J Electroanal Chem, 72 (1976) 117--119 117 © Elsevier Sequoia S .A, Lausanne -- Printed m The Netherlands

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POLAROGRAPHIC BEHAVIOUR OF SOME 4°HYDROXY PYRIMIDINES

V.K. MAHESH, R.N GOYAL and OM PRAKASH

Department of Chemzstry, University of Roorkee, Roorkee (U P ) (India)

(Received 29th September 1975, in revised form 4th December 1975)

In troduction

Polarographic reduction of pyrimidines has been extensively studied [1--5]. The present study was devoted to 5-substituted 4-hydroxy-2-methyl thio- pyrimidines.

Experimen~l

All the pyrimidines were synthesised in the laboratory by the reported method [6--8] and the purity of the compounds was tested by nitrogen esti- mation and t.l.c.

Polarograms were recorded on a Cambridge pen recording polarograph. The capillary characteristics were 3.75 mg 2/3 s- l /2 at 25°C and h = 40 cm. A satu- rated calomel electrode was used as reference electrode and triply distilled mercury was used.

Britton--Robinson buffers [ 9] of different pH (2.0--11.0) were prepared for these studies and their pH measurements were recorded by a pH meter model ELICO-LI-10 after due standardization.

Stock solutions of all the six pyrimidines, concentration 1 X 10 - 3 M, were prepared in 10% purified alcohol [10]. 0.1 M KC1 was used as supporting electrolyte. The polarograms for all the pyrimidines were recorded at various heights, viz., 30, 35, 40, 45, 50 cm, etc. and at various concentrations: 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 X 10 -4 M. To ensure complete deaeration purified nitrogen gas was passed for 10 to 20 min before recording the polarogram. A constant temperature of 25 -+ 0.2°C was maintained by a thermostatic water bath. The temperature coefficient was calculated by Nejedly's method [11]. Due correction for residual current was made in all cases.

Controlled potential electrolysis was carried out in a three compartment H-cell using a mercury pool as cathode; the value of n, the number of electrons involved in the reduction, was calculated by the method of DeVries and Kroon [12].

118

T A B L E 1

Half-wave potent ia l s o f 4 - h y d r o x y pyr imidines at pH 5.0

No R --EI/2/V AE1/2/V td//AA

1 H 1 1 8 0 - - 0 75 2 CH 3 1 175 0 . 0 0 5 0 65 3 C2H 5 1 . 1 7 0 0 0 1 0 0 65 4 C6H 5 1 1 8 6 - - 0 . 0 0 5 0 .65 5 p-CH3C6H4 1 . 1 6 0 0 0 2 0 0 80 6 p - O C H 3 C 6 H 4 1 1 5 0 0 . 0 3 0 0 .60 7 C1 1 1 9 0 - - 0 . 0 1 0 0 .70

Results and discussion

All compounds studied (Table 1) gwe a single, two-electron, diffusion-con- trolled wave. The limiting current of this wave is pH-independent up to pH 7.0 and decreases with increasing pH, so that at about pH 8.5 it cannot be observed. The half-wave potential shifts with increasing pH to more negative values by about 0.05 V/pH (Fig. 1). The reduction thus proceeds according to an overall scheme (where Pyr is one of the possible tautomeric forms of 4-hydroxy-2- methyl thiopyrimidines)

Pyr + H ÷ ~ PyrH ÷ (1)

PyrH + + 2e-* products (2)

I _ R = H 2 - R = C H 3

['- 3 - R = C2H5 | 4 - R = C6Hs / 5 - R = p-CH3C6H /7"

% / 6 - R = p-OCH3C6H 5

/ j

/ 12- ( / ( m ,

,/" / / /

-1.0 ' ~ f f

1 0 :"" I I I I 2 3 4 5 co 7

pH

,9 ¢_

,2o

1.2

• 15

11

-11

10

-1(

0 . 9 -

I I 0.! 9 10

1.9. ¢~ >* t -

L

v v

10

O 9

O8

Fig 1. Plot o f El~ 2 vs. pH for 4 - h y d r o x y pyrimidines .

119

By analogy it is possible to assume that the 1,2--C= N 1=3nd undergoes re- duction.

The observed substituent effect, i.e. that the introduction of alkyl groups results in a shift to more positive potentials, whereas that of a phenyl group to more negative potentials, is the opposite of that for the majority of substi- tuents (cf. ref. 13). The possibility of steric effects cannot be excluded, but without separation of the effects on step (1) and (2} discussion is difficult.

Acknowledgement

Thanks are due to C.S.I.R. for a Junior Research Fellowship to one of us (O.P.).

REFERENCES

1 J C. Heath, Nature, 156 (1946) 23. 2 B Janik and P.J Elving, J. Electrochem. Soc., 116 (1969) 1087. 3 B. Jamk and P.J. Elvmg, J. Amer. Chem. Soc., 92 (1970) 235 4 D L. Smith and P.J Elving, J. Amer. Chem. Soc., 84 (1962} 1412 5 P.J Elving, Abhandl. Deut. Akad Wlss. (Berlin), Kl. Med., (1966} 485. 6 H W Barrett, I Goodman and K Dlttmer, J Amer Chem. Soc., 70 (1948) 1753. 7 J Johnson, J. Biol Chem., 3 (1907) 299. 8 A Robert and H.W Barrelt, J. Amer Chem. Soc., 76 (1954) 3146 9 H.T.S Brltton, Hydrogen Ions, Vol. I, D. Van Nostrand, New York, 1956, pp. 360--365.

10 A I Vogel, A Text Book of Practical Organic Chemistry, Longmans Green, 1957, p. 166. 11 V. Nejedly, Collect. Czech. Chem. Commun., 1 (1922) 319. 12 T. DeVries and J.L. Kroon, J. Amer Chem Soc., 75 (1953) 2484. 13 P. Zuman, Substituent Effects in Organic Polarography, Plenum Press, New York,

1967, p. 32.