ELECTROCHEMICALLY COMMUNICATION

GENERATED i. REDUCTION

A. V. Ilryasov, Yu. Ya. A. Levin, I. D. N. N. Sotnikova, V. and R. T. Safin

OF

FREE

CARBOXYLIC

RADICALS ACID

DERIVATIVES

M. Kargin, Morozova, Kh. Ivanova,

UDC

541.138 + 541.515 + 547.39

We had earlier demonstrated that derivatives of the carboxylic acid series are capable of being reduced on a mercury electrode, with the formation of anion radicals in certain cases [i]. For a detailed determination of the possibilities and conditions of electrochemical generation of anion radicals of these compounds, an evaluation of the stepwise nature of the process of electron transfer and the stability of the primary products of the electrode reaction, we investigated the reduction of certain derivatives of carboxylie acids in an aprotonic medium by methods of classical polarography, microcoulometry, alternating current polarography, cyclic voltammetry with a stationary electrode, and a switching method. EXPERIMENTAL The LP-60 polarograph of the Heyrovsky system was used to record the classical poiarograms. The dropping mercury electrodes with forced breakoff of the drops possessed m = 0.120 and 0.366 mg/sec and t = 0.50 sec. The electrolytic cell with a jacket for thermostatic control did not differ significantly from that described in [2]. The temperature was kept equal to 25 ~ 0.i ~ Commercial dimethylformamide (DMFA), used as the solvent, was purified by the well-known methods [3]. All the solutions were prepared only in freshly redistilled DMFA and were stored for no more than one day. The concentration of the depolarizer was 1 9 10 -3 M in all cases. The basic electrolyte was tetraethylammonium iodide with a concentration of 0.081 M. The bottom mercury served as the reference electrode; its potential was taken as zero. The values of El/~' were given without considering the potential drop in the solution, since it did not exceed 2 inV. The dissolved oxygen was removed with a stream of electrolytic hydrogen. CoulometTic measurements were conducted according to the procedure of [4]. The vector polarograph of the Central Laboratory of Automation was used to record the alternating current curves; the amplitude of the sinusoidal voltage was 4 mV; frequency 50 Hz, lag 2 sec; m = 1.43 mg/sec; rate of development 2 mV/sec. Cyclic polarograms were recorded on the PO-4 polarograph (,VRadiometer" Denmark) with a hanging mercury electrode (according to Kemula) at rates of change of the potential of 0.i and 0.8 V/rain. The height of the cathodic and anodic peaks was measured according to [5]. The switched polarograms were recorded with a Kalousek switch, connected according to circuits I or II [6]. The investigated compounds (commercial or synthesized according to standard procedures) were purified to constancy of the melting points by recrystallization and sublimation under a vacuum of 10 -3 tot, while the liquids were purified by redistillation under suitable vacuum until constancy of the index of refraction and density, the values of which corresponded to the literature data. DISCUSSION

OF

RESULTS

Table 1 presents the characteristics of the reduction waves of the investigated compounds. Physical polarograms are given in Fig. la. A comparison of the half-wave potentials EI/~ of the first step of reduction, characterizing the ease of addition of the first electron to the molecule, permits a qualitative estimation of the influence of the chemical structure upon the electrochemical reactivity. Thus, lengthening the chain of conjugation facilitates reduction (dimethyl oxalate -- dimethyl maleate, dimethyl fumarate, dimethyl terephthalate; methyl benzoate -- methyl cinnamate). The participation of an olefinic bond in the chain of conjugation facilitates the reduction nqore than conjugation with a benzene ring (dimethyl fumarate -- dimethyl terephthalate, methyl acrylate -- methyl benzoate, dimethyl maleate -- dimethyl phthalate, maleic anhydride-phthalic anhydride). Amides of acids are reduced with greater difficulty than esters. In the series of esters, the electron is accepted more readily, the less electron donor the ester group (terephthalates, benzoates)~ A. E. Arbuzov Institute of Organic and Physical Chemistry, lated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, article submitted July 17, 1967.

Academy of Sciences of the USSR. TransNo. 4, pp. 736-739, April, 1908. Original

711

T A B L E 1. C h a r a c t e r i s t i c s of the R e d u c t i o n W a v e s

Compound

CH.O0::--~>--COOCII~ i-C,U,OOC--~=~--COOC~I,-i

,-C.H,O0r C,H,-t C,H,O0C--<~)-~C00Col-15 CJ,,,005--<~>-- C00C.H,, NII~--CO--<2~}--CO--NH,

9 E~/,

"V

- Y - -AE ---V--

Alg~

0,058 0,063 0,059

)' --E;, V

t, 140 1, t 47 i,f80 t,245 0,950 1,t75 1,476

0,058 0,057 0,055

1,695 i,692 i,728 i,750 1,325 * 1,680 1,772

i ,43t

0,064

t,748

i ,440

0,064

t,780

1,450

0,055

t,760

i ,442

0,064

2,032

i, 70i t,755

2,15 2,t6 2,003

i,913 t,267

0,063 0,057 0,056 0,060 0,059

0,720

0,06i

1,580

0,840

0,065

T

COOCtI~

~/\COOCH3

COOC2H~

COOC~.Hs / / \ / C00C~H~ %/\

C00C~I-I~ C0OCH~ / \

COOCH~

<--->--COOCH~

<--)--C00C3H,-i --COOC6H,

i,537

1,711

co /2\/ )o N/'-, co CHa00C--CH f~ OH--C00CHa HC--COOCH. f~ HC--C003H3 HC--CO

I,,f!

CH3OOC--COOCH3 C~H~00C--C00C~H5 CHz=CH--C00CH~ CHz~C--COOCH~

0,980

i,570

0,3i

t,22

1,265 i ,311 1,600 1,735

0,062 0,069 0,090

I CPI3

*n=3. ~ A f t e r the 1st s t a g e of r e d u c t i o n , s o m e d r o p in the c u r r e n t , and then a r i s e , a r e o b s e r v e d . The c a u s e s of this will be d i s c u s s e d below. The i n t r o d u c t i o n of a s e c o n d c a r b a l k o x y l g r o u p -- a s t r o n g e l e c t r o n a c c e p t o r -- into the benzene r i n g s u b s t a n t i a l l y f a c i l i t a t e s r e d u c t i o n . In this c a s e , t e r e p h t h a l a t e s with conjugated c a r b o n y l g r o u p s a r e r e d u c e d m o r e r e a d i l y than isophthalate. In p h t h a l a t e s , evidently, s t e r i c h i n d r a n c e s to conjugation have an effect, and t h e r e f o r e they a r e r e d u c e d with g r e a t e r difficulty than t e r e p h t h a l a t e s . Analogous p h e n o m e n a a l s o o c c u r in the aliphatic s e r i e s , w h e r e d i m e t h y l f u m a r a t e , which p o s s e s s e s no s t e r i c h i n d r a n c e s , a c c e p t s the f i r s t e l e c t r o n c o n s i d e r a b l y m o r e r e a d i l y than d i m e t h y l m a l e a t e . All t h e s e o b s e r v a t i o n s a r e in s a t i s f a c t o r y a g r e e m e n t with the c o n c e p t s of t h e o r e t i c a l o r g a n i c c h e m i s t r y and with the v a l u e s of the e n e r g i e s of the lower f r e e levels, c a l c u l a t e d a c c o r d i n g to the m o l e c u l a r o r b i t a l method. The quantitative a s p e c t of the influence of the c h e m i c a l s t r u c t u r e upon the e l e c t r o c h e m i c a l a c t i v i t y of the c o m p o u n d s studied will be d i s c u s s e d later.

712

!;aA

-Z,~J

2,g

i

"<0

I a

-ge

f,v

-J.~

c

~0

Fig. 2. Dependence of the active (1) and c a pacitance (2) components of alternating c u r r e n t upon the p o tential in a solution of diisopropyl terephthalate (i0 -a M).

-2,a

Fig. 1. Dependenee of the eathodic (a) and anodie (b) reduction c u r r e n t s on the potential (10 -a M, DMFAS: 15 dimethy[ terephthalate; 2) dimethyl phthalate; 3) dimethyl oxalate; 4) methyl methacrylate,

For all the compounds studied (with the exception of dimethyl tetrachlorophthalateS, at the first stage of the reduction (F.t/2'), there is a transfer of one electron. In certain cases, the product thereby formed accepts a second electron at more negative potentials (El/2"). For diphenyl terephthalate and dimethyI fumarate, the second step evidently is more complex. We estimated the reversibility of the first step according to the Mope of a logarithmic graph (E versus log i/id -- i), which for reversible oneelectron processes should comprise 0.059 V (25 ~ (see Table 15, according to the presence of the anodic oxidation current on the switched polarograms (typical curves are presented in Fig. ib), and in the case of terephthalates -- according to alternating-current polarograms (Fig. 2). From these data it follows that for esters of all the investigated aromatic acids, as well as fumaric acid, giving switched curves i and 2 (see Fig. ib), the first step in the electrode reaction may be represented by the sehe'me A+e~-A

,

The reversible nature of these processes means that during the time of polarization (0.5, 0.02, and 0.i sec in polarography, alternating-current polarography, and with a switch, respectively), the influence of irreversible subsequent reactions can be neglected. For amides of aromatic carboxylic acids and esters of oxalic and methacrylic acids, the switched anodic waves possess a relatively lower height (curve 3 in Fig. ib) or are practically absent (curve 4). In these cases, the first step of the reduction can be described by the scheme Irreversible A @ e ~_-~A-" - - - > Productsof further conversions Rapid The second step in the reduction is most often i r r e v e r s i b l e . Evidently the cause of this i r r e v e r s i b i l i t y , just as in the case of the f i r s t step, is not electron t r a n s f e r itself, but an i r r e v e r s i b l e chemical reaction, deactivating the reduction product [7]. This hypothesis is supported by the fact that the product formed at the second step is not oxidized at potentials ~ 1 V m o r e positive than Et/2" , although the angular coefficient of the waves does not differ appreciably f r o m 0.059 V. For dimethyi phthalate (curve 15, the r a t e of deactivation of the product of the second step of the reduction, we might a s s u m e , is insufficiently large, so that close to the s u r f a c e of the electrode some amount of the product is p r e s e r v e d , and the anodie wave does not drop to zero. Matters are substantially different for terephthalates, for which the second step of the r e d u c tion is r e v e r s i b l e (curve 1). This can be explained by the fact that for terephthalates there is a stabilization of the p r i m a r y product on account of the f o r m a t i o n of a methytene-quinoid s t r u c t u r e [8, 9]

O-

RO

\c--/~--c

/

-0

OR

713

with a high d e g r e e of delocalization of the negative charge. Steric hindrances to a coplanar a r r a n g e m e n t of

the two = C / ~ groups in the product of two-electron reduction of dimethyl phthalate (ortho-quinoid struc\OR ture) lower the degree of delocalization of the negative charge and the stability of the dianion. CONCLUSIONS 1. The e l e c t r o c h e m i c a l reduction of a number of derivatives of carboxylic acids was studied on a m e r c u r y e l e c t r o d e in dimethylformamide by methods of polarography, a l t e r n a t i n g - c u r r e n t polarography, the Kalousek method, and coulometry. 2. Schemes of the e l e c t r o d e r e a c t i o n s w e r e proposed. 3. The influence of the chemical s t r u c t u r e upon the ease of reduction was considered. LITERATURE

CITED

1.

A. V. II'yasov, Yu. M. Kargin, Ya. A. Levin, and V. Kh. Ivanova, Izv. Akad. Nauk SSSR, Set. Khim.,

2.

S. G. Mairanovskii and F. S. Titov, Zh. Analiticheskoi Khim., 15, 121 (1960). A. B. Thomas and E. G. Rochow, J. Amer. Chem. Soc., 79, 1843 (1965). Yu. M. Kargin and K. V. Nikonorov, Izv. Akad. Nauk SSSR, Set. Khim., 1966, 1902. R. Nicholson and J. Shain, Analyt. Chem., 3__~7,178 (1965). J. Heyrovsky and J. Kuta, Fundamentals of Polarography [Russian translation], "Mir" (1965), p. 448. A. C. Aten, C. Biithker, and C. J. Hoijtink, T r a n s . F a r a d a y Soc., 55, 324 (1959); A. C. Aten and G. J. Hoijtink, Z. Phys. Chem., 2.1, 192 (1959). R. H. Philp, J r . , R. L. F l u r r y , and R. A. Day, J r . , J. E l e c t r o c h e m . Soc., 111, 328 (1964). Yu. Kargin, O. Manousek, and P. Zuman, J. Electroanalyt. Chem., 1__.99,443 (1966).

1966, 5 ss. 3. 4. 5. 6. 7. 8.

9.

714