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Kinetics of the reaction between hydrazobenzene and iodine in aqueous ethanol solutions May, John Walter 1959

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KINETICS OP THE REACTION BETWEEN HYDRAZOBENZENE AND IODINE IN AQUEOUS ETHANOL SOLUTIONS by JOHN WALTER MAY B.A., U n i v e r s i t y o f B r i t i s h Columbia, 1957 A THESIS SUBMITTED IN PARTIAL FULFILMENT OP THE REQUIREMENTS FOR THE DEGREE OP MASTER OP SCIENCE i n the Department of CHEMISTRY 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 t o the r e q u i r e d s t a n d a r d THE UNIVERSITY OP BRITISH COLUMBIA November, 1959 In presenting t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements 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 Columbia, I agree that the L i b r a r y s h a l l make . i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree th a t permission f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . I t i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission. Department of The U n i v e r s i t y of B r i t i s h Columbia, Vancouver 8,' Canada. Date h/0j/^H6^K; i i ABSTRACT The k i n e t i c s of the o x i d a t i o n of hydrazobenzene by i o d i n e i n buffered ethanol-water s o l u t i o n s were studied between 0°C. and 2 5 ° C . A measurable rate was obtained by depressing the Iodine concentration by the a d d i t i o n of excess i o d i d e , so that most of the iodine was transformed to t r i i o d i d e . The e f f e c t s of i n i t i a l concentrations, a c i d i t y , solvent composition and bu f f e r I o n i c strength were determined, as w e l l as that of adding quinone and several metal s a l t s . Both io d i n e and t r i i o d i d e were found to o x i d i z e hydrazo-benzene, but iodi n e I s about 1 0 0 , 0 0 0 times more r e a c t i v e than t r i i o d i d e . k l I 2 + CgH^NHNHCgH,- ±-> CgH^NCgH^ + 2 HI k 2 I_ + C^NHNHC,^ CrHcN=NCrHI- + 2 HI + I 3 6 5 6 5 6 5 6 5 The k i n e t i c s suggest that both r e a c t i o n s have a simple. bimolecular mechanism, expressed by the rat e law dC6H5N=NC6H^/dt = k l LI 2][C 6H 5NHNHC 6H 5] '+ k g t l ' ] [CgH^HNHCg^) The constants k-^  and kg were evaluated i n 6 0 volume per cent ethanol and b u f f e r i o n i c strength 0 . 3 1 as , 11 o v i n 1 2 - 1 1 , 6 0 0 / R T n . - 1 - 1 k-^  = 4 . D x 1 0 e ' l.mol. sec. • , Q o v i n 1 2 - 1 7 , 8 0 0 / R T , , - 1 - 1 kg = 2 . 3 x 1 0 e ' l.mol. sec. In connection w i t h t h i s work i t was found necessary to evaluate the e q u i l i b r i u m constant of formation of t r i i o d i d e i n the solvents used. The heat and entropy of formation from io d i n e and i o d i d e i n 6 0 volume per cent ethanol were found to be - 9 - 5 + 0 . 5 kcal.mol. 1 and - 1 3 . 2 e.u. r e s p e c t i v e l y . i i i TABLE OP CONTENTS Page PART I . THE TRIIODIDE-IODINE EQUILIBRIUM IN 60 VOLUME PER CENT ETHANOL I n t r o d u c t i o n 1 E x p e r i m e n t a l Technique 3 R e s u l t s 4 1. T r i i o d i d e S p e c t r a . 4 2 . The E q u i l i b r i u m C o n s t a n t o f F o r m a t i o n i n V a r i o u s E t h a n o l - w a t e r S o l v e n t s a t 15°C. . . 4 3 . The D e t e r m i n a t i o n o f the Heat and E n t r o p y o f F o r m a t i o n o f T r i i o d i d e i n 60 Volume P e r Cent E t h a n o l from I o d i n e and Iodide:. . . . 8 D i s c u s s i o n 9 PART I I . THE KINETICS OP THE OXIDATION IN SOLUTION OF HYDRAZOBENZENE BY IODINE I n t r o d u c t i o n 13 E x p e r i m e n t a l 18 1. M a t e r i a l s 18 2 . G e n e r a l Remarks 18 3 . A n a l y t i c a l P r o c e d u r e and K i n e t i c Measurements 19 R e s u l t s and D i s c u s s i o n 22 1. S t o i c h i o m e t r y o f the r e a c t i o n 22 2. K i n e t i c s o f the R e a c t i o n 24 3 . The e f f e c t o f pH on the R e a c t i o n 29 4 . E f f e c t o f the S o l v e n t C o m p o s i t i o n 32 5 . E f f e c t o f Temperature 39 6. I n f l u e n c e o f the B u f f e r I o n i c S t r e n g t h on k 42 7 . EffectPof Quinone and M e t a l S a l t s on the Rate 48 C o n c l u s i o n s 51 i v Page APPENDIX. THE OXIDATION IN SOLUTION OF HYDRAZOBENZENE BY MOLECULAR OXYGEN I n t r o d u c t i o n and E x p e r i m e n t a l Technique 5^ R e s u l t s 55 C a t a l y s i s by Sodium Hy d r o x i d e 62 E f f e c t of S o l v e n t . . . 62 Other O b s e r v a t i o n s 62 D i s c u s s i o n : The P r e s e n t S t a t e o f the Problem 62 BIBLIOGRAPHY 66 V LIST. OF TABLES PART I . Table 1-1 M o l a r E x t i n c t i o n C o e f f i c i e n t s o f T r i i o d i d e and I o d i n e i n S e v e r a l Aqueous E t h a n o l S o l v e n t s , a t the T r i i o d i d e Maximum Page 1-2 Data f o r the D e t e r m i n a t i o n o f the E q u i l i b r i u m C o n s tant o f F o r m a t i o n o f T r i i o d i d e i n E t h a n o l -w ater S o l v e n t s a t 15°C. 11 1- 3 Data t o determine A H ° f o r the t r i i o d i d e E q u i l i b r i u m C o n s t a n t o f F o r m a t i o n i n 60 Volume Per Cent E t h a n o l 12 PART I I . Table 2-1 E v i d e n c e f o r the S t o i c h i o m e t r y o f the R e a c t i o n between I o d i n e and Hydrazobenzene 23 2 - 2 Independence o f k on the I n i t i a l Hydrazo-benzene C o n c e n t r a f i B n 26 2-3 The Independence o f k e X p on the "pH" 33 2-4 The E f f e c t o f S o l v e n t C o m p o s i t i o n on k . . . . . 36 ^ exp 2-5 The E f f e c t o f V a r y i n g the I o d i d e C o n c e n t r a t i o n i n Three S o l v e n t s 37 2-6 The V a r i a t i o n o f k.-^ and kg w i t h the S o l v e n t . . . 38 2-7 The E f f e c t o f V a r y i n g the I o d i d e C o n c e n t r a t i o n a t S e v e r a l Temperatures i n 60 Volume P e r Cent E t h a n o l 43 2-8 The V a r i a t i o n i n 60 Volume P e r Cent E t h a n o l o f k^ and kg w i t h temperature 44 2-9 The E f f e c t o f I o n i c S t r e n g t h 47 2-10 The E f f e c t on k^ v r i o f Adding Quinone and M e t a l APPENDIX S a l t s . . . . . 50 T a b l e 3-1 P s e u d o - f i r s t Order Rate C o n s t a n t s f o r the A u t o x i -d a t i o n o f Hydrazobenzene. Summary o f the R e s u l t s o f S e v e r a l Workers 63 v i L IST OF FIGURES F i g u r e Page 1-1 The S p e c t r a o f T r i i o d i d e i n S e v e r a l S o l v e n t s . . . 6 1-2 K as a f u n c t i o n o f S o l v e n t C o m p o s i t i o n 7 1- 3 L ° g 1 0 K V e r s u s l / T 10 2- 1 S p e c t r a o f Trans-azobenzene i n V a r i o u s S o l v e n t s . . 20 2-2 T y p i c a l Rate P l o t s : V a r y i n g I n i t i a l Hydrazobenzene C o n c e n t r a t i o n . . 25 2 - 3 k O V T_ V e r s u s I n i t i a l T o t a l I o d i n e C o n c e n t r a t i o n . . 27 2-4 T y p i c a l Rate P l o t s : V a r y i n g I o d i d e C o n c e n t r a t i o n , 30 2-5 T y p i c a l P l o t : k g x p a g a i n s t l / [ l " ] f . . . . . . . . 31 2-6 k V e r s u s S o l v e n t C o m p o s i t i o n . 34 2 -7 k e x p V e r s u s l / [ l ~ ] T f o r Thr,ee S o l v e n t s a t 15°C . . 35 2-8 k V e r s u s l / [ I ~ ] m a t F i v e Temperatures i n 60 exp ' L JT Volume P e r Cent E t h a n o l 40 2- 9 L o g ^ t k j / K ] and l o g 1 0 k 2 V e r s u s l / T . . 4 l 2-10 k V e r s u s l / [ I ~ ] f o r two B u f f e r I o n i c S t r e n g t h s 46 3 - 1 T y p i c a l Rate P l o t s : F o r m a t i o n o f Azobenzene and hydrogen p e r o x i d e 56 3-2 F i r s t Order Dependence o f dA/dt and dHgOg/dt on the I n i t i a l Hydrazobenzene C o n c e n t r a t i o n 57 3-3 F o r m a t i o n o f Azobenzene and Hydrogen P e r o x i d e i n B a s i c S o l u t i o n : Added I n i t i a l [HgOg] 59 3-4 T y p i c a l Rate P l o t s : Base C a t a l y z e d F o r m a t i o n o f Azobenzene and Hydrogen P e r i x i d e . . . 60 3-5 Rate of F o r m a t i o n o f Azobenzene V e r s u s [NaOH] 2 . . 6 l 3-6 Rate o f F o r m a t i o n o f Azobenzene and S o l v e n t C o m p o s i t i o n " 64 ACKNOWLEDGEMENT The author s i n c e r e l y thanks Dr. J . Halpern f o r h i s support and f i r m guidance throughout the course of t h i s work. The author i s a l s o indebted to Dr.. G. Po r t e r and Dr. J.F. Harrod f o r t h e i r h e l p f u l d i s c u s s i o n s and c r i t i c i s m s . PREFACE This t h e s i s has been d i v i d e d i n t o two Parts and an Appendix. Part I describes the measurement of the e q u i l i b r i u m constant of formation of the t r i i o d i d e i o n i n s e v e r a l s o l -vents, and the measurement of the heat and entropy of form-a t i o n from i o d i n e and i o d i d e i n one of them. The determ-i n a t i o n of these q u a n t i t i e s was found to be necessary to c a l c u l a t e the values of k i n e t i c constants measured i n Part I I . Part I I , though d e s c r i b i n g the main work of t h i s t h e s i s , was placed second so that the p r e s e n t a t i o n of r e s u l t s would f o l l o w a l o g i c a l order. I t describes a k i n e t i c study of the o x i d a t i o n i n s o l u t i o n of hydrazo-benzene by i o d i n e . The appendix i s a d e s c r i p t i o n of some e a r l i e r work, and i s an incomplete k i n e t i c study of the o x i d a t i o n i n s o l u t i o n of hydrazobenzene by molecular oxygen.;,. PART I . THE TRIIODIDE-IODINE EQUILIBRIUM IN ETHANOL-WATER SOLUTIONS INTRODUCTION As i s w e l l known, Iodine i n i o d i d e s o l u t i o n s forms the t r i i o d i d e i o n i n an e q u i l i b r i u m I 2 + I " s K * I ~ Recently K has been measured i n water by Awtrey and Connick ( l ) who found a value of AH° of formation of -5.1 + 0 . 4 kcal/mol. Their data y i e l d e d a value f o r A S° of -3.9 e.u. A necessary and Important adjunct to the present k i n e t i c study was the determination of K i n s e v e r a l aqueous ethanol s o l v e n t s , and the determination of A H° i n one of them. The method used, was a' m o d i f i c a t i o n of the spectro-photometry technique of Awtrey and Connick, which takes advantage of the very l a r g e molar e x t i n c t i o n c o e f f i c i e n t of t r i i o d i d e compared, to that of i o d i n e . Their method con-s i s t e d e s s e n t i a l l y of knowing the t o t a l i o d i n e and t o t a l i o d i d e i n i t i a l l y , and c a l c u l a t i n g K d i r e c t l y a f t e r measuring the e q u i l i b r i u m t r i i o d i d e c o n centration. Their knowledge of the t o t a l i o d i n e concentration, however, was based on a value of K i n water p u b l i s h e d by Jones and Kaplan ( 2 ) . ( i t i s d i f f i c u l t to know the t o t a l i o d i n e d i r e c t l y when i t s con-c e n t r a t i o n must be of the order of 10 ^M.) The modified method of Awtrey and Connick used i n t h i s work allows the t o t a l i o d i n e concentration to be deter-mined d i r e c t l y by adding a measured small volume of 1 2 c o n c e n t r a t e d i o d i d e s o l u t i o n t o a known volume o f e q u i l i b r i u m s o l u t i o n , c o n v e r t i n g e s s e n t i a l l y a l l the i o d i n e t o t r i i o d i d e . The method a l s o h e l p s t o reduce e r r o r s e x p e c t e d from random a i r o x i d a t i o n o f i o d i d e i n the a l c o h o l i c media, w h i c h i s f a i r l y r a p i d under normal c o n d i t i o n s . The t o t a l i o d i n e c o n c e n t r a t i o n need not be known i n i t i a l l y , and i t s v a l u e , as w e l l as t h a t o f the e q u i l i b r i u m t r i i o d i d e c o n c e n t r a t i o n can be det e r m i n e d r e g a r d l e s s o f o x i d a t i o n o c c u r r i n g b e f o r e measurements are made. The i n i t i a l l y known t o t a l i o d i d e can be a d j u s t e d by e s t i m a t i n g a i r o x i d a t i o n ( r e l a t i v e l y s m a l l ) w i t h a b l a n k . A n o t h e r b l a n k i s r e q u i r e d f o r the f i n a l c o n c e n t r a t e d i o d i d e s o l u t i o n , but t h i s c o r r e c t i o n i s s m a l l i f the added s t r o n g i o d i d e i s aqueous. U s i n g these i d e a s , an e x p r e s s i o n f o r K may be de v e l o p e d , u s i n g the f o l l o w i n g n o m e n c l a t u r e : S u b s c r i p t s o r S u p e r s c r i p t s : 1 r e f e r s t o the i n i t i a l or e q u i l i b r i u m s o l u t i o n , f r e f e r s t o the f i n a l o r s t r o n g i o d i d e s o l u t i o n . T means t o t a l ox means b l a n k due t o o x i d a t i o n . Symbols: A means absorbance e means molar e x t i n c t i o n c o e f f i c i e n t v means volume ( I )rp means I n i t i a l t o t a l i o d i d e c o n c e n t r a t i o n . "The e q u i l i b r i u m c o n s t a n t K i s d e f i n e d [I"]° x + [I2]°/ 3 An expression f o r each, of the terms f o l l o w s : 1) [ I ~ ] ± = A[I~]±/e1 2) [ I 2 ] ± = v f / v ± ^ [ I ~ ] f + [ I 2 ] f - v f / v i . e f ^ A [ l ~ ] f - A[l"]° x J( 1 + 1 / K [ l " ] f ) - A [ l " ] 1 / e i 3) [ l " ] i = [ I " ] T - [ l - ] i - 2A[l"]° X = [ I " ] T - A [ I - ] ± / e 1 - 2A[l"]° x/e i K was r e c a l c u l a t e d u n t i l s e l f - c o n s i s t e n c y was obtained. EXPERIMENTAL TECHNIQUE Absorbances were measured by a Beckman DU spectro-photometer, us i n g a Cary 1 cm. quartz c e l l h e l d i n a Beckman c e l l holder whose end w a l l had been cut away. The narrow-necked c e l l , f i t t e d w i t h a ground stopper, kept a i r away from the s o l u t i o n i n s i d e during experimental manipulations. S o l u -t i o n s were admitted to the c e l l by hypodermic syringe (5 ml. or a 1 m l . t u b e r c u l i n t y p e ) . The c e l l compartment was thermo-sta t e d w i t h c i r c u l a t i n g water which had been cooled i n an i c e bath, and the temperature i n s i d e measured by a thermometer i n s e r t e d through a hole i n the c e l l compartment cover and dipping i n t o an or d i n a r y Beckman c e l l c o n t a i n i n g water. O x i d a t i o n of i o d i d e was kept to a minimum by s t r i c t l y observing three precautions, namely 1) A l l solvents were purged w i t h n i t r o g e n . 2) A l l prepared s o l u t i o n s were stored, i n a r e f r i g e r a t o r , 3) The s o l u t i o n s were kept away from l i g h t . 4 The s o l u t i o n s i n w h i c h K was measured c o n t a i n e d the same b u f f e r c o n c e n t r a t i o n as used i n most o f the k i n e t i c runs i n P a r t I I . A l l c h e m i c a l s were a n a l y t i c a l r e a g e n t grade, and b o t h e t h a n o l and water were r e d i s t i l l e d . RESULTS 1) T r i i o d i d e S p e c t r a These were measured u s i n g b l a n k s c o n t a i n i n g no added i o d i n e . A summary f o r the molar e x t i n c t i o n c o e f f i c i e n t s f o r i o d i n e and t r i i o d i d e a t the t r i i o d i d e maximum i s g i v e n i n Table 1-1. The s o l u t i o n s c o n t a i n e d 17*3 a c e t i c a c i d - p o t a s s i u m a c e t a t e b u f f e r {J*- = 0 . 3 1 ) . T y p i c a l s p e c t r a are i l l u s t r a t e d i n F i g u r e 1-1. 2) The E q u i l i b r i u m C o n s t a n t o f F o r m a t i o n o f T r i i o d i d e i n  V a r i o u s E t h a n o l - w a t e r S o l v e n t s a t 15° C. E x p e r i m e n t a l r e s u l t s are summarized i n Table 1-2. The f i n a l e x t i n c t i o n c o e f f i c i e n t e^ was e v a l u a t e d by i n t e r -p o l a t i o n f r o m T a b l e 1-1. I n F i g u r e 1-2 i s shown a p l o t o f K as a f u n c t i o n o f the s o l v e n t c o m p o s i t i o n . U s i n g Awtrey and Co n n i c k ' s thermodynamic d a t a , the mean v a l u e o f K o b t a i n e d i n wa t e r a t l 6 . 8 ° c . , 8 l 4 , i s c o r -r e c t e d t o 857 a t 15° C T h e i r v a l u e i s 946 a t 1 5°C, i n f a i r agreement. 5 TABLE 1-1 The molar e x t i n c t i o n c o e f f i c i e n t s of t r i i o d i d e and Iodine  In s e v e r a l aqueous ethanol solvents at the t r i i o d i d e maximum Volume per cent ^ t \ a v l f l ^ ethanol \ max (T ) V-T0 e t r i i o d i d e i o d i n e 0 352 2.65 + .03 50 356 2 . 8 4 + .05 55 356 (2 .70 + .06) 60 357 2.72 + .06 65 357 2.62 + .07 75 358 2 . 48 + .08 0 353 2 . 64 Data of Awtrey and Connick ( l ) . 300 320 340 360 }P>0 400 WAVELENGTH - MILLIMICRONS FIGURE 1-1 . Spectra of t r i i o d i d e i n several solvents Conditions s buffer i o n i c strength = O .31 , T = 15 C. O = 50 volume i= ethanol, • =75 volume 1° ethanol, /\ = water. 3) The Determination of the Heat and Entropy of Formation of  T r i i o d i d e i n 60 Volume Per Cent Ethanol from Iodine and  Iodide , • I t was observed that a small p r o p o r t i o n of the t o t a l i o d i n e i n an e q u i l i b r i u m s o l u t i o n decomposed during the long ( h a l f an hour) wait f o r temperature e q u i l i b r i u m to be e s t a b l i s h e d , e s p e c i a l l y at the higher temperatures. The r e a c t i o n i n v o l v e d i s thought to be CH 3CH 20H + I g > CH^CHO + 2 HI I n b a s i c s o l u t i o n the r e a c t i o n occurs as a step i n the halo-form r e a c t i o n . The e f f e c t i v e pH of the s o l u t i o n s i s higher than the value of 4 c a l c u l a t e d assuming the solvent to be water. Necessary then was a s l i g h t m o d i f i c a t i o n of the procedure already o u t l i n e d . A measure of the ex t r a i o d i d e produced was obtained by measuring the t o t a l i o d i n e before and a f t e r the e q u i l i b r i u m measurements, and was used as a cor-r e c t i o n to the known t o t a l i o d i d e concentration. The b u f f e r strength was halved as some im p u r i t y i n i t could be a c a t a l y s t f o r the iodi n e decomposition ( B u f f e r i o n i c s t r e n g t h reduced to 0 . 1 5 ) . A fundamental assumption was made that reducing the i o n i c s t r e n g t h would not a f f e c t AH°, although the r e s u l t s show a s l i g h t increase i n K at 15°C. i n the solvent of lower i o n i c s t r e n g t h . Complete data are tabul a t e d i n Table 1-3. The amount of io d i n e decomposition i s represented by i t s cor-responding t r i i o d i d e absorbance i n the l e f t hand column. 9 Figure 1-3 shows a p l o t of l°g-j_Q (K) against I/T, which gave values f o r AH° and A S° of t r i i o d i d e formation of - 9 . 5 + 0 . 5 kcal./mol. and -13-2 e.u. r e s p e c t i v e l y . DISCUSSION I t i s i n t e r e s t i n g to compare the thermodynamic data f o r the formation of t r i i o d i d e i n the two s o l v e n t s , water and 60 volume per cent ethanol. Awtrey and Connick reported A H° as -5 . 1 + 0 . 4 kcal./mol. i n water, and As° was c a l -c u l a t e d from t h e i r r e s u l t s as - 3 . 9 e.u. Thus there i s a net change i n As° of - 9 . 3 e.u. going from water to 60 volume per cent ethanol. I t has been s t a t e d that about f i v e entropy u n i t s are a s s o c i a t e d w i t h a completely s o l v a t e d water mole-cule ( 3 ) ) so that the change i n AS° i n d i c a t e s a l e s s e r degree of s o l v a t i o n of i o d i d e r e l a t i v e to t r i i o d i d e i n the a l c o h o l i c s o l v e n t , by about two solvent molecules (or more i f they are only weakly bound). As support to t h i s i d e a , a value of AH° equal to -7.0 kcal.mol. 1 i n the a l c o h o l i c solvent was c a l -c u l a t e d s o l e l y from the d i f f e r e n c e i n K between the two s o l -vents at 1 5°C, assuming no change i n the entropy of form-a t i o n . The d i f f e r e n c e of - 2 . 5 k c a l . m o l . - 1 between the mea-sured value and t h i s may be i n t e r p r e t e d as the e x t r a heat l i b e r a t e d i n the formation of t r i i o d i d e by the two solvent molecules, or about 1 k c a l . per mole per solvent molecule (or l e s s f o r a greater number of more weakly bound solvent molecules). 1 0 F I G U R E 1-3.Log-.QK versus r e c i p r o c a l absolute temperature.. C o n d i t i o n s ; 6 0 volume per cent e t h a n o l , b u f f e r i o n i c s t r e n g t h = 0 . 1 5 . TABLE 1 - 2 Data f o r the d e t e r m i n a t i o n of the e q u i l i b r i u m c o n s t a n t of f o r m a t i o n o f t r i i o d i d e i n e t h a n o l - w a t e r s o l v e n t s a t 1 5 C. A c e t i c a c i d -potassium a c e t a t e b u f f e r ', i o n i c s t r e n g t h = 0 . 1 5 Volume % e t h a n o l T °C. [ I ~ ] x l 0 4 mol 7".!- 1 [ I ] _ x l 0 2 m o l . l " 1 A [ I ~ ] f A [ I ~ ] f : A[I"]° X - 4 K x l O 1 .mol. 0 1 6 . 8 1 6 . 8 2 0 . 0 1 8 . 9 0 . 2 6 0 0 . 5 3 7 0 . 3 6 5 0 . 7 4 9 0 0 0 0 . 0 0 3 0 . 0 8 1 7 0 . 0 8 1 2 5 0 1 4 . 8 1 5 . 0 2 . 0 0 0 3 . 1 4 0 . 8 6 0 0 . 4 7 0 0 . 9 9 8 0 . 5 4 0 0 . 0 0 3 0 . 0 0 5 0 . 0 0 8 0 . 0 0 8 2 . 0 9 2 . 0 1 5 5 1 5 . 0 1 5 . 0 1 . 6 0 0 3 . 1 4 0 . 8 2 4 0 . 5 5 2 1 . 0 0 2 0 . 6 6 0 0 . 0 0 3 o . o o 4 0 . 0 0 9 0 . 0 1 1 2 . 2 9 2 . 3 7 6 0 1 4 . 8 1 4 . 9 1 . 2 1 0 3 . 1 4 0 . 8 0 3 0 . 4 1 6 1 . 0 1 6 0 . 5 3 0 0 . 0 0 1 0 . 0 0 2 0 . 0 0 5 0 . 0 1 1 2 . 7 1 ( 2 . 4 2 ) 6 5 1 4 . 9 1 4 . 9 1 . 0 7 2 3 . 1 4 1 . 1 7 5 0 . 8 1 1 1 . 6 1 0 1 . 0 6 4 0 . 0 0 1 0 . 0 0 2 o . o o 4 o . o o 4 3 . 2 2 3 . 0 3 7 5 1 4 . 9 1 4 . 9 1 . 0 5 2 3 . 1 4 1 . 3 6 1 0 . 9 2 0 1 . 8 2 6 1 . 1 3 9 0 0 0 . 0 0 1 0 . 0 0 3 4 . - 3 0 4 . 2 5 Note: v, = 2.-80 ml. v- = 3 . 2 0 m l . The f o l l o w i n g v a l u e s o f e. and e^ were used: 1 I I n i t i a l % EtOH -4 e ± x l O e f x l O 5 0 2 . 8 3 2 . 9 1 5 5 2 . 7 6 2 . 8 5 6 0 2 . 6 9 2 . 7 9 6 5 2 . 6 2 2 . 7 3 7 5 2 . 4 8 2 . 6 0 - 4 1 2 TABLE 1 - 3 Data to determine A H ° f o r the t r i i o d i d e e q u i l i b r i u m constant  of formation i n 6 0 volume per cent ethanol Decomp. A [ I - ] . A [ I 3 ] f [ I ] T x l 0 4 M . uncorr. T ° C . - 4 K x 1 0 _ ^ 1.mol.~ 1 0 g 1 0 [ K ] T 1 x l 0 3 [ ° K H 0 . 0 4 3 0 . 3 5 6 O . 6 3 8 O . 9 8 5 3 2 . 3 1 . 1 9 4 . 0 7 6 3 . 2 7 4 0 . 1 5 9 0 . 2 3 4 0 . 3 5 8 1 . 0 2 8 2 9 . 0 1 . 3 6 4 . 1 3 3 3 . 3 0 9 0 . 0 1 9 0 . 4 1 1 0 . 6 6 2 0 . 9 8 5 2 7 . 4 1 . 5 8 4 . 1 9 9 3 - 3 2 8 0 . 0 2 9 0 . 4 4 5 O . 6 5 2 • 0 . 9 8 5 2 2 . 8 2 . 0 2 4 . 3 0 6 3 . 3 7 9 0 . 0 3 3 0 . 4 8 6 0 . 6 4 8 O . 9 8 5 1 7 . 9 2 . 7 1 4 . 4 3 3 3 . 4 3 6 0 0 . 4 3 1 0 . - 5 1 7 1 . 0 2 8 1 2 . 4 3 . 4 8 4 . 5 4 2 3 . 5 0 1 0 . 0 4 4 0 , 5 1 3 0 . 6 3 7 0 . 9 8 5 1 1 . 9 3 . 5 1 4 . 5 4 6 3 . 5 0 9 B u f f e r = 0 . 1 5 v 1 = 2 . 8 0 ml. e. = 2 . 6 9 x 1 0 v f = 3 - 2 0 ml. e f = 2 . 7 9 x 1 0 PART I I . THE KINETICS OP THE OXIDATION IN SOLUTION OP HYDRAZOBENZENE BY IODINE INTRODUCTION The o x i d a t i o n o f hydrazobenzene ( 1 , 2 ' d i p h e n y l h y d r a z i n e ) has l o n g been known t o o c c u r e a s i l y i n s o l u t i o n . " 1 " The o x i d a n t removes the N-bonded hydrogen atoms i n the t h e r m o d y n a m i c a l l y f a v o u r -a b l e f o r m a t i o n o f azobenzene. At the t u r n o f the c e n t u r y , Herzog and Manchot (4) n o t e d the a u t o x i d a t i o n ( o x i d a t i o n by m o l e c u l a r oxygen) and found the p r o d u c t s t o be azobenzene and hydrogen p e r o x i d e . + ° 2 0 " N = N 0 +" H 2 ° 2 ( I ) (AH 2) (A) T h i s u n u s u a l r e a c t i o n was the f i r s t known a u t o x i d a t i o n t h a t p r o -2 duced hydrogen p e r o x i d e , and i n q u a n t i t a t i v e y i e l d . S u b s e q u e n t l y s e v e r a l k i n e t i c s t u d i e s have been made on the r e a c t i o n (5^6,7) , and some knowledge of the k i n e t i c s has a l s o been o b t a i n e d i n t h i s L a b o r a t o r y . (See the appendix t o t h i s t h e s i s . ) The mechanism,, however, i s s t i l l not u n d e r s t o o d . A h i g h l y i n t e r e s t i n g f e a t u r e o f the o x i d a t i o n by m o l e c u l a r oxygen i s the p o s s i b i l i t y t h a t i t o c c u r s v i a a s i m p l e b i m o l e c u l a r mechanism, not r u l e d out t o d a t e , and w h i c h i n v o l v e s t h e " s i m u l t a n e o u s " c l e a v a g e o f b o t h N-H bonds and the accompanying t r a n s f e r o f the two hydrogen atoms t o a m o l e c u l e o f oxygen. 1 e.g. Azobenzene may be made i n the l a b o r a t o r y by o x i d i z i n g AHg w i t h hypobromite, p e r i o d a t e , l e a d t e t r a a c e t a t e e t c . ... 2 There has been i n d u s t r i a l a p p l i c a t i o n o f the r e a c t i o n ( 3 5 ) . 13 14 The work so f a r done i n t r y i n g to e s t a b l i s h the mechanism f o r the a u t o x i d a t i o n has produced a c o n f l i c t i n g p i c t u r e of se v e r a l proposed mechanisms. I n an attempt to c l a r i f y the s i t u a t i o n some-what the present work was undertaken u s i n g another molecular oxidant, i o d i n e , and the study of the k i n e t i c s of i t s r e a c t i o n w i t h hydrazobenzene made the main t o p i c of t h i s t h e s i s . The s t o i c h i o m e t r i c equation f o r the r e a c t i o n was expected to be (using the n o t a t i o n i n d i c a t e d i n equation I) AH 2 . + I 2 > A + 2 HI ( I I ) A simple bimolecular mechanism could a l s o apply to t h i s r e a c t i o n , w i t h the added p o i n t of great i n t e r e s t that should t h i s be the case, there would be a s t r i k i n g resemblance to the w e l l known r e a c t i o n i n the gas phase between hydrogen and i o d i n e . The c l a s s i c work by Bodenstein (8) showed that the mechanism i s t r u l y b i molecular, and l a t e r t h e o r e t i c a l c a l c u l a t i o n s by Wheeler, Topley and E y r i n g (9) have emphasized the nature of the f o u r - c e n t e r a c t i v a t e d complex. That a f r e e r a d i c a l mechanism i s a l s o p o s s i b l e f o r the iod i n e o x i d a t i o n of hydrazobenzene w i l l now be discussed. Not only do many au t o x i d a t i o n s of hydrogen atom donors l i k e hydrazobenzene occur by r a d i c a l chains, but a l s o i t has been shown that hydrazobenzene can be r e a d i l y attacked by d i p h e n y l p l -c r y l h y d r a z y l (10) and •triphenylmethyl ( l l ) , complete r e a c t i o n being almost instantaneous at room temperature. The r a d i c a l s dehydrogenate hydrazobenzene to produce azobenzene. Whalley, Evans and Winkler (12) have studied the k i n e t i c s i n s o l u t i o n of the o x i d a t i o n of hydrazobenzene w i t h persulphate, and found that a r a d i c a l - i o n mechanism explained t h e i r observations. They suggest the f o l l o w i n g r e a c t i o n scheme: S-Og + AH 2 HSO^ + AH* + SOJ] _ L 2 A H * ^ A + AH, S0_| + AH 2 HSO^ + AH* i , - -SgOg + AH' —^> HSO^ + SO^ + A Omitting 2 SO^ + AH* > HSO^ + A The o v e r a l l s t o i c h i o m e t r y was S 20g + AH 2 > 2HS0^ + A A p p l i c a t i o n of the usual steady s t a t e approximation gave the f o l l o w i n g r a t e law, v e r i f i e d by the i n v e s t i g a t o r s . dA/dt - k^AHgHSgOg] _ _ _ _ _ + 1 k.jk 4LAH 2J ( H I ) I t i s very u s e f u l here to compare other r e a c t i o n s i n which two hydrogen atoms are t r a n s f e r r e d ( i n the o v e r a l l stoichiometry) from a donor to an acceptor. I n the a u t o x i -d a t i o n i n s o l u t i o n of hydrazine, G i l b e r t ( l 3 ) and Au d r i e t h and Mohr ( l 4 ) have shown that H 20 2 i s an intermediate and can be i s o l a t e d from the r e a c t i o n mixture. Another r e a c t i o n of i n t e r e s t i s the a u t o x i d a t i o n i n s o l u t i o n of duroquinone by James, S n e l l and Weissberger ( 1 5 ) : 16 OH 0 They p o s t u l a t e that the mechanism of peroxide formation i n v o l v e s a two-electron t r a n s f e r from the doubly i o n i z e d hydroquinone to a molecule of oxygen. A complementary r e a c t i o n i s the re d u c t i o n of quinones by s u i t a b l e hydrogen atom donors, symbol-i z e d by RH 2 + Q > R + QH 2 (V) Braude, Brook and Linstead. ( l 6 ) studied three donors and found t h e i r r e a c t i v i t i e s to be i n the order t e t r a l i n , acenaphthene and d i b e n z y l . F i n a l l y , the i n t e r e s t i n g r e a c t i o n of io d i n e w i t h hydrazine has been reported by Baxendale (17) who has proposed the f o l l o w i n g steps N 2H^ + I 2 — ' > N 2H 2 + 2 HI N 2H 2 + I 2 > N 2 + 2 HI S u r p r i s i n g l y l i t t l e work has been done on ox i d a t i o n s by molecular i o d i n e , e s p e c i a l l y of organic molecules, although some i n t e r e s t i n g mechanisms have been put forward f o r some Inorganic r e a c t i o n s . The o x i d a t i o n of t i t a n i u m ( i l l ) has a r a t e -c o n t r o l l i n g one e l e c t r o n t r a n s f e r to i o d i n e , g i v i n g I 2 , as proposed by Johnston and Winstein ( l 8 ) , and Awtrey and Connick (19) e x p l a i n the o x i d a t i o n by i o d i n e of t h i o s u l f a t e .as going by means of the slow step 17 K s 2 o 3 + i 2 x N s 2 o 3 i + I or the e f f e c t i v e t r a n s f e r of I + to t h i o s u l f a t e . Not only i s i t hoped that the present work w i l l enlarge the area of what i s known about o x i d a t i o n s by molecular . i o d i n e , but a l s o that i t w i l l add to the knowledge of the r e a c t i o n s of hydrogen atoms bonded at n i t r o g e n centers. The r e a c t i o n s at n i t r o g e n centers are g e n e r a l l y f a s t e r than the analogous r e a c t i o n s at carbon centers, but the d i f f e r e n c e i n r e a c t i v i t i e s i s probably not due to a d i f f e r e n c e i n bond ener-g i e s . (The bond d i s s o c i a t i o n energy of H^C-H i s about the same as HgN-H ( 2 0 ) . ) The d i f f e r e n c e may be a r e f l e c t i o n of the greater ease i n which the n i t r o g e n compounds form planar pro-ducts a f t e r o x i d a t i o n ( 1 6 ) . EXPERIMENTAL 1) M a t e r i a l s P r a c t i c a l grade hydrazobenzene from Eastman Kodak was repeatedly r e c r y s t a l l l z e d from ethanol-water s o l u t i o n s u n t i l white c r y s t a l s , M.P. 127°C. were obtained. These were stored i n darkness under vacuum. A n a l y t i c a l reagent potassium i o d i d e was r e c r y s t a l l l z e d by d i l u t i n g a hot concentrated aqueous s o l u t i o n w i t h ethanol, d r i e d at 110°C. f o r twenty four hours and stored i n the dark. Baker and Adamson a n a l y t i c a l reagent formic a c i d and C P . grade g l a c i a l a c e t i c a c i d from C.I.L. were p u r i f i e d f u r t h e r by r e d i s t i l l a t i o n . C r y s t a l l i n e i o d i n e of a n a l y t i c a l reagent grade was used as provided by the M a l l i n c k r o d t Chemical Works. Water was d o u b l e - d i s t i l l e d and 95^ ethamol was p u r i f i e d by a s i n g l e d i s t i l l a t i o n through a poi n t column. (B.P. range 7<3-80°C.) A l l other chemicals used were of reagent grade and were not f u r t h e r p u r i f i e d . 2) General Remarks A Model DU Beckman spectrophotometer was used f o r a l l s pectrophotometry measurements, but mainly to analyze quenched r e a c t i o n samples f o r azobenzene. The spectrum of t r a n s -azobenzene, which i s formed i n the r e a c t i o n , changes consider-a b l y w i t h the solvent composition i n the regi o n of i t s 18 19 a b s o r p t i o n o f v i s i b l e l i g h t (\ = 440 m i l l i m i c r o n s ) . F i g u r e max 2-1 shows the v a r i a t i o n i n the molar e x t i n c t i o n c o e f f i c i e n t s o f t r a n s - a z o b e n z e n e w i t h the c o m p o s i t i o n o f the aqueous e t h a n o l s o l v e n t , a t s e v e r a l w a v e l e n g t h s . I t was e s t a b l i s h e d t h a t s o l u t i o n s o f tran s - a z o b e n z e n e obeyed the Beer-Lambert law w i t h i n the e x p e r i m e n t a l c o n c e n t r a t i o n range, and t h a t the spectrum was i n s e n s i t i v e t o the pr e s e n c e o f o r d i n a r y s a l t s . S p e c i e s i n the quenched r e a c t i o n s o l u t i o n s o t h e r t h a n a z o -benzene had a n e g l i g i b l e absorbance i n the r e g i o n . A l l s o l u t i o n s c o n t a i n i n g t r a n s - a z o b e n z e n e were s h i e l d e d f r om l i g h t t o p r e v e n t the f o r m a t i o n o f the c i s isomer (21) , whose spectrum i s more i n t e n s e i n the v i s i b l e r e g i o n t h a n t h a t o f the t r a n s . (22, 23) The mixed s o l v e n t o f e t h a n o l - w a t e r was chosen so t h a t hydrazobenzene, azobenzene and p o t a s s i u m i o d i d e were a l l r e a d i l y s o l u b l e . 3) A n a l y t i c a l Procedure and K i n e t i c Measurements K i n e t i c e x p e r i m e n t s were made i n open narrow-necked 250 m l . d a r k b o t t l e s p l a c e d i n a t h e r m o r e g u l a t e d (+ 0.03°C.) water b a t h . ( C o o l i n g o f the b a t h was e f f e c t e d by c i r c u l a t i n g c o l d w a t e r t h r o u g h a submerged c o i l o r by a d d i n g i c e . ) An e x p e r i -ment was begun by. w e i g h i n g p o t a s s i u m i o d i d e i n t o the r e a c t i o n v e s s e l , f o l l o w e d by the a d d i t i o n o f the d e s i r e d amounts o f s t o c k i o d i n e and b u f f e r s o l u t i o n s , and n i n e t y p e r cent o f the 100 m l . r e a c t i o n volume made up w i t h the a p p r o p r i a t e s o l v e n t . J I I I I L_ I 90 80 70 60 50 40 VOLUME PER CENT ETHANOL FIGURE 2-1. Spectra of trans-azobenzene i n various solvent compositions, room temperature. Wavelength, millimicronss A = 480,0 =460 • = 440, y = 420,Q = 400 . 21 The v e s s e l was then placed i n the bath i n company w i t h a stoppered volumetric f l a s k c o n t a i n i n g a s o l u t i o n of hydrazo-benzene i n ethanol. At a s p e c i f i e d time 10 ml. of the hydrazo-benzene s o l u t i o n were p i p e t t e d i n t o the r e a c t i o n v e s s e l , the mixture v i g o r o u s l y a g i t a t e d , and the v e s s e l replaced i n the bath. A l i q u o t samples of ^ ml. were delivered, by a f a s t - d r a i n i n g p i p e t t e ( ^ 9 5 per cent d e l i v e r y i n 3 seconds) i n t o dark b o t t l e s c o n t a i n i n g each 1 ml. of aqueous 1.5 N. sodium t h i o s u l f a t e s o l u t i o n , which quenched the r e a c t i o n by reducing the unreacted i o d i n e . When sampling was completed., the quenched, s o l u t i o n s were analyzed f o r azobenzene at 440 and 460 m i l l i m i c r o n s . A few days f o l l o w i n g a given run, a sample was again taken and the f i n a l azobenzene concen t r a t i o n measured. I t s value was used i n the k i n e t i c c a l c u l a t i o n s as a measure of the e f f e c t i v e I n i t i a l hydrazobenzene concentration, which i n the slower runs was up to three per cent smaller than expected, presumably because some of the hydrazobenzene underwent the benzidine rearrangement i n the weakly a c i d medium. Most of the experiments were conducted at 15° C. i n 60 Volume per cent ethanol s o l u t i o n s . The i o n i c s t r e n g t h c o n t r i -b u t i o n of the b u f f e r was u s u a l l y 0.31.> and produced a "pH" i n the r e a c t i o n mixture of 3-99 c a l c u l a t e d on the b a s i s of a pure water s o l v e n t . Iodine stock s o l u t i o n was prepared by d i s s o l v i n g i n a volumetric f l a s k a known weight of io d i n e i n ethanol. The stock b u f f e r s o l u t i o n was u s u a l l y prepared from 17 p a r t s 22 aqueous a c e t i c a c i d to three p a r t s potassium acetate s o l u t i o n of the same conc e n t r a t i o n . When necessary, the h y d r i o d i c a c i d concentration was determined by po t e n t i o m e t r i c t i t r a t i o n . For the most p a r t , the k i n e t i c runs were f a s t , r e a c t i o n g e n e r a l l y being n e a r l y complete i n 1000 seconds, yet rat e s could be reproduced to an accuracy of + 5$. RESULTS AND DISCUSSION Stoichiometry of the Reaction Iodine was found to react w i t h hydrazobenzene accord-i n g to the s t o i c h i o m e t r i c equation H H I - + C 6H 5N-NC 6H 5 > 2 HI + _6H-N=NC6H- ( i ) Evidence to support t h i s s t o i c h i o m e t r y i s that the f i n a l azobenzene co n c e n t r a t i o n was never l e s s than n i n e t y - s i x per cent of the i n i t i a l hydrazobenzene concentration ( i n i t i a l [ l g ] i n i t i a l [AHg].) A reason f o r the l a c k of complete azobenzene formation i n slow runs has already been put f o r -ward (p. 2.1 ) . Further evidence was obtained when both the azobenzene and the corresponding h y d r i o d i c a c i d concentrations were measured i n two unbuffered runs. (Table- 2-1) W i t h i n e x p e r i -mental e r r o r , twice as much h y d r i o d i c a c i d was produced as azobenzene during the runs. 23 TABLE 2-1 E v i d e n c e f o r the s t o i c h i o m e t r y o f the r e a c t i o n ,  between i o d i n e and hydrazobenzene 2 Runs : 1 5°C, 60 volume p e r cent e t h a n o l I n i t i a l C o n c e n t r a t i o n s [A] x 10J M. [HI] x 10D M [ H I ] / [ A ] [AH 2] - 1.20 x 10~ 3 M. 0.701 1.56 2.23 [ I 2 1 T - 4.10 x 10~ 3 M. 0.895 1.86 2.08 [ I " ] - 0 .25 M. 1.015 2.03 2.00 0.590 1.34 2.27 [AH 2] - 1.03 x 10~ 3 M. 0.763 1.60 V 2.10 [ I 2 ] T - 7.02 x 10~ 3 M. O.850 1.79 2.11 [ I " ] - 0.643 M. 0.915 1.82 1.99 0.945 1.84 1.95 * Note: The t i t e r o f 0.01 N. NaOH was always l e s s t h a n 2 ml. f o r a 10 ml. r e a c t i o n sample, so t h a t e x p e r i m e n t a l e r r o r i n d e t e r m i n i n g the h y d r i o d i c a c i d c o n c e n t r a t i o n was f a i r l y l a r g e . 1 24 K i n e t i c s of the Reaction The r a t e of r e a c t i o n between i o d i n e and hydrazo-benzene was found to be too r a p i d to measure w i t h the tech-nique described unless the i o d i n e concentration was suppressed by the a d d i t i o n of potassium i o d i d e , which transforms the greater p a r t of the i o d i n e i n t o the t r i i o d i d e i o n . The measurement of the e q u i l i b r i u m constant of t r i i o d i d e formation has been discussed i n Part I of t h i s t h e s i s . On the b a s i s of the bimolecular stoichiometry, an experimental second order rate constant was defined at the outset by the r a t e law -d[AH 2]/dt = d[A]/dt = k e x p ( l 2 ) T ( A H 2 ) (II) where [ I 2 ] T i s the t o t a l i o d i n e concentration and [AH 2] the hydrazobenzene concentration. n 2] T = ti2] + [ 1 3 ] (nir The data f o r each experiment gave a good second order s t r a i g h t l i n e p l o t of log-^Q [^Jrp/tAELp] vs time up to the equivalent of about n i n e t y per cent r e a c t i o n , the slope of which y i e l d e d k_„_. T y p i c a l r a t e p l o t s are shown i n Figure 2 - 2 . k"exp was found to be independent of the i n i t i a l hydrazobenzene concentration (Table 2 - 2 ) , but decreased s l i g h t l y w i t h i n c r e a s i n g t o t a l i o d i n e concentration, l e v e l l i n g o f f when the i n i t i a l r a t e of formation of azobenzene —6 —1 —1 exceeded 5 x 10 m o l . l . sec. (Figure 2 - 3 ) . The increase of 2.0 1 0 G 1 0 [ I 2 ] T / [ A H 2 ] 26 TABLE 2-2 Independence of k on the i n i t i a l hydrazobenzene concentration ^ exp  [AH 2] x 1CT M. 0 . 6 6 4 0.92 0 . 9 H 0.996 1.20 0.984 1.52 0.995 1.82 1.12 2.66 0.987 3.03 1.03 k exp 1 .mol -1 -1 sec. Conditions: [ I 2 ] T = 5 .46 x 10~D M. [ I - ] = 0 .25 M. 15° C , 60 Volume <fo ethanol "pH" = 3 . 9 9 , B u f f e r = 0 . 3 1 I o n i c Strength I I I I I I ! 1 I 0 1 2 3 ,4 5 _ T 6 7 [ I - ] T x 10-5 - MOL.L. FIGURB 2-3. P l o t of k versus i n i t i a l t o t a l iodine concen-t r a t i o n . Conditions j e x p _ = 15 C.,buffer i o n i c strength =0.31 "pH" = 3.99, 60 volume # ethanol,[I~]-=.25 M.,[AH-]=1.20 x 10-2 8 k- v_ a t lo w e r t o t a l i o d i n e c o n c e n t r a t i o n s i s b e l i e v e d due t o some c o n t r i b u t i o n f r om a i r o x i d a t i o n o f hydrazobenzene, t h a t i s k a p p a r e n t * ^  I 2 ^ T = k a i r o x i d a t i o n + k e x p ^ I2-'T ° r ^apparent = k . J ^ • / [ l „ ] m + k ^ a i r o x i d a t i o n * 2 J T exp Support was l e n t t o t h i s c o n c l u s i o n when k a p p a r e n t d e c r e a s e d i f n i t r o g e n was p a s s e d d u r i n g r e a c t i o n t h r o u g h r e a c t i o n mix-t u r e s c o n t a i n i n g low t o t a l i o d i n e c o n c e n t r a t i o n s . On the as s u m p t i o n t h a t b o t h i o d i n e and t r i i o d i d e i can o x i d i z e hydrazobenzene, the r a t e law o f e q u a t i o n ( i i ) may be r e s t a t e d . dA/dt = k ^ I g ] [AHg] + k 2 [ l ~ ] [AH 2] ( I V ) The s t o i c h i o m e t r y o f the t r i i o d i d e r e a c t i o n i s assumed t o be I ~ + AH 2 > A + 2 HI + I " (V) I n the r e g i o n i n v e s t i g a t e d . , the i o d i d e c o n c e n t r a t i o n was s u f f i c i e n t l y l a r g e so t h a t [ I ^ ] ^ ~ [I-^j 1 a n d *~ [ l l ^ -The e q u i l i b r i u m i o d i n e c o n c e n t r a t i o n i s g i v e n by [ I 2 ] = [ l - ] / K [ l " ] ( V I) U s i n g t h e s e a p p r o x i m a t i o n s and combining e q u a t i o n s ( i i ) , ( I V ) and ( V I ) , r e s u l t s i n the e x p r e s s i o n k e x p = k i A ^ " ] T + k 2 ( ™ ) Good l i n e a r p l o t s o f k vs l/fl""]™ were o b t a i n e d whose i n t e r c e p t s and s l o p e s gave v a l u e s o f kg and k-j/K r e s -s p e c t l v e l y . A r e p r e s e n t a t i v e p l o t i s shown i n F i g . 2 - 5 , and the c o r r e s p o n d i n g r a t e p l o t s shown i n F i g . 2 r 4 . I n a n o t h e r s e r i e s the i n i t i a l t o t a l i o d i n e c o n c e n t r a t i o n was v a r i e d as a check on the k i n e t i c s (see Table 2 - 5 ) . The E f f e c t o f pH on the R e a c t i o n The a c i d i t y o f the a l c o h o l i c s o l u t i o n s i n the s e e x p e r i m e n t s Slower t h a n i n d i c a t e d by the "pH", w h i c h was always c a l c u l a t e d as i f water were the s o l v e n t . k"exp d i d not v a r y s i g n i f i c a n t l y when the "pH" was l o w e r e d o v e r the range 4 . 0 2 down t o 3 . 1 5 > u s i n g f o r m i c a c i d -sodium formate b u f f e r e d s o l u t i o n s o f c o n s t a n t i o n i c s t r e n g t h , ^yU - 0 . 5 5 . The new b u f f e r system was chosen so as t o keep k e x p s m a , l 1 e n ° u g h t o measure o v e r a r e a s o n a b l e v a r i a t i o n i n "pH". B e n z i d i n e was formed when u n r e a c t e d hydrazobenzene r e a r r a n g e d i n the quenched s o l u t i o n s , but was never seen i n the r e a c t i o n m i x t u r e . S o l u t i o n s were c e n t r i f u g e d b e f o r e a n a l y z i n g f o r azobenzene. E x p e r i m e n t a l e r r o r r a n h i g h e r t h a n f o r o t h e r e x p e r i m e n t s . ( T a b l e 2 - 3 ) The f a c t t h a t k e x p i s a c i d independent i n d i c a t e s t h a t the mechanism i n a c i d s o l u t i o n does not i n v o l v e i o n i z a t i o n o f hydrazobenzene b e f o r e the r a t e - c o n t r o l l i n g s t e p , as suggested by Hinshelwood and B l a c k a d d e r ( 6 ) f o r the a u t o x i d a t i o n o f hydrazobenzene i n s t r o n g l y b a s i c " s o l u t i o n . 30 ^ 100 200 300 400 500 600 700 TIME - SECONDS FIGURE 2 - 4 . Typical,-, r a t e p l o t s s v a r y i n g i o d i d e c o n c e n t r a t i o n . Conditions i I = 15 C . , ' / b u f f e r = 0 . 3 1 , "•DII" = 3-99,75 volume per cent e t h a n o l , [ A H ] = 1.2^'x 1 0 ~ 3M.,[I 0 1 5.46 x 10~3M. 3 1 0 1 2 3 4 ^ 5 FIGURE 2-5. 1 / [ I ~ ] T - L.MOI. Typical p l o t : k v.s. r e c i p r o c a l totaliodide.Conditions: T = 15 C , buffer i o n i c strength = 0.3l," pH" = 3.99, 75 volume per cent ethanol,[AH„] = 1.20 x 10~3M.,[I ] = 5.46 x 10-3M. D D 1 3 2 Some e x p e r i m e n t s i n n e u t r a l and b a s i c s o l u t i o n were attem p t e d , but the r a t e was too f a s t t o measure by the p r e -sent t e c h n i q u e , w h i c h i n d i c a t e s t h a t the r e a c t i o n i s base c a t a l y z e d a t h i g h e r pH. E f f e c t o f the S o l v e n t C o m p o s i t i o n On d e c r e a s i n g the e t h a n o l p r o p o r t i o n o f the s o l v e n t f r om e i g h t y - f i v e t o f o r t y - f i v e volume p e r c e n t , k i n c r e a s e d a t f i r s t s l o w l y and t h e n more markedly. The i n c r e a s e i n k g x p amounted t o about f o u r t e e n f o l d o ver the range. (See F i g u r e 2 - 6 and Table 2 - 4 ) . The e f f e c t on k, and k 0 was found by p l o t t i n g k a g a i n s t l / [ I ] T ( T a b l e 2 - 5 ) f o r t h r e e s e t s o f ex p e r i m e n t s i n f i f t y , s i x t y and s e v e n t y - f i v e volume p e r cent e t h a n o l a t 15°C. ( F i g u r e 2 - 7 ) . T h e i r v a l u e s a r e l i s t e d i n Table 2 - 6 , u s i n g the v a l u e s o f K p r e v i o u s l y d e t e r m i n e d ( p. l i ) t o c a l c u l a t e k^. The l a r g e i n c r e a s e i n k 2 as the p r o p o r t i o n o f water i n the s o l v e n t i n c r e a s e s i s e v i d e n c e t h a t the t r i i o d i d e o x i d -a t i o n of hydrazobenzene has a b i m o l e c u l a r mechanism. A charged a c t i v a t e d complex i s v i s u a l i z e d w h i c h becomes p r o -g r e s s i v e l y more s t a b i l i z i e d as the p o l a r i t y o f the medium i n c r e a s e s . By s i m i l a r r e a s o n i n g the s m a l l e f f e c t on k^ i n d i c a t e s an uncharged a c t i v a t e d complex f o r the r e a c t i o n of hydrazobenzene w i t h i o d i n e . 3 3 TABLE 2 - 3 The independence of k _ _ n^uii exp on the pH "pH"* k , - 1 - 1 e x p , 1 . m o l . s e c . 4 . 0 2 0 . 6 2 3 . 9 3 0 . 7 6 3 . 7 5 O . 6 5 3 . 5 8 0 . 6 6 3 . 3 9 0.64 3 . 1 5 0 . 6 8 C o n d i t i o n s : [ I 2 ] T = 4.48 x 1 0 ~ 3 M. [AHg] = 1 . 2 0 x 1 0 ~ 3 M. [I"] = 0 . 2 5 M. 1 5 ° C , 6 0 Volume % e t h a n o l JUL B u f f e r = 0 . 3 0 ( F o r m i c A c i d - Sodium Formate) C a l c u l a t e d on the b a s i s o f a water s o l v e n t . 35 0 1 . 2 , 3 4 l / [ I ] T - L.MOL. FIGURE^ 2-7. k f o r three solvents,plotted against r e c i p r o c a l t o t a l iodide. ^Conditions: T = 15 C.,buffer i o n i c strength = 0. "pH" = 3-99 , i n i t i a l [AH-] = ( 1 . 2 0 x 10"3M. Solvent composition : volume % ethanol: • =50 O =60 _ =75 36 TABLE 2-4 The e f f e c t of s o l v e n t c o m p o s i t i o n on k exp Volume Per Cent E t h a n o l k j l . m o l . s e c . exp J 45 3.4 50 2.02 55 1.36 6o 0.984 65 0.775 70 0.610 75 0.472 80 0.342 85 0.248 C o n d i t i o n s : [ I 2 ] T = 5.46 x 10 3 M. [AHg] = 1.20 x 10~ 3 M. 15°C. [ I ] = 0 .25 M. "pH" = 3-99, B u f f e r = 0.31 V o l Jo e t h a n o l o n l y 3 7 TABLE 2 - 5 The e f f e c t o f v a r y i n g the I o d i d e c o n c e n t r a t i o n I n t h r e e s o l v e n t s Volume p e r cent [ A H p ] x l 0 3 M . [ I p ] _ x l 0 3 M . e t h a n o l . d 1 [ 1 ] M. k exp- .". _^ L m o l T 1 s e c . 5 0 1 . 2 0 5 . 4 6 0 . 2 5 2 . 0 2 0 . 3 7 5 1 . 4 8 0 . 5 1 . 1 9 0 . 6 2 5 1 . 0 4 0 . 7 5 0 . 9 0 9 4 . 4 8 0 . 2 5 1 . 0 2 7 . 0 6 0 . 2 5 0 . 9 9 7 6 . 1 8 0 . 2 5 1 . 0 0 4 . 4 8 0 . 3 7 5 0 . 7 2 4 7 . 0 6 0 . 3 7 5 0 . 7 3 2 4 . 4 8 0 . 5 0 . 5 4 6 7 . 0 6 0 . 5 0 . 5 6 4 4 . 4 8 0 . 6 2 5 0 . 4 8 6 6 . 1 5 0 . 6 2 5 0 . 4 6 8 7 . 0 6 0 . 6 2 5 0 . 4 7 1 4 . 4 8 0 . 7 5 0 . 4 3 2 6 . 1 5 0 . 7 5 0 . 4 1 8 7 . 0 6 0 . 7 5 0 . 4 1 0 7 5 1 . 2 0 5 . 4 6 0 . 1 5 0 . 7 7 5 0 . 2 5 0 . 4 7 2 0 . 2 5 0 . 4 6 7 0 , 3 7 5 0 . 3 2 6 0 . 5 0 . 2 5 1 0 . 6 2 5 0 . 1 9 5 0 . 7 5 0 . 1 7 2 C o n d i t i o n s : "pH = 3 - 9 9 J 1 B u f f e r = 0 . 3 1 1 5 ° C 3 8 TABLE 2 - 6 The v a r i a t i o n o f k^ and k^ w i t h the s o l v e n t Volume p e r cent e t h a n o l k 2 i i - l • - 1 l . m o l . s e c . k /K x - 1 s e c . * _ 4 K x 1 0 l . m o l . 1 k^ x 1 0 3 i - l - 1 .mol. s e c . 7 5 0 . 0 2 1 0 . 1 1 3 4 . 2 8 4 . 8 6 0 0 . 0 8 5 0.248 2 . 6 8 6 . 7 5 0 0 . 3 8 0 . 4 1 3 2 . 0 4 8 . 4 C o n d i t i o n s : y ^ B u f f e r 0 . 3 1 *pH" = 3 . 9 9 1 5 ° C I n t e r p o l a t e d f r om F i g u r e 1 - 2 , 3 9 E f f e c t of Temperature I n f i v e s e r i e s of e x p e r i m e n t s i n w h i c h the i o d i d e c o n c e n t r a t i o n was v a r i e d , p l o t s o f k a g a i n s t l / [ l ]m were obtained, a t f i v e t e m p e r a t u r e s between 0°C. and 2 5 ° C . ( T a b l e 2 - 7 , F i g u r e 2 - 8 ) . The p l o t s y i e l d e d v a l u e s f o r k^/K and kg l i s t e d i n . T a b l e 2 - 8 , where v a l u e s f o r k^ are a l s o l i s t e d . To c a l c u l a t e k^, a c o r r e c t i o n f a c t o r was n e c e s s a r y t o the v a l u e s o f K a t d i f f e r e n t t e m p e r a t u r e s i n t e r p o l a t e d f rom F i g u r e 1 - 3 . These v a l u e s were o b t a i n e d i n s o l u t i o n s o f h a l f the b u f f e r i o n i c s t r e n g t h o f the r e a c t i o n m i x t u r e s , w h i l e the v a l u e s o f K i n d i f f e r e n t s o l v e n t s ( F i g u r e . 1 - 2 ) were mea-sur e d i n s o l u t i o n s o f the p r o p e r b u f f e r c o n c e n t r a t i o n . As i t was assumed t h a t A H° o f t r i i o d i d e f o r m a t i o n was independ-ent o f i o n i c s t r e n g t h ( p . 8 ) t h e n the e f f e c t a t 1 5 ° C was t a k e n t o a p p l y o ver the range o f temperature studied.. That i s , t o determine K i n the r e a c t i o n m i x t u r e s , the v a l u e s i n t e r -p o l a t e d f rom F i g u r e 1 - 3 were m u l t i p l i e d by O . 8 7 . F i g u r e 2 - 9 shows l o g ^ C k-^/K] and l a g - ^ k g p l o t t e d a g a i n s t the r e c i p r o c a l a b s o l u t e t e m p e r a t u r e . S i n c e A H° f o r K was known t o be - 9 . 5 + 0 . 5 k c a L m o l . 1 (p. 9 ) i t was p o s s i b l e t o c a l c u l a t e the a c t i v a t i o n e n e r g i e s and p r e -e x p o n e n t i a l f a c t o r s f o r b o t h k^ and kg. The d a t a f o r k-^  are q u i t e u n c e r t a i n , no doubt because the e x p e r i m e n t a l techniqueowas under c o n s i d e r a b l e s t r a i n . I n 6 0 volume p e r cent e t h a n o l , A-Quffer = ° - 3 1 , 40 0 1 _ 2 3 4 1/.[I ] T - L. MOL. FIGURE 2-8 (A)« k versus r e c i p r o c a l t o t a l iodide concentration at f i v e temperatures i n 60 volume per cent ethanol.Conditions : buffe r i o n i c strength = n0 .31 , "pH" = 3.99 . A Temperatures : A = 25 C. • = 20°C. O = 15 C. 41 4 2 k x = 4 . 8 x 1 0 1 2 e " 1 1 ^ 0 0 ^ l . m o l ^ s e c T 1 k 2 = 2 . 3 x 1 0 1 2 e " 1 ^ 8 0 0 / ^ l . m o l ^ s e c . " 1 A p p l i c a t i o n o f the s t a n d a r d e q u a t i o n s o f the A b s o l u t e R e a c t i o n Rate t h e o r y r e s u l t e d i n h e a t s o f a c t i v a t i o n , AH of 1 1 . 0 + 2 . 0 kcal.mol." 1" f o r the i o d i n e o x i d a t i o n o f h y d r a z o -benzene and 1 7 . 2 + 0 . 4 k c a l . m o l . 1 f o r the t r i i o d i d e o x i -d a t i o n a t 2 8 8 ° K . The c o r r e s p o n d i n g e n t r o p i e s o f a c t i v a t i o n , A S + were - 2 . 6 + 7 - 0 e.u. and - 4 . 0 + 1 . 4 e.u. r e s p e c t i v e l y , a t 2 8 8 ° K . k x = [ k T / h J e ^ ' ^ e - H ^ ^ / ^ l . m o l ^ s e c : 1 k, = [ k T / h l e - 4 - 0 / ^ - 1 ^ ^ / ^ . ^ ! ; ^ ^ ; ! The s m a l l n e g a t i v e e n t r o p i e s of a c t i v a t i o n s u p p o r t the i d e a s a l r e a d y put f o r w a r d on the mechanisms of the two r e a c t i o n s . I n b o t h cases the v a l u e s o f AS* i n d i c a t e t h a t t h e r e i s no s u b s t a n t i a l change i n t o t a l charge o r t o t a l s o l v a t i o n i n p r o c e e d i n g from r e a c t a n t s t o a c t i v a t e d complex. The magnitudes o f the e n t r o p i e s o f a c t i v a t i o n f o r b o t h k^ and k 2 a r e normal f o r t h i s type o f r a t e - c o n t r o l l i n g s t e p , a l t h o u g h t h a t f o r k^ i s u n c e r t a i n . (See F i g u r e 2 - 9 ) . I n f l u e n c e o f the B u f f e r I o n i c S t r e n g t h on k exp At a c o n s t a n t i o d i d e c o n c e n t r a t i o n o f 0 . 2 5 M., the t o t a l i o n i c s t r e n g t h was i n c r e a s e d from 0 . 4 0 t o 0 . 7 1 by i n c r e a s i n g the b u f f e r c o n t r i b u t i o n from 0 . 1 5 t o 0 . 4 6 , the b u f f e r b e i n g a c e t i c a c i d - p o t a s s i u m a c e t a t e , k was TABLE 2 - 7 The e f f e c t o f v a r y i n g the i o d i d e c o n c e n t r a t i o n a t ^  s e v e r a l t e m p e r a t u r e s i n 60 Volume p e r cent e t h a n o l "PH" = 3 . 9 9 ^ B u f f e r = 0 . 3 1 k Temperature M. [ A H 2 ] x 1 0 3M. [ I 2 ] T x 1 0 3 M exp - 1 - 1 °C l . m o l . s e c . 2 5 0 . 2 5 1 . 2 0 3 . 3 3 4 . 2 6 0 . 2 9 1 3 . 6 8 0 . 3 7 5 2 . 8 6 0 . 5 0 2 2 . 2 3 0 . 6 2 5 1 . 8 6 • 0 . 7 5 1 . 5 9 2 0 0 . 2 5 1 . 2 0 3 . 7 3 2 . 9 8 0 . 2 9 2 2.40 0 . 3 7 5 1 . 9 2 0 . 5 1.46 O . 6 2 5 1 . 2 1 0 . 7 5 1 . 0 2 1 0 O . 2 5 1.40 5 . 9 0 0 . 6 1 4 0 . 3 7 5 1 . 2 0 5 . 9 0 0 . 4 3 4 0 . 5 1.40 5 . 9 0 0 . 3 5 7 O . 6 2 5 1.40 8 . 8 5 0 . 2 3 9 0 . 7 5 1.40 8 . 8 5 0 . 2 3 9 0 0 . 2 5 2 . 4 0 1 0 . 0 7 0.214 0 . 2 9 4 0 . 1 7 7 0 . 3 7 5 0 . 1 5 3 0 . 5 0 . 1 1 4 0 . 6 2 5 O . 0 9 6 0 . 7 5 O . 0 8 7 *See T a b l e 2 - 5 f o r d a t a a t 1 5 ° C . 4 4 TABLE 2 - 8 The v a r i a t i o n i n 6 0 Volume % e t h a n o l o f k^ and k^ w i t h t e m p e r a t u r e _ _ = 0 . 3 1 B u f f e r Temperature °C. 1 .mol. s e c . * 1 - 1 s e c . K x l O c o r r . l . m o l . " 4 - 3 k x 1 0 3 l . m o l . s e c . 2 5 0 . 2 3 1 . 0 0 1 . 5 7 1 6 2 0 0 . 1 4 O . 6 5 8 2 . 0 3 1 3 1 5 0 . 0 8 5 0 . 2 4 8 2 . 6 9 6 . 7 1 0 0 . 0 4 8 0 . 1 4 2 3 . 6 0 5 . 1 0 0 . 0 1 5 0 . 0 5 0 6 . 5 9 3 . 3 See page 3 9 . 4 5 was a p p r o x i m a t e l y d o u b l e d . ( T a b l e 2 - 9 ) The s e p a r a t e e f f e c t s on k^/K and kg are seen i n F i g u r e 2 - 1 0 , where p l o t s o f k v e r s u s 1 / [ I ]m a r e shown a t two b u f f e r i o n i c s t r e n g t h s o f 0 . 4 6 and 0 . 3 1 . The e f f e c t , i f any, o f chan g i n g the p o t a s s i u m i o d i d e i o n i c s t r e n g t h over each s e r i e s i s assumed, f o r t h e sake o f comparison, t o be the same f o r b o t h . At b u f f e r i o n i c s t r e n g t h o f 0 . 4 6 , k^/K = 0 . 2 9 9 s e c ? 1 and kg = 0 . 1 3 5 1 .mol ."^sec. 1, and a t b u f f e r i o n i c s t r e n g t h 0 . 3 1 , = 0 . 2 4 8 s e c . - 1 and kg = O..O85 1 .mol. "'"sec. 1. The d e t a i l e d s i g n i f i c a n c e o f thes e r e s u l t s must remain open as no ex p e r i m e n t s were done w i t h an i n e r t e l e c t r o -l y t e such as KNO^ ., w h i c h might have i n d i c a t e d i f the i o n i c s t r e n g t h o f p o t a s s i u m i o d i d e a f f e c t e d the r a t e . I n view o f the h i g h i o d i d e c o n c e n t r a t i o n s r e q u i r e d i n t h i s work, t h e r e was no good way o f k e e p i n g the i o n i c s t r e n g t h c o n s t a n t when the i o d i d e c o n c e n t r a t i o n was varied.. The p o t a s s i u m i o d i d e and b u f f e r c o n c e n t r a t i o n s were always f a r i n ex c e s s o f those o f the r e a c t a n t s . The r e s u l t s , however, suggest t h a t any i o n i c s t r e n g t h e f f e c t due t o i o d i d e i s not l a r g e , as e x c e l l e n t p l o t s o f k g X p a g a i n s t l / [ K I ] were o b t a i n e d over a wide range of c o n d i t i o n s , and the v a l u e s of kg produced a good A r r h e n i u s p l o t . •Thus kg has been shown t o be s t r o n g l y a f f e c t e d by the b u f f e r i o n i c s t r e n g t h , w h i l e the e f f e c t on k-j/K i s s m a l l . The e f f e c t on k^ may w e l l be n e g l i g i b l e , as K has been shown t o i n c r e a s e s l i g h t l y w i t h i n c r e a s i n g b u f f e r i o n i c s t r e n g t h . These r e s u l t s a r e i n t e r p r e t e d t o sup p o r t the proposed b i m o l e c u l a r mechanisms f o r b o t h the i o d i n e and t r i i o d i d e o x i d a t i o n of h y d r a -zobenzene . 1 . 2 o i —i i • o 0.8 P 4 CD 0 . 4 0 0 1 / [ I " ] T _L.M0L -1 versus r e c i p r o c a l t o t a l iodide concentration,for two - - 4 . 1 . _ n ~ » * 4 + i ™ e , . T = i5 UC.,"pH" = 3«99 ,60 volume F I G U R E 2-10. k buffer i o n i c e x p s t r e n g t h s . C o n d i t i o n s . -per cent ethanol. Buffer i o n i c strength : V = ° ' 4 6 ' U " " ' ^ * 4 7 TABLE 2 - 9 The e f f e c t of I o n i c s t r e n g t h [ A H 2 ] x l 0 3 M . [ l 2 ] T x l 0 3 M . [I ] M. y ^ t o t a l / 1 b u f f e r k* ' exp - i - l ' - 1 l . m o l r s e c . 1 . 2 0 6 . 3 5 0 . 2 5 0 . 4 0 0 . 1 5 O . 6 5 8 0 . 9 H 5 . 4 6 0 . 2 5 O . 5 6 0 . 3 1 0 . 9 9 6 1 . 2 0 6 . 3 5 0 . 2 5 0 . 7 1 0 . 4 6 1 . 1 3 1 . 2 0 7 . 0 9 0 . 3 7 5 0 . 8 3 5 0 . 4 6 0 . 7 8 6 1 . 2 0 7 . 0 9 0 . 5 O . 9 6 0 . 4 6 0 . 6 4 0 1 . 2 0 7 - 0 9 O . 6 2 5 I . 0 8 5 0 . 4 6 0 . 5 2 8 1 . 2 0 7 . 0 9 0 . 7 5 8 1 . 2 2 0 . 4 6 0 . 4 6 2 C o n d i t i o n s : 6 0 Volume % e t h a n o l , 1 5 C. "pH" = 3 . 9 9 Note: See Table 2 - 5 f o r d a t a w i t h v a r y i n g [I ]^ a t ^ B u f f e r = ° ' 3 1 48 E f f e c t o f Quinone and M e t a l S a l t s on the Rate S m a l l c o n c e n t r a t i o n s o f quinone i n the r e a c t i o n m i x t u r e had no e f f e c t on k , but . 0 0 0 1 M. c o b a l t o u s p e r c h l o r -exp a t e and . 0 0 0 1 M. f e r r o u s s u l f a t e d e p r e s s e d the r a t e about t e n p e r cent and twenty p e r cent r e s p e c t i v e l y . I n c r e a s i n g the c o b a l t ( i i ) c o n c e n t r a t i o n f i v e f o l d d i d n o t , however, produce any f u r t h e r r a t e d e c r e a s e . ( T a b l e 2 - 1 0 ) The a d d i t i o n o f . 0 0 0 1 M. c o n c e n t r a t i o n s o f m e r c u r i c a c e t a t e , u r a n y l a c e t a t e and chromic p e r c h l o r a t e had no e f f e c t on k . P r o b a b l y because the m e r c u r i c i o n c o n c e n t r a t i o n was exp ° r e d u c e d t o an i n s i g n i f i c a n t l y s m a l l v a l u e by the f o r m a t i o n of complex a n i o n s l i k e Hgl7| , a c a t a l y t i c c y c l e c o u l d not be ++ + o b s e r v e d , a l t h o u g h e t h a n o l i c s o l u t i o n s o f Hg and Ag were q u a l i t a t i v e l y o b s e r v e d t o be i m m e d i a t e l y reduced t o the f r e e m e t a l on the a d d i t i o n o f a s o l u t i o n of hydrazobenzene. On the h y p o t h e s i s t h a t the o x i d a t i o n of hydrazobenzene by i o d i n e c o u l d o c c u r by a f r e e r a d i c a l mechanism, one would expect the a d d i n g o f a good r a d i c a l scavenger l i k e quinone ( 2 4 , 2 5 ) o r c a t a l y s t l i k e c o b a l t ( i i ) o r i r o n ( I I ) ( 2 6 ) t o a f f e c t the r a t e d r a s t i c a l l y ; f o r by c a l c u l a t i o n about n i n e t y p e r cent of the measured r a t e can be a s c r i b e d t o the i o d i n e r e a c t i o n w i t h hydrazobenzene, and o n l y t e n p e r cent t o the t r i i o d i d e r e a c t i o n . That quinone was i n e f f e c t i v e i s i n c o n -c l u s i v e . Hydrazobenzene has been used as an a n t i o x i d a n t ( 2 7 , 2 8 ) and has been shown t o be r e a d i l y a t t a c k e d by the r a d i c a l s d i p h e n y l p i c r y l h y d r a z y l ( 1 0 ) and t r i p h e n y l m e t h y l ( l l ) , so i t may be a more e f f i c i e n t r a d i c a l scavenger t h a n quinone. The 49 i n e f f e c t i v e n e s s of i r o n ( i i ) and c o b a l t ( I I ) as c a t a l y s t s i s more c o n c l u s i v e e v i d e n c e of a s i m p l e b i m o l e c u l a r mechanism, as the f o l l o w i n g proposed c a t a l y t i c c y c l e d i d not o c c u r . I t i s based on the well-known r e a c t i o n o f these i o n s w i t h o r g a n i c p e r o x i d e s (26) p r o d u c i n g RO* r a d i c a l s . F e + + + I - > Fe +" H~ + 1* + i " F e " ^ + AH 2 > Fe**' + AH* + H + That no c a t a l y s i s o c c u r r e d s u p p o r t s the b i m o l e c u l a r mechanism f o r the a t t a c k o f i o d i n e on hydrazobenzene. That k was independent o f b o t h the t o t a l i o d i n e -and h y d r a z o -exp ^ benzene c o n c e n t r a t i o n s , as w e l l as the f a c t t h a t good second o r d e r k i n e t i c s were obse r v e d t o o v e r n i n e t y p e r cent r e a c t i o n , a l s o s u p p o r t s t h i s mechanism. 5 0 TABLE 2-10 The e f f e c t on k o f a d d i n g quinone and m e t a l s a l t s [ A H 2 ] x l 0 3M. [ I 2 ] T X 1 0 3 M . Added 1 k exp .mol. s e c . 0.9H 5 .46 - 0.996 1.52 - 0.995 2 . 66 - 0.997 1 . 8 0 8.47 .0001 M.Co [C10 i | ] 2 0.912 • 1.80 .0005 M. C o [ C 1 0 4 ] 2 0.900 1.80 5.65 .0001 M. Quinone 0.989 1.80 .0002 M. Quinone 1.04 1 . 2 0 5.32 .0001 M. FeSO^ 0.815 1.20 .0001 M. U 0 2 [ A c ] 2 0.984 1 1 . 2 0 .0001 M. C r [ C 1 0 4 ] 3 0.996 1.20 .0001 M. H g [ A c ] 2 O.982 C o n d i t i o n s : [I ] = 0 . 2 5 M. 6 0 Volume fo e t h a n o l , 1 5 ° C . "PH" = 3 . 9 9 Jk B u f f e r = 0 . 3 1 51 CONCLUSIONS The e x p e r i m e n t a l e v i d e n c e s t r o n g l y s u g gests t h a t b o t h the i o d i n e and t r i i o d i d e r e a c t i o n s w i t h hydrazobenzene have s i m p l e b i m o l e c u l a r mechanisms. Each obeys a b i m o l e c u l a r r a t e law and each has a t y p i c a l b i m o l e c u l a r e n t r o p y of a c t i v a t i o n f o r the type o f r e a c t i o n i n which the charge on the a c t i v a t e d complex i s the t o t a l o f the r e a c t a n t c h a r g e s . A f r e e r a d i c a l mechanism f o r the i o d i n e hydrazobenzene r e a c t i o n has not been d i s p r o v e d , but has been shown t o be u n l i k e l y . None of the e x p e r i m e n t a l e v i d e n c e s u g g e s t s t h a t f r e e r a d i c a l s a r e i n v o l v e d . I o d i n e i s a p p r o x i m a t e l y 100,000 tim e s more r e a c t i v e a t 15°C. t h a n t r i i o d i d e as an o x i d a n t o f hydrazobenzene ( T a b l e 2-9), the d i f f e r e n c e i n r e a c t i v i t i e s b e i n g almost e n t i r e l y due t o the d i f f e r e n c e i n a c t i v a t i o n e n e r g i e s f o r the two r e a c t i o n s . I t i s r e a s o n a b l e t o propose f o r the r e a c t i o n between an i o d i n e m o l e c u l e and one o f hydrazobenzene an a c t i v a t e d complex t h a t resembles I i I t i s s i g n i f i c a n t t h a t the a c t i v a t i o n energy of 11.0 k c a l . m o l . 1 f o r the i o d i n e r e a c t i o n i s v e r y low i n d e e d f o r a r e a c t i o n i n w h i c h t h r e e c o v a l e n t bonds must be broken and t h r e e 5 2 new bonds formed, even though the o r i g i n a l N-H bonds are not s t r o n g . I t su g g e s t s t h a t a s u b s t a n t i a l p o r t i o n o f the resonance energy g a i n e d i n the f o r m a t i o n o f azobenzene appears i n the a c t i v a t e d complex and s t a b i l i z e s i t . Though c a l c u l a t i o n s o f the resonance e n e r g i e s o f trans-azobenzene and hydrazobenzene have not been made, i t i s r e a s o n a b l e t o compare the two com-pounds, t r a n s - s t i l b e n e and 1 , 2 d i p h e n y l e t h a n e w h i c h have resonance e n e r g i e s o f 7 8 . 0 and 7 1 . 2 k c a l . m o l . 1 r e s p e c t i v e l y ( 3 0 ) . I t i s l i k e l y t h a t the d o n a t i o n o f the n i t r o g e n " l o n e p a i r s " t o the c o n j u g a t e d IT" e l e c t r o n system i n azobenzene produces as much s t a b i l i z a t i o n ( o v e r and above the resonance energy i n the c o n j u g a t e d system) as the c o n t r i b u t i o n o f the f o l l o w i n g k i n d o f r e s o n a t i n g s t r u c t u r e s i n hydrazobenzene. The c o n c l u s i o n i s t h a t the resonance energy o f t r a n s -azobenzene i s i n e x c e s s over t h a t o f hydrazobenzene by about - 1 ' 7 k c a l . m o l . , and s u p p o r t s the suggested s t a b i l i z a t i o n o f the a c t i v a t e d complex. The a c t i v a t e d complex r e s u l t i n g f r om a t t a c k o f the l i n e a r t r i i o d i d e i o n on hydrazobenzene i s v i s u a l i z e d t o be 53 The h i g h e r a c t i v a t i o n energy of 1 7 . 2 k c a l . m o l . 1 may be a t t r i b u t e d t o two p o s s i b i l i t i e s , v i e w i n g t r i i o d i d e as i o d i n e c o o r d i n a t e d by i o d i d e . 1 . The c o o r d i n a t i o n o f the e x t r a i o d i d e i o n d e c r e a s e s the a v a i l a b i l i t y o f the o r b i t a l s o f i o d i n e f o r bonding w i t h the hydrogen atoms of hydrazobenzene. T h i s i m p l i e s a more weakl y bound a c t i v a t e d complex, hence one of h i g h e r energy. 2 . The Ig-I® bond i s c o n s i d e r a b l y s t r e t c h e d d u r i n g the f o r m a t i o n of the a c t i v a t e d complex, w i t h a c o n s i d e r a b l e l o c a l i z a t i o n of charge away from the r e a c t i o n c e n t e r , as i m p l i e d i n the diagram. The e x t r a 6 . 2 k c a l . m o l . 1 o f a c t i v a t i o n energy o v e r t h a t o f the i o d i n e r e a c t i o n i s a s u b s t a n t i a l p a r t of the energy r e q u i r e d t o remove i o d i d e c o m p l e t e l y from i o d i n e . (The heat of d i s s o c i a -t i o n o f t r i i o d i d e was d e t e r m i n e d i n P a r t I t o be 9 . 5 k c a l . m o l . 1 ) I t i s not thought t h a t t r i i o d i d e s h o u l d have t o o v e r -come s i g n i f i c a n t l y more s t e r i c h i n d r a n c e by the benzene r i n g s t h a n i o d i n e i n f o r m i n g the a c t i v a t e d complex. Any such e f f e c t i s p r o b a b l y s m a l l because the N-bonded hydrogen atoms are e a s i l y a c c e s s i b l e f o r r e a c t i o n . APPENDIX THE OXIDATION IN SOLUTION OP HYDRAZOBENZENE BY MOLECULAR OXYGEN INTRODUCTION As p o i n t e d out i n the i n t r o d u c t i o n some e a r l i e r work was an i n v e s t i g a t i o n o f the o x i d a t i o n o f hydrazobenzene by m o l e c u l a r oxygen i n s o l u t i o n . The s t u d y was abandoned when a t h e s i s (7) appeared d e s c r i b i n g a f a i r l y e x t e n s i v e i n v e s t i -g a t i o n of the r e a c t i o n . However i t seems u s e f u l t o g i v e a g e n e r a l account o f the work done and the r e l a t i o n o f the r e s u l t s o b t a i n e d t o those of o t h e r workers ( 5 ^ 7 ^ 3 1 ) . The s t o i c h i o m e t r y o f the r e a c t i o n was r e p o r t e d by Walton and F i l s o n ( 5 ) as AH 2 + 0 2 > A + H 2 0 2 T h i s was c o n f i r m e d by Zubyk (7) but not i n the p r e -sent work, where the amount o f H 2 0 2 was always l e s s t h a n s t o i c h i o m e t r i c , i n d i c a t i n g t h a t some of the p e r o x i d e formed e i t h e r decomposed o r was f u r t h e r reduced by hydrazobenzene. A c e t o n i t r i l e - w a t e r was chosen as the s o l v e n t . The r e a c t i o n was c a r r i e d out by b u b b l i n g oxygen a t 1 Atmosphere p r e s s u r e and p r e s a t u r a t e d w i t h s o l v e n t t h r o u g h a s o l u t i o n of hydrazobenzene s h i e l d e d from the l i g h t . (The absence o f l i g h t was n e c e s s a r y t o p r e v e n t the f o r m a t i o n o f ci.s-Azobenzene ( 2 l ) whose'spectrum d i f f e r s somewhat f r o m the t r a n s f orm ( 2 2 , 2 3 ) . ) F o r m a t i o n of azobenzene was det e r m i n e d s p e c t r o p h o t o m e t r i c a l l y u s i n g the 440 ~"yi band, and p e r o x i d e was a l s o d e t e r m i n e d s p e c t r o p h o t o m e t r i c a l l y u s i n g the y e l l o w complex ( A max, 410iy 5 5 w h i c h i t forms w i t h T i t a n i u m ( I V ) i n .concentrated s u l f u r i c a c i d ( 3 2 ) . The e x p e r i m e n t a l method comprised f i r s t a n a l y z i n g the s o l u t i o n f o r azobenzene a t 4 2 0 t y * . To a n o t h e r sample o f the s o l u t i o n was added a q u a n t i t y o f T i g t S O j ^ r e a g e n t ( 3 3 , 3 4 ) , and a f t e r c e n t r i f u g a t i o n (-to remove rearrangement p r o d u c t s o f hydrazobenzene, b e n z i d i n e and. d i p h e n y l i n e ) i t s absorbance was measured a t two w a v e l e n g t h s , and the p e r o x i d e c o n c e n t r a t i o n c a l c u l a t e d by d i f f e r e n c e . I t was e s t a b l i s h e d t h a t b o t h a z o -benzene and the p e r o x i d e - T l ( I V ) complex obeyed the B e e r -Lambert law. RESULTS T y p i c a l r a t e p l o t s f o r azobenzene and hydrogen p e r o x -i d e f o r m a t i o n a r e shown i n F i g u r e 3 - 1 and are seen t o be l i n e a r i n the e a r l y s t a g e s . The r e a c t i o n i s f i r s t o r d e r i n hydrazobenzene, as may be seen i n F i g u r e 3 - 2 i n w h i c h the s l o p e dA/dt v e r s u s [AH 2 f] g i v e s a p s e u d o - f i r s t o r d e r r a t e c o n s t a n t of 1 6 . 3 x 1 0 4 min 1 a t 2 5 ° C . i n 8 0 mole p e r cent a c e t o n i t r i l e . (A v a l u e of 1 4 . 8 x 1 0 4 min 1 was found i n 5 0 mole p e r cent a c e . b o n i t r i l e ) . Table 3 - 1 compares these r e s u l t s w i t h t h o s e o f o t h e r w o r k e r s . dHgOg/dt was always found t o l e s s t h a n dA/dt by a r o u g h l y c o n s t a n t f a c t o r 0 . 7 4 and the r a t i o o f the f i n a l p r o d u c t c o n c e n t r a t i o n s , [UpO^J/tA], had a mean v a l u e o f 0 . 7 3 i n 8 0 mol % a c e t o n i t r i l e a t 2 5 ° C . T h i s e f f e c t was n o t e d by S o u k o r e f f ( 3 1 ) 57 FIGURE 3-2.First order dependence of dA/dt (open points) and dH 20p/dt (shaded points) on the i n i t i a l [AH?]'„ Conditions : T = 25 C , 0- pressure = 1 atm., 80 raol.% o A n -f r \ n i ' + v i - i 1 Q 5 8 i n 5 0 mol % e t h a n o l , ( [ H - 0 2 ] / [ A ] 0.7), and i m p l i e s t h a t one o r b o t h o f the f o l l o w i n g r e a c t i o n s must o c c u r : H 2 0 2 + AH 2 2 H 2 0 + A H 2 ° 2 •* H 2 ° + ^ ° 2 I t was not p o s s i b l e u s i n g our t e c h n i q u e t o d i f f e r e n -t i a t e between th e s e two. I t was d e f i n i t e l y shown, however, t h a t the second r e a c t i o n d i d o c c u r . S o l u t i o n s o f p e r o x i d e de-composed r a p i d l y under the e x p e r i m e n t a l c o n d i t i o n s , and i n i t i a l p e r o x i d e added t o the r e a c t i o n m i x t u r e decayed f a s t e r t h a n azobenzene b u i l t up. (See F i g u r e 3 - 3 ) . I t was not c l e a r why the i n i t i a l decay o f added p e r o x i d e was f o l l o w e d by a subsequent b u i l d - u p and d e c o m p o s i t i o n . The most l i k e l y e x p l a n -a t i o n i s t h a t the i n i t i a l decay was caused by some c a t a l y t i c i m p u r i t y . Zubyk ( 7 ) r e p o r t e d t h a t no r e a c t i o n o c c u r r e d between HgOg and hydrazobenzene i n dim e t h y l f o r m a m i d e , and by u s i n g t h i s s o l v e n t a v o i d e d many o f the c o m p l i c a t i o n s e n c o u n t e r e d i n o u r work. C a t a l y s i s by Sodium Hy d r o x i d e A d d i t i o n o f sodium h y d r o x i d e produced a l a r g e i n c r e a s e i n t he r a t e . T y p i c a l r a t e p l o t s f o r base c a t a l y z e d f o r m a t i o n o f azobenzene a r e shown i n F i g u r e 3 - 4 . The r a t e was found 1. t o v a r y as [NaOH] 2, a r e s u l t s u b s t a n t i a t e d by Zubyk. F i g u r e 1. 3 t o 5 shows a p l o t o f dA/dt v e r s u s [NaOH] 2. The f a c t t h a t the 59 0 10 20 30 TIME - MINUTES FIGURE 3~3 •» Formation of azobenzene (open points) and Hp0 9(shaded p o i n t s ) i n basic s o l u t i o n : added i n i t i a l [H 90 1.44Qx 10 J M . , i n i t i a l [AHp] = 3.51 x 10 Conditions I = 25 C , 0 9 pressure = 1 a tin., 50 mol.fi a c e t o n i t r i l e . [NaOH] : Q = .006 M. A= . 0 0 5 M. 60 0' 5 10 15 20 25 TIME - MINUTES F I G U R E 3~4. T y p i c a l r a t e p l o t s ; "base c a t a l y z e d f o r m a t i o n o f azobenzene (open p o i n t s ) a n d H „ 0 p (shaded p o i n t s ) . C o n d i t i o n s : T = 25 C , 0^ p r e s s u r e = 1 atm . , 5 0 m o l . % a c e t o n i t r i l e , i n i t i a l [ A H ?] = 3*51 x 10~3 M. [ NaOH ], M. s O = .004"M.f _= .002 M. 0 1 i 2 i _ i 3 [NaOH] 2 - MOL? I . 2 i FIGURE 3-5. Rate of f o r m a t i o n o f azobenzene v e r s u s [ N a O H ] a . C o n d i t i o n s 0 ? p r e s s u r e = 1 atm.,T = 25°C.,50 M o l . % a c e t o n i t r i l e , / < = .01 , i n i t i a l h y r a z o b e n z e n e c o n c e n t r a t i o n = 3.51 x 10 3 M. p l o t does not pass t h r o u g h the o r i g i n s u g g e s t s t h a t t h e r e are two independent p a t h s , one base independent. Of i n t e r e s t i s t h a t Zubyk found a f i r s t o r d e r r a t e dependence on sodium e t h o x i d e . E f f e c t o f S o l v e n t V a r i a t i o n o f the s o l v e n t f rom 2 5 t o 1 0 0 mole p e r cent a c e t o n i t r i l e produced a r a t e i n c r e a s e o f about f i f t y p e r c e n t . F i g u r e 3 - 6 shows a p l o t o f dA/dt a g a i n s t mol p e r cent a c e t o -n i t r i l e . The s m a l l s o l v e n t e f f e c t s u g g e s t s a n o n - i o n i c r a t e c o n t r o l l i n g s t e p I n the u n c a t a l y z e d r e a c t i o n . Other O b s e r v a t i o n s N e i t h e r exposure t o o r d i n a r y l i g h t n o r a d d i t i o n o f an i n e r t e l e c t r o l y t e [ 0 . 0 1 M. NaClO^] produced s i g n i f i c a n t changes i n the r a t e . DISCUSSION; THE PRESENT STATE OF THE PROBLEM At the p r e s e n t time t h e r e remains much work t o be done i n f i n d i n g out the r e a l k i n e t i c s and mechanism o f the r e a c t i o n i n t h i s system, e s p e c i a l l y i n r e s o l v i n g the d i f f i c u l t i e s o v e r the n o n - s t o i c h i o m e t r i c p r o d u c t i o n o f hydrogen p e r o x i d e . i t i s w o r t h m e n t i o n i n g f e a t u r e s o f the r e a c t i o n brought t o l i g h t by v a r i o u s w o r k e r s . A l l have found a f i r s t o r d e r dependence o f the r a t e on the hydrazobenzene c o n c e n t r a -t i o n . Zubyk found a f i r s t o r d e r dependence on the p a r t i a l 6 3 TABLE 3 - 1 Pseudo f i r s t o r d e r r a t e c o n s t a n t s f o r the a u t o x i d a t i o n of hydrazobenzene Summary of the r e s u l t s o f s e v e r a l w o r kers Oxygen p r e s sure assumed 1 atmosphere S o l v e n t T°C Pseudo 1s t ' r a t e c o n s t minT 1 x 1 0 o r d e r ant Method R e f e r e n c e i - p r o p a n o l 3 0 4 . 2 C o l o r i m e t r i c 5 e t h a n o l 3 . 4 9 5 ^ . e t h a n o l 1 1 5 0 mol.% e t h a n o l 3 0 2 0 C o l o r i m e t r i c 3 1 dimethyIformamide 3 8 3 5 2 5 5 5 4 4 2 6 Gasometric 7 e t h a n o l 3 5 2 1 -5 5 raol.% e t h a n o l 3 5 1 5 - 9 8 0 rn.o1.fo a c e t o n i t r i l e 5 0 m o l a c e t o n i t r i l e 2 5 2 5 1 6 . 3 1 4 . 8 C o l o r i m e t r i c T h i s work 6 5 p r e s s u r e o f oxygen, and no a p p r e c i a b l e o x i d a t i o n of AH"2 by p e r o x i d e i n d i m e t h y l f o r m a m l d e . However S o u k o r e f f ( 3 l ) found the o r d e r w i t h r e s p e c t t o oxygen somewhat l e s s t h a n 1 , but a l s o f o u n d n o n - s t o i c h i o m e t r i c p e r o x i d e f o r m a t i o n . In' h i g h l y b a s i c s o l u t i o n , B l a c k a d d e r and Hinshelwood ( 6 ) found the r a t e independ-ent of the oxygen p a r t i a l p r e s s u r e above a s m a l l c o n c e n t r a t i o n , and Zubyk ( 7 ) a l s o n o t e d an i n h i b i t i o n o f the r a t e a t h i g h pH. The i n v e r s e dependence of the r a t e on the aqueous component o f the s o l v e n t f o u n d by us i s i n agreement w i t h Zubyk but con-t r a r y t o W a l ton and E i l s o n . ( 5 ) ( T a b l e 3 - 1 ) . The e f f e c t may be e x p l a i n e d by the h i g h e r s o l u b i l i t y i n g e n e r a l o f oxygen i n o r g a n i c media t h a n i n water. Other i n t e r e s t i n g r e s u l t s found by Zubyk (7) i n c l u d e a d e p r e s s i o n o f the r a t e by about a h a l f u s i n g N , N 1 d i d e u t e r o -hydrazobenzene and changes i n t h e r a t e by s u b s t i t u t i o n i n the benzene r i n g s o f hydrazobenzene. E l e c t r o n w i t h d r a w i n g groups i n the p-p 1 p o s i t i o n s de.creased the r a t e i n the o r d e r o f t h e i r e l e c t r o n e g a t i v i t i e s , N0 2 >^ CI >^ B r . There was a r a t e i n c r e a s e w i t h e l e c t r o n r e p e l l i n g m e t h y l groups i n the p-p' p o s i t i o n s . The complex base dependence found by us and Zubyk ( 2 9 ) h i n t a t c o m p l i c a t e d mechanisms f o r t h e s e p r o c e s s e s . C a t a l y s i s by m e t a l i o n s i n s o l u t i o n s of pH 1 3 has been r e p o r t e d by B l a c k a d d e r and Hinshelwood ( 2 9 ) . 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