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Electrochemical behaviour of dihydroxyfumaric acid in methanol at a mercury electrode
J. ElectroanaL Cherry, 185 (1985) 305-313
305
Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
ELECTROCHEMICAL BEHAVIOUR OF DIHYI)ROXYFUMARIC METHANOL AT A MERCURY ELECTRODE
ACID
IN
D. SAZOU, P. K A R A B I N A S and D. J A N N A K O U D A K I S
Laborato~. of Physical Chemistr); Department of Chemistry, University of ThessalonikL Thessaloniki (Greece) (Received 31st July 1984; in revised form 22nd November)
ABSTRACT The electrochemical-behaviour of dihydroxyfumaric acid has been investigated by de polaxography, cyclic voltammetry and controlled-potential electrolysis, at HE., in methanolic solutions. The cyclic vollammogram involves a two-electron wave corresponding to the oxidation of dihydroxyfuma_,-ic acid to
diketosuecinic acid and two successive one-el~tron reduction waves due to deposition of the two hydrogen of the acid. The electrooxidafion of dihydro.xyfumarie acid in methanol is an irreversible electrode reaction. The .corresponding rate constant of the electrode process has been calculated.
(1) I N T R O D U C T I O N
• The electrochemical behaviour of dihydroxyfumaric acid (DHF) has been studied in a q u e o u s s o l u t i o n s b y several a u t h o r s [ 1 - 5 ] . It h a s b e e n f o u n d t h a t D H F is o x i d i z e d to d i k e t o s u c c i n i e acid ( D K S ) , w h i c h is r a p i d l y c o n v e r t e d to it.g h y d l a t e d f r o m , n a m e l y d i h y d r o x y t a r t a r i c acid ( D H T ) . I n s t r o n g acidic s o l u t i o n s (0.75 3./" H 2 S O 4 ) , D H F a l s o e x h i b i t a t w o - e l e c t r o n r e d u c t i o n w a v e c o r r e s p o n d i n g to t a r t a r i c acid. I n acid a n d a l k a l i n e s o l u t i o n s , D H F exists in e q u i l i b r i u m w i t h its k e t o f o r m , w h i c h u n d e r g o e s d e c a r b o x y l a t i o n as a f l - k e t o a c i d [2,4,6]. T h e d e c o m p o s i t i o n r a t e o f D H F a n d its m o n o a n d d i - a n i o n f o r m s h a s a l s o b e e n m e a s u r e d at d i f f e r e n t p H values [4]. T h e k e t o - e n o l e q u i l i b r i u m o f D H F is p H - d e p e n d e n t a n d it is t : t a l l y s h i f t e d to the e u o l f o r m in n e u t r a l s o l u t i o n s , w h e r e n o decarboxy_ l a t i o n o f t~e a c i d occurs. M o s t e l e c t r o c h e m i c a l s t u d i e s w e r e c a r r i e d o u t in this p H region, w h e r e the acid exists in its e n o l f o r m a n d is t h e r e f o r e stable. T h e p u r p o s e o f this i n v e s t i g a t i o n Was to s t u d y the e l e c t r o e h e m l e a l behaviou,-, o f D H F in m e t h a n o l , ha w h i c h the acid is also stable. T h e effect o f s t r o n g a c i d s a n d b a s e s o n the r e d u c t i o n w a v e is d i s c u s s e d mad a c o m p a r i s o n is m a d e w i t h o t h e r d i c - a r b o x y l i c acids. T h e o x i d a t i o n - r e d u c t i o n m e c h a n i s m c o n s i s t e n t w i t h the e x p e r i m e n t a l d a t a is a l s o discussed. 0022-0728./85/$03.30
© 1985 Elsevier Sequoia S.A.
306 (II) E X P E R I M E N T A L
Cyclic v o l t a m m e t r i c i - E c u r v e s were c a r r i e d o u t o n a H M D E ( P A R 9323) with a s u r f a c e a r e a o f 3.513 x 1 0 - 2 c m 2. T h e e x p e r i m e n t a l s e t - u p i n c l u d e d ~a G . B a n k E l e c t r o n i c P C A - 7 2 L p o t e n t i o s t a t , a G . B a n k E l e c t r o n i c V S G - 7 2 L f u n c t i o n g e n e r a t o r a n d a H e w l e t t P a c k a r d 7045 A X - Y recorder. D e p o l a r o g r a m s w e r e t a k e n w i t h a P o l a r i t e r P O 4 R a d i o m e t e r . A P t sheet w a s used as the c o u n t e r - e l e c t r o d e a n d the p o t e n t i a l s w e r e m e a s u r e d a g a i n s t a n a q u e o u s c a l o m e l e l e c t r o d e ( S C E ) s a t u r a t e d w i t h N a C l . B o t h the c o u n t e r a n d the r e f e r e n c e e l e c t r o d e s w e r e s e p a r a t e d f r o m the w o r k i n g e l e c t r o d e c o m p a r t m e n t b y glassy d i a p h r a g m s . T h e r e a g e n t s used w e r e m e t h a n o l ( " a b s o l u t u n d a c e t o n f r e i p u r i s s p . a . ' ) , l i t h i u m p e r c h l o r a t e (" p u r u m p . a . " ) a n d t o l u o l - 4 - s u l f o n i c acid ( " p u r i s s p.a."), all f r o m F l u k a A . G . (Switzerland). D i h y d r o x y f u m a r i c acid, a l s o o b t a i n e d f r o m F l u k a , w a s r e c r y s t a l lized twice f r o m a b s o l u t e e t h a n o l . A s t o c k s o l u t i o n o f 10 -2 M C H 3 O N a w a s p r e p a r e d b y d i s s o l v i n g N a in M e O H . In o r d e r to a v o i d p o s s i b l e a u t o - o x i d a t i o n o f D H F in a s t o c k s o l u t i o n , the a p p r o p r i a t e a m o u n t o f solid D H F w a s d i s s o l v e d a f t e r d e - a e r a t i o r i o f the s u p p o r t i n g e l e c t r o l y t e (0.2 M LiCIO4 in M e O H ) . H i g h - p u r i t y A r w a s used for o x y g e n r e m o v a l a n d all m e a s u r e m e n t s w e r e t a k e n in A r a t m o s p h e r e . T h e e x p e r L ~ e n t s w e r e c a r r i e d o u t at a c o n s t a n t t e m p e r a t u r e o f 25 _ 0 . i ° C . (III) R E S U L T S A N D D I S C U S S I O N
( H L 1) A n o d i c wave
I n m e t h a n o l i e solutions, D H F gives o n a H M D E a t w o - e l e c t r o n o x i d a t i o n w a v e at p o s i t i v e p o t e n t i a l s a n d t w o successive o n e - e l e c t r o n r e d u c t i o n w a v e s at high n e g a t i v e p o t e n t i a l s , as s h o w n in Fig. 1. All these w a v e s are e n t i r e l y d i f f u s i o n - c o n trolled since the p e a k c u r r e n t is p r o p o r t i o n a l to the s q u a r e r o o t o f the p o t e n t i a l s w e e p rate (inset in Fig. 1). T h e p e a k c u r r e n t is ,also p r o p o r t i o n a l to the b u l k c o n c e n t r a t i o n o f D H F in all cases since the s l o p e o f the log / p - l o g c d i a g r a m is unity. T h e t w o - e l e c t r o n o x i d a t i o n w a v e c o r r e s p o n d s to the f o r m a t i o n o f D K S , w h i c h we tried to d e t e c t b y c o n t r o l l e d - p o t e n t i a l electrolysis at the p l a t e a u o f the o x i d a t i o n wave. Since the m e t h a n o l i c s o l u t i o n is a n u n b u f f e r e d one, the p r o t o n s released, a c c o r d ing to the r e a c t i o n HOOC- C-OH
II
HO- C-COOH
--,
t~-C-COOH [ O=C-COOH
+2 H++
2 e-
(I)
i n f l u e n c e the o x i d a t i o n p o t e n t i a l o f D H F . A f t e r a c e r t a i n t i m e o f electrolysis, the o x i d a t i o n w a v e o f D H F w a s shifted to m o r e p o s i t i v e p o t e n t i a l s , s o t h a t it o v e r l a p p e d
307
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7
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o v V a / ( m V s-~) vz
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/ b
I -1.t~
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I
-1.2
- 0.8
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E/V~.s.cm
0
1
0.4
----*"
Fig. 1. C y c l i c v o l t a m m o g r a m s o f 2.5 × 1 0 - 3 M D H F in M e O H + 0.2 M L i C I O 4 a t d i f f e r e n t ~ r a t e s : (1) 36, (2) 64, (3) 100, (4) 144, (5) 196, (6) 256, (7) 324 m V s - I . Inset.: P l o t O f i p vs. 01/2- for t h e a n o d i c w a v e ( O x ) a n d the t w o c a t h o d i c w a v e s (a a n d b).
with the m e r c u r y dissolution. F o r this reason, a s o l u t i o n o f D H F - d i a n i o n s was electrolyzed a c c o r d i n g to r e a c t i o n ( I l l ) (see S e c t i o n 111.4). T h e best p r o c e d u r e to g e n e r a t e D H F - d i a n i o n s has b e e n p r o v e d to be e x h a u s t i v e electrolysis o f a D H F s o l u t i o n at -- 1.7 V. A f t e r w a r d s , a n e w electro!ysis was c a r r i e d o u t at + 0.25 V for a b o u t 2 h. A t the e n d o f the s e c o n d electrolysis, the s o l u t i o n was t r e a t e d with p h e n y l h y d r a x i n e a c e t a t e in o r d e r to p r e c i p i t a t e the c o r r e s p o n d i n g o s a z o n e . T h e p h e n y l h y d r a z o n e isolated w a s r e c r j s t a i l i v e d f r o m e t h a n o l ( m p 2 0 8 - 2 1 0 ° C ) a n d i d e n t i f i e d b y m a s s s p e c t r o s c o p y as the d i - o s a z o n e o f m e s o x a l i c acid s e m i a l d e h y d e ; this s e m i a l d e h y d e results f r o m D K S b y loss o f C O 2. I f the o x i d a t i o n is an irreversible process, the p e a k c u r r e n t is given b y : it, = 3.01 × 10SA (an)l/ZncD1/2vl/2
(1)
w h e r e a s for reversible c h a r g e t r a n s f e r it is given by: it, = 2.72 ×
lOSAnaP-Dl/2co ~/2
(2)
w h e r e A is the e l e c t r o d e surface, n is the n u m b e r o f electrons, D is the d i f f u s i o n coefficient, o is the voltag,; scan rate a n d ~t is the t r a n s f e r coefficient.
308
T h e theoretical d e p e n d e n c e of ip on vt/z- for the above :two cases and the c o r r e s p o n d i n g experimental values are given in Fig. 2. F r o m this c o m p a r i s o n it is concluded that the experimental data agree reasonably well with an irreversible process. In addition, logarithmic analysis of the c o r r e s p o n d i n g pol arographi e waves gave straight lines with slopes of 92 m V / d e c a d e . ~ In Fig. 3 the variation of the peak potential of the oxidation wave with l o g o shows that the system behaves reversibly at low scan rates and irreversibly at high scan rates [7]. F o r an anodic electrode reaction under irreversible conditions, i.e. when the rate o f the reverse reaction is negligible throughout the potential region studied, Ep is related to the polarographic half-wave potential E I ~ by [8]: Ep = E , ~ + b[0.52 -- 1 / 2 l o g ( b / D ) -- log kh + 1/'2 log v] (3) where kh is the heterogeneous rate constant. F r o m eqn. (3) we have: d E p / d log v = b / 2 and from Fig. 3 a Tafel slope of 90 mV is obtained. F o r the case where the system is in equilibrium, Ep is related to the polarographic Ea/z by: E.p = El~ 2 .-k 1 . 1 R T / n F
too 1--
,~
8 0
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.
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v'/2/(m y s-~)v2 F i g . 2. C o m p a r i s o n o f i p vs. u z/2 p l o t f o r t h e o x i d a t i o n w a v e o f 2 . 5 x 1 0 - 3 M D H F i n M e O H + 0 . 2 .,~ L i C I O 4, w i t h t w o m o d e l s o f t h e e l e c t r o d e p r o c e s s : (1) a r e v e r s i b l e twe~-electron . t r a n s f e r ; (2) a n i r r e v e r s i b l e t w o - e l e c t r o n t r a n s f e r , an,~ = 1. T h e p o i n t s a r e e x p e r i m e n t a l d a t a .
309 I
t
I
I
I
I
42o
E
380 O
Y
i
O-,.'"
36O
340 0.01
J '.Ue O.1
I 1.0 v/V
s -1
Fig. 3. Peak potential variation with scan rate for the o x i d a t i o n wave of D H F M e O H +0.2 M LiCiO4.
(2.5×10 -3 M)
in
A t the sweep rate vc, at w h i c h the s y s t e m d e p a r t s f r o m e q u i l i b r i u m , we finally h a v e f r o m eqns. (1) a n d (3) log k h --- 0.52 -- 1 / 2 I o g ( b / D )
+ log v c - - 1 . 1 R T / n F b
The diffusion coefficient of cltronoamperometric curves and F r o m these d a t a a n d eqn. (5), ~ g r e e m e n t with the theoretically r e a c t i o n [9].
(5)
D H F in m e t h a n o l has b e e n c a l c u l a t e d f r o m the a value of 5.4 x 1 0 - 6 c m 2 s - 1 h a s b e e n o b t a i n e d . k h = 1.63 × 10 - 4 c m s - t was o b t a i n e d , w h i c h is in p r e d i c t e d values for an irreversible c h a r g e - t r a n s f e r
(111.2) C a t h o d i c w a v e s
A s has b e e n p o i n t e d o u t in m e t h a n o l i c s o l u t i o n s D H F ~ v e s at a H M D E two successive o n e - e l e c t r o n , d i f f u s i o n - c o n t r o l l e d r e d u c t i o n waves. T h e r e are two p o s s i b l e i n t e r p r e t a t i o n s w h i c h c a n d e s c r i b e the n a t u r e o f these waves. O n e o f these c o u l d b e d o u b l e - b o n d r e d u c t i o n , as o c c u r s in the case o f o t h e r u n s a t u r a t e d acids in M e O H , s u c h as f u m a r i c acid a n d m a l e i c acid. T h e r e d u c t i o n p r o d u c t w o u l d b e tartaric acid as in the case o f a q u e o u s acid s o l u t i o n s (0.75 M H 2 S O 4 ) [4]. I n that case, the t w o - s t e p r e d u c t i o n c a n b e e x p l a i n e d b y the c a t a l y t i c effect o f the p r o t o n s o f the fin'st D H F d i s s o c i a t i o n . A s has b e e n p r o v e d in the case o f n i t r o c o m p o u n d s [10] a n d a z o c o m p o u n d s [11], the a d d i t i o n o f an e q u i v a l e n t qtiantity o f p r o t o n d o n o r s in M e O H results in the a p p e a r a n c e o f a pre-wave. This p r e - w a v e is o n e q u a r t e r o r o n e h a l f o f the w h o l e w a v e in the case o f a f o u r - e l e c t r o n o r a t w o - e l e c t r o n r e d u c t i o n , respectively. I n o u r case, h a l f the a m o u n t o f D H F c o u l d be r e d u c e d at - - 1 . 5 V, c o n s u m i n g the p r o t o n s o f the first dissociation, a n d the
310 r e m a i n i n g half o f D H F , as the m o n o - a n i o n , c o u l d b e r e d u c e d at ~ 1.7 V, u s i n g the p r o t o n s of the s e c o n d d i s s o c i a t i o n a c c o r d i n g to t h e r e a c t i o n s : HOOC-C-OH II HO-C-COOH
HO-CHCOO+ e- -, ½ [ HO-CHCOOH
+
-OOC'C-OH II HO-C-COOH
½
E=
-1.5 V
and 1
-OOC-C-OH II HO-C-COOH
+½
HO-CHCO0I HO-CHCOOH
+e-~
HO-CHCO0I HO-CHCOO-
E=
--1.7V
T h e s e c o n d i n t e r p r e t a t i o n c o u l d be the successive d e p o s i t i o n o f D H F p r o t o n s at different negative potentials, d u e to the d i f f e r e n t d i s s o c i a t i o n c o n s t a n t s o f the two c a r b o x y l i c groups. In o r d e r to distinguish which o f the a b o v e t w o m e c h a n i s m s takes place, the effect o f s t r o n g acids a n d bases o n the c a t h o d i c wave was studied.
(111.3) Effect of proton donors In Fig. 4 the h y d r o g e n d e p o s i t i o n o f p - t o l u o i - s u l f o ~ c acid ( p - T S ) , as well as the influence o f this acid o n the r e d u c t i o n waves o f D H F in M e O H , is s h o w n . W i t h i n c r e a s i n g p - T S c o n c e n t r a t i o n , o n l y the first c a t h o d i c w a v e g r o w s while the s e c o n d o n e r e m a i n s u n a f f e c t e d . This c a n be e x p l a i n e d a s s u m i n g that p - T S a n d the first c a r b o x y l i c g r o u p o f D H F have a p p r o x i m a t e l y the s a m e s t r e n g t h in M e O H , so tbat the d e p o s i t i o n o f H ÷ takes place at the s a m e potential. If the c a t h o d i c w a v e s were d u e to the r e d u c t i o n o f the d o u b l e b o n d o f D H F , then with a d o u b l e q u a n t i t y o f p - T S the w h o l e w a v e w o u l d a p p e a r in o n e step at a less negative p o t e n t i a l t h a n the pre-wave, as o c c u r s in the case o f f u m a r i e acid (Fig. 5).
(IlL 4) Effect of strong bases F u r t h e r m o r e , the influence o f C H 3 O N a a n d L i O H o n b o t h the a n o d i c a n d c a t h o d i c waves was studied. A s Fig. 6 shows, a n equJ.valent c o n c e n t r a t i o n o f base results in the d i s a p p e a r a n c e o f the first wave, where~ts a d o u b l e q u a n t i t y o f base causes b o t h c a t h o d i c waves to d i s a p p e a r . T h i s b e h a v i o u r c a n be e x p l a i n e d b y the successive n e u t r a l i z a t i o n o f the t w o h y d r o g e n c a t i o n s o f the acid. O n the o t h e r h a n d , the o x i d a t i o n o f D H F m 0 n o - a n i o n s a n d d l a n i o n s takes p l a c e at different p o t e n t i a l s b e c a u s e o f the + I i n d u c t i v e effect o f the - C O O ' - g r o u p s . T h e u n d i s s o c i a t e d acid is oxidized at + 0.4 V (vs. S C E ) to D K S (Fig. 6, c u r v e a). T h e D H F m o n o - a n i o n s (curve b) are oxidized in t w o stages, the first b e i n g at + 0 . 2 v (vs. SCE). A t this potential, half the a m o u n t o f D H F gives D K S a n d the r e m a i n i n g half is c o n v e r t e d to the u n d i s s o c i a t e d f o r m a c c o r d i n g to the r e a c t i o n :
311 _
0
I
I
--10
I
S
--20 "'..
~-30 °
m
--40
--50
-60
-70
I --1.8
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-- 1.6
I
-1.4
-- 1.2
I
I
--1.0
I
-0.8
- 0-6
1
E / V ¢ S.C.E.) Fig. 4. Cyclic voltan'u-aograms o f 2 . 5 × 1 0 - a M D H F (curve a), 2.5X 10 - 3 Mp-TS (curve b) a n d m i x t u r e o f D H F a n d p - T S at different m o l a r ratios: (c) 1 : 1, (d) 1 : 2 a n d (e) 1 : 3.
- OOC- C-OH
II
O=C-COOH
---' ½
HO- C-COOH
HOOC- C-OH
I
+ ½
C~--C-COOH
II
+e-
(II)
HO- C-COOH
T h i s u n d i s s o c i a t e d a m o u n t u n d e r g o e s o x i d a t i o n at a m o r e p o s i t i v e p o t e n t i a l (0.4 V ) . F i n a l l y , t h e d i - a n i o n s o f D H F ( F i g . 6, c u r v e c) a r e a l s o o x i d i z e d o n o n e w a v e : - OOC- C-OH I HO- C-C00-
---"
O=C-COOH I O=C-COOH
+ 2 e-
(III)
(11L 5) Controlled-potential electrolysis The above mechanism can be further verified by controlled-potenti~ electrolysis a t the p l a t e a u o f t h e s e c o n d r e d u c t i o n w a v e . I f t h e e l e c t r o l y s i s p r o d u c t w e r e t a r t a r i c
312
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I
it
b_---"
-10 l
/
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T--20 ,¢ --30
i[/i 1
--40
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--50
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-1.7
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-1.5
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-1.3
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l
--1.1 --0.9 E/V(S.C.F__)
I
--0.7 I~
-0.5
Fig. 5. Cyclic v o h a m m o g r a m s o f 2.5 x 1 0 - 3 bar f u m a r i c acid (curve a), 2.5 × 1 0 - 3 M p - T S (curve b) a n d a m i x t u r e of f u m a r i c acid and p - T S at different m o l a r ratios: (c) 1 : 1, (d) 1 : 2 a n d (e) 1 : 3.
acid, n o o x i d a t i o n wave w o u l d a p p e a r at the end o f the electrolysis in the positive scan direction. O n the c o n t r a r y , if the r e d u c t i o n c u r r e n t is d u e to the d e p o s i t i o n o f h y d r o g e n ions o f D H F , at the e n d o f electrolysis the a n o d i c wave o f d i - a n i o n s o f
D H F must appear. Actually, when 1 electron per molecule is passed through the cell the cyclic v o l t a m m o g r a m is identical to c u r v e b a n d b y e x h a u s t i v e electrolysis to c u r v e c (Fig. 6). All these e x p e r i m e n t a l d a t a suggest that the r e d u c t i o n p r o c e s s o f D H F in M e O H is the d e p o s i t i o n o f the t w o h y d r o g e n ions, in c o n t r a s t to o t h e r u n s a t u r a t e d acids w h i c h u n d e r g o r e d u c t i o n o f the d o u b l e b o n d . T h i s b e h a v i o u r m u s t be c o n n e c t e d with s o m e s t e r e o c h e m i c a l p r o t e c t i o n o f the d o u b l e b o n d o f the D H F m o l e c u l e b y two h y d r o g e n b o n d s :
OH !
o~ :
C~
c/O~H II c 6 I
OH
313 i
30 25
b
1
20 15 10
=t_ o--
5 0
0
--5 -10 --15 --20
! -1.5
! -1.0
!
- 0.5
0
0.5
E / V(s.c.E.)
Fig. 6. Cyclic voltammograms o f 2.5 x 10 - 3 M D H F (curve a) in the presence of different concentrations o f CH3ONa: (b) 2.5 × 10- 3 M and (e) 5 × 10-3 M.
These internal h y d r o g e n bonds cause steric h i n d r a n c e of the D H F d o u b l e bond, which is not reduced but facilitates the deposition ot the carboxylic group p r o t o n s because of the resulting electron-withdrawal. REFERENCES 1 2 3 4 5 6 7 8 9 10 11
K. W;-esner, Chem. Listy, 38 (1944) 91. M. H e - r y , Compt. Rend., 252 (1961) 1931. M. Dominquez and E. Valera, Electrochim. Acta, 25 (1980) 833. D. Sazou, P. Karabinas and D. Jannakoudakis, J. Electroanal. Chem., 176 (1984) 225. D. Sazou, P. Karabinas and D. Jannakoudakis, to be published. L. Hough and J.K_N. Jones, Nature (London), 167 (1951) 180. E. Gileadi, E. Kirowa-Eisner and J. Penciner, Interracial Electrochemistry, Addison-Wesley, London, 1975. R. Nicholson and I. Shain, Anal. Chem., 36 (1964) 706. J. HcyrovskS, and J. Khta, Principles of Polarography, Academic Press, New York, London, 1966. D. Jannakoudakis and A. Wfldenau, Z. Natutforsch., 22 (1967) 118. L. Holleck, D. Jannakoudakis and A. Wildenau, Eleetrochim. Acta, 12 (1967) 1523.
305
Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
ELECTROCHEMICAL BEHAVIOUR OF DIHYI)ROXYFUMARIC METHANOL AT A MERCURY ELECTRODE
ACID
IN
D. SAZOU, P. K A R A B I N A S and D. J A N N A K O U D A K I S
Laborato~. of Physical Chemistr); Department of Chemistry, University of ThessalonikL Thessaloniki (Greece) (Received 31st July 1984; in revised form 22nd November)
ABSTRACT The electrochemical-behaviour of dihydroxyfumaric acid has been investigated by de polaxography, cyclic voltammetry and controlled-potential electrolysis, at HE., in methanolic solutions. The cyclic vollammogram involves a two-electron wave corresponding to the oxidation of dihydroxyfuma_,-ic acid to
diketosuecinic acid and two successive one-el~tron reduction waves due to deposition of the two hydrogen of the acid. The electrooxidafion of dihydro.xyfumarie acid in methanol is an irreversible electrode reaction. The .corresponding rate constant of the electrode process has been calculated.
(1) I N T R O D U C T I O N
• The electrochemical behaviour of dihydroxyfumaric acid (DHF) has been studied in a q u e o u s s o l u t i o n s b y several a u t h o r s [ 1 - 5 ] . It h a s b e e n f o u n d t h a t D H F is o x i d i z e d to d i k e t o s u c c i n i e acid ( D K S ) , w h i c h is r a p i d l y c o n v e r t e d to it.g h y d l a t e d f r o m , n a m e l y d i h y d r o x y t a r t a r i c acid ( D H T ) . I n s t r o n g acidic s o l u t i o n s (0.75 3./" H 2 S O 4 ) , D H F a l s o e x h i b i t a t w o - e l e c t r o n r e d u c t i o n w a v e c o r r e s p o n d i n g to t a r t a r i c acid. I n acid a n d a l k a l i n e s o l u t i o n s , D H F exists in e q u i l i b r i u m w i t h its k e t o f o r m , w h i c h u n d e r g o e s d e c a r b o x y l a t i o n as a f l - k e t o a c i d [2,4,6]. T h e d e c o m p o s i t i o n r a t e o f D H F a n d its m o n o a n d d i - a n i o n f o r m s h a s a l s o b e e n m e a s u r e d at d i f f e r e n t p H values [4]. T h e k e t o - e n o l e q u i l i b r i u m o f D H F is p H - d e p e n d e n t a n d it is t : t a l l y s h i f t e d to the e u o l f o r m in n e u t r a l s o l u t i o n s , w h e r e n o decarboxy_ l a t i o n o f t~e a c i d occurs. M o s t e l e c t r o c h e m i c a l s t u d i e s w e r e c a r r i e d o u t in this p H region, w h e r e the acid exists in its e n o l f o r m a n d is t h e r e f o r e stable. T h e p u r p o s e o f this i n v e s t i g a t i o n Was to s t u d y the e l e c t r o e h e m l e a l behaviou,-, o f D H F in m e t h a n o l , ha w h i c h the acid is also stable. T h e effect o f s t r o n g a c i d s a n d b a s e s o n the r e d u c t i o n w a v e is d i s c u s s e d mad a c o m p a r i s o n is m a d e w i t h o t h e r d i c - a r b o x y l i c acids. T h e o x i d a t i o n - r e d u c t i o n m e c h a n i s m c o n s i s t e n t w i t h the e x p e r i m e n t a l d a t a is a l s o discussed. 0022-0728./85/$03.30
© 1985 Elsevier Sequoia S.A.
306 (II) E X P E R I M E N T A L
Cyclic v o l t a m m e t r i c i - E c u r v e s were c a r r i e d o u t o n a H M D E ( P A R 9323) with a s u r f a c e a r e a o f 3.513 x 1 0 - 2 c m 2. T h e e x p e r i m e n t a l s e t - u p i n c l u d e d ~a G . B a n k E l e c t r o n i c P C A - 7 2 L p o t e n t i o s t a t , a G . B a n k E l e c t r o n i c V S G - 7 2 L f u n c t i o n g e n e r a t o r a n d a H e w l e t t P a c k a r d 7045 A X - Y recorder. D e p o l a r o g r a m s w e r e t a k e n w i t h a P o l a r i t e r P O 4 R a d i o m e t e r . A P t sheet w a s used as the c o u n t e r - e l e c t r o d e a n d the p o t e n t i a l s w e r e m e a s u r e d a g a i n s t a n a q u e o u s c a l o m e l e l e c t r o d e ( S C E ) s a t u r a t e d w i t h N a C l . B o t h the c o u n t e r a n d the r e f e r e n c e e l e c t r o d e s w e r e s e p a r a t e d f r o m the w o r k i n g e l e c t r o d e c o m p a r t m e n t b y glassy d i a p h r a g m s . T h e r e a g e n t s used w e r e m e t h a n o l ( " a b s o l u t u n d a c e t o n f r e i p u r i s s p . a . ' ) , l i t h i u m p e r c h l o r a t e (" p u r u m p . a . " ) a n d t o l u o l - 4 - s u l f o n i c acid ( " p u r i s s p.a."), all f r o m F l u k a A . G . (Switzerland). D i h y d r o x y f u m a r i c acid, a l s o o b t a i n e d f r o m F l u k a , w a s r e c r y s t a l lized twice f r o m a b s o l u t e e t h a n o l . A s t o c k s o l u t i o n o f 10 -2 M C H 3 O N a w a s p r e p a r e d b y d i s s o l v i n g N a in M e O H . In o r d e r to a v o i d p o s s i b l e a u t o - o x i d a t i o n o f D H F in a s t o c k s o l u t i o n , the a p p r o p r i a t e a m o u n t o f solid D H F w a s d i s s o l v e d a f t e r d e - a e r a t i o r i o f the s u p p o r t i n g e l e c t r o l y t e (0.2 M LiCIO4 in M e O H ) . H i g h - p u r i t y A r w a s used for o x y g e n r e m o v a l a n d all m e a s u r e m e n t s w e r e t a k e n in A r a t m o s p h e r e . T h e e x p e r L ~ e n t s w e r e c a r r i e d o u t at a c o n s t a n t t e m p e r a t u r e o f 25 _ 0 . i ° C . (III) R E S U L T S A N D D I S C U S S I O N
( H L 1) A n o d i c wave
I n m e t h a n o l i e solutions, D H F gives o n a H M D E a t w o - e l e c t r o n o x i d a t i o n w a v e at p o s i t i v e p o t e n t i a l s a n d t w o successive o n e - e l e c t r o n r e d u c t i o n w a v e s at high n e g a t i v e p o t e n t i a l s , as s h o w n in Fig. 1. All these w a v e s are e n t i r e l y d i f f u s i o n - c o n trolled since the p e a k c u r r e n t is p r o p o r t i o n a l to the s q u a r e r o o t o f the p o t e n t i a l s w e e p rate (inset in Fig. 1). T h e p e a k c u r r e n t is ,also p r o p o r t i o n a l to the b u l k c o n c e n t r a t i o n o f D H F in all cases since the s l o p e o f the log / p - l o g c d i a g r a m is unity. T h e t w o - e l e c t r o n o x i d a t i o n w a v e c o r r e s p o n d s to the f o r m a t i o n o f D K S , w h i c h we tried to d e t e c t b y c o n t r o l l e d - p o t e n t i a l electrolysis at the p l a t e a u o f the o x i d a t i o n wave. Since the m e t h a n o l i c s o l u t i o n is a n u n b u f f e r e d one, the p r o t o n s released, a c c o r d ing to the r e a c t i o n HOOC- C-OH
II
HO- C-COOH
--,
t~-C-COOH [ O=C-COOH
+2 H++
2 e-
(I)
i n f l u e n c e the o x i d a t i o n p o t e n t i a l o f D H F . A f t e r a c e r t a i n t i m e o f electrolysis, the o x i d a t i o n w a v e o f D H F w a s shifted to m o r e p o s i t i v e p o t e n t i a l s , s o t h a t it o v e r l a p p e d
307
I 6 0 r- !
~o eo
4O
o aa
t
I
I
I
7
~ ~R*dCb) r
5
2O
II~ .~.0 :3..
~
lC.
o v V a / ( m V s-~) vz
0
-:20
/ b
I -1.t~
I
I
-1.2
- 0.8
I - 0.4
E/V~.s.cm
0
1
0.4
----*"
Fig. 1. C y c l i c v o l t a m m o g r a m s o f 2.5 × 1 0 - 3 M D H F in M e O H + 0.2 M L i C I O 4 a t d i f f e r e n t ~ r a t e s : (1) 36, (2) 64, (3) 100, (4) 144, (5) 196, (6) 256, (7) 324 m V s - I . Inset.: P l o t O f i p vs. 01/2- for t h e a n o d i c w a v e ( O x ) a n d the t w o c a t h o d i c w a v e s (a a n d b).
with the m e r c u r y dissolution. F o r this reason, a s o l u t i o n o f D H F - d i a n i o n s was electrolyzed a c c o r d i n g to r e a c t i o n ( I l l ) (see S e c t i o n 111.4). T h e best p r o c e d u r e to g e n e r a t e D H F - d i a n i o n s has b e e n p r o v e d to be e x h a u s t i v e electrolysis o f a D H F s o l u t i o n at -- 1.7 V. A f t e r w a r d s , a n e w electro!ysis was c a r r i e d o u t at + 0.25 V for a b o u t 2 h. A t the e n d o f the s e c o n d electrolysis, the s o l u t i o n was t r e a t e d with p h e n y l h y d r a x i n e a c e t a t e in o r d e r to p r e c i p i t a t e the c o r r e s p o n d i n g o s a z o n e . T h e p h e n y l h y d r a z o n e isolated w a s r e c r j s t a i l i v e d f r o m e t h a n o l ( m p 2 0 8 - 2 1 0 ° C ) a n d i d e n t i f i e d b y m a s s s p e c t r o s c o p y as the d i - o s a z o n e o f m e s o x a l i c acid s e m i a l d e h y d e ; this s e m i a l d e h y d e results f r o m D K S b y loss o f C O 2. I f the o x i d a t i o n is an irreversible process, the p e a k c u r r e n t is given b y : it, = 3.01 × 10SA (an)l/ZncD1/2vl/2
(1)
w h e r e a s for reversible c h a r g e t r a n s f e r it is given by: it, = 2.72 ×
lOSAnaP-Dl/2co ~/2
(2)
w h e r e A is the e l e c t r o d e surface, n is the n u m b e r o f electrons, D is the d i f f u s i o n coefficient, o is the voltag,; scan rate a n d ~t is the t r a n s f e r coefficient.
308
T h e theoretical d e p e n d e n c e of ip on vt/z- for the above :two cases and the c o r r e s p o n d i n g experimental values are given in Fig. 2. F r o m this c o m p a r i s o n it is concluded that the experimental data agree reasonably well with an irreversible process. In addition, logarithmic analysis of the c o r r e s p o n d i n g pol arographi e waves gave straight lines with slopes of 92 m V / d e c a d e . ~ In Fig. 3 the variation of the peak potential of the oxidation wave with l o g o shows that the system behaves reversibly at low scan rates and irreversibly at high scan rates [7]. F o r an anodic electrode reaction under irreversible conditions, i.e. when the rate o f the reverse reaction is negligible throughout the potential region studied, Ep is related to the polarographic half-wave potential E I ~ by [8]: Ep = E , ~ + b[0.52 -- 1 / 2 l o g ( b / D ) -- log kh + 1/'2 log v] (3) where kh is the heterogeneous rate constant. F r o m eqn. (3) we have: d E p / d log v = b / 2 and from Fig. 3 a Tafel slope of 90 mV is obtained. F o r the case where the system is in equilibrium, Ep is related to the polarographic Ea/z by: E.p = El~ 2 .-k 1 . 1 R T / n F
too 1--
,~
8 0
t
i
(4)
t
I
I
--
--
"-~60
40
--
20~
--
O
I 0
4
8
12
1G
.
20
v'/2/(m y s-~)v2 F i g . 2. C o m p a r i s o n o f i p vs. u z/2 p l o t f o r t h e o x i d a t i o n w a v e o f 2 . 5 x 1 0 - 3 M D H F i n M e O H + 0 . 2 .,~ L i C I O 4, w i t h t w o m o d e l s o f t h e e l e c t r o d e p r o c e s s : (1) a r e v e r s i b l e twe~-electron . t r a n s f e r ; (2) a n i r r e v e r s i b l e t w o - e l e c t r o n t r a n s f e r , an,~ = 1. T h e p o i n t s a r e e x p e r i m e n t a l d a t a .
309 I
t
I
I
I
I
42o
E
380 O
Y
i
O-,.'"
36O
340 0.01
J '.Ue O.1
I 1.0 v/V
s -1
Fig. 3. Peak potential variation with scan rate for the o x i d a t i o n wave of D H F M e O H +0.2 M LiCiO4.
(2.5×10 -3 M)
in
A t the sweep rate vc, at w h i c h the s y s t e m d e p a r t s f r o m e q u i l i b r i u m , we finally h a v e f r o m eqns. (1) a n d (3) log k h --- 0.52 -- 1 / 2 I o g ( b / D )
+ log v c - - 1 . 1 R T / n F b
The diffusion coefficient of cltronoamperometric curves and F r o m these d a t a a n d eqn. (5), ~ g r e e m e n t with the theoretically r e a c t i o n [9].
(5)
D H F in m e t h a n o l has b e e n c a l c u l a t e d f r o m the a value of 5.4 x 1 0 - 6 c m 2 s - 1 h a s b e e n o b t a i n e d . k h = 1.63 × 10 - 4 c m s - t was o b t a i n e d , w h i c h is in p r e d i c t e d values for an irreversible c h a r g e - t r a n s f e r
(111.2) C a t h o d i c w a v e s
A s has b e e n p o i n t e d o u t in m e t h a n o l i c s o l u t i o n s D H F ~ v e s at a H M D E two successive o n e - e l e c t r o n , d i f f u s i o n - c o n t r o l l e d r e d u c t i o n waves. T h e r e are two p o s s i b l e i n t e r p r e t a t i o n s w h i c h c a n d e s c r i b e the n a t u r e o f these waves. O n e o f these c o u l d b e d o u b l e - b o n d r e d u c t i o n , as o c c u r s in the case o f o t h e r u n s a t u r a t e d acids in M e O H , s u c h as f u m a r i c acid a n d m a l e i c acid. T h e r e d u c t i o n p r o d u c t w o u l d b e tartaric acid as in the case o f a q u e o u s acid s o l u t i o n s (0.75 M H 2 S O 4 ) [4]. I n that case, the t w o - s t e p r e d u c t i o n c a n b e e x p l a i n e d b y the c a t a l y t i c effect o f the p r o t o n s o f the fin'st D H F d i s s o c i a t i o n . A s has b e e n p r o v e d in the case o f n i t r o c o m p o u n d s [10] a n d a z o c o m p o u n d s [11], the a d d i t i o n o f an e q u i v a l e n t qtiantity o f p r o t o n d o n o r s in M e O H results in the a p p e a r a n c e o f a pre-wave. This p r e - w a v e is o n e q u a r t e r o r o n e h a l f o f the w h o l e w a v e in the case o f a f o u r - e l e c t r o n o r a t w o - e l e c t r o n r e d u c t i o n , respectively. I n o u r case, h a l f the a m o u n t o f D H F c o u l d be r e d u c e d at - - 1 . 5 V, c o n s u m i n g the p r o t o n s o f the first dissociation, a n d the
310 r e m a i n i n g half o f D H F , as the m o n o - a n i o n , c o u l d b e r e d u c e d at ~ 1.7 V, u s i n g the p r o t o n s of the s e c o n d d i s s o c i a t i o n a c c o r d i n g to t h e r e a c t i o n s : HOOC-C-OH II HO-C-COOH
HO-CHCOO+ e- -, ½ [ HO-CHCOOH
+
-OOC'C-OH II HO-C-COOH
½
E=
-1.5 V
and 1
-OOC-C-OH II HO-C-COOH
+½
HO-CHCO0I HO-CHCOOH
+e-~
HO-CHCO0I HO-CHCOO-
E=
--1.7V
T h e s e c o n d i n t e r p r e t a t i o n c o u l d be the successive d e p o s i t i o n o f D H F p r o t o n s at different negative potentials, d u e to the d i f f e r e n t d i s s o c i a t i o n c o n s t a n t s o f the two c a r b o x y l i c groups. In o r d e r to distinguish which o f the a b o v e t w o m e c h a n i s m s takes place, the effect o f s t r o n g acids a n d bases o n the c a t h o d i c wave was studied.
(111.3) Effect of proton donors In Fig. 4 the h y d r o g e n d e p o s i t i o n o f p - t o l u o i - s u l f o ~ c acid ( p - T S ) , as well as the influence o f this acid o n the r e d u c t i o n waves o f D H F in M e O H , is s h o w n . W i t h i n c r e a s i n g p - T S c o n c e n t r a t i o n , o n l y the first c a t h o d i c w a v e g r o w s while the s e c o n d o n e r e m a i n s u n a f f e c t e d . This c a n be e x p l a i n e d a s s u m i n g that p - T S a n d the first c a r b o x y l i c g r o u p o f D H F have a p p r o x i m a t e l y the s a m e s t r e n g t h in M e O H , so tbat the d e p o s i t i o n o f H ÷ takes place at the s a m e potential. If the c a t h o d i c w a v e s were d u e to the r e d u c t i o n o f the d o u b l e b o n d o f D H F , then with a d o u b l e q u a n t i t y o f p - T S the w h o l e w a v e w o u l d a p p e a r in o n e step at a less negative p o t e n t i a l t h a n the pre-wave, as o c c u r s in the case o f f u m a r i e acid (Fig. 5).
(IlL 4) Effect of strong bases F u r t h e r m o r e , the influence o f C H 3 O N a a n d L i O H o n b o t h the a n o d i c a n d c a t h o d i c waves was studied. A s Fig. 6 shows, a n equJ.valent c o n c e n t r a t i o n o f base results in the d i s a p p e a r a n c e o f the first wave, where~ts a d o u b l e q u a n t i t y o f base causes b o t h c a t h o d i c waves to d i s a p p e a r . T h i s b e h a v i o u r c a n be e x p l a i n e d b y the successive n e u t r a l i z a t i o n o f the t w o h y d r o g e n c a t i o n s o f the acid. O n the o t h e r h a n d , the o x i d a t i o n o f D H F m 0 n o - a n i o n s a n d d l a n i o n s takes p l a c e at different p o t e n t i a l s b e c a u s e o f the + I i n d u c t i v e effect o f the - C O O ' - g r o u p s . T h e u n d i s s o c i a t e d acid is oxidized at + 0.4 V (vs. S C E ) to D K S (Fig. 6, c u r v e a). T h e D H F m o n o - a n i o n s (curve b) are oxidized in t w o stages, the first b e i n g at + 0 . 2 v (vs. SCE). A t this potential, half the a m o u n t o f D H F gives D K S a n d the r e m a i n i n g half is c o n v e r t e d to the u n d i s s o c i a t e d f o r m a c c o r d i n g to the r e a c t i o n :
311 _
0
I
I
--10
I
S
--20 "'..
~-30 °
m
--40
--50
-60
-70
I --1.8
I
I
-- 1.6
I
-1.4
-- 1.2
I
I
--1.0
I
-0.8
- 0-6
1
E / V ¢ S.C.E.) Fig. 4. Cyclic voltan'u-aograms o f 2 . 5 × 1 0 - a M D H F (curve a), 2.5X 10 - 3 Mp-TS (curve b) a n d m i x t u r e o f D H F a n d p - T S at different m o l a r ratios: (c) 1 : 1, (d) 1 : 2 a n d (e) 1 : 3.
- OOC- C-OH
II
O=C-COOH
---' ½
HO- C-COOH
HOOC- C-OH
I
+ ½
C~--C-COOH
II
+e-
(II)
HO- C-COOH
T h i s u n d i s s o c i a t e d a m o u n t u n d e r g o e s o x i d a t i o n at a m o r e p o s i t i v e p o t e n t i a l (0.4 V ) . F i n a l l y , t h e d i - a n i o n s o f D H F ( F i g . 6, c u r v e c) a r e a l s o o x i d i z e d o n o n e w a v e : - OOC- C-OH I HO- C-C00-
---"
O=C-COOH I O=C-COOH
+ 2 e-
(III)
(11L 5) Controlled-potential electrolysis The above mechanism can be further verified by controlled-potenti~ electrolysis a t the p l a t e a u o f t h e s e c o n d r e d u c t i o n w a v e . I f t h e e l e c t r o l y s i s p r o d u c t w e r e t a r t a r i c
312
O
I
I
I
it
b_---"
-10 l
/
/
i!
T--20 ,¢ --30
i[/i 1
--40
/
""
I11 l"
~ ~
iII d
--50
I I
/
I
e "~./ I
1
-1.9
-1.7
I
-1.5
I
-1.3
I
l
--1.1 --0.9 E/V(S.C.F__)
I
--0.7 I~
-0.5
Fig. 5. Cyclic v o h a m m o g r a m s o f 2.5 x 1 0 - 3 bar f u m a r i c acid (curve a), 2.5 × 1 0 - 3 M p - T S (curve b) a n d a m i x t u r e of f u m a r i c acid and p - T S at different m o l a r ratios: (c) 1 : 1, (d) 1 : 2 a n d (e) 1 : 3.
acid, n o o x i d a t i o n wave w o u l d a p p e a r at the end o f the electrolysis in the positive scan direction. O n the c o n t r a r y , if the r e d u c t i o n c u r r e n t is d u e to the d e p o s i t i o n o f h y d r o g e n ions o f D H F , at the e n d o f electrolysis the a n o d i c wave o f d i - a n i o n s o f
D H F must appear. Actually, when 1 electron per molecule is passed through the cell the cyclic v o l t a m m o g r a m is identical to c u r v e b a n d b y e x h a u s t i v e electrolysis to c u r v e c (Fig. 6). All these e x p e r i m e n t a l d a t a suggest that the r e d u c t i o n p r o c e s s o f D H F in M e O H is the d e p o s i t i o n o f the t w o h y d r o g e n ions, in c o n t r a s t to o t h e r u n s a t u r a t e d acids w h i c h u n d e r g o r e d u c t i o n o f the d o u b l e b o n d . T h i s b e h a v i o u r m u s t be c o n n e c t e d with s o m e s t e r e o c h e m i c a l p r o t e c t i o n o f the d o u b l e b o n d o f the D H F m o l e c u l e b y two h y d r o g e n b o n d s :
OH !
o~ :
C~
c/O~H II c 6 I
OH
313 i
30 25
b
1
20 15 10
=t_ o--
5 0
0
--5 -10 --15 --20
! -1.5
! -1.0
!
- 0.5
0
0.5
E / V(s.c.E.)
Fig. 6. Cyclic voltammograms o f 2.5 x 10 - 3 M D H F (curve a) in the presence of different concentrations o f CH3ONa: (b) 2.5 × 10- 3 M and (e) 5 × 10-3 M.
These internal h y d r o g e n bonds cause steric h i n d r a n c e of the D H F d o u b l e bond, which is not reduced but facilitates the deposition ot the carboxylic group p r o t o n s because of the resulting electron-withdrawal. REFERENCES 1 2 3 4 5 6 7 8 9 10 11
K. W;-esner, Chem. Listy, 38 (1944) 91. M. H e - r y , Compt. Rend., 252 (1961) 1931. M. Dominquez and E. Valera, Electrochim. Acta, 25 (1980) 833. D. Sazou, P. Karabinas and D. Jannakoudakis, J. Electroanal. Chem., 176 (1984) 225. D. Sazou, P. Karabinas and D. Jannakoudakis, to be published. L. Hough and J.K_N. Jones, Nature (London), 167 (1951) 180. E. Gileadi, E. Kirowa-Eisner and J. Penciner, Interracial Electrochemistry, Addison-Wesley, London, 1975. R. Nicholson and I. Shain, Anal. Chem., 36 (1964) 706. J. HcyrovskS, and J. Khta, Principles of Polarography, Academic Press, New York, London, 1966. D. Jannakoudakis and A. Wfldenau, Z. Natutforsch., 22 (1967) 118. L. Holleck, D. Jannakoudakis and A. Wildenau, Eleetrochim. Acta, 12 (1967) 1523.