 1961. A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of C h e m i s t r y We a c c e p t t h i s t h e s i s as c o n f o r m i n g to the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA August, 1963. I n p r e s e n t i n g 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 o f the requ irements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h C o l u m b i a , 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 f o r r e f e r e n c e and s t u d y . I f u r t h e r agree that p e r -m i s s i o n for ex tens ive c o p y i n g o f 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 r e p r e s e n t a t i v e s . , I t i s unders tood that copying, or p u b l i -c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l lowed without my w r i t t e n p e r m i s s i o n . 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. Date IP i A3STPAGT A study has been made of the potassium permanganate oxidat ion of three organic compounds: pivalaldehyde, p-nitrobenzaldehyde and p-n i t rophenyl t r i f luoromethyl -c a r b i n o l . The permanganate-pivalaldehyde reac t ion has been invest igated i n the pH range 1 to 13. The resu l t s show that the reac t ion i s f i r s t order i n permanganate and aldehyde, that the oxidat ion by manganate i s slower than that of permanganate by a fac tor of twenty f i v e , and that the oxidat ion i s general ac id-ca ta lyzed . The resu l t s i n a lka l i ne so lu t ion show some increase i n rate but are not reproducible , which i s most l i k e l y due to r a d i c a l decomposition of the pivalaldehyde. Three mechanisms are discussed: F i r s t l y , a t y p i c a l general a c id - ca t a ly s i s scheme, secondly, one i n v o l v i n g a tetragonal t r a n s i t i o n state and t h i r d l y , one based on a hydride t ransfer . Of these, only the f i r s t two are able to accommodate the experimental evidence found e a r l i e r i n the permanganate oxidat ion of benzaldehyde by Wiberg and Stewart ( 1 ) . The autocata lys is exhibi ted by the permanganate benzaldehyde reac t ion has been studied at low pH. The autocata lys is has also been invest igated wi th i i p - n i t r o b e n z a l d e h y d e i n o r d e r t o see i f t h e r e i s any s u b s t i t u e n t e f f e c t . R e s u l t s show t h a t t h e p - n i t r o a l d e h y d e has a l o n g e r i n d u c t i o n p e r i o d . M o reover, i t has been shown t h a t o v e r o x i d a t i o n o f the benzene r i n g r a t h e r t h a n t r u e a u t o c a t a l y s i s has o c c u r r e d . A r a d i c a l c h a i n mechanism has been put f o r w a r d i n an attempt t o e x p l a i n t h i s o b s e r v a t i o n . The permanganate o x i d a t i o n o f p - n i t r o p h e n y l t r i -f l u o r o m e t h y l c a r b i n o l has been s t u d i e d i n 0.1 M sodium h y d r o x i d e i n o r d e r t o d e t e r m i n e t h e mechanism o f t h e r e a c t i o n . A l a r g e enhancement i n r a t e o b s e r v e d com-pared t o r a t e s found by S t e w a r t and Van d e r L i n d e n ( 2 ) i n the permanganate o x i d a t i o n of o t h e r s u b s t i t u t e d p h e n y l t r i f l u o r o m e t h y l c a r b i n o l s i s good e v i d e n c e f o r a r e a c t i o n p a t h i n v o l v i n g hydrogen atom a b s t r a c t i o n f r o m the a l k o x i d e t o t h e permanganate i o n . Such a mechanism i s c o n s e q u e n t l y f a v o u r e d . i i i ACKNOWLEDGEMENT Sincere thanks are extended to my research d i r e c t o r , Professor Ross Stewart, for h i s invaluable help and suggestions during the course of t h i s i n v e s t i g a t i o n . TABLE OF CONTENTS Page INTRODUCTION 1 OBJECT 07 E.ESKARCH......a...... <« ......... • •.. 18 EXP ERIMENTAL 20 1. Preparation and I d e n t i f i c a t i o n of a) Pivalaldehyde. 20 b) Benzaldehyde and p-N i t r o b e n z a l d e h y d e . . . . . . . . . . . . 26 c) Phenyl t r i f luoromethylcarbinol and p -Ni t ropheny l t r i f luo ro -methylcarbinol . . . . . . . . 26 2 . K i n e t i c Method . . 29 RESULTS 32 1. Permanganate Oxidat ion of P i v a l a l d e -hyde 34 a) Nature of the s u b s t r a t e . . . . . . 34 b) Oxidat ion i n the pH r e g i o n . . . 35 c) General a c id - ca t a ly s i s 41 d) Oxidat ion i n a l k a l i n e so lu t ion 48 e) Effect of i on i c strength 50 f ) Oxidation by manganate 51 2 . Permanganate Oxidat ion of Benzaldehyde and p-Nitrobenzaldehyde 54 a) Effect of pH 54 b) The effect of added manganous S 3 . l t * * « o « o e e » « e o o o « o e o o e 0 e e o f l 64 V Page c) Permanganate oxidat ion of benzoic ac id „ 65 3. Permanganate Oxidat ion of p-Ni t rophenyl t r i f luoromethylcarbinol 67 a) Oxidation i n 0.1 M sodium b) Oxidat ion i n weakly a l k a l i n e so lu t ion 73 c) The Kammett p lo t 77 DISCUSSION OF RESULTS 81 1. Permanganate Oxidat ion of Pivalaldehyde 81 2 . Permanganate Oxidat ion of Benzalde-hyde and p-Nitrobenzaldehyde 88 3. Permanganate Oxidat ion of p-Ni t rophenyl t r i f luoromethylcarbinol 9 3 LIST OF R E F E R E N C E S . . . . . . . . . . > . « « . « . . . . . . . . . . 97 TABLES v i Table Page I The Absorption of Pivalaldehyde i n SoXlltXOTlo « « » » o o o o » e o « » » o « e a i S o o o « 0 4 * « o a « Q * 23 I I Decomposition of a Typ ica l So lu t ion of Pivalaldehyde 25 I I I The permanganate Oxidat ion of P i v a l a l d e -. . . hyde - A Typica l K i n e t i c R u n . . . . . . . . . . . 37 IV The Permanganate Oxidat ion of P i v a l a l d e -hyde - V a r i a t i o n of Rate Constant wi th pH 40 V Oxidation of Pivalaldehyde at Constant pH V a r i a t i o n of the Rate Constant w i th Buffer43 VI Oxidat ion of Pivalaldehyde at Constant Buffer - V a r i a t i o n of the Rate Constant wi th pH 44 VII Oxidation of Pivalaldehyde at Constant pH - V a r i a t i o n of the Rate Constant wi th Pyrophosphate 46 VI I I Oxidat ion of Pivalaldehyde, Influence of . . . Ionic Strength . . . . . 50 IX The Manganate Oxidation of Pivalaldehyde 52 X The Permanganate Oxidat ion of Benzalde-hyde - Dependence of the Rate on p H . . . . 57 XI Permanganate Oxidat ion of p-Nitrobenzalde-hyde - Differences i n I n i t i a l Rate from XI I Overoxidation of Benzaldehyde and p-Nitrobenzaldehyde. - Experimental D i f f e r -ences Found i n F i n a l V o l u m e s . . . . . . . . . . . . . 59 X I I I The Permanganate Oxidat ion of Benzaldehyde Effect of added Manganous Sulphate on the Rate 65 v i i Table Page XIV The Permanganate Oxidation, of Benzoic A c i d - Change i n I n i t i a l Volume of Permanganate wi th Time 66 XV Permanganate Oxidat ion of P h e n y l t r i -fluoromethylcarbinols - Oxidations i n 0.1 M Sodium Hydroxide 72 XVI Permanganate Oxidat ion of P h e n y l t r i -f luoromethylcarbinols - Oxidations XVII Oxidat ion of p -Ni t ropheny l t r i f luo ro -. . methylcarbinol - Rate Changes wi th pH During the Course of R e a c t i o n . . . . 76 XVIII Permanganate Oxidat ion of P h e n y l t r i -. . - f luoromethylcarbinols - The Hammett P l o t for 0.1 M Sodium Hydroxide 78 at a pH of 9.17 74 v i i i FIGURES Page 1. The Permanganate Oxidat ion of P i v a l a l d e -hyde - A t y p i c a l ra te p l o t . . . . . . . . 38 2 . The Permanganate Oxidat ion of P i v a l a l d e -hyde - Dependence of the rate on p H . . . . 39 3. The Permanganate Oxidation of P i v a l a l d e -hyde - Dependence of rate on orthophos-phate cone 42 4. The Permanganate Oxidation of P i v a l a l d e -hyde - Dependence of rate on pH at con-stant buffer 45 5. The Permanganate Oxidat ion of P i v a l a l d e -hyde - Dependence of rate on buffer cone 47 6. The Permanganate Oxidation of P i v a l a l d e -hyde - A t y p i c a l rate p lo t i n a l k a l i n e s o l u t i o n . . 49 7. The Manganate Oxidat ion of Pivalaldehyde 53 8. The Permanganate Oxidat ion of Benzalde-hyde - Effect of pH on the autocataly-9. The Permanganate Oxidat ion of Benzalde-hyde - I n i t i a l rate at a pH of 1 . 40 . . . . 56 10. The Permanganate Oxidat ion of Benzalde-hyde - Effect of pH on the i n i t i a l rate 58 11. Autoca ta lys is i n the Permanganate Oxida-t i o n of Benzaldehyde and p-Nitrobenzalde-hyde . . . . . 62 12. The Permanganate Oxidat ion of Benzalde-hyde - Autoca ta lys is corrected for overoxidation 63 13. The Permanganate Oxidation of P h e n y l t r i -f luoromethylcarbinol 68 i x Page 14. The Permanganate Oxidation of p-Nit rophenyl-t r i f luoromethylcarb inol - A t y p i c a l rate p l o t . . . . . . . . . . . . . « « . « . * . o « a . . o . . « » « . . o . o . . o 69 15. The Permanganate Oxidat ion of p-Nitrophenyl-t r i f luoromethylcarb inol - A sigmoid ra te plot 71 16. Oxidat ion of Phenyl t r i f luoromethylcarbinols-The Hammett p lo t 79 INTRODUCTION Oxidations of organic compounds by potassium permanganate are usua l ly mult i -s tage processes. The degradation of organic molecules proceeds v i a the breakage of d iscree t covalent bonds whi le manganese undergoes reduction to lower valency states v i a a number of e lec t ron t r a n s i t i o n s . With ce r t a in r e -ductants, i n strongly a c i d i c so lu t ions , an o v e r a l l f i v e e lec t ron change may occur which i s represented by: MnOj + 8 I + 5 e ^—- Mil 2 + 4 H 20 In weakly a c i d i c , neu t r a l , and weakly a l k a l i n e so lu t ions , the permanganate can undergo a three e lec t ron change as shown by: + Mn0 4 + 4 H + 3 e - Mn0 2 + H 2 0 and i n a l k a l i n e solut ions (pH 12-14), permanganate reduction may be arrested at the manganate stage w i t h -out any further noticeable reduction of the manganate: MnO^ + e *Mn0 4 General ly , inorganic substrates appear to reduce ac id permanganate to i t s lowest valency state (Mn ) , where-as organic substrates containing oxid izable hydrogen stop - 2 -the reduction at the manganese dioxide stage, In f a c t , Mocek (3) found manganese dioxide p r e c i p i t a t i n g i n the oxidat ion of f l u o r a l i n s i x t y percent sulphuric a c i d , which i s c l e a r l y i n d i c a t i v e of arres t at t h i s stage. The s t a b i l i t y of the oxy-anions of manganese i s of major importance, as many reac t ion mechanisms have been formulated wi th reac t ive species formed from the decomposition of permanganate. Potassium permanganate i t s e l f i s stable for months i n neut ra l so lu t ion when kept i n the dark (4 ) , but decomposition i s noticeable i n ac id (5) or i n a l k a l i n e solut ions ( 6 ) . The purple , te t rahedral permanganate ion i s known to decompose i n a l k a l i n e so lu t ion according to the equation: 4 MnO^ + 4 OH * 4 Mn0~ + 2 H 20 + 0 2 The rate of t h i s reac t ion was f i r s t studied by Fergusson and co-workers, ( 6 ) , under various condit ions of temperature and concentration of a l k a l i , but no mechanistic in terpre ta t ions were made. La te r , Stamm (7) suggested that the equ i l ib r ium, MnO" + OH , "OH + MnO^ occurs i n a l k a l i n e so lu t i on , w i th oxidat ion of a sub-s t ra te taking place by free hydroxy-radica ls . - 3 -K i n e t i c s t u d i e s on t h e a l k a l i n e d e c o m p o s i t i o n o f permanganate have been made by Symons ( 8 ) who f o l l o w e d t h e r a t e o f e v o l u t i o n o f oxygen. He p r e s e n t e d t h e f o l l o w i n g mechanism i n v o l v i n g a s e r i e s o f e l e c t r o n and p r o t o n t r a n s f e r s . MnO£ + OHt 'OH + MnO^ •OH + 0_H = ± : *0 + H2Cv MnO~ + »0H + '0 .. MnO^ + H0~ Mn04 + HO2 1—- Mn04 + H0 2 H0 o + Q"H ^ 0~ + H O Mn0"4 + 0 2 * MnO^ + 0 2 Symons s t a t e s t h a t t h e u s u a l mode o f o x i d a t i o n o f a l k a l i n e permanganate s o l u t i o n s i s by a t t a c k on t h e s u b s t r a t e by h y d r o x y l r a d i c a l s o r h y d r o x y - r a d i c a l i o n s . He g i v e s good e v i d e n c e f o r t h i s by i n i t i a t i n g p o l y -m e r i z a t i o n s o f a c r y l o n i t r i l e and s t y r e n e i n a l k a l i n e s o l u t i o n s o f permanganate g r e a t e r t h a n t h r e e m o l a r , whereas i n l e s s b a s i c s o l u t i o n , o n l y o x i d a t i o n was found t o o c c u r . T h i s c o n c l u s i o n i s doubted by Drummond and Waters ( 9 ) , who showed t h a t d i e t h y l e t h e r and d i o x a n e , w h i c h a r e r a p i d l y o x i d i z e d by f r e e h y d r o x y -- 4 -r ad i ca l s (10) , are not attacked by a l k a l i n e permanganate. They further showed that t e r t i a r y butanol , p rop ionic , succ in ic and ad ip ic acids are attacked by hydroxyl r ad i ca l s but not by a l k a l i n e permanganate; whi le con-ve r se ly , acetone, malonic ac id and fumaric ac id are r ap id ly attacked by a l k a l i n e permanganate but not by free hydroxyl r a d i c a l s . However, they point out that i n some of these cases inherent differences i n a c i d i t y may account for differences i n r e a c t i v i t y . I t i s worthwhile mentioning some of the react ions of the lower oxidat ion states of manganese which are often he lp fu l i n e luc ida t ing the mechanisms of perman-ganate ox ida t ion . Undoubtably, the t ransfer of more than two electrons simultaneously would be considered energe t ica l ly u n l i k e l y and thus i t seems reasonable that permanganate should pass through some intermediate stage before being reduced to i t s f i n a l s ta te . The behaviour, s t a b i l i t y , and o x i d i z i n g power of manganate were invest igated extensively by Waters and co-workers (11 ,9 ) . Manganate i s stable only i n strong a l k a l i and solut ions which are less than one molar i n base slowly disproport ionate in to potassium permanganate and manganese dioxide (4 ,12) . 2 H 20 + 3 MnO^ * Mn0 2 + 2 MnO^ + 4 OH - 5 -However, t h e pH a t w h i c h t h i s o c c u r s has n e v e r been p r e c i s e l y d e f i n e d o r i n v e s t i g a t e d . The b l u e hypomanganate i o n , MnO~, was f i r s t p r e -p a red by Lux ( 1 3 ) and i t s o x i d i z i n g p r o p e r t i e s and s t a b i l i t y have been i n v e s t i g a t e d by Pode and Waters ( 1 1 ) . T h i s i o n d i s p r o p o r t i o n a t e s i n t o manganate and manganese d i o x i d e , i n s o l u t i o n s l e s s t h a n e i g h t m o l a r i n sodium h y d r o x i d e , as shown by: 2 H 2 0 + 2 MnO^ ^ MnO~ + Mn0 2 + 4 OH I t i s i n t e r e s t i n g t o n o t e t h a t a . v e r y f a s t r e a c t i o n o c c u r s between hypomanganate and permanganate i o n s when the two a r e mixed ( 4 ) and t h i s i s t h e r e a s o n f o r the i n a b i l i t y t o o b s e r v e any i n t e r m e d i a t e s i n v o l v i n g Mn V i n permanganate o x i d a t i o n s i n a l k a l i n e s o l u t i o n ( 1 1 ) : M n 6 4 + MnO~ - 2 Mn0~ The o x i d i z i n g power o f manganese o x y - a n i o n s o f the t y p e MnO^ d e c r e a s e s i n t h e o r d e r MnO^ > MnO^ ^ MnO^. T h i s i s e x p l a i n e d on t h e b a s i s t h a t t h e o x i d a t i o n p r o c e s s i n v o l v e s a c c e p t a n c e o f e l e c t r o n s by t h e a n i o n , whereby t h e movement of e l e c t r o n s i s c o n t r a r y t o t h e charge on t h e a n i o n ( 1 2 ) . M o r e o v e r , t h e c h a r a c t e r i s t i c g r e en c o l o u r of t h e manganate i o n can be seen t o accumulate i n many o x i d a t i o n m i x t u r e s o f a l k a l i n e - 6 -permanganate and i s a f u r t h e r i n d i c a t i o n o f i t s weaker o x i d i z i n g power. W i b e r g and S t e w a r t ( L ) however, f o u n d t h a t b o t h manganate and permanganate o x i d i z e s u b s t i t u t e d b e n z a l d e h y d e s a t t h e same r a t e . S i n c e t h e f o r m a t i o n o f h y d r o x y l r a d i c a l s i s p o s t u l a t e d i n t h e o x i d a t i o n p r o c e s s , i t may w e l l be t h a t i n d u c e d o x i d a t i o n i s o c c u r r i n g w h i c h would a c c o u n t f o r t h e same r a t e f o r b o t h manganese s p e c i e s . D e c o m p o s i t i o n by manganous s a l t s i s known as Guyards r e a c t i o n (14) and o c c u r s i n n e u t r a l and a l k a l i n e s o l u t i o n . 2 MnO^ + 3 Mn 2 + 4 OH - 5 Mn0 2 + 2 H 20 The mechanism o f t h i s r e a c t i o n i s f a r f r om e s t a b l i s h e d ( 1 5 ) . The main d i f f i c u l t y appears t o be t h e f a c t t h a t one d e a l s w i t h a heterogeneous r e a c t i o n i n v o l v i n g s o l i d manganese d i o x i d e and a l s o t h a t o t h e r manganese s p e c i e s of i n t e r m e d i a t e v a l e n c y a r e formed i n t h i s r e a c t i o n . The c a t i o n Mn +3 i s i n v o l v e d i n t h e r a t e d e t e r m i n i n g s t e p o f many r e a c t i o n s i n a c i d s o l u t i o n , b u t i s s t a b l e o n l y i n 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 ( 1 6 ) , w i t h f a s t d i s p r o p o r t i o n a t i o n o c c u r r i n g a t l o w e r a c i d i t i e s : 2 M n + 3 ^ M n + A + M n + 2 I t i s poss ib le , however, to s t a b i l i z e solut ions of Mn wi th complexing agents such as pyrophosphates, which makes them more sui table as o x i d i z i n g agents i n the pH reg ion . In f ac t , Waters and h i s col labora tors (17,18) have extensively invest igated the Mn I I I oxidations of a number of organic substrates using t h i s technique. Manganese dioxide has long been used as an o x i d i z i n g agent. These reactions are usua l ly ca r r i ed out i n the presence of sulphuric ac id or i n aprot ic solvents and are of heterogeneous nature. No homo-geneous oxidations invo lv ing Mn IV are as yet known (13) . I t was pointed out from analogy wi th other t r a n s i t i o n metal hydroxides that Mn(OH)^ would be too weak an ac id to ex i s t i n the ionized form i n so lu t ion and therefore dehydrates to the more thermodynamically stable manganese dioxide (12) . Examples of Potassium Permanganate Oxidations: 1. The oxidat ion of a l i p h a t i c aldehydes has received some study. Tronov (19) i n 1927 determined the rate of reac t ion between potassium permanganate and a number of alcohols and aldehydes under a l i m i t e d range of condi t ions , but he d id not attempt any - a -m e c h a n i s t i c i n t e r p r e t a t i o n s . H o l l u t a and M u t s c h i n ( 2 0 ) o x i d i z e d formaldehyde w i t h permanganate and found the r e a c t i o n t o be b a s e - c a t a l y z e d . 2. Drummond and Waters ( 9 ) have i n v e s t i g a t e d t h e permanganate o x i d a t i o n o f a v a r i e t y o f a l i p h a t i c a l c o h o l s , a l d e h y d e s , k e t o n e s and a c i d s ; b u t th e y have p r e s e n t e d l i t t l e d i s c u s s i o n i n terms o f mechanism. They found t h a t s a t u r a t e d c a r b o x y l i c a c i d s were i n e r t t o permanganate, a r e s u l t t h e y s t a t e as c o n s i s t e n t w i t h t h e s t a b i l i t y o f G—K and G—G l i n k s i n p a r a f f i n c h a i n s . W i b e r g and Fox (21) have v e r y r e c e n t l y o x i d i z e d a nunber of branched c h a i n c a r b o x y l i c a c i d s u s i n g a l k a l i n e permanganate. I n eve r y c a s e t h e y p o s t u l a t e r a d i c a l a b s t r a c t i o n by h y d r o x y l r a d i c a l s of a t e r t i a r y hydrogen atom as an i n i t i a t i o n s t e p i n t h e r e a c t i o n . E a r l i e r work i n t h i s f i e l d was done by Kenyon and Symons ( 2 2 ) who found a l m o s t q u a n t i t a t i v e c o n v e r s i o n o f a number of branched c h a i n c a r b o x y l i c a c i d s t o t h e i r c o r r e s p o n d i n g hydroxy a c i d s . The l a t t e r w o r k e r s a l s o p o s t u l a t e d a mechanism i n v o l v i n g r a d i c a l a b s t r a c t i o n o f a t e r t i a r y hydrogen atom by h y d r o x y l r a d i c a l s . The m a j o r i t y o f a l c o h o l s , a l d e h y d e s and k e t o n e s s t u d i e d i n a l k a l i n e media by Drummond and Waters ( 9 ) - 9 -were oxidized beyond the stage of t he i r corresponding carboxyl ic acids even though, as stated above, the acid was stable to permanganate. Extensive degradation was found i n base and the authors suggest that such oxidations proceed, i n par t , by way of the enols , which l i k e o l e f i n s , could be attacked at the double bond and so be oxidized beyond the o r i g i n a l carbonyl s i t e . 3 . F l u o r a l Hydrate - The mechanism of the oxidat ion of t h i s a lcohol and i t s deuteriuai analogue has recent ly been studied extensively by Mocek (3) throughout the pH reg ion . Compared wi th 2,2,2-t r i f luoroe thano l and other subst i tuted alcohols which ion ize i n the pH region, f l u o r a l hydrate i s unique i n that i t has two a c i d i c hydrogens, both of which can i o n i z e . The pK of the f i r s t i o n i z a t i o n process was found to be 10.1. Deuteri\im isotope effects were found to change wi th the nature of the substrate, being a maximum of ten for the monoanion. Furthermore, a c t i v a t i o n para-meters and a pos i t ive s a l t effect show that the t r an-s i t i o n state i s formed from species of l i k e charge. These resu l t s are i nd i ca t i ve of hydride ion transfer to the permanganate ion i n the rate determining step. The fo l lowing mechanism i s proposed: - 10 -OH 0 I - K | GF Q —C—H +OH - GFp— G —H + H o0 3 I 3 I 2 OH OH 0 0 1 k I  GFQ— G —H + MnO * G. + H MnO, 3 | 4 / GF q 4 OH HO d _ _ fas t H MnO, + MnO, * 2 MnOT + H 4 4 4 In the strongly a lka l ine region, (pH 1.2-14), the rate determining step would be: GF„ GFQ - I 3 k | 3 0 — G— H + MnO * G + H MnO. I 4 / ' \ 4 0 0' — K) The evidence presented by Mocek does not al low an unequivocal in te rp re ta t ion of the k i n e t i c data. I t i s k i n e t i c a l l y impossible (23) to d i s t i ngu i sh be-tween a mechanism invo lv ing the aldehydrol anion as a d iscree t intermediate and a concerted mechanism i n -vo lv ing hydroxyl ion and aldehyde hydrate. Hence, termolecular mechanisms are also considered, an example of which i s : - 1 1 -OH OH 4. Benzaldehyde - The permanganate oxida t ion of t h i s aldehyde was f i r s t examined by Tompkins (24) who reported a l i n e a r increase i n rate wi th hydroxide ion concentration over a very l i m i t e d range, although i n basic so lu t ion the rate i n a given run was found to drop wi th t ime. He a t t r ibu ted the increase i n rate to the formation of the "aldehydate ion" -0~ I , G H — G—OH 6 I -H , The drop i n rate wi th time was a t t r ibu ted to the Gannizzaro r eac t ion , which i s known to be catalyzed by o x i d i z i n g agents (25) . However, t h i s c a t a l y s i s has since been shown to apply to heterogeneous react ions on ly . He postulated the fo l lowing mechanisms: H fast | CJH,. GHO + MnO. > C.H — C —0Mn0Q 6 5 4 6 5 j 3 _0 H •I k C 6 H 5 - C — OMnOg ^ G 6 H 5 G0 2 H + MnOg 0 - 12 -fast 2 MnO~ + H 20 * H Mn04 + OH + Mn02 fas t H MnO 7 + G HcCHO * C ^ C O - H + OH + MnO-*+ 6 5 o i> 2 2 A more de ta i l ed study was undertaken by Wiberg and Stewart (1) using deuterium and oxygen-18 l a b e l l i n g techniques. A subs tant ia l isotope effect was found i n weakly ac id i c regions; t h i s effect decreased wi th increas ing pH as d id the oxygen-18 t ransfer from per-manganate to substrate. The reac t ion was also found to be general ac id-ca ta lyzed . The fo l lowing mechanism was proposed: K l GgH^GHO + Hg+0 c C6H5CH0H + HO C J r GH$H + MnO, - C.H —G-O-MnO 6 5 4 6 5 | 3 H OH U k C,H — G—H * :B * G>-HcC0oH + HB + MnO 6 5 |p 6 5 2 3 0. 'r± • ' ^MnOg fast 3 MnOq + H^O 2 MnO + MnO, + 2 0_H o 2 2 4 A Hammett p lo t for the reac t ion i n t h i s region showed a good l i n e a r r e l a t i o n wi th a small negative - 13 -^ va lue . One would expect the rate determining step to have a pos i t ive ^ since an e lec t ron withdrawing group would f a c i l i t a t e the abst ract ion of a proton i n t h i s step. Hence, the equi l ib r ium forming the i n -termediate ester i s proposed as having a negative ^ which i s l a rger i n magnitude than that for the rate determining step. A s i m i l a r s i t u a t i o n i s known to ex i s t i n the chromic ac id oxidat ion of a lcohols (26) . In a l k a l i n e solut ions the rate was found to depend on the square root of the hydroxyl concentration and most, i f not a l l , of the oxygen introduced in to the aldehyde was derived from the solvent . The deuterium isotope effect was small and was found to decrease wi th increas ing pH. This suggested that the reac t ion may no longer involve cleavage of the aldehyde carbon-hydrogen bond i n the rate determining step. Fur ther-more, the reac t ion has a large pos i t ive ^ value (+1.83) i n contrast to the small negative value .found i n neutra l s o l u t i o n . The o r i g i n a l proposal of Tompkins that the "aldehydate ion" i s formed f i t s some of the f a c t s . This ion could react w i th the permanganate ion by a hydride sh i f t and the effect of substituents on the rate would be i n agreement wi th the s t a b i l i t y of the - 14 -i o n . However, i n order to expla in the dependence of the rate on the square root of the hydroxyl concentra-t i o n , a free r a d i c a l chain mechanism invo lv ing hydroxyl r ad i ca l s was suggested. Furthermore, manganate and permanganate were found to react wi th benzaldehyde wi th rates of the same order of magnitude. This supports a r a d i c a l mechanism, as one would expect a hydride sh i f t to be much slower for manganate i n view of the double negative charge on the i o n . A t y p i c a l mechanism considered i s shown below: k-. I n i t i a t i o n : MnO, + OH — - — » MnO + HO* 4 4 0* Propagation: HO' + GgH^GHO * CgELz—C—OH H MnO, 11 •* C H CO H + Mn0„ + HO* k 3 6 5 2 3k 4 fas t Termination: 2 HO* —-—> H 0 =-** 0 + H MnO . 2 2 MnO^ 2 . 1 5 . Benzhydrol - This substrate on oxidat ion y i e l d s a product stable to further reac t ion and was examined thoroughly by Stewart ( 2 7 ) . The use of deuterium subs t i tu t ion and oxygen-18 exchange enabled - 15 -him to show that the carbon-hydrogen bond i s broken i n the rate determining step and that none of the oxygen i s introduced in to the substrate by the per-manganate. The rate showed a dependence on the hydroxyl ion concentration and a pos i t ive s a l t e f fec t . On the basis of these observations, a mechanism invo lv ing a hydride ion transfer from the benzhydrylate anion to the permanganate ion was postulated: Ph CHOH + OH ~» Ph GHO + HO fast 2 2 2 Ph GHO + MnO. ^PhoG=0 + H MnO, slow 2 4 2 4 H MnO" + MnO" + OH *2 MnO^ + H O fast 4 4 4 2 6. Phenyl t r i f luoromethylcarbinols - As a further study on the oxidat ion of a lcoho ls , a series of a r y l t r i f luoromethylcarb inols proved to be sa t i s fac tory since they are h igh ly ionized i n tenth normal base and are oxidized c leanly to the corresponding ketones by potassium permanganate. ArCHOHCFg + 2 Mn0 4 + 2 OH- *~ArCOCFg + 2 MnOj + 2 H 20 Stewart 's work on benzhydrol given above has i n -dicated that the alcoholate anion i s the reac t ive species i n the ox ida t ion . I f t h i s i s so, then i t should be - I m -possible to observe a l e v e l l i n g - o f f of the rate i n the region where the a lcohol i s almost f u l l y ionized and moreover, the rate should be determined by the alcoholate i o n present. This has i n fact been confirmed (2 ,28) . Further , the observation of a pos i t ive s a l t e f fec t , a large negative entropy of a c t i v a t i o n and a large deuterium isotope effect (k^/kp =16) are i n d i c a t i v e of a mechanism which involves a hydride ion t ransfer . ArCHOHCFg + OH T " ArCHOCFg + H ? 0 0 r 0 o 1 _ T ii 1 Ar—G—H + MnO -I . . . . « GF CF,Ar-G — H— OMnO 3 ^ArCCFg + H Mn0 4 Normal deuterium isotope e f fec t s , which r e su l t from a loss of carbon-hydrogen s t re tch ing modes i n the t r a n s i t i o n s ta te , are less than h a l f t h i s s ize at the same temperature ( 8 ) . The large effect observed here i s postulated as due to the loss of bending as w e l l as s t re tching modes i n the t r a n s i t i o n s ta tes . However, the p o s s i b i l i t y of quantum mechanical tunne l l ing i s not excluded. The effect of nuclear subs t i tu t ion was found to be s l i g h t and th i s i s not i n accord wi th the above mechanism. A hydride transfer from the alkoxide ion to permanganate should r e su l t i n a negative - 17 -value , i . e . a p-methoxy substituent should acceler-ate and a p -n i t ro group re tard the process. However, the reac t ion rate was found to be only s l i g h t l y affected by nuclear subs t i tu t ion and furthermore, the small va r i a t ions i n rate which d id occur were not r e -la ted l i n e a r l y to the Hammett substi tuent constant ^ . The fo l lowing explanations have been presented. F i r s t l y , a simple hydride transfer may i n fact be occurring wi th the e lec t ron ic effect of a d i s tan t group i n the molecule being unimportant. Secondly, two d i f fe ren t processes wi th d i f fe ren t e lec t ron ic requirements may be occurr ing , one wi th a pos i t i ve and the other wi th a negative ^ va lue . T h i r d l y , one of several termolecular mechanisms may be taking place, an example of which i s shown below: Ar O ^ I < ^ - k I H 9 ° 9~° M n 0 z * H 3 0 + A r ~ - ~ G F 3 + M n 0 ^ G F 3 0* fast Ar—C—GF + MnO^ > products - 18 -OBJECT OF RESEARCH This project was undertaken wi th a view to strengthening some previously reported mechanisms for permanganate oxidations determined i n t h i s and other l abora to r i e s . Stewart and Wiberg CD have studied extensively the permanganate oxidat ion of benzaldehyde and subst i tuted benzaldehydes i n the pH range 5 to 13. In order to see i f the mechanism they proposed i s app l i ca t ive to other aldehydes of s i m i l a r nature, the a l i p h a t i c analog of benzaldehyde, pivalaldehyde, was oxidized wi th permanganate. Since nei ther substrate contains an o( -hydrogen, i t was of in te res t to deter-mine i f both reacted v i a s i m i l a r paths. Furthermore, Wiberg and Stewart (1) also reported that an auto c a t a l y t i c effect i n the oxidat ion of benzaldehyde by permanganate occurred when the pH was lowered much below f i v e . As a further extension of t h e i r research, i t was decided to study th i s effect i n the pH region 1 to 4, i n an attempt to e lucidate the nature of the auto c a t a l y s i s . Benzaldehyde i t s e l f was studied i n th i s connection, as w e l l as p-ni t robenz-aldehyde, i n order to see i f there was any substi tuent e f fec t . - 19 -Stewart and Van der Linden (2) have postulated three possible mechanisms for the permanganate oxida t ion of phenyl t r i f luoromethylcarbinols . However, more r e -cen t ly , Stewart (29) has postulated a mechanisnfor the oxidat ion of f luoroalcohols which involves hydrogen atom abst rac t ion from the anion to the permanganate i o n . In order to check the p o s s i b i l i t y of such a mechanism i n the oxida t ion of subst i tuted phenyl-t r i f luoromethylcarbinols , a study of the permanganate oxidat ion of p-ni t rophenyl t r i f luoromethyl ca rb ino l i n tenth normal sodium hydroxide was undertaken. I t can be immediately seen that i f such a mechanism i s operat ive, then the greatest s t a b i l i z a t i o n of a r a d i c a l intermediate would be afforded by a p -n i t ro subst i tuent , as the fo l lowing resonance structures ind ica te : - 20 -EXPERIMENTAL A . Preparat ion, P u r i f i c a t i o n , and I d e n t i f i c a t i o n of Substrates L . Pivalaldehyde Pivalaldehyde was obtained from Columbia Organic Chemicals L t d . I t was p u r i f i e d before use by vapour phase chromatography using a ten foot "Carbowax" column at a temperature of about 30° . Stock solut ions of 0.02 molar were prepared by p ipe t t ing an accurate volume of the pu r i f i ed aldehyde ( ^\ pipet tes were used) in to a volumetric f l a sk and making up to the mark wi th warm d i s t i l l e d water. The water had been previously bo i l ed i n order to remove d issolved oxygen, but had to be cooled to about 45° i n order to minimize vapor iza t ion losses , s ince pivalaldehyde b o i l s i n the range 71-4° (30) . The concentration of the aldehyde could be accurately determined since the density was known (.793 g. per m l . ) . However, i t was found that pivalaldehyde undergoes both autoxida-t i o n and decomposition upon standing, and other means had to be employed to determine i t s concentrat ion. The decomposition of pivalaldehyde to i s o -butane and carbon monoxide and i t s autoxidat ion to - 21 -p i v a l i c ac id have both been shown to be catalyzed by l i g h t and/or peroxides (31) . However, when stored i n a cool dark place, both these effects have been r e -ported as being markedly reduced (30) . Stock solut ions of pivalaldehyde were then prepared i n blackened volumetries and stored i n the r e f r i g e r a t o r . Solut ions prepared and kept i n t h i s way were checked p e r i o d i c a l l y for autoxidat ion by t i t r a t i o n wi th 0.02 N sodium hydroxide. I t was found that under such condi t ions , oxidat ion i s n e g l i g i b l e . This i s to be contrasted wi th samples stored at room temperature, where as much as twenty-five percent oxida t ion was encountered over a two month per iod . Qua l i t a t i ve evidence for the decomposition can be gained by simply loosening the cap of a sample bo t t l e stored at room temperature and n o t i c i n g the r e -l a t i v e l y large release of pressure due to the isobutane and carbon monoxide. Furthermore, i t i s worth noting that a sample c e l l l e f t on the bench overnight showed no absorption the fo l lowing morning. This r e su l t i s presumably due to the l i g h t catalyzed decomposition. A l s o , stock solut ions made up by the add i t ion of warm water could l i k e l y cause some v o l a t i l a t i o n of the pivalaldehyde due to i t s high vapour pressure. These troublesome cha rac te r i s t i c s made a spectroscopic - 22 -d e t e r m i n a t i o n o f t h e c o n c e n t r a t i o n mandatory, i n t h a t s t o i c h i o m e t r i c r a t i o s i n the o x i d a t i o n by p o t a s s i u m permanganate be a s s u r e d . The f o l l o w i n g p r o c e d u r e was adopted. One hundred raicrolitre samples of f r e s h l y p u r i f i e d p i v a l a l d e h y d e were added t o 50 m l . b l a c k e n e d v o l u m e t r i c s and i m m e d i a t e l y d i s s o l v e d i n hexane, met h a n o l , d i o x a n e and w a t e r . U s i n g a GARY-14 r e c o r d i n g s p e c t r o p h o t o m e t e r , t h e maximum wave l e n g t h f o r a b s o r p -t i o n was found f o r each o f t h e s e and u s i n g t h e Beckman D.U. S p e c t r o p h o t o m e t e r , t h e i r o p t i c a l d e n s i t i e s were a c c u r a t e l y d e t e r m i n e d . A v a l u e f o r t h e o p t i c a l den-s i t y o f p i v a l a l d e h y d e i n aqueous s o l u t i o n was found t o be much l e s s t h a n i n hexane and d i o x a n e , as shown i n T a b l e I . T h i s d i f f e r e n c e was i n t e r p r e t e d as a l o s s due t o v a p o r i z a t i o n i n p r e p a r i n g the s o l u t i o n , r a t h e r t h a n t o f o r m a t i o n o f t h e h y d r a t a . F u r t h e r m o r e , v i r t u a l l y no a b s o r p t i o n o c c u r r e d i n m e t h a n o l , presumably due t o the f o r m a t i o n of t h e h e m i a c e t a l . The average o p t i c a l d e n s i t y of p i v a l a l d e h y d e i n hexane and d i o x a n e was t a k e n and t h e e x t i n c t i o n c o -e f f i c i e n t was found t o be 17.0. U s i n g t h i s v a l u e t o d e t e r m i n e t h e c o n c e n t r a t i o n o f the a l d e h y d e i n aqueous s o l u t i o n , as much as t h i r t y - f i v e p e r c e n t d e v i a t i o n - 23 -T a b l e I A b s o r p t i o n o f P i v a l a l d e h y d e i n S o l u t i o n O p t i c a l S o l v e n t ) s (mA) D e n s i t y max r M e t h a n o l 295 0.06 Hexane 295 0.290 Dio x a n e 290 0.262 Water 285 0.209 - 24 -from that expected by weight c a l c u l a t i o n was found. As a further check on the value of the ex t i nc t i on co-e f f i c i e n t , a reac t ion was done i n which potassium permanganate was added i n f i f t y percent excess of pivalaldehyde and l e t stand overnight. Upon t i t r a t i o n , th i s y ie lded a concentration of pivalaldehyde which should have resul ted from an ex t i nc t i on coe f f i c i en t of 16 .3 . An average value of 16.7 was then decided upon as the ex t i nc t i on coe f f i c i en t of pivalaldehyde i n order to determine i t s concentration accura te ly . This gives good agreement wi th a value of 16.5 obtained i n hexane by other workers (32) . The concentration of the aldehyde was determined each day from the o p t i c a l density found on the Beckman D . U . Spectrophotometer, before any k i n e t i c runs were s ta r ted . The o p t i c a l density was found to be constant over a period of about three weeks but then began to decrease. At t h i s point the stock so lu t ion was d i s -carded and a fresh one prepared. No k i n e t i c runs were done when the so lu t ion gave evidence of decay as the prospect of free r ad ica l s i n t e r f e r i n g wi th the oxida t ion was not des i r ab le . A t y p i c a l decay scheme i s shown i n Table I I . - 25 -T a b l e I I D e c o m p o s i t i o n of a T y p i c a l S o l u t i o n o f P i v a l a l d e h y d e Time O p t i c a l ( d a y s ) D e n s i t y 1 .198 2 .204 4 .201 9 .203 17 .204 23 .179 25 .173 32 .163 40 .156 58 .146 73 .154 75 .162 78 .152 97 .156 141 .134 - 26 -2 . Benzaldehyde and p-Nitrobenzaldehyde. Benzaldehyde was d i s t i l l e d over ni trogen imme-d i a t e l y before use. A 0.01 molar so lu t ion was pre-pared by p ipe t t ing an accurate volume in to a volumetric f l a sk and d i s s o l v i n g immediately i n b o i l i n g d i s t i l l e d water. Stewart and Wiberg (1) have mentioned the s t a b i l i t y of aqueous solut ions of benzaldehyde to autoxidat ion and no check for the presence of benzoic ac id was undertaken. p-Nitrobenzaldehyde, obtained from Eastman Kodak Company, was r e c r y s t a l l i z e d from a s i x to one water to methanol mixture to a constant melt ing poin t , m.p. 107° ( L i t , , 1 0 6 . 5 ° ) . A sample was weighed out and dissolved i n b o i l i n g d i s t i l l e d water i n a v o l u -metric f l a sk to give a 0.002 molar s o l u t i o n . The water s o l u b i l i t y of th i s aldehyde i s not expected to be much more. 3. Phenyl t r i f luoromethylcarbinol and p- Ni t rophenyl t r i f luoromethylcarb ino l . A sample of phenyl t r i f luoromethylcarbinol obtained from Columbia Organic Chemicals L t d . , was pu r i f i ed before use by vapour phase chromatography using a ten foot "Uycon polar" column at a temperature of about 135? - 27 -As before, aqueous solut ions were prepared by p ipe t t ing accurate volumes and d i s s o l v i n g i n b o i l i n g d i s t i l l e d water i n a volumetric f l a s k . Solut ions of the order of 0.006 molar were prepared i n th i s way. The density of t h i s a lcohol could not be found i n the l i t e r a t u r e . I t was determined both by p ipe t t ing )^  volumes accura-t e l y and weighing by di f ference , on two occasions, and weighing a pippetted volume from a m i c r o - l i t r e syringe in to a small container on one other occasion. A l l determinations of the density gave good agree-ment and y ie lded an average value of 1.307 g. ml ^ . I t i s worth noting that the gravimetric balance showed an increase i n weight wi th time for t h i s a l coho l , i n d i c a t i n g that i t may be hygroscopic. p-Ni t rophenyl t r i f luoromethylcarbinol was prepared i n good y i e l d by Mr. L . J . Muenster. The fo l lowing i s a b r i e f out l ine of h i s experimental procedure: The acetate was f i r s t prepared by adding the ca rb ino l dropwise to ace t ic anhydride. Fuming n i t r i c ac id was added to the ace ty la t ion mixture. The reac t ion mixture was then poured onto i ce water and l e f t standing overnight. The p-ni troacetate was separated and hydrolyzed wi th ethanol and hydrochlor ic ac id and af ter solvent evaporation, a r e s idua l brown l i q u i d - 28 -s o l i d i f i e d t o a c r y s t a l l i n e mass. T h i s was r e c r y s t a l l -i z e d t o a c o n s t a n t m e l t i n g p o i n t f rom a s i x t o one w a t e r t o methanol m i x t u r e . C o l o r l e s s c r y s t a l s o f p - n i t r o p h e n y l t r i f l u o r o m e t h y l c a r b i n o l were o b t a i n e d , m.p. 133-134°. A n a l y s i s gave e x c e l l e n t agreement w i t h c a l c u l a t e d v a l u e s : C a l c d . f o r CgHgNOgFg: C, 43.45; H, 2.74; N, 6.34 Found f o r CgHgNOgFg r C, 43.50; H, 2.63; N, 6.40 I n f r a r e d spectrum showed t h e e x p e c t e d a b s o r p t i o n bands. O x i d a t i o n w i t h p o t a s s i u m permanganate r e s u l t e d i n p - n i t r o b e n z o i c a c i d w h i c h was c o n f i r m e d by mixed m e l t i n g p o i n t d e t e r m i n a t i o n and i n f r a r e d spectrum. Aqueous s o l u t i o n s o f t h e p - n i t r o p h e n y l t r i f l u o r o -m e t h y l c a r b i n o l were p r e p a r e d by w e i g h i n g and d i s s o l v i n g i n b o i l i n g d i s t i l l e d w a t e r . Even though b o i l i n g w a t e r must a i d i n i t s s o l u b i l i t y , t h i s a l c o h o l was f o u n d t o be v e r y i n s o l u b l e as s o l u t i o n s c o u l d n o t be made up much i n e x c e s s of 0.001 m o l a r . A l k a l i n e s o l u t i o n s can be made much more c o n c e n t r a t e d as p - n i t r o p h e n y l -t r i f l u o r o m e t h y l c a r b i n o l has been shown by S t e w a r t and Van d e r L i n d e n (33 ) t o have a p l ^ o f 11.1. However, good k i n e t i c s a r e n o t o b t a i n e d when t h e a l c o h o l i s f i r s t d i s s o l v e d i n a l k a l i n e s o l u t i o n . - 29 -B. K i n e t i c Method An i o d o m e t r i c method f o r f o l l o w i n g the r a t e o f d i s a p p e a r a n c e o f t h e permanganate i o n was found s u i t a b l e f o r f o l l o w i n g t h e o x i d a t i o n o f p i v a l a l d e h y d e . Wiberg and S t e w a r t ( 1 ) and S t e w a r t and Van d e r L i n d e n (28) had p r e v i o u s l y found t h e same method s a t i s f a c t o r y f o r s t u d y i n g t h e permanganate o x i d a t i o n o f s u b s t i t u t e d b e nzaldehydes and p h e n y l t r i f l u o r o m e t h y l c a r b i n o l s r e s -p e c t i v e l y , and i t was a g a i n employed i n t h e s e f u r t h e r s t u d i e s . The method i s e s s e n t i a l l y t h a t d e s c r i b e d by Tompkins ( 2 4 ) ; namely, t h e c o u r s e o f t h e r e a c t i o n was f o l l o w e d by q u e n c h i n g a l i q u o t s i n a c i d i f i e d p o t a s s i u m i o d i d e s o l u t i o n and t i t r a t i n g t h e l i b e r a t e d i o d i n e w i t h sodium t h i o s u l p h a t e . I n most cases t h e p e r m a n g a n a t e - s u b s t r a t e r e a c t i o n was found t o be q u i t e f a s t and i t was n o t p o s s i b l e t o t i t r a t e each a l i q u o t i m m e d i a t e l y a f t e r q u e n c h i n g . I n s t e a d , a l i q u o t s were removed c o n t i n u o u s l y and quenched one a f t e r a n o t h e r and t h e f l a s k s s e t a s i d e i n a darkened p l a c e t o be l a t e r t i t r a t e d . A f t e r t h e l a s t a l i q u o t had been quenched, t h e p r e v i o u s l y s e t a s i d e f l a s k s were t i t r a t e d i n a random o r d e r and a l l t i t r a t i o n s were completed w i t h i n f i f t e e n m i n u t es a f t e r t h e i n i t i a l l i b e r a t i o n o f i o d i n e . - 30 -Due to the decomposition of pivalaldehyde by l i g h t (an effect which has been previously discussed) , a l l reactions were ca r r ied out i n darkened f l a s k s . The same procedure was used for a l l other oxida t ions . A t y p i c a l k i n e t i c run was car r ied out as fo l lows : 0.2 M sodium hydroxide (21.8 m l . ) , d i s t i l l e d water (20.1 ml . ) and pivalaldehyde (3.30 ml . ) were added to a 125 m l . darkened erlenmeyer f l a s k . This reac t ion mixture was allowed to equ i l ib ra t e i n a constant temperature bath of 25.2 * ,05°G. The oxida t ion was then i n i t i a t e d by the add i t ion of 4.78 m l . of potassium permanganate (standardized against sodium oxalate) at the same temperature from a fas t de l ive ry pipet te ( t i p removed) and a timer immediately s ta r ted . A l l react ions were based on a 50 m l . t o t a l volume. Before the addi t ion of potassium permanganate, 50 m l . e r l en -meyer f lasks were made ready wi th d i l u t e sulphuric ac id to which was added a mixture of s o l i d potassium iodide and sodium bicarbonate. The l a t t e r reagent was added to el iminate possible a i r oxidat ion of the i od ide . Af te r the evolut ion of carbon dioxide had ceased, potassium permanganate was added to the r e -ac t ion mixture, the vesse l sw i r l ed , and a l iquots r e -moved one af ter another and quenched i n the a c i d i f i e d i od ide . These were set aside and. la ter t i t r a t e d wi th - 31 -sodium thiosulphate from a 5 m l . capacity burette using . thyodene as i n d i c a t o r . Overa l l concentrations used here were: sodium hydroxide 0.1 M, substrate 0.000555 M, permanganate 0.00111 M. Here the r a t i o of permanganate to substrate i s two to one since the permanganate i s reduced to manganate i n a l k a l i n e so lu t ions . For reactions i n weakly basic and a c i d i c so lu t ions , a two to three r a t i o of permanganate to substrate was used as the permanganate i s reduced to manganese dioxide at pH*s less than 11.5 . Dipotassium hydrogen phosphate was used as a buffer wi th add i t ion of sulphuric ac id or sodium hydroxide to vary the pH. Potassium sulphate was used to maintain the i o n i c s t rength. I t was found that k i n e t i c plots were most accurate i n th i s region and rates were reproducible to w i t h i n f i v e percent. - 32 -RESULTS Although three d i f fe ren t substrates were employed i n t h i s work, a l l were two equivalent reductants and as such consumed equal amounts of permanganate. Molar r a t i o s were consequently the same for a l l three sub-s t ra tes . A b r i e f ou t l ine of the k i n e t i c expressions and the s toichiometr ies used i s i n order here. In the pH range 1 to about 10.5 the Mn VII i s reduced to Mn IV . Hence, a three equivalent e lec t ron change occurs. Since the oxidat ion involves a two equivalent change, th i s sets the r a t i o of the sub-s t ra te to permanganate at 3:2 as the fo l lowing equation ind ica tes : + 2 MnO^ + 3 RGHO + 2 H *2 Mn0 2 + 3 RGOOH + 4 H 2 0 In more strongly a l k a l i n e so lu t ions , above pH 12, the permanganate i s reduced to manganate which accumulates and reacts w i th the substrate at a slower rate compared to that of oxidat ion by permanganate. Since here, a one equivalent change occurs, the r a t i o of substrate to permanganate now becomes 1:2, as shown: 2 MnOl + RGHO + 2 OH »• 2 MnO7 + RGOOH + 2 H^O - 33 -In the region between pH 10.5 and 12, some i n t e r -mediate r a t i o app l i e s . Manganate i s stable above pH 12 and completely unstable below pH 10.5; however, i n the intermediate range the d ispropor t ionat ion of manganate appears to be both time and pH dependent and therefore precise information about the oxidat ion rate cannot be obtained. Mocek gives a good account of t h i s i n h i s thesis ( 3 ) . Stoichiometr ic r a t i o s of the substrate to permanganate of 3:2 and 1:2 below pH 11 and above pH 12 respec t ive ly were used through-out t h i s i n v e s t i g a t i o n . Integrated rate expressions for a second order reac t ion were derived by Stewart (1,27) for perman-ganate oxidations i n aqueous so lu t ion where t h i o -sulphate i s used i n an iodometric t i t r a t i o n and the s to ichiometr ic concentration of the reactants i s present throughout the ox ida t ion . In a l k a l i n e s o l u -t i o n s , the rate expression i s given by: 1 substrate x o where V o i n i t i a l volume of thiosulphate at t=0 volume of thiosulphate used at time t 4/5 V = f i n a l volume at t ' o - 34 -t = time i n seconds subs t ra te j Q = i n i t i a l concentration of sub-s t ra te at t=0 k 2 = second order rate constant i n 1 mole"-'-sec. A s i m i l a r expression i s found for the r a t i o 3:2 and i s given by: 1 V o " V t k 2 = = =r X [substrate] V T - 2/5 V Q A p lo t of (V Q - V t^'V t - 4/5 V Q) and ( V Q - V T ^ ( V T - 2/5 V Q ) v s . time gave good s t ra ight l i n e s passing through the o r i g i n . A . The Permanganate Oxidat ion of Pivalaldehyde 1. Nature of the Substrate I t i s w e l l established that t r ichloroacetaldehyde ex is t s i n aqueous so lu t ion i n the hydrated form. S i m i l a r l y , t r i f luoroacetaldehyde takes up water to form the hydrate ( 3 ) . In the experimental part of th i s thes is the d i f f i c u l t y encountered i n determining the concentration of pivalaldehyde i n aqueous so lu t ion i s discussed. In that study, the o p t i c a l density of - 35 -s o l u t i o n s p r e p a r e d i n hexane, w a t e r and methanol ( T a b l e I ) d e c r e a s e d i n t h e o r d e r hexane ") w a t e r ^ m e t h a n o l . The d e c r e a s e i n o p t i c a l d e n s i t y i n w a t e r was a t t r i b u t e d t o v o l a t i l i z a t i o n o f t h e p i v a l a l d e h y d e upon p r e p a r a t i o n o f t h e s o l u t i o n r a t h e r t h a n t o f o r m a t i o n o f t h e h y d r a t e . T h i s a s s u m p t i o n i s s u p p o r t e d by t h e d a t a g i v e n i n T a b l e I I , w h i c h shows a c o n s t a n t o p t i c a l d e n s i t y f o r about t h r e e weeks* b e f o r e d e c o m p o s i t i o n o c c u r s . Such a r e s u l t w ould n o t be c o n s i s t e n t w i t h f o r m a t i o n of a h y d r a t e . F u r t h e r m o r e , h y d r a t e f o r m a t i o n would be i n -c o n s i s t e n t w i t h t h e e l e c t r o n i c n a t u r e o f t h e m e t h y l groups when one c o n s i d e r s t h e mechanism o f h y d r a t e f o r m a t i o n w h i c h i n v o l v e s a p r o t o n t r a n s f e r ( 3 4 ) ( 3 5 ) . I t i s w o r t h w h i l e t o comment upon t h e o p t i c a l d e n s i t y of a s o l u t i o n p r e p a r e d i n methanol w h i c h shows v i r t u a l l y no a b s o r p t i o n ( T a b l e I ) . T h i s c o u l d o n l y be due t o t h e f o r m a t i o n of t h e h e m i a c e t a l , w h i c h i s c o n s i s t e n t w i t h the w e l l known h e m i a c e t a l f o r m a t i o n o f a c e t a l d e h y d e i n methanol ( 3 6 ) . 2. O x i d a t i o n o f P i v a l a l d e h y d e i n the R e g i o n  pH 1-12.5 C o n s t a n t i o n i c s t r e n g t h c o n d i t i o n s (/A-= 0.6) were used f o r a l l runs t h r o u g h o u t t h i s r e g i o n . A t c o n s t a n t - 36 -pH and i o n i c strength the rate of oxidat ion of p i v a l -aldehyde i s given by the expression: Good second order ra te plots have been obtained for a l l experiments i n t h i s r eg ion . No k i n e t i c runs were done i n the region pH 10.5 to 12.0 . The resu l t s for a t y p i c a l run are given i n Table I I I , w i t h the cor re -sponding s t ra ight l i n e p lo t shown i n Figure 1. A dependence on pH i s observed, wi th the rate r i s i n g below a pH of 4.5 and beyond 10.5 and pH i n -dependent between these two values , as shown i n Figure 2 . By inspec t ion , the changes i n rate are shown as l i n e a r , although such may not be the case, e spec ia l ly i n the basic reg ion . This p lo t i s not un l ike that found by Wiberg and Stewart (1) i n the pH region 5 to 13 for the permanganate oxidat ion of benzaldehyde, although the rate of oxidat ion of pivalaldehyde i s observed to be ten times f a s t e r . The resu l t s corresponding to Figure 2 are given i n Table IV. = k~ MnO Pivalaldehyde dt T a b l e I I I The Permanganate O x i d a t i o n o f P i v a l a l d e h y d e  A T y p i c a l K i n e t i c Run 7t. ( m l . ) V -V t ( m l . ) V t.-2/5V 0 (ml.) 3.812 3.620 3.470 3.278 3.200 3.105 3.011 2.885 0.417 0.609 0.759 0.951 1.029 1.124 1.218 1.344 2.121 1.929 1.779 1.587 1.509 1.414 1.320 1.194 Vfc.-2/5V0 0.196 0.316 0.427 0.650 0.682 0.795 0.921 1.125 time ( s e e s . 33 55 73 98 117 134 157 196 P i v a l a l d e h y d 2 J ° MnO" V 2/5 V Q = 1.512 x 10 ° Mole 1 -1 1.008 x 1 0 ~ 3 Mole l . " L = 4.229 ml, = 1.691 ml, 1 .04-Vo-Vf V t - | V o FIGURE 1 A typical rate plot Oxidation of Pivalaldehyde 054-pH = 10.37 yU= 0-6 50 teo time in sec. 150 - 40 -T a b l e IV Permanganate O x i d a t i o n o f P i v a l a l d e h y d e  V a r i a t i o n of R a t e C o n s t a n t w i t h pH 1 r a t e , n pH ( 1 . mole" 1.65 9.2 2.70 8.6 3.45 .7.6 4.60 5.7 5.50 4.6 6.65 5.1 7.18 5.05 8.05 5.1 9.13 4.8 10.38 4.6 12.50 21.4 Temperature = 25.00 t 0.05° I o n i c S t r e n g t h = 0.6 mole 1.7 - 41 -3. General A c i d - c a t a l y s i s To demonstrate that general a c i d - c a t a l y s i s does occur, rates were determined i n solut ions containing varying amounts of buffer at constant i o n i c strength and pH and hence containing varying amounts of un-dissocia ted a c i d . A c a t a l y t i c effect i s apparent as shown i n Figure 3, although rates are only about twenty percent enhanced at pH's studied for a tenfold change i n phosphate concentrat ion. The r e su l t s fo r Figure 3 are given i n Table V . This i s to be com-pared wi th a s i m i l a r p lo t of Stewart 's (1) i n the permanganate oxidat ion of benzaldehyde which also shows a twenty percent increase i n rate over the same buffer concentration range. In add i t i on , several runs were done at constant buffer concentration while varying the pH by the addi t ion of concentrated sulphuric a c i d . Aga in , th i s resul ted i n varying amounts of undissociated a c i d . Results for these runs are given i n Table V I . A general a c i d - c a t a l y s i s effect i s evident again i n Figure 4. In order to give further evidence for general a c i d - c a t a l y s i s , several runs were done wi th sodium FIGURE 3 Oxidation of Pivalaldehycje 4 0 -0.1 02 [HPO;] 03 '0.4 M . - 43 -T a b l e V O x i d a t i o n of P i v a l a l d e h y d e a t C o n s t a n t pH V a r i a t i o n o f t h e R a t e C o n s t a n t w i t h B u f f e r C o n c e n t r a t i o n — - i PH k 2 _ i t m o l e I . - 1 ) ( 1 . mole" 5.0 0.04 4.98 5.0 0.20 5.45 5.0 0.40 5.98 2.90 0.08 6.48 2.90 0.20 6.65 2.90 0.28 6.90 2.90 0.34 7.02 2.90 0.40 7.18 1.40 0.04 7.31 1.40 0.20 7.75 1.40 0.40 8.05 s e c . " ) - 44 -T a b l e V I O x i d a t i o n o f P i v a l a l d e h y d e a t C o n s t a n t B u f f e r C o n c e n t r a t i o n  V a r i a t i o n o f t h e R a t e C o n s t a n t w i t h pH HPO, (mole l . " 1 ) pH . \ _]_ _ i C I . mole s e c . ) 0.2 1.65 9.10 0.2 2.70 8.10 0.2 3.25 7.30 0.2 4.60 5.60 0.2 5.50 4.50 0.6 1.65 10.6 0.6 2.90 9.40 0.6 3.00 9.00 0.6 6.00 6.00 0.6 6.73 5.25 FIGURE 4 Oxidat ion of Pivalaldehyde Dependence of rate on pH, constant orthophosphate concentrat ion - 46 -pyrophosphate b u f f e r s . R e s u l t s f o r t h e s e r u n s a r e g i v e n below i n T a b l e V I I and a r e shown i n F i g u r e 5, a l o n g w i t h a r e p r e s e n t a t i v e p l o t f o r o r t h o p h o s p h a t e c a t a l y s i s a t t h e same pH. T a b l e V I I O x i d a t i o n ^ o f P i v a l a l d e h y d e by Permanganate a t C o n s t a n t pH  V a r i a t i o n ' o f t h e R a t e C o n s t a n t w i t h P y r o p h o s p h a t e ' ' C o n c e n t r a t i o n -41 pH [p2©;._ (•moles> 1~^) (l»Kiaole~^ sec-...~''") k-2 5...0 0.02 6.1 5.0 0.08 8..0 '5,0 0.14 9.4 The l o w e r s o l u b i l i t y . o f t h e sodium pyrophosphate d i d n o t a l l o w , s o l u t i o n s o f t h e same m o l a r i t y as a v a i l a b l e f o r o r t h o p h o s p h a t e t o be prepared,. The pyrophosphate shows a marked, c a t a l y t i c e f f e c t * ' o v e r " t h e p o o r e r c a t a l y -t i c s p e c i e s o r t h o p h o s p h a t e ; t h i s was a l s o o b s e r v e d i n the p e r m a n g a n a t e 1 o x i d a t i o n 1 o f benzaldehyde by'Wiberg and S t e w a r t ( 1 ) , B o t h t h e o r t h o - and pyrophosphate p l o t s s h o w n " i n " F i g u r e 5 a p p e a r ' t o approach t h e same FIGURE 5 Oxidat ion of Pivalaldehyde Dependence of r a t e on buffer concent ra t ion , pH = 5.0 < * 1 1 1 0.1 0.2 0.3 0.4 Buffer Conc.(M.) - 48 -ra te at zero concentration of phosphate. This i s to be expected i n view of the •uncatalyzed* rate opera-t i v e i n the permanganate oxida t ion of pivalaldehyde, as shown i n Figure 2 . 4 . Oxidat ion i n a l k a l i n e s o l u t i o n . As can be seen from Figure 2, the rate appears to show a marked dependence on the hydroxyl ion con-cen t ra t ion . Consequently., several oxidations of pivalaldehyde by permanganate were ca r r i ed out i n the range 0.02 M to 0.20 M sodium hydroxide. The mangan-ate formed i n t h i s : r e g i o n , as a r e su l t of permanganate reduct ion, i s s table and reacts wi th the•pivalaldehyde s lowly . A t y p i c a l rate p lo t for these reactions i s shown i n Figure 6. Unfortunately, large differences i n rate were often observed for the same concentration of base. As a r e s u l t , not too much can be sa id of the oxidat ion i n th i s region as rates were very e r r a t i c and fa r from reproducible•. This could perhaps be due to some r a d i c a l decomposition of the pivalaldehyde, as Symons (8) has pointed out that a l k a l i n e permangan-ate undergoes decomposition to manganate v i a a r a d i c a l path and i t has already been shown i n the experimental sec t ion of t h i s thesis that pivalaldehyde i s susceptible to r a d i c a l decomposition. 2.0 J. Vo-Vt FIGURE 6 Oxidation of Pivalaldehyde Typical ra te plot in alkaline solution 1.04. O. O [OH] =0.02 = 0.20 -+-5 0 100 t ime in sec. 150 - 50 -5. Effect of i o n i c s trength. The effect of i o n i c strength on the reac t ion rate was found to be n e g l i g i b l e , as the resu l t s shown i n Table VI I I i nd ica t e s . Table V I I I The Permanganate Oxidat ion of Pivalaldehyde  Influence on the Rate of Ionic Strength pH k 2 (mole l . " 1 ) (mole 1. 'L) ( 1 . mole--*- sec. ^) 0.2 10.37 1.04 4.60 4.60 0.2 10.37 0.60 4.60 3.90 0.2 5.50 0.60 5.41 4.60 0.2 5.00 1.20 5.44 5.44 Although there i s a s l i g h t difference i n pH for one of these values , the rate has already been shown to be pH independent i n th i s region (Figure 2 ) . The two values for the rate shown for each i o n i c strength were obtained wi th d i f fe ren t solut ions of p i v a l a l d e -hyde and some i n d i c a t i o n of an i o n i c strength effect i s apparent from the resu l t s shown i n the l a s t column above. However, d i f f i c u l t y was experience! throughout - 51 -t h i s work i n determining exact ly the concentration of the pivalaldehyde. As a consequence, rates found wi th d i f fe ren t solut ions at the same pH were often as much as twenty percent d i f f e r e n t . Nevertheless, rates determined on the same so lu t ion were reproducible to w i t h i n f i ve percent at the same pH. Furthermore, experimental trends were not appreciably changed even when d i f fe ren t solut ions were used. For t h i s reason i t i s f e l t that there i s a n e g l i g i b l e i o n i c strength effect i n the permanganate oxidat ion of pivalaldehyde. 6 . Oxidat ion by Manganate. Even though manganate i s seen to accumulate during the permanganate oxidat ion i n a l k a l i n e s o l u t i o n , i t was advisable to determine the magnitude of the rate constant for the manganate r eac t ion . Some potassium manganate had been previously prepared i n t h i s labora-tory and th i s was used i n these determinations. The concentration of the manganate was determined on the Beckman D.U. spectrophotometer, since the ex t i nc t i on coe f f i c i en t and the maximum wave length for absorption are accurately known ( 3 7 ) . The rate was followed by the iodometric method i n the same manner used for the permanganate oxida t ions . Linear second order rate - 52 -plots were obtained and rate constants were determined from the expression: k 2 = 7= ~ x V 0 " v t [pivalaldehyde] 0 V t - j ^ o K i n e t i c data for the runs done are shown below i n Table IX and a t y p i c a l ra te p lo t i s given i n Figure 7. The resu l t s show that the oxidat ion of p iva l a lde -hyde by permanganate occurs twenty four times slower than the permanganate oxidat ion at the same a l k a l i n i t y , Table IX Manganate Oxidat ion of Pivalaldehyde i n A l k a l i n e Solu t ion [OH] k 2 (mole 1."^") ( 1 . mo le^sec . 0.1 .890 0.1 .870 [pivalaldehyde] Q = 9.56 x 1 0 " 4 M. [Manganate| = 9.56 x 1 0 ~ 4 M. Temperature = 25.0 * 0 . 0 5 ° . Ionic Strength = 0.10 mole l . " 1 " . 0.5-L FIGURE 7 Oxidat ion of Pivalaldehyde by Manganate Vo-Vt  \4-iVo O 0.25 4 O [OH] = 0.1 M ^ = 0.1 O + 100 200 3 0 0 t ime in sec. 4 0 0 - 54 -B. The Permanganate O x i d a t i o n o f Benzaldehyde and  p - N i t r o b e n z a l d e h y d e . 1. E f f e c t o f pH The a u t o c a t a l y t i c e f f e c t i n t h e permanganate o x i d a t i o n o f b e n z a l d e h y d e was found t o be m a r kedly dependent on t h e pH, w i t h v e r y f a s t r a t e s a t low pH. I n a d d i t i o n , t h e i n i t i a l r a t e i n t h e permanganate o x i d a t i o n o f benzaldehyde was f o u n d t o be dependent i n t h e same way on the pH, b e i n g f a s t e r a t low pH. The e f f e c t o f pH on the a u t o c a t a l y s i s i s shown i n F i g u r e 8. The e f f e c t o f pH on the i n i t i a l r a t e o f o x i d a t i o n i s shown i n T a b l e X. The f i r s t t h r e e r a t e s i n t h i s t a b l e were found i n t h i s work and a r e r e -p r e s e n t a t i v e o f i n i t i a l r a t e s a t low pH. These gave e x c e l l e n t s t r a i g h t l i n e p l o t s , an example of w h i c h i s shown i n F i g u r e 9. T h i s i s t o be compared w i t h t h e upper c u r v e of F i g u r e 8 i n o r d e r t o a p p r e c i a t e t h e e f f e c t of t i m e on the o x i d a t i o n , , The r e m a i n i n g d a t a shown i n T a b l e X i s t a k e n f r o m the work of W i b e r and S t e w a r t ( 1 ) , whose r a t e s have been c o r r e c t e d f o r d i f f e r e n c e s i n b u f f e r c o n c e n t r a t i o n . The o v e r a l l e f f e c t o f pH on t h e permanganate o x i d a t i o n o f b e n z a l d hyde i s shown i n F i g u r e 10. FIGURE 8 Ox idat ion of Benzaldehyde Ef fect of pH on autocatalysis time in sec. 0.5 -FIGURE 9 Oxidation of Benzaldehyde Initial ra te at pH = 1.40 * V o - V t \ 0.25-| ' 5 0 '100 time in sec. '150 - 57 -T a b l e X The Permanganate O x i d a t i o n o f Benzaldehyde  Dependence o f Rate on t h e pH pH k 2 ( 1 . mole"-*- s e c . " " ) 1.40 1.64 2.90 0.890 5.00 0.586 5.85* 0.477 6.80* 0.448 7.22* 0.448 9.35* 0.444 11.06* 0.480 12.43* 0.733 13.00* 0.865 These r a t e s were t h o s e found by W i b e r g and S t e w a r t ( 1 ) and have been c o r r e c t e d f o r i o n i c s t r e n g t h e f f e c t , [Benzaldehyde] Q = 1.622 x 1 0 ~ 3 mole I . " 1 §In0 4] o = 1.082 x 1 0 " 3 mole l . ' 1 I o n i c S t r e n g t h = 1.20 mole l . " 1 2.01 3 10 O E XL 1.01 FIGURE 10 Oxidation of Benzaldehyde Ef fec t of pH on initial rate O +— 8 1 — 10 12 - 59 -p - N i t r o b e n z a l d e h y d e was found t o o x i d i z e a t a s l o w e r r a t e t h a n b e n z a l d e h y d e . I t a l s o e x h i b i t e d a u t o c a t a l y s i s and b o t h t h e a u t o c a t a l y t i c and t h e i n i t i a l r a t e s were reduced by a f a c t o r o f about was s t u d i e d o n l y a t a pH o f 1.40. The d i f f e r e n c e s i n i n i t i a l r a t e s o f benza l d e h y d e and p - n i t r o b e n z a l d e h y d e a r e g i v e n i n T a b l e X I w h i l e t h e d i f f e r e n c e s i n r a t e s of a u t o c a t a l y s i s of t h e s e two a l d e h y d e s a r e shown i n F i g u r e 11. Marked o v e r o x i d a t i o n was obser v e d f o r b o t h b e n z a l d e h y d e and p - n i t r o b e n z a l d e h y d e as e v i d e n c e d by th e e x p e r i m e n t a l v a l u e s o f Vco g i v e n i n T a b l e X I I . Over O x i d a t i o n o f Benzaldehyde and p - N i t r o b e n z a l d e h y d e  E x p e r i m e n t a l D i f f e r e n c e s Found i n F i n a l Volumes f o u r compared t o b e n z a l d e h y d e . The p - n i t r o a l d e h y d e T a b l e X I I C a l c u l a t e d E x p e r i m e n t a l S u b s t r a t e Vcft = 2/5 V Q ( m l . ) V*° ( m l . ) C.H[-CH0 6 5 1.818 1.165 pN0 2C 6H 4CH0 1.847 1.551 - 60 -Table XI The Permanganate Oxidat ion of p-Nitrobenzaldehyde  Differences i n I n i t i a l Rate From Benzaldehyde pH Substrate k 2 ( l .mole ''"sec. ^) 1.40 G6H5GH0 1.64 1.40 pN02G6H4GH0 0.470 FIGURE 11 Autocatalysis in the ox idat ion of Benzaldehyde and p-Nitrobenzaldehyde Vo-Vt 204-16 + 12 + 81 41 O C U H X H O b D Q p N Q 2 C 6 H 4 C H O 100 t ime in min. 150 - 62 -Di f f e r e n c e s shown between experimental and c a l c u l a t e d are f a r to great to be accounted f o r by any decomposi-t i o n of manganese d i o x i d e i n t o oxygen and lower o x i -d a t i o n s t a t e s of manganese, which could account f o r small changes i n t i t r e volume. Over o x i d a t i o n was a l s o observed i n k i n e t i c runs c a r r i e d out w i t h the permanganate present i n excess of aldehyde. The lar g e d i f f e r e n c e s found are i n t e r p r e t e d as over o x i d a t i o n and are suggested as being due to decomposi-t i o n of the benzene r i n g . This has been p r e v i o u s l y postulated by C h i l l i s and Ladbury (39) i n the o x i d a t i o n of toluene, which they say e x h i b i t s over o x i d a t i o n at the benzaldehyde stage. In order to see i f the ' a u t o c a t a l y t i c ' e f f e c t observed at low pH i n the permanganate o x i d a t i o n of benzaldehyde was completely a r e s u l t of over o x i d a t i o n or i f i n f a c t some t r u e a u t o c a t a l y s i s was o p e r a t i v e , the experimental value of V oo was used i n determining the r a t e p l o t a t a pH of 1.40. This r e s u l t i s shown i n F i g u r e 12 along w i t h the o r i g i n a l p l o t shown i n Fig u r e 8. I t seems to be apparent that i n spite of over o x i d a t i o n , some t r u e a u t o c a t a l y s i s i s o c c u r r i n g , but such an e f f e c t i s d i f f i c u l t to separate. FIGURE 12 Oxidat ion of Benzaldehyde Autocatalysis c o r r e c t e d for overoxidat ion 200 4 0 0 6 0 0 8 0 0 1000 t ime in sec. - 64 -2 . The Effect of Added Manganous S a l t . The add i t ion of manganous sulphate was found to reduce both the i n i t i a l and autocata lys is rates for the permanganate oxidat ion of benzaldehyde. This suggests that the ac t ive oxidant i n the reac t ion i s permanganate and not some intermediate manganese species. That th i s i s true can be r e a l i z e d from the reductions i n rate observed above which are a t t r ibu ted to immediate decomposition of the permanganate to manganese d iox ide . This reduces the concentration of permanganate and hence reduces the r a t e . Such an occurrence can be r e a d i l y understood from the reac t ion shown below: 2 Mn6"4 + 3 Mn 2 + 4 OH *5 Mn0 2 + 2 H 2 0 Rate differences are shown i n Table X I I I . I t was found that percent reductions i n rate were of the same order as; predicted from a considerat ion of the s toichiometr ies of the above r eac t ion . - 65 -Table X I I I The Permanganate Oxidation of Benzaldehyde  Effect of MnSO^ on the Reaction Rate at pH = 1.40 r ++i [Mn J k 2 % reduction i n % reduction (mole 1."^) ( 1 . mole ^"secT )^ rate found ' predicted 0 1.64 0 0 1.62 x 10" 4 1.48 10 10 4.40 x 10~ 4 1.35 20 27 3. Permanganate Oxidat ion of Benzoic A c i d . In order to further determine the nature of the observed over oxidat ion of benzaldehyde, i t was of in te res t to see i f benzoic ac id underwent any oxidat ion by permanganate. After ' r eac t ion times ten f o l d i n excess of that required for au toca ta lys i s , the amount of oxidat ion of benzoic ac id was found to be n e g l i g i b l e . The degree of oxida t ion of benzoic ac id was determined only q u a l i t a t i v e l y by observing changes i n the i n i t i a l volume of permanganate. Furthermore, the effect of added manganous sulphate on the permanganate oxida t ion of benzoic ac id was studied and th i s a lso was found to be n e g l i g i b l e . Results are shown i n Table XIV. - 66 -Table XIV The Permanganate Oxidat ion of Benzoic A c i d  Change i n I n i t i a l Volume of Permanganate With Time Time (min.) V t (wi thout MnS0 4) (ml . ) V t ( w i t h MnS0 4) (ml.) 20 55 70 110 120 130 140 150 4.585 4.400 4.495 4.362 4.362 4.368 4.408 4.410 4.545 4.558 4.518 4.475 4.522 4.490 4.370 4.460 V, Benzoic ac id [MnO,"] 4jJ o = 4.618 m l . l] 0 = 1.665 x 10" 3 mole L . " 1 = 1.110 x 10" 3 mole l . " 1 pH = 1.40 Ionic Strength = 1.20 mole 1 -1 - 67 -C. The Permanganate Oxidat ion of p -Ni t ropheny l t r i f l uo ro -methylcarbinol . 1. Oxidat ion i n 0.1 M Sodium Hydroxide. The permanganate oxida t ion of the unsubstituted a lcohol was f i r s t studied i n order to have a basis for comparison i n the oxidat ion of p -n i t r opheny l t r i f l uo ro -methylcarbinol,. . E x c e l l e n t r s t r a i g h t ' l i n e plots,were obtained as shown :in Figure 13 and rates were found to give very good, agreement wi th those obtained e a r l i e r by Stewart and Van der Linden (2)... The rate , of o x i -da t ion , of the p -n i t ro alcohol-was found to be almost three times as fast as that of the unsubstituted a l c o h o l . 1 A t y p i c a l rate p lo t i s shown i n Figure 14. When compared to the p lo t shown i n Figure 13, the scat ter of points ' i s ha rd ly . sa t i s f ac to ry but under-standable when one considers the small volume changes encountered i n the t i t r a t i o n . I t i s i n t e r e s t i ng to note the k i n e t i c s found when the. p-ni t rophenyl t r i f luoromethylcarbinol 'was d issolved i n 0.2 M sodium hydroxide ( i n ' o r d e r to en-hance : i t s s o l u b i l i t y ) and allowed to stand i n t h i s so lu t ion before being oxidized,v Experiments performed FIGURE 13 Oxidat ion of Phenyl t r i f luoromethy l -carbinol t ime in sec. FIGURE 14 t ime in sec. - 70 -wi th these solut ions gave very e r r a t i c r esu l t s and sigmoid rate curves, from which a true rate constant could not be determined. An example of such k i n e t i c s i s shown i n Figure 15. These sigmoid curves are suggested as due to hydrolys is occurring i n a l k a l i n e so lu t i on , l eav ing p-nitrobenzaldehyde which then under-goes the Cannizzaro r eac t ion . The benzyl a lcohol produced would then undergo a two stage oxidat ion to benzoic ac id and t h i s could account for the sigmoid rate p l o t , Stewart and Van der Linden (2) have studied the permanganate as ida t ion of a number of subst i tuted phenyl tr i f luoromethylcarbinols i n 0.1 M sodium hydroxide. Their rates are shown i n Table XV along wi th the r e su l t s found i n th i s work. I t i s f e l t that a v a l i d comparison ex i s t s between the corrected rates for one hundred percent i o n i z a t i o n for the p -n i t ro a lcohol and others since the rates found for the u.n-subst i tuted a lcohol were the same i n both Van der Linden 's (28) and th i s work* I t can be seen then f.von. Table XV that the effect of substi tuents i s neg-l i g i b l e on the r a t e , save for the p -n i t ro group which shows a large increase i n r a t e . t ime in sec. Table XV The Permanganate O x i d a t i o n of P h e n y l t r i f l u o r o m e t h y l c a r b i n o l s Oxidations i n 0.1 M NaOH Sub s t i t u e n t k 2 k 2 c o r r e c t e d to ( 1 . mole'^secT''') 100% i o n i z a t i o n M-N02* 8.7 8.9 M-Br* 7,.4 7.6 P-CHg* 7.1 7.9 H 7.7 8.3 P-N0 o 21.2 21.4 These r a t e s were found by Stewart and Van der Linden ( 2 ) . The r a t e of o x i d a t i o n of the u n s u b s t i t u t e d a l c o h o l was found t o be 7.6 1. mole'^'secT''" by Van der Linden ( 2 8 ) . MnoJ = 1.11 x 10" 3 Mole l . " 1 PN0 oC 6H 4CH0HGFgJ Q = 5.55 x 10~ 4 Mole l . " L C6H5CHOHCF^J o = 5.55 x 10~ 4 Mole l . " 1 l o n i c Strength = 0.1 mole l.""*" - 73 -2. Oxidation i n Weakly A l k a l i n e S o l u t i o n . In order to see i f the p -n i t ro a lcohol reacted fas te r at lower pH, the oxidat ion of the unsubstituted and the p-ni t rophenyl t r i f luoromethylcarbinol was studied at a pH of 9.17. The reac t ion i s expected to be much slower here since Stewart and Van der Linden (2) have shown that the rate determining step involves the anion. The resu l t s are shown i n Table XVI along wi th values found for other substi tuents at a pH of 9.10 by Van der Linden (28) . As i n Table XV, the rates have been corrected for one hundred percent i o n i z a t i o n and again i t i s f e l t that a v a l i d comparison can be made between the corrected rate for the p -n i t ro a lcohol and others since the rate of oxidat ion of the unsubsti tuted a lcohol i n t h i s work was found to be i d e n t i c a l wi th that fottnd by Van der Linden (28) . The rates of oxidat ion of the m-bromo and m-nitro alcohols were studied by Van der Linden (28) at a pH of 9.10 and one might expect a s l i g h t increase i n rate for a pH of 9.17, as was used i n t h i s work. However, any increase would not b r ing the oxidat ion rate up to that found for the p -n i t ro a l c o h o l . That the difference - 74 -T a b l e XVI The Permanganate O x i d a t i o n of P h e n y l t r i f l u o r o m e t h y l c a r b i n o l s  O x i d a t i o n s a t pH = 9.17 S u b s t i t u e n t k-2 k 2 c o r r e c t e d t o ( 1 . mole sec7 ) 100% i o n i z a t i o n M-N0 2* 0.019 2.58 M-Dr* 0.0061 1.54 H 0.0051 2.76 P-NCs 0.039 3.41 R e s u l t s o b t a i n e d by Van d e r L i n d e n ( 2 8 ) a t pK=9.10. The r a t e o f o x i d a t i o n o f t h e u n s u b s t i t a t t e d a l c o h o l was found t o be 0.0053 a t a pH=9.10 by Van d e r L i n d e n . MnO ,1 = 5.57 x 1 0 ~ 4 Mole l . " 1 •ij o PN0 2G 6H 4GHOHGF 3 J Q = 8,35 x 1 0 ~ 4 Mole l . " 1 l14n0 4 J o = 1.11 x 1 0 " 3 Mole l . " L rG6H5CHOHGF3~| = 1.665 x 1 0 - 3 Mole l . " L -1 I o n i c S t r e n g t h = 0.35 mole 1. - 75 -i n i o n i c strength i n the two studies (0.35 v s . 0.20) might account for increases i n rate can be refuted by the fact that the unsubstituted a lcohol gives i d e n t i c a l rates i n both cases. The rate of oxidat ion of p -n i t r opheny l t r i f l uo ro -methylcarbinol at a pH of 9.17 i s not nearly as en-hanced as was found at a pH of 13, but nevertheless i s s t i l l fas ter than the m-nitro a lcohol which has comprable a c i d i t y . I t i s suggested that a much bet ter basis for comparison i s i n the rates of oxidat ion i n 0.1 M sodium hydroxide, where the alcohols are a l l appreciably i o n i z e d . At low a l k a l i n i t i e s , the rate i s markedly dependent on the pH. In f ac t , the pH was found to change over the course of reac t ion i n the oxidat ion p-ni t rophenyl t r i f luoromethylcarbinol and th i s is shown i n Table X V I I , along wi th cor re-sponding changes i n rate that one would expect. I t should be noted that the reac t ion invo lv ing the un-subst i tuted a lcohol underwent no change i n pH during the course of the r eac t ion . That the rates of oxidat ion of these alcohols should y i e l d d i f fe ren t rate constants for one hundred percent i o n i z a t i o n seems rather puzz l i ng . The - 76 -Table XVII Permanganate Oxidat ion of p-Nit rophenyl t r i f luoromethyl ' ca rb inol Rate Changes wi th pH During Course of Reaction time pH k 2 corrected to (min.) 100% i o n i z a t i o n ( 1 . mole ^"sec.^) 0 9.17 3.41 107 9.13 3.77 229 9.10 4.03 334 9.09 4.12 478 9.05 4.51 570 9.01 4.94 715 8.99 5.17 826 8.99 5.17 - 77 -d i s p a r i t y i s l i k e l y the r e s u l t of a combination of e f fects due to d i f f e r e n t reac t ion condit ions at low and high pH and to departure from i d e a l i t y i n the measurement of hydroxyl ion concentrat ion by the glass e lec trode . Stewart and Van der Linden (33) measured the pK f l values of t h e i r subst i tuted phenyl-t r i f luoromethylcarbinols i n the s trongly basic region and the pH was obtained d i r e c t l y from the concentrat ion of hydroxide present . I t was not measured on a pH meter. Furthermore, the s to ichiometries are d i f f e r e n t at low and high pH and consequently d i f f e r e n t con-centrat ions of a lcohol were used, s ince the perman-ganate concentrat ion was kept constant. The con-centrat ion of a lcohol var ied by about t h i r t y percent over the two r a t i o s and th i s could contr ibute to changes i n the rate constant since e f fects of th i s order are commonly observed i n k i n e t i c s . 3. The Hammett P l o t Stewart and Van der Linden (2) have previous ly shown the Hammett p lo t i n the ox idat ion of subst i tuted phenyl tr i f luoromethylcarbinols i n 0.2 M sodium hydroxide to be a shallow curve. Table XVIII gives the data f o r the Hammett p lo t which i s shown i n Figure 16. The - 78 -Table XVIII The Permanganate Oxidat ion of Phenyl t r i f luoromethylcarbinols The Hammett P lo t at 0.1 M NaOH substi tuent cr value ko corrected for l o g k 2 100% i o n i z a t i o n -1 -1 ( 1 . mole sec. ) M-N0 2 0.71 8.9 0.949 M-Br* 0.39 7.6 0.881 p-CH g * -0.17 7.9 0.898 H 0 8.3 0.919 p-N0 2 -0.78 21.4 1.33 These rates were found by Van der Linden (28) FIGURE 16 Oxidation of Phenyltr i f luoromethyl-carbinols The Hammett plot - 80 -points shown are for one hundred percent i o n i z a t i o n i n 0.1 M sodium hydroxide and i t can be c l e a r l y seen that the p -n i t ro group shows an enhanced e f fec t . General ly , a p lo t of log k versus cr gives a s t ra igh t l i n e , the slope of which gives the reac t ion constant ^ . The s l i g h t curve shown i n Figure 16 as a dotted l i n e has been interpreted by Stewart and Van der Linden ( 2 ) as due to two d i f fe ren t processes occurr ing i n the reac t ion wi th d i f fe ren t e lec t ron ic requirements, one wi th a pos i t ive and the other wi th a negative va lue . However, t h i s w i l l not be pursued, since the degree of curvature i s small and should be in terpreted as a s t ra ight l i n e , as indicated i n the diagram. This s t ra igh t l i n e i s then representative of a normal Hammett p l o t , having ^ approximately equal to zero, which shows that the rate i s independent of nuclear subs t i t u t i on . The exception i s the p -n i t ro group, which shows a marked departure from a l l the other points shown i n Figure 16. - 81 -DISCUSSION OF RESULTS A . The Permanganate Oxidation of Pivalaldehyde From the pH p r o f i l e shown i n Figure 2 , i t can be seen that the rate i s independent of pH i n the neutra l and weakly a l k a l i n e reg ion . I t i s obvious then that an uncatalyzed or ' n e u t r a l ' r eac t ion i s operat ive, which i s very l i k e l y due to the general-acid water. I t i s apparent from the increases i n rate observed at low pH that the reac t ion fol lows some dependence on hydrogen i o n concentration and indeed general ac id c a t a l y s i s has been shown to occur. The rate of the reac t ion was also found to be proport ional to the f i r s t powers of permanganate and pivalaldehyde concentrat ions. Furthermore, although no experimental evidence i s a v a i l a b l e , i t seems a reasonable assumption that the aldehyde carbon-hydrogen bond i s broken i n the rate determining step, e spec ia l ly i n l i g h t of the amount of evidence ava i lab le on a va r i e ty of substrates which supports such a step. I t has been stated that one of the objects of - 82 -t h i s work was to see i f pivalaldehyde underwent oxida-t i o n v i a a s i m i l a r path as i t s aromatic analog benzaldehyde. In that study, Stewart and Wiberg (1) have shown that the oxygen introduced in to the aldehyde was derived mainly from the o x i d i z i n g agent, i nd i ca t i ng that a bond was at some time formed between the aldehyde carbon and an oxygen of the o x i d i z i n g agent. Furthermore, these workers were able to show that a normal isotope e f fec t , kjj/k D=7.0, exis ted and hence the rate determining step must involve cleavage of the aldehyde carbon-hydrogen bond. A reasonable mechanism then for the oxidat ion of pivalaldehyde by permanganate i s the fo l lowing : v OH * K l 'I RGHO + Hg5 ~ RGH + HO OH + K | RGHOH + MnO, . * R—C—OMnO, 4 H OH I /v /* - Ifc R—G-j-0 MnOQ + A —* RCOJ3 + HA + MnO. |7 6 2 3 H fas t 2 MnO 3 + H 20 * 2 Mn0 2 + Mn0 4 + 2 OH - 83 -T h i s mechanism accommodates t h e e x p e r i m e n t a l e v i d e n c e found i n t h i s work as w e l l as i n c o r p o r a t i n g t h a t found by S t e w a r t ( 1 ) i n t h e permanganate o x i d a t i o n o f b e n z a l d e h y d e . The Hammett ^ v a l u e i n t h e p e r -manganate o x i d a t i o n o f benzaldehyde i s found t o be n e g a t i v e (-0.25) and hence e l e c t r o n d o n a t i n g groups w i l l a i d t h e r e a c t i o n . T h i s was a l s o found a t low pH i n t h i s work when t h e o x i d a t i o n o f p - n i t r o b e n z a l d e -hyde was found t o be f o u r f o l d s l o w e r t h a n t h e o x i d a t i o n of b e n z a l d e h y d e . Hence i t seems a r e a s o n a b l e e x t e n -s i o n t o e x p e c t t h a t e l e c t r o n d o n a t i n g groups on t h e p i v a l a l d e h y d e w i l l enhance t h e r e a c t i o n r a t e . However, o t h e r mechanisms s h o u l d be c o n s i d e r e d , such as t h e one shown below: 0 R — C i - 0 — - i + MnO, H i-i—o—i R G—0 MnO o 3 H RC + MnO q \ 3 OH f a s t 2 Mn0 3 + H 20 2 Mn0„ + MnO. + 2 OH 2 4 A g a i n , t h i s mechanism would i n c o r p o r a t e t h e f a c t s found by W i b e r g and S t e w a r t ( 1 ) i n t h e permanganate o x i d a t i o n of benzaldehyde s i n c e b o t h oxygen-18 t r a n s f e r would - 84 -occur and the carbon-hydrogen bond would be weakened i n the t r a n s i t i o n s ta te . Such a reac t ion path could a lso accommodate general a c id - ca t a ly s i s found i n t h i s work i n that complexing could occur wi th the permangan-ate and so a id i n i t s reduction to lower valency s ta tes . That t h i s should cause an increase i n rate can be r ea l i z ed from the fact that permanganic ac id i s a stronger oxidant than permanganate i t s e l f and hence one would expect a hydrogen bond formed wi th per-manganate to also r e su l t i n a stronger o x i d i z i n g agent. One would not predic t such an effect to show large ca t a ly s i s and i n fact only a twenty percent increase i n rate has been observed for a tenfold change i n buffer concentrat ion. In order to expla in the enhancement i n rate observed for pivalaldehyde over benzaldehyde i n terms of the above mechanism, one can consider that i n going from the t r i g o n a l aldehyde to a tetra-coordinated t r a n s i t i o n s ta te , a loss of conjugation must occur. That conjugation effects can be important i n aromatic systems i s w e l l known, whereas s i m i l a r effects are n e g l i g i b l e i n a l i p h a t i c saturated systems. Hence, i t i s reasonable to postulate that benzaldehyde would - 85 -undergo a greater loss of conjugation i n forming the t r a n s i t i o n state than pivalaldehyde and correspond-i n g l y , one would expect a fas ter reac t ion for the pivalaldehyde. This ' l o s s of conjugation' concept can moreover be included i n the former mechanism postulated, since the abst ract ion of a proton by the general base i n the rate determining step would r e su l t i n the fo l lowing t r a n s i t i o n s tate: i - OH I 5-R — C — 0 MnO H \ AS-The t r i g o n a l aldehyde has now approached a t e t r a -coordinated t r a n s i t i o n state w i th consequent loss i n conjugation which would be more pronounced for benzaldehyde than pivalaldehyde. Aga in , t h i s would r e su l t i n greater rates of oxida t ion for the l a t t e r aldehyde, i n accordance wi th what has been observed. On the other hand, there i s no assurance that the oxygen introduced in to the pivalaldehyde ar ises from the o x i d i z i n g agent and may i n fact come from - 86 -the solvent . Furthermore, the oxidat ion of p i v a l a l d e -hyde by manganate occurs twenty f i v e times slower than that by permanganate, whereas Stewart (1) found the same rates of oxidat ion for both wi th benzaldehyde. This could most e a s i l y be interpreted i n terms of a mechanism invo lv ing hydride t ransfer , which would be slower to manganate than to permanganate i n view of the extra negative charge on the manganate. A l s o , the induct ive effect of the t e r t i a r y bu ty l group would serve to weaken the carbon-hydrogen bond and so enhance a hydride t ransfer to permanganate. In view of the w e l l known e lec t ron withdrawing effect of the phenyl group (the Taft 0~ ( 39) values for t e r t i a r y bu ty l and phenyl are -0.300 and +0.600 respec t ive ly) t h i s could a id i n expla ining the en-hancement i n rate observed for pivalaldehyde over benzaldehyde. The fo l lowing mechanism then i s not u n l i k e l y : 0 + MnO 4 G =0 + H MnO 4 \ + H 0 fast + R—G + - 87 -0 0 II ^ s t || R—C =s: R—G—OH + H A H H + f a s t 3 H MnO, + 5 H » MnO + 2 MnO + 4 HJ3 4 4 2 2 The above mechanism i s i n no way meant to be c o n c l u s i v e , and i n f a c t s u f f e r s from a number of drawbacks. F i r s t of a l l , the Hatnmett ^ value i n the permanganate o x i d a t i o n of benzaldehyde i s known to be small and negative whereas one would expect a l a r g e negative ^ i f hydride t r a n s f e r was o c c u r r i n g i n the r a t e determining step. I f pivaldehyde and benzaldehyde r e a c t v i a d i f f e r e n t paths, then the value observed f o r benzaldehyde would be of no im-portance i n d i s c u s s i n g the mechanism of the o x i d a t i o n of p i v a l a l d e h y d e , but i t seems p e r f e c t l y p l a u s i b l e that the two should r e a c t by the same path. Secondly, conjugation e f f e c t s i n the s t a b i l i z a t i o n of the carbonium i o n formed would be very important and phenyl could provide much more i n t h i s d i r e c t i o n than t e r t -i a r y b u t y l , whose resonance e f f e c t would be n e g l i g i b l e . - 88 -I t i s not proposed that the i n d u c t i v e e f f e c t of the t e r t i a r y b u t y l group would be more important than the conjugative e f f e c t of the phenyl group. I n summary, then, there i s no c l e a r cut s i n g l e mechanism which e x p l a i n s a l l the f a c t s i n the per-manganate o x i d a t i o n of p i v a l a l d e h y d e . In order to determine t h i s , more work would have to be done to f i n d whether the oxygen i s introduced v i a the solvent or the permanganate, whether the o x i d a t i o n by man-ganate i s s i g n i f i c a n t , and i f i n f a c t the carbon-hydrogen bond i s i n v o l v e d i n the r a t e determining step, although t h i s does not seem u n l i k e l y . B. The Permanganate O x i d a t i o n of Benzaldehyde and p-Nitrobenzaldehyde Although t h i s work was undertaken w i t h the view to studying the nature of the a u t o c a t a l y t i c e f f e c t observed by Wiberg and Stewart (1) below a pH of 5, i t was a l s o worthwhile to study the i n i t i a l r a t e s of o x i d a t i o n at low pH. As the r e s u l t s show i n F i g u r e 10, the o x i d a t i o n shows some dependence on hydrogen i o n c o n c e n t r a t i o n , although i t c e r t a i n l y does not e x h i b i t s p e c i f i c c a t a l y s i s . This i s not s u r p r i s i n g - 89 -i n view of the general a c i d - c a t a l y s i s which the system e x h i b i t s . Furthermore, the i n i t i a l r a t e of o x i d a t i o n of p-nitrobenzaldehyde i s reduced by almost f o u r f o l d compared to benzaldehyde at the same pH. This i s i n accord w i t h the negative ^ value found by Wiberg and Stewart (1) i n the permanganate o x i d a t i o n of s u b s t i t u t e d benzaldehydes. I t seems obvious t h a t the experimental r e s u l t s gained at low pH i n t h i s work serve to strenghthen the mechanism already proposed by Wiberg and Stewart (1) at h i g h pH. That mechanism i s as f o l l o w s : G H CHO + H*0 1 - C.HcCHQH + HO '6 5 3 "* 6"5V 2 OH G H K C H 6 H + MnO, - 2 •* G H — C — 0 —MnO_ 6 5 4 6 5 1 3 H OH I - K G^-Hj-—G—0—Mn0 o + A =—>G,H cC0 oH + HA + Mn0 o 6 5 | 3 o 5 o H f a s t 3 MnO 0 + H 0 s» 2 MnO + MnO, + 2 OH 3 2 2 *+ However, i n view of the enhancement i n r a t e observed w i t h p i v a l a l d e h y d e , an analogous mechanism can be proposed f o r the permanganate o x i d a t i o n of - 90 -benzaldehyde. The f o l l o w i n g mechanism then a l s o accommodates the experimental f a c t s found by Wiberg and Stewart (1) although the ^ of the r e a c t i o n would be d i f f i c u l t to p r e d i c t . 0 k || _ f a s t C ,H_CH0 + MnO, C-H—6-0—MnO„ * G H_C0„H+Mn0 •6 5 4 6 5 \ / 3 6 5 2 v H 3 MnO. + H o0 :—» 2 MnO„ + MnO. + 2 OH 3 2 f a s t 2 4 The a u t o c a t a l y t i c e f f e c t was a l s o found to be reduced by almost f o u r f o l d f o r the permanganate o x i d a t i o n of p-nitrobenzaldehyde compared to benz-aldehyde. For both cases, since experimental evidence has shown tha t the system e x h i b i t s o v e r o x i d a t i o n , i t i s p o s t u l a t e d that the benzene r i n g i s undergoing decomposition at l a t e r stages i n the r e a c t i o n . The mechanism of t h i s i s f a r from c e r t a i n , but i t seems reasonable to suggest a r a d i c a l r e a c t i o n path. I t seems l i k e l y that the permanganate o x i d a t i o n of benzaldehyde, and a l s o p-nitrobenzaldehyde, goes by two paths; i . e . a major one which i s r e s p o n s i b l e f o r the normal o x i d a t i o n , g i v i n g good second order k i n e t i c s and a minor path which i n v o l v e s a very small - 91 -percent of a r a d i c a l species formed from hydrogen atom abst rac t ion by the permanganate i o n . This r a d i c a l then i n i t i a t e s rupture of the benzene r i n g , l i k e l y v i a a chain mechanism. The observed reduction i n the overoxidation of p-nitrobenzaldehyde i s d i f f i c u l t to e x p l a i n . G u l l i s and Ladbury (38) have observed a s i m i l a r effect i n the oxidat ion of p-ni trotoluene compared to toluene i t s e l f . That the e lec t ron withdrawing effect of the p -n i t ro group should re tard formation of a r a d i c a l species does not seem p l a u s i b l e . Perhaps once the r a d i c a l i s formed there i s su f f i c i en t s t a b i l i z a t i o n due to the p -n i t ro group to re tard the chain reac t ion which resu l t s i n r i n g degradation. Such s t a b i l i z a t i o n would r e su l t from the fo l lowing resonance s t ructures: 0 k G H—G=0 + HMnO 6 5 . r etc. - 92 -However, i n view of the short l i f e t i m e of most r a d i c a l species (approximately 10"^ s e c ) , t h i s a lso does not seem l i k e l y . Further inexp l icab le experimental evidence i s presented by the marked dependence on pH for the overoxidation of both benzaldehyde and p-ni t robenz-aldehyde, as shown i n Figure 8. I t might be sug-gested that some r a d i c a l reac t ion invo lv ing the conjugate of the aldehyde i n i t i a t e s r i n g rupture but th i s seems extremely u n l i k e l y as t h i s species would be present to about one part i n a b i l l i o n . Intermediate species of manganese are known to be stable i n a c i d i c solut ions (16) and i t may be that such a species i s complexing wi th the benzaldehyde i n some way which enhances the r a d i c a l induced over-ox ida t ion . However, again t h i s seems u n l i k e l y , since no increase i n overoxidation was observed upon the addi t ion of manganous i ons . A l l that can be said then wi th any degree of ce r ta in ty i s that the overoxidat ion wi th permanganate exhibi ted by benzaldehyde at low pH i s a r e su l t of r i n g rupture. As to the mechanism of th i s occurrence, not enough experimental evidence has been gathered - 93 -to allow the p r e s e n t a t i o n of any concrete proposals. I t would be i n t e r e s t i n g , f o r example, to study the e f f e c t of other s u b s t i t u e n t s on the o v e r o x i d a t i o n and a l s o to see i f o x i d a t i o n by other oxidants r e -s u l t s i n r i n g r u p t u r e . C. The Permanganate O x i d a t i o n of p - N i t r o p h e n y l t r i -f l u o r o m e t h y l c a r b i n o l I t has been c o n c l u s i v e l y shown i n the r e s u l t s of t h i s t h e s i s t h a t p - n i t r o p h e n y l t r i f l u o r m e t h y l c a r b i n o l shows an enhanced r a t e of o x i d a t i o n by potassium permanganate over a l l other s u b s t i t u t e d p h e n y l t r i -f l u o r o m e t h y l c a r b i n o l s s t u d i e d by Stewart and Van der Linden ( 2 ) . Stewart and Van der Linden ( 2 ) have shown that although the prospect of a hydride t r a n s f e r o c c u r r i n g i n the r a t e determining step seemed l i k e l y , they found no s u b s t i t u e n t e f f e c t ; i . e . p-methoxy gave e s s e n t i a l l y the same r a t e as m-nitro. Moreover, the enhancement i n r a t e observed f o r the p - n i t r o a l c o h o l i s f u r t h e r evidence against a hydride t r a n s -f e r o c c u r r i n g . Stewart and Van der Linden ( 2 ) a l s o l i s t e d three termolecular mechanisms as p o s s i b l e - 94 -paths f o r the r e a c t i o n , but only one of these seems l i k e l y s i n c e the other two r e q u i r e termolecular s o l u t e c o l l i s i o n s and w i l l t h e r e f o r e not be considered. The f o l l o w i n g mechanism they proposed does not r e q u i r e a termolecular s o l u t e c o l l i s i o n s : 0 _ I ArCHOH G F o + OH A r — G H — C F ^ 3 3 H^O ( H ) - G —0 MnO, — ^ — 2 — ^ H o 0 Ar-C—CF„ + MnO, + '4 "" J3 ( G F 3 0* 0 I _ fast || A r — G —GF + MnO, * ArC—CF„ + MnO, What i s i n v o l v e d here i s a simultaneous process, e l e c t r o n a b s t r a c t i o n by the permanganate and proton a b s t r a c t i o n by the s o l v e n t . I t seems e n e r g e t i c a l l y more favourable however, to have a one stage process i n the t r a n s f e r of a hydrogen atom from the a l k o x i d e i o n to the permanganate i o n . T h i s i s more l i k e l y than the e a r l i e r mechanism shown above and i n a d d i t i o n accommodates a l l the ex-perimental evidence gathered by Stewart and Van der Linden ( 2 ) , i n c l u d i n g the n e g l i g i b l e s u b s t i t u e n t e f f e c t . Moreover, the enhancement i n r a t e observed - 95 -f o r t h e p - n i t r o a l c o h o l f i t s v e r y w e l l i n t o t h i s mechanism i n t h a t t h e p - n i t r o group would a f f o r d enhanced r a d i c a l s t a b i l i z a t i o n . I t has been found i n t h e p a s t t h a t p - n i t r o s u b s t i t u e n t s enhance r a d i c a l r e a c t i o n s (40) as e v i d e n c e d by t h e l a r g e d i s s o c i a t i o n c o n s t a n t o f p - n i t r o h e x a p h e n y l e t h a n e o v e r a l l o t h e r s u b s t i t u e n t s . A mechanism i n v o l v i n g hydrogen atom a b s t r a c t i o n by permanganate from t h e a l k o x i d e i o n i s t h u s f a v o u r e d i n v i e w o f t h e r e s u l t s found i n t h i s work on t h e permanganate o x i d a t i o n o f p - n i t r o p h e n y l t r i f l u o r o m e t h y l -c a r b i n o l . I n f a c t , i t seems r e a s o n a b l e t o p o s t u l a t e t h e same mechanism as o c c u r r i n g i n t h e p o t a s s i u m permanganate o x i d a t i o n o f a l l s u b s t i t u t e d p h e n y l t r i -f l u o r o m e t h y l c a r b i n o l s and i n d e e d a l c o h o l s i n g e n e r a l . That t h e p - n i t r o group s h o u l d be e x p e c t e d t o g i v e an enhancement i n r a t e can be r e a l i z e d f r o m the r e s o n -ance s t r u c t u r e s shown w i t h t h e mechanism f o r t h e o x i d a t i o n below: 0" f pN0 2ArCH0HCF 3 + OH • pN0 2-AR-GH-GF 0" p N0 2—Ar- C — GF 3 + HMhO + H 20 0-P N O ^ r — C ^ - H 1 + GF, MnO, - 97 -LIST OF REFERENCES 1. K. B. Wib e r g and R. S t e w a r t , J . Am. Chem. S o c , 77. 1786 ( 1 9 5 5 ) . 2. R. Van d e r L i n d e n and R. S t e w a r t , Faraday Soc. D i s c , 29, 211 ( 1 9 6 0 ) . 3. M. Mocek, Ph.D. T h e s i s , U n i v e r s i t y o f B r i t i s h C o l u m b i a , Vancouver, 1962. 4. M. C. R. Symons and A. C a r r i n g t o n , J . Chem. S o c , 3373 ( 1 9 5 6 ) . 5. D. V e o r l a n d e r , G. B l o u , and T. W a l l i s , Ann., 345. 261 ( 1 9 0 6 ) . 6. R. H. F e r g u s s o n , W. L e r c h , and J . F. Day, J . Am. Ghem. Soc., 53, 126 ( 1 9 3 1 ) . 7. H. Stamm, "Newer Methods o f V o l u m e t r i c A n a l y s i s , " t r a n s , by Oesper, Van N o s t r a n d and Co., N.Y., 1938. 8. M. C. R. Symons, J . Ghem. S o c , 3956 ( 1 9 5 3 ) . 9. A. Y. Drummond and W. A. Waters, J . Ghem. S o c , 435 ( 1 9 5 3 ) . 10. M. Merz and W. A. Wa t e r s , J . Ghem. S o c , S 15 ( 1 9 4 9 ) . 11. J . S. F. Pode and W. A. W a t e r s , J . Ghem. S o c , 717 ( 1 9 5 6 ) . 12. W. A. Waters, Q u a r t . Revs., 12, 277 ( 1 9 5 9 ) . 13. H. L u x , Z. N a t u r f o r s c h . , 1, 281 ( 1 9 4 6 ) . 14. A. Guyard, B u l l . Soc. Ghim., F r a n c e , 6, 89 ( 1 8 6 4 ) . 15. J . W. Ladbury and G. F. G u l l i s , Ghem. Revs., 58, 403 ( 1 9 5 8 ) . 16. A. R. J . P. Ubbelohde, J . Ghem. S o c , 1605 ( 1 9 3 5 ) . 17. A. Y. Drummond and W. A. Waters, J . Ghem. S o c , 440 ( 2 8 3 6 ) ; 3119 ( 1 9 5 3 ) ; 2456 ( 1 9 5 4 ) ; 497 ( 1 9 5 5 ) . - 98 -18. H. Land and W. A. W a t e r s , J . Chem. Soc., 4312 ( 1 9 5 7 ) ; 2129 ( 1 9 5 8 ) . 19. B. V. Tronov, J . Russ. P h y s . Chem., 59, 1155 ( 1 9 2 7 ) . 20. J . H o l l u t a and A. M u t s c h i n , Z. P h y s i k Chem., 150, 381 ( 1 9 3 0 ) . 21. K. B. Wib e r g and A. F o x , J . Am. Chem. Soc., i n p r e s s . 22. J . Kenyon and M. C. R. Symons, J . Chem. S o c , 3580 ( 1 9 5 3 ) . 23. C. G. Swain, R. A. W i l l , and R. F. W. Bader, J . Am. Ghem. Soc., 83, 1945 ( 1 9 6 1 ) ; 83, 1951 ( 1 9 6 1 ) . 24. F. G. Tompkins, T r a n s . Faraday S o c , 39, 280 ( 1 9 4 5 ) . 25. M. S. K h a r a s c h and M. Foy, J . Am. Ghem. S o c , 57, 1510 ( 1 9 3 5 ) . 26. H. Kwart and P. S. ^ r a n c i s , paper p r e s e n t e d a t 126th M e e t i n g of Am. Ghem. S o c , New Y o r k , N.Y., S e p t . , 1954; c f . A b s t . , 1 2 6 t h M e e t i n g . 27. R. S t e w a r t , J . Am. Ghem. S o c , 79, 3057 ( 1 9 5 7 ) . 28. P>.. Van d e r L i n d e n , Ph.D. T h e s i s , U n i v e r s i t y of B r i t i s h C o l u m b i a , Vancouver, 1960. 29. R. S t e w a r t , " O x i d a t i o n Mechanisms: A p p l i c a t i o n t o Or g a n i c C h e m i s t r y , " W. A. Benjamin Co., New Y o r k , N.Y., 1963. 30. A. Bra n d s t r o m , A c t a . Ghem. Scand., 13^, 611 ( 1 9 5 9 ) , 31. J . B. Gonat, G. N. Webb and W. G. Mendum, J . Am. Ghem. S o c , 1246 ( 1 9 2 9 ) . 32. H . C o n r a d - B i l l r o t h , Z. P h y s i k Ghem., B23, 315 ( 1 9 5 3 ) . 33. R. S t e w a r t and R. Van d e r L i n d e n , Can. J . Ghem., 38, ( 1 9 6 0 ) . 34. R. P. B e l l e t a l . , P r o c . Roy. S o c , 197A, 141 ( 1 9 4 9 ) . 35. G. G. Swain, J . Am. Ghem. S o c , 72, 4578 ( 1 9 5 0 ) . - 99 -36. M. Gohn and H. G. Urey, J . Am. Ghem. S o c , 60, 679 (1938). 37. G. L. Zimmerman, Ph.D, T h e s i s , U n i v e r s i t y of Chicago, 1949. . . . . 38. G. F. C u l l i s and J . W. Ladbury, J . Ghem. S o c , 4186 (1955). 39. M. S. Newman, " S t e r i c E f f e c t s i n Organic Chemistry," John Wiley and Sons Inc., pg. 591, 1956. 40. E. S. Gould, "Mechanism and S t r u c t u r e i n Organic Chemistry," H. H o l t and Co., New York, N.Y., ph. 675, 1959. . 

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