ELECTROCHEMICALLY COMMUNICATION OF ANION

RADICALS

GENERATED 2. ELECTROCHEMICAL OF DERIVATIVES

FREE

RADICALS

GENERATION OF AROMATIC

AND

STABILITY

CARBOXYLIC

ACIDS UDC

A. V. II'yasov, Yu. M. Kargin, Ya. A. Levin, I. D. Morozova, N. N. Sotnikova, V. Kh. Ivanova, and N. I. Bessolitsyna

541.138 + 541.515 + 547.58

The method of e l e c t r o c h e m i c a l generation of anion r a d i c a l s , in contrast to the method of reduction by alkali metals, p e r m i t s a smooth variation of the reducing ability of the electrode within broad limits. M o r e over, in many c a s e s subsequent chemical conversions, difficult to take into consideration, can be avoided, which p e r m i t s the use of the method of e l e c t r o c h e m i c a l generation for many c l a s s e s of organic compounds. In addition, the use of solvents with a high d i e l e c t r i c constant and voluminous t e t r a a l k y l a m m o n i u m cations weakens the tendency of the anion r a d i c a l to f o r m ion pairs, which disrupt the s y m m e t r y of the molecular orbital of the unpaired electron. In most of the published works, the interpretation of the EPR s p e c t r a of the r a d i c a l s produced by e l e c t r o c h e m i c a l reduction is based upon the assumption that the s p e c t r a c o r r e s p o n d to the anion r a d i c a l A-', differing f r o m the initial particle A only in the p r e s e n c e of an e x t r a electron. However, there is no doubt that the appearance of p a r a m a g n e t i s m still does not prove the c o r r e c t n e s s of this hypothesis, since in view of the possible instability of A-" ( i s o m e r i s m , fragmentation, interaction with A or with the solvent, s e c o n d a r y e l e c t r o c h e m i c a l conversions, etc.), other r a d i c a l s or their mixtures may be formed in solution. Ignoring this frequently leads to e r r o r s in the interpretation of the EPR s p e c t r a . In the case when the p r i m a r y p r o c e s s is A + e ~-A-"

(1)

the stability of A-" must be estimated within a time sufficient for the accumulation of the radical and the r e cording of its EPR s p e c t r u m . It should be mentioned that the r e q u i r e m e n t of a o n e - e l e c t r o n p r o c e s s of r e duction at the f i r s t stage, frequently taken as the n e c e s s a r y (and sufficient) condition for the formation of A _ is not always n e c e s s a r y . The anion r a d i c a l A - can also be formed in the volume of the solution a c c o r d ing to the following s c h e m e [1, 2] A q- 2e ~-A=;

A= + A~-2A-"

(2)

Other pathways of formation of A-" also a r e not excluded. These considerations r e q u i r e a special study of the nature and stability of p a r a m a g n e t i c p a r t i c l e s . In our previous communication [3], we demonstrated that e s t e r s of a r o m a t i c carboxylic acids a r e i r r e v e r s i b l y reduced on a m e r c u r y electrode in aprotonic medium, and no instability of the product of o n e - e l e c t r o n reduction is detected in short segments of time (less than 1 sec). This work was devoted to a study of the stability of anion r a d i c a l s during long intervals of tirae and to the e l e c t r o c h e m i c a l generation of anion r a d i c a l s in o r d e r to investigate them by the EPR method. The c o m bination of e l e c t r o c h e m i c a l and r a d i o s p e c t r o s c o p i c investigations, in our opinion, yields m o r e reliable information on the nature of p a r a m a g n e t i c p a r t i c l e s and the p r o c e s s of e l e c t r o l y s i s . EXPERIMENTAL Stability of Anion Radicals. To evaluate the stability of anion r a d i c a l s of e s t e r s of benzoic and i s o m e r i c phthalic acids, formed according to s c h e m e (1) [3], we used the methods of polarographic couloA. E. Arbuzov Institute of Organic and Physical Chemistry, A c a d e m y of Sciences of the USSR. T r a n s lated f r o m I z v e s t i y a Akademii Nauk SSSR, Seriya Khimicheskaya, Noo 4, pp. 740-743, April, 1968. Original article submitted July 17, 1967.

715

1

TABLE

0.I V/min

0.8 V/rain

Compound

i~, ~A

ia, ~A

AE c

m~' .(H:~C):IICO

ig, p.A

~a, y.A

OCH (CH3)..

\ c --~_~_/--'.% //~% ~ / // O O H~CO

1,80

t,72

80

0,69

0,78

t,88

t,92

90

0,89

0,95

t,88

1,85

60

0,82

0,87

2,0

1,56

100

0,88

0,94

2,14

2,16

80

0,93

t,03

2,12

2,20

90

t ,02

1,10

OCH3

\//c -\~/w~

c% / O

O tlsC~O

OC~H~ z~--

l--c,

0

0

[QCO \~ OCH~ //~\.J--% ~/ 0 \ ~ / - - "'%0

o \

C--OCH~

ff

O

/~% %~/-

c/OCH3 %0

~-2

/

..i /} ;\. >

',t
..g

0

/,0

~V Fig. i

i

1,7

/,I

1,7,[/,,t~ t 2 I,# s V

t8

Fig. 2

Fig. 1. P o l a r o g r a m of diisopropyl terephthalate (10-3 M) in DMFA: 1) background (0.08 M Et4NI); 2) b e f o r e e l e c t r o l y s i s ; 3) 2 h after the beginning of e l e c t r o l y s i s . Fig. 2. Cyclic p o l a r o g r a m s of dimethyl phthalate (1), diethyl terephthalate (2), and methyl benzoate (3). Rate of change of the potential 0.8 V/min. m e r r y and cyclic polarography. Thus, for example, after e l e c t r o l y s i s of a solution of diisopropyl terephthalate with a dropping m e r c u r y electrode in a m i c r o c e l l [4] for s e v e r a l hours, a single anodic-cathodic p o l a r o g r a m was obtained (Fig. 1). In this case, the d e c r e a s e in the cathodic c u r r e n t was equal to the increase in the anodic current. It is r a t h e r improbable that the s e c o n d a r y product formed f r o m A - as a r e s u l t of further conversion, would be oxidized at the s a m e potentials at which A - is oxidized, and would thus be e l e c t r o chemically indistinguishable f r o m A-.* F r o m this it follows that the anion r a d i c a l A-" is stable for at least s e v e r a l hours. The stability of the p r i m a r y reduction product can also in principle be evaluated according to its oxidation c u r r e n t in the case when the e l e c t r o c h e m i c a l equilibrium on the electrode is not sufficiently labile (a single anodic-cathodic wave is not obtained). Figure 2 p r e s e n t s cyclic p o l a r o g r a m s obtained f r o m a s t a t i o n a r y m e r c u r y electrode according to Kemula. The c h a r a c t e r i s t i c s of the p o l a r o g r a m s a r e cited in Table 1. F r o m these data it is evident that at r a t e s of change of the potential of 0.1 and 0.8 V/rain (duration of e l e c t r o l y s i s ,~ 5 and 0.5 rain), the s y m m e t r y of the cyclic curves with r e s p e c t to the potentials and heights of the peaks is p r e s e r v e d . The i n c r e a s e in the height of the anodic peaks in c o m p a r i s o n with the heights of the cathodic peaks at a r a t e of change of the potential of 0.1 V/rain can be explained by the influence of convection upon the r a t e of delivery of the d e p o l a r i z e r to the electrode. The difference between the potentials of the peaks somewhat exceeds the value c h a r a c t e r i s t i c of r e v e r s i b l e o n e - e l e c t r o n p r o c e s s e s in the case of * At the s a m e time, if a nonequilibrium mixture of e l e c t r o c h e m i c a l l y indistinguishable r a d i c a l s were formed during e l e c t r o l y s i s , then the nature of the EPR s p e c t r u m would change with time, which was not observed.

716

........ - ~ C~--" /}F Z

linear diffusion in a flat e l e c t r o d e (0.058 V). This is evidently a consequence of the influence of c u r v a t u r e of the e l e c t r o d e s u r f a c e upon the diffusion p r o c e s s .

t0il I/

If

-]

8-,,

8\

~

2i/

--

~

_

7

d

(~ l

Procedure for Electrochemical Generation of the A n i o n R a d i c a 1. The b a s i c r e q u i r e m e n t s that should be satisfied by the cell for 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 in their i n v e s t i gation by the EPR method can be formulated in the following way: I) the possibility of monitoring the system of electrolysis; 2) separation of the cathodic and anodic spaces; 3) rapid and complete electrolysis of the entire depolarizer; 4) deoxygenation of the solution; 5) the possibility of rapid investigation of the radical obtained by the EPR method. The fulfillment of these requirements permits a comparison of the data obtained by the EPR method with the polarographic and other electrochemical data, and also increases the stability of the radicals. The completeness of the reduction facilitates a treatment of the mechanism of radical formation, eliminates exchange reactions of the radical with the initial compound, and improves the resolution of the hyperfine struct~me.

J

Numerous designs of cells for the electrochemical generation of anion radicals are known [i, 2, 5-11]. In part of them, these requirements are satisfied at the expense of a substantial complication of the apparatus [6, 7]. In simpler designs, these requirements are usually not entirely fulfilled, especially with respect to control of the system of electrolysis and completeness of reduction. We have designed a relatively simple cell for the electrochemical generation of free radicals, satisfying all the requirements outlined above and permitting a complete reduction in 3-5 rain (Fig. 3). The radicals were generated on the electrode i at the potentials of the limiting current of the first wave. The potential of this electrode was measured during electrolysis with respect to a mercury reference electrode 3 with a high-voltage tube voltmeter. The polarization curves obtained with our cell corresponded to the classical polarograms within 0.i V. To accelerate electrolysis, the solution near the electrode 1 was mixed with magnetic mixture 7. The completeness of the r e d u c t i o n was e s t i m a t e d a c c o r d i n g to the drop in the e l e c t r o l y s i s c u r r e n t to the background value. E l e c t r i c a l contact between the cathodic and anodic s p a c e s was a c c o m p l i s h e d by a bridge of g l a s s fiber 6, i m p r e g n a t e d with the solution. In this case, the r e s i s t a n c e of the cell containing a d i m e t h y l f o r m a m i d e solution with 1 9 10 -3 M d e p o l a r i z e r and 8 910 -2 M t e t r a e t h y l - or t e t r a m e t h y l a m m o n i u m iodide, evacuated to 10 -5 t o t , was 2 k ~ . After the end of e l e c t r o l y s i s , the solution of the f r e e r a d i c a l was poured over into c a p i l l a r y 9, which was placed in the r e s o n a t o r of a s p e c t r o m e t e r for r e c o r d i n g of the EPR spectrum.

Fig. 3. Design of electrochemical cell: i) mercury electrode for generation of radicals; 2) 2nd mercury current electrode; 3) mercury reference electrode; 4 (5)) cathodic space; 5 (4)) anodic space; 6) currentconducting glass fiber bridge; 7) m a g n e t i c mixer; 8) opening; 9) c a p i l l a r y for f r e e r a d i cals placed in s p e c t r o m e t e r r e s o n a t o r ; 10) v a c u u m s t o p cock; 11) to v a c u u m pump.

F o r all the e s t e r s of benzoic and i s o m e r i c phthalic acids that we studied, and c e r t a i n Other compounds, anion r a d i c a l s w e r e produced by 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 in this cell; their EPR s p e c t r a will be d i s c u s s e d in our following communication. The nature of the o b s e r v e d EPR s p e c t r a does not depend upon the solvent and b a s i c e l e c t r o l y t e . Solutions of anion r a d i c a l s of t e r e p h t h a l a t e s p o s s e s s a purple color, while those of b e n z o a t e s , phthalates, isophthalates, and phthalic anhydride a r e yellow or yellowish brown. CONCLUSIONS EPR

i. A new cell was proposed for the electrochemical method.

generation of radicals in their investigation by the

2. It was shown e l e c t r o c h e m i c a l l y and by the EPR method that in the e l e c t r o l y s i s of e s t e r s of a r o m a t i c c a r b o x y l i c acids, r a t h e r stable anion r a d i c a l s a r e f o r m e d , which a r e p r i m a r y products of o n e - e l e c tron reduction. LITERATURE i.

J.E. Harriman

and A. H. MaM,

J. Chem.

CITED

Phys., 3_~9, 778 (1963).

717

2. 3. 4. 5. 6. 7. 8. 9. i0. ii.

718

B.I. Shapiro, V. M. Kazakova, and Ya. K. Syrkin, Zh. Strukt. Khim., 6, 540 (1965). A.V. IPyasov, Yu. M. Kargin, Ya. A. Levin, I. D. Morozova, N. N. Sotnikova, V. Kh. Ivanova, and R. T. Satin, Izv. Akad. Nauk SSSR, Set. Khim., 1968, 736. Yu. M. Kargin and K. V. Nikonorov, Izv. Akad. Nauk SSSR, Ser. Khim., 1966, 1902. D.H. Geske and A. H. Maki, J. Amer. Chem. Soc., 82, 2671 (1960). P.H. Rieger, I. Bernal, W. H. Reinmuth, and G. K. Fraenkel, J. Amer. Chem. Soc., 85, 683 (1963). J.R. Bolton and G. K. Fraenkel, J. Chem. Phys., 40, 3307 (1964). Advances in Magnetic Resonance, Ed. by J. S. Waugh, New York (1965), p. 317. B. Kastening, Electroehim. acta, 9, 241 (1964). L.H. Pierre, P. Ludwig, and R. N. Adams, J. Amer. Chem. Soe., 83, 3909 (1961). R. Ao Gavar, Yao Po Stradyn', and S. A. Giller, Zavodo Lab~ 31, 41 (1965).