ELECTROCHEMICALLY COMMUNICATION PROCESS

AND

GENERATED 5. ACTIVATION SUCCESSIVE

A. V. II'yasov, Ya. A. Levin,

FREE

ENERGY

CHEMICAL

OF

THE

RADICALS CURRENT-DETERMINING

REACTIONS

UDC 541.13 + 541.515

Yu. M. Kargin, and B. V. Mel'nikov

The m e t h o d s of studying the m e c h a n i s m and d i a g n o s i n g v a r i o u s s c h e m e s of e l e c t r o c h e m i c a l p r o c e s s e s a r e of undoubted i n t e r e s t . C h e m i c a l and e l e c t r o c h e m i c a l c o n v e r s i o n s of the p r i m a r y r e d u c t i o n p r o d u c t s have been e x t r e m e l y little i n v e s t i g a t e d . And yet, t h e y m u s t be taken into c o n s i d e r a t i o n f o r a p r o p e r u n d e r s t a n d i n g of the c o u r s e of the e l e c t r o c h e m i c a l p r o c e s s , in e l e c t r o s y n t h e s i s [1] and in the e l e c t r o c h e m i c a l g e n e r a t i o n of f r e e r a d i c a l s [2]. In this c o m m u n i c a t i o n we p r o p o s e a m e t h o d of studying the m e c h a n i s m of the e l e c t r o c h e m i c a l p r o c e s s and quantitative e v a l u a t i o n of the kinetic p a r a m e t e r s of an included o r s u b s e q u e n t c h e m i c a l r e a c t i o n by m e a s u r i n g the a c t i v a t i o n e n e r g y of the limiting c u r r e n t in p o l a r o g r a p h y and switching polarography. In the s i m p l e s t c a s e , when t h e r e a r e no kinetic l i m i t a t i o n s on the limiting c u r r e n t on a c c o u n t of c o n jugated c h e m i c a l and e l e c t r o c h e m i c a l r e a c t i o n s , the a c t i v a t i o n e n e r g y of the limiting diffusion c u r r e n t Qd can be found [3] f r o m the slope of the g r a p h o f l o g i d v e r s u s 1 / T Qd--

h lg i d 2R A O/T) "0.~3

-

w h e r e A log i d = (log id)T2 - (log id)T1 and A ( l / T ) = 1 / T 1 - : I / T 2. Let us c o n s i d e r an e l e c t r o d e p r o c e s s c o n s i s t i n g of t h r e e b a s i c s t e p s E'I]~, i d

A -+- eZ ---~. A-'

(1)

A - __+B + . . .

(2)

E"V~ , i

w h e r e id and i a r e the l i m i t i n g c u r r e n t s of s t e p s (1) and (3), r e s p e c t i v e l y . It s e e m s to us that s c h e m e s (1)(3), with v a r i o u s d i f f e r e n c e s in d e t a i l s , will be c h a r a c t e r i s t i c of m a n y o r g a n i c c o m p o u n d s , r e d u c e d in a p r o tonic m e d i u m with the f o r m a t i o n of r e l a t i v e l y unstable anion r a d i c a l s . Depending on the r a t i o b e t w e e n the values of E 't/2 and E " I / : and on the d e g r e e of r e v e r s i b i l i t y * of s t e p (3), we can isolate a n u m b e r of p a r t i c u l a r c a s e s . Let us c o n s i d e r in g r e a t e r detail the c a s e s when E ' ~ / 2 ~ E " t / 2 o r E ' I / 2 < E " I / 2 . The a c t i v a t i o n e n e r g y of the c u r r e n t - d e t e r m i n i n g p r o c e s s Qk in this c a s e can be c a l c u l a t e d a n a l o g o u s l y to Qd. If we c o n s i d e r that the o c c u r r e n c e of r e a c t i o n s (2) and (3) inc r e a s e s the diffusion c u r r e n t i d by f t i m e s , i . e . , i l i m = idf (tables of the values of f as a function of kt w e r e c o m p i l e d by Nicholson et al. [4]), then f r o m this it follows that * We have in mind the influence of the i r r e v e r s i b l e c h e m i c a l r e a c t i o n (k 0, which l o w e r s the c o n c e n t r a t i o n of the s u b s t a n c e C, c a p a b l e of being oxidized in the a c c e s s i b l e r e g i o n of potentials, and not the r a t e c o n s t a n t of t r a n s f e r of an e l e c t r o n . A. E. A r b u z o v Institute of O r g a n i c and P h y s i c a l C h e m i s t r y , A c a d e m y of S c i e n c e s of the USSR. T r a n s l a t e d f r o m I z v e s t i y a A k a d e m i i Nauk SSSR, S e r i y a K h i m i c h e s k a y a , No. 9, pp. 1979-1983, S e p t e m b e r , 1970. Original a r t i c l e s u b m i t t e d N o v e m b e r 14, 1968. 0 1971Cons'ultants Bureau, a divisior~ of Plenum PublisMng Corporation, 227 West 17th Street, New York, N. Y. 10011. 4ll rights reserved. This article cannot be reproduced for any purpose whatsoever without permission of the publisher. A copy of this article is available from the publisher for $15.00.

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Lg ilim

t 9 ~'1o z

C

/,8 -

t~S

- #,6

O ~g k t

4#

.

~6

4#

40

~,o

I/ T. IO~

Fig. 2 Fig. 1 Fig. 1. Graph of the dependence of log f on log kt, constructed according to the data of [4]. Fig. 2. Graph of the dependence of log ill m on 1 / T for an e l e c trode p r o c e s s proceeding according to s c h e m e s (1)-(3), at E'1/2 E"I/2 and E'I/2 < E"I/2.

Q~ ~

h lg ilim A (l/T)

2R A lg f 0.43 = Q~ + ~ "

2R ~

= Q~ + AQ

where h Ig /

2R

A (t-ff~ o . - ~ =

(lg I)T~ -- (lg f)T,

2R

t l T 1 - i/T2

"o-~

is denoted by AQ. If the activation e n e r g y Q of the chemical reaction (2) is positive, then f i n c r e a s e s with i n c r e a s i n g t e m p e r a t u r e , and AQ > 0. Thus, the i n c r e a s e in the experimental activation e n e r g y Qk in c o m p a r i s o n with the value of Q~ c h a r a c t e r i s t i c of purely diffusion p r o c e s s e s (k = 0) is a diagnostic c r i t e r i o n of the o c c u r r e n c e of a subsequent chemical reaction with reduction of its product. The case with kt > 30 cannot be detected according to the value of Qk, since in this case the limiting c u r r e n t is limited by the rate of diffusion; however, the value of the limiting c u r r e n t c o r r e s p o n d i n g to the t r a n s f e r of two o r m o r e electrons is a weighty a r g u m e n t in support of an included chemical reactiocL. * The method of detecting and studying an included chemical reaction according to the t e m p e r a t u r e dependence of the limiting c u r r e n t is useful when the value of ill m does not differ significantly f r o m i d (n = 1). As a r e s u l t of the i n a c c u r a c y in the determination of the diffusion coefficient D (by the method of c o m p a r i s o n with a model compound or calculation according to the S t o k e s - E i n s t e i n equation), it cannot be stated with a s s u r ance whether the limiting c u r r e n t of the compound studied is elevated, and, consequently, whether the m e c h a n i s m of the p r o c e s s is d e s c r i b e d by scheme (1) or (1)-(3). As it follows f r o m the dependence of log f on log kt (Fig. 1), the graph of log ill m v e r s u s 1 / T will have three linear portions within a r a t h e r broad t e m p e r a t u r e interval (Fig. 2). In the region of low (a) and high (c) t e m p e r a t u r e s , the rate of the included chemical reaction will be too low (a), and its influence on i d can be neglected, or k will be too large (c), and f will p r a c t i c a l l y not change with the t e m p e r a t u r e . The slope of these portions c o r r e s p o n d s to the value of Qd. In the region of medium t e m p e r a t u r e s (portion b), the influence of the chemical reaction (2) will be the g r e a t e s t . F r o m the slope of this straight line we can find the activation e n e r g y of the included chemical reaction dlgf 2R Q' = (~ ~ ~ 0.43 ,

dlgf dig/ dlgk d l g f = 0.18 (found graphically) d(l/T) = d - - ~ ' d ( t / T ) ; d l g k dtgk 0.430 d l g ( l / T ) -~ R ; Q=2.78(Qk-Qa) In the t r a n s i t i o n region (a-b o r b-c), the dependence of log ilim on 1 / T is nonlinear; however, on account of 9 It is a s s u m e d that the t r a n s f e r of each s u c c e s s i v e e l e c t r o n o c c u r s with an appreciably g r e a t e r negative potential, if the molecule has one reaction center.

1863

# 2,0

fo

,

o

g5

r

s

~,~

-I/Z

kt

[n Cathodic

//glpotential

Fig. 3 Fig. 4 F i g . 3. G r a p h s of t h e d e p e n d e n c e of Q on (kt) a t c o n s t a n t v a l u e s of Qk - Qd ( k c a l / m o l e ) . F i g . 4. S c h e m a t i c d e p i c t i o n of p o l a r o g r a m s and s w i t c h i n g p o t e n t i a l s in t h e c a s e w h e n E ' I / 2 ~ E " I / 2 (a) a n d E ' I / 2 > E"I/2 (b).

TABLE

I

step (3) Irreversible

Reversible

~','/,

~',,,,> ~','~?

Q~>Qa Q~.a~.Qd

Q~>Qd Qg a~.Qd

Q/~= Qdb Q ~ Qde

Qk>Qa Qk.a=Qa

Qk>Qa Qk a < Qde

~;/, = ~,"],

6, <

Qk. a <~ Qad Q~ = Qah

t h e e x p e r i m e n t a l e r r o r s and t h e s m a l l t e m p e r a t u r e i n t e r v a l in p r a c t i c e , i t c a n be a p p r o x i m a t e d by s t r a i g h t l i n e s . T h e i r s l o p e is a f u n c t i o n of kt and Q. V a r i a t i o n of the d r o p p i n g p e r i o d of t h e m e r c u r y e l e c t r o d e w i l l be u s e f u l f o r r e s o l v i n g the q u e s t i o n of t h e p o r t i o n o f t h e c u r v e in F i g . 2 (a-b o r b) to w h i c h t h e e x p e r i m e n t a l d e p e n d e n c e of log i l l m upon 1 / T c o r responds.

Q~ > Qae Q~. a < Qad

If i t is e x p e r i m e n t a l l y shown t h a t Qk > Qd, i. e . , the p r i m a r y p r o d u c t of e l e c t r o c h e m i c a l r e d u c Qk. a - - q dg t i o n A'- i s s u b j e c t e d to a s u b s e q u e n t c h e m i c a l and electrochemical conversion, useful information can Note: a)E'I/2 E"t/2 means that E'l/z is more negative than c h e m i c a l r e a c t i o n . * A t the l o w e s t t e m p e r a t u r e (Tt), E'I/2. t h e c o n t r i b u t i o n of i to the v a l u e of i l l m c a n be n e g b) At the potentials of the limiting current of the 1st wave. l e c t e d , and u s i n g the i n d e p e n d e n t l y found v a l u e of c) Pertains to the summary height of 2 waves. Qd, id c a n be c a l c u l a t e d f o r the h i g h e s t t e m p e r a t u r e d~g) Methods of recording of the anodic switched waves (Eaux is the potential of the auxiliary electrode; Erec is the potential (T2). F r o m t h e t a b l e s of [4 ] we c a n o b t a i n a v a l u e of at which the current is recorded): e) Erec = Ez; Eaux = Ei-Ea (kt) T , c o r r e s p o n d i n g to ( i l i m / i d ) T2. The a c t i v a t i o n 2 (Fig. 4a); f) Erec = El; Eaux = E2-Es (Fig. 4a); d) Erec = El; Eaux e n e r g y Q of t h e i n c l u d e d c h e m i c a l r e a c t i o n i s d e t e r = El- E2 (Fig. 4b); g) Erec = El; Eaux = El- ~, summary height m i n e d f r o m g r a p h s of Q v e r s u s kt ( F i g . 3), w h i c h we (Fig. 4b). c o n s t r u c t e d f o r v a r i o u s v a l u e s of Q k - Q d . To r e d u c e t h e e r r o r s in t h e d e t e r m i n a t i o n of k and Q, i t i s a d v i s a b l e t o p e r f o r m the m e a s u r e m e n t s of ( i l i m ) T t and (ilim)T2 with m i n i m u m and m a x i m u m d r o p p i n g p e r i o d s r e s p e c t i v e l y . Q~ a = Qd f

The i n c l u d e d c h e m i c a l r e a c t i o n (2) c h a n g e s t h e flux of p a r t i c l e s A" t o w a r d t h e e l e c t r o d e and s h o u l d a l s o be r e f l e c t e d in t h e v a l u e of the a n o d i c c u r r e n t of t h e c o m m i t t e d w a v e i l i m . a (the c a s e w h e n C i s not o x i d i z e d in t h e s e l e c t e d r e g i o n of p o t e n t i a l s ) . The c o n t r i b u t i o n of the c h e m i c a l r e a c t i o n to t h e c h a n g e in ilim. a will increase with the temperature. The q u a n t i t a t i v e t r e a t m e n t of t h i s e f f e c t w i l l r e q u i r e an i n d e p e n d e n t c o n s i d e r a t i o n , b u t it i s e v i d e n t t h a t t h e a c t i v a t i o n e n e r g y of t h e l i m i t i n g a n o d i c c u r r e n t Qk. a < QdT h e r e s u l t s of a n a l y s i s of v a r i o u s p a r t i c u l a r c a s e s , b o t h with r e s p e c t to t h e c a t h o d i c and w i t h r e s p e c t to t h e a n o d i c l i m i t i n g c u r r e n t s , a r e p r e s e n t e d in T a b l e 1. A s i s s h o w n b y t h i s t a b l e , the c a s e s of t h e a p p e a r a n c e of s e c o n d a r y p r o d u c t s B a n d C c a n be d e t e r m i n e d f r o m t h e r a t i o b e t w e e n the v a l u e s o f Qk, Qk. a, and Qd. In g e n e r a l , t h e y m a y be p a r a m a g n e t i c , and t h i s s h o u l d be t a k e n into c o n s i d e r a t i o n in i n t e r p r e t i n g t h e E P R s p e c t r a of e l e c t r o c h e m i c a l l y g e n e r a t e d a n i o n r a d i c a l s . * T h i s p e r t a i n s o n l y to r e a c t i o n s d e s c r i b e d by a f i r s t - o r d e r

1864

kinetic equation.

We used the method p r o p o s e d in this w o r k to study the m e c h a n i s m of the reduction and generation of anion r a d i c a l s of a n u m b e r of organophosphorus and carbonyl compounds, and t h e s e r e s u l t s will be published in o u r following communications. CONCLUSIONS 1. A method was p r o p o s e d for studying e l e c t r o d e p r o c e s s e s with a subsequent c h e m i c a l r e a c t i o n on the b a s i s of a m e a s u r e m e n t of the activation e n e r g y of the c u r r e n t - d e t e r m i n i n g p r o c e s s . 2. Functions w e r e obtained p e r m i t t i n g the e x t r a c t i o n of quantitative information on the kinetics of the included reaction. LITERATURE 1, 2. 3. 4.

CITED

Ao 1D. Tomilov, S. G. Mairanovskii, M. Ya. Fioshin, and V. A. Organic Compounds [in Russian], Khimiya (1968). A. V. I I ' y a s o v , Yu. M. Kargin, Ya. A. Levin, I. D. Morozova, and N. I. B e s s o l i t s y n a , Izv. Akad. Nauk SSSR, Ser. K h i m . , 740 A. Vi~ek, Collection, 24, 3538 (1959). R. Nicholson, J. Wilson, and M. Olmstead, Analyt. C h e m . , 38,

Smirnov, The E l e c t r o c h e m i s i ' r y of N. N. Sot~zikova, V. Kh. Ivanova, (1968). 542 (1966).

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