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Polarographic investigations of some arylazopyrimidines
J. Electroanal. Chem., 86 (1978)429--432
429
© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
Short communication POLAROGRAPHIC INVESTIGATIONS OF SOME ARYLAZOPYRIMIDINES
V.K. MAHESH, R.N. GOYAL and RAMA GUPTA
Department of Chemsitry, University of Roorkee, Roorkee (India) (Received 6th December 1976; in revised form 3rd February 1977)
Introduction The polarography of pyrimidines at the dropping mercury electrode has been extensively studied by several workers [1--4]; however, no attempt has been made so far in determining the electrochemical behaviour of arylazopyrimidines. In the present communication we report the results of our investigations on the 14 arylazopyrimidines (A). As all these pyrimidines were water insoluble, investigations were carried out in dimethylformamide. OH
R
l_lS-~? N = N ~ (A) Experimental The substituted 5-arylazo-2-thio-4-hydroxy-6-methyl pyrimidines (I--XIV) were synthesised in the laboratory by the method developed [ 5] in this laboratory. The purity of these compounds was ascertained by recrystallisation and by t.l.c. The solutions (1 X 10 - 3 M) of all these pyrimidines were prepared in methanol (A.R.). Dimethylformamide (A.R.) was used throughout these investigations and 0.1 M LiC1 (A.R.) in dimethylformamide was used as supporting electrolyte. Triply distilled mercury was used for the DME.
Apparatus. Polarographic measurements were made with a Cambridge pen recording polarograph. The capillary had a flow rate of 0.91 mg s -1 and drop time 4.02 s at h = 45 cm in 1 M KC1 solution. A H-type cell (thermostated at 30 + 0.1 ° C) was used for recording the polarograms. The number of electrons involved in the reduction was determined by the method of DeVries and Kroon [6] using a mercury pool cathode. The value of the temperature coefficient was determined by Nejedly's method [7].
Procedure. Working solutions were prepared by taking 8.0 ml of dimethylformamide, 1.0 ml of arylazopyrimidine and 1.0 ml of 1.0 M lithium chloride
430
solution. The solution was then thoroughly mixed by the stream of nitrogen for 15 min and the polarographic curves were then recorded.
Results and discussions All the arylazopyrimidines (Table 1) gave a single two-electron transfer, well defined cathodic wave in dimethylformamide (typical polarograms are shown in Fig. 1). Keeping in view the feasibilities of the sites of the reduction it may be concluded that the possible reduction is reduction of --N=N--, and the pyrimidine nucleus exhibits no reduction wave at the dropping mercury cathode. Furthermore this wave can be safely assigned to the reduction of --N=N-- as --N=N-- reduction often occurs at positive potential, --C=N usually at more negative. The waves were found to be diffusion controlled as shown b y the linear plots of id VS. x/h and id VS: concn, of depolarizer. The low value of the temperature coefficient further confirmed the diffusion controlled nature of the waves. The values of the diffusion current constant (I) were also f o u n d to lie in the same range (Table 1). The polarograms of all the arylazopyrimidines were recorded in the concentration range 1.0 X 10 - 4 --3.0 X 10 - 4 M and it was found that the half-wave potentials are not constant b u t become more positive with increasing concentration of the azopyrimidines (Table 1). This fact confirmed the irreversible nature of the waves which was again confirmed by the log plots [8]. A mechanism for the reduction of the azogroup similar to that suggested by Nygard [9] and others [10--12] can be proposed for these compounds.
Effect o f substituents. Table 1 gives the values of polarographic characteristics for arylazopyrimidines (I--XIV) and it can be seen that the value of the diffusion current constant (I) and the transfer coefficient (a) are reasonably constant. Therefore, it was considered worthwhile to correlate the half-wave poten-
TABLE 1 P o l a r o g r a p h i c c h a r a c t e r i s t i c s of a r y l a z o p y r i m i d i n e s in d i m e t h y l f o r m a m i d e No.
R
--EII21V
I II III IV V VI VII VIII IX X XI XII XIII XIV
H 2-OCH 3 4-OCH 3 3-C1 4-C1 3-CH 3 4-CH 3 2-NO 2 3-NO 2 4-NO2 2-Br 4-I 3,5-(CH3) 2 2,4-(OCH3) 2
0.60 0.60 0.62 0.57 0.56 0.60 0.60 0.58 0.56 0.56 0.56 0.59 0.60 0.60
AEI/2/V 0.00 0.00 --0.02 +0.03 +0.04 0.00 0.00 +0.02 +0.04 +0.04 +0.04 +0.01 0.00 0.00
id/t.tA
tin
I
4.50 4.50 4.25 4.25 4.25 4.65 4.25 5.00 4.25 4.50 5.00 3.50 4.50 4.75
0.417 0.433 0.417 0.433 0.417 0.390 0.433 0.433 0.417 0.433 0.410 0.410 0.433 0.390
0.61 0.61 0.58 0.58 0.58 0.64 0.58 0.70 0.58 0.61 0.70 0.47 0.61 0.64
431
I
-0.2.
I
-O.G
I
I
-I.0
POTEIVTIA / / V
=-
Fig.
1. T y p i c a l p o l a r o g r a m s o f a r y l a z o p y r i m i d i n e s at C = 1.0 × 10 - 4 M. ( 1 ) R = H, h = 4 5 c m ; ( 2 ) R = H, h = 6 0 c m ; ( 3 ) R = 3 - N O 2 , h = 4 5 c m ; ( 4 ) R = 4 - C H 3 , h = 45 c m ; ( 5 ) R = 3-CH3, h = 4 5 c m .
tial of these pyrimidines with the Hammett substituent constant [13] to express the effect of substituent quantitatively. The values of o were taken from obenzoic acids. Figure 2 represents a linear plot of --Ez/2 vs. o for arylazopyrimidines in dimethylformamide. The values for o r t h o derivatives, viz., 2-OCHa, 2-NO2, 2-Br, 2-4-di-OCH3, were found to show deviation from the regression line and from the values of meta and para derivatives, only the 4-C1 derivative shows deviation. The value of specific reaction constant (p) was found to be 0.08 V, which is comparable with the reported values for similar systems [15]. The El/2 value of di-substituted compounds (XIII and XIV) indicated that the 2,4-disubstituted derivative does not follow the additivity principle [16]
I ~ = 01.~hoder.iveHves }
O.GO
--
"~'o.~5,e-~°'58 I -O.G
Fig.
~ or -0.3
0.0 O--
+0.2
°z~z~ +O.&
2. P l o t o f - - E l ~ 2 vs. O f o r a r y l a z o p y r i m i d i n e s in d i m e t h y l f o r m a m i d e .
~0.$
432
and shows a smaller shift of 0.02 V, whereas the 3,5-disubstituted derivative shows a reasonable additivity of 3 and 5 shifts. The smaller shift of 0.02 V in the case of the ortho derivative may be attributed to the steric hindrance to coplanarity [ 17 ]. REFERENCES
1 P.J. Elving, W.A. Struck and D.L. Smith, Mises Point Chim., Anal. Org. Pharm. Bromotol., 14 (1965) 141. 2 D.L. Smith and P.J. Elving, J. Amer. Chem. Soc., 84 (1962) 2741. 3 V.K. Mahesh, R.N. Goyal and Om Prakash, J. Electroanal. Chem., 72 (1976) 117. 4 B. Janik and P.J. Elving, J. Amer. Chem. S o c , 92 (1970) 235. 5 V.K. Mahesh, R.N. Goyal and R. Gupta, J.I.C.S., accepted. 6 T. Deries and J.L. Kroon, J. Amer. Chem. Soc., 75 (1953) 2484. 7 V. Nejedly, Collect. Czech. Chem. Commun., 1 (1922) 319. 8 L. Meites, Polarographic Techniques, Interscience, New York, 1967, p. 232. 9 B. Nygard, Arkiv Kemi, 26 (1966) 167. 10 C.R. Castor and H.J. Saylor, J. Amer. Chem. Soc., 75 (1953) 1427. 11 G.S. Hortley, J. Chem. Soc., (1938) 633. 12 P.J. Hilson and P.P. Birubaun, Trans. Faraday Soe., 48 (1952) 478. 13 L.P. H a m m e t t , J. Amer. Chem. Soc., 59 (1937) 96. 14 H.H. Jaffe, Chem. Rev., 53 (1953) 191. 15 P. Zuman, Substituent Effects in Organic Polarography, Plenum Press, New York, 1967. 16 R.W. Brockman and D.E. Pearson, J. Amer. Chem. Soc., 74 (1952) 4128. 17 M. Charton and B.I. Charton, J. Org. Chem., 36 (1971) 260.
429
© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
Short communication POLAROGRAPHIC INVESTIGATIONS OF SOME ARYLAZOPYRIMIDINES
V.K. MAHESH, R.N. GOYAL and RAMA GUPTA
Department of Chemsitry, University of Roorkee, Roorkee (India) (Received 6th December 1976; in revised form 3rd February 1977)
Introduction The polarography of pyrimidines at the dropping mercury electrode has been extensively studied by several workers [1--4]; however, no attempt has been made so far in determining the electrochemical behaviour of arylazopyrimidines. In the present communication we report the results of our investigations on the 14 arylazopyrimidines (A). As all these pyrimidines were water insoluble, investigations were carried out in dimethylformamide. OH
R
l_lS-~? N = N ~ (A) Experimental The substituted 5-arylazo-2-thio-4-hydroxy-6-methyl pyrimidines (I--XIV) were synthesised in the laboratory by the method developed [ 5] in this laboratory. The purity of these compounds was ascertained by recrystallisation and by t.l.c. The solutions (1 X 10 - 3 M) of all these pyrimidines were prepared in methanol (A.R.). Dimethylformamide (A.R.) was used throughout these investigations and 0.1 M LiC1 (A.R.) in dimethylformamide was used as supporting electrolyte. Triply distilled mercury was used for the DME.
Apparatus. Polarographic measurements were made with a Cambridge pen recording polarograph. The capillary had a flow rate of 0.91 mg s -1 and drop time 4.02 s at h = 45 cm in 1 M KC1 solution. A H-type cell (thermostated at 30 + 0.1 ° C) was used for recording the polarograms. The number of electrons involved in the reduction was determined by the method of DeVries and Kroon [6] using a mercury pool cathode. The value of the temperature coefficient was determined by Nejedly's method [7].
Procedure. Working solutions were prepared by taking 8.0 ml of dimethylformamide, 1.0 ml of arylazopyrimidine and 1.0 ml of 1.0 M lithium chloride
430
solution. The solution was then thoroughly mixed by the stream of nitrogen for 15 min and the polarographic curves were then recorded.
Results and discussions All the arylazopyrimidines (Table 1) gave a single two-electron transfer, well defined cathodic wave in dimethylformamide (typical polarograms are shown in Fig. 1). Keeping in view the feasibilities of the sites of the reduction it may be concluded that the possible reduction is reduction of --N=N--, and the pyrimidine nucleus exhibits no reduction wave at the dropping mercury cathode. Furthermore this wave can be safely assigned to the reduction of --N=N-- as --N=N-- reduction often occurs at positive potential, --C=N usually at more negative. The waves were found to be diffusion controlled as shown b y the linear plots of id VS. x/h and id VS: concn, of depolarizer. The low value of the temperature coefficient further confirmed the diffusion controlled nature of the waves. The values of the diffusion current constant (I) were also f o u n d to lie in the same range (Table 1). The polarograms of all the arylazopyrimidines were recorded in the concentration range 1.0 X 10 - 4 --3.0 X 10 - 4 M and it was found that the half-wave potentials are not constant b u t become more positive with increasing concentration of the azopyrimidines (Table 1). This fact confirmed the irreversible nature of the waves which was again confirmed by the log plots [8]. A mechanism for the reduction of the azogroup similar to that suggested by Nygard [9] and others [10--12] can be proposed for these compounds.
Effect o f substituents. Table 1 gives the values of polarographic characteristics for arylazopyrimidines (I--XIV) and it can be seen that the value of the diffusion current constant (I) and the transfer coefficient (a) are reasonably constant. Therefore, it was considered worthwhile to correlate the half-wave poten-
TABLE 1 P o l a r o g r a p h i c c h a r a c t e r i s t i c s of a r y l a z o p y r i m i d i n e s in d i m e t h y l f o r m a m i d e No.
R
--EII21V
I II III IV V VI VII VIII IX X XI XII XIII XIV
H 2-OCH 3 4-OCH 3 3-C1 4-C1 3-CH 3 4-CH 3 2-NO 2 3-NO 2 4-NO2 2-Br 4-I 3,5-(CH3) 2 2,4-(OCH3) 2
0.60 0.60 0.62 0.57 0.56 0.60 0.60 0.58 0.56 0.56 0.56 0.59 0.60 0.60
AEI/2/V 0.00 0.00 --0.02 +0.03 +0.04 0.00 0.00 +0.02 +0.04 +0.04 +0.04 +0.01 0.00 0.00
id/t.tA
tin
I
4.50 4.50 4.25 4.25 4.25 4.65 4.25 5.00 4.25 4.50 5.00 3.50 4.50 4.75
0.417 0.433 0.417 0.433 0.417 0.390 0.433 0.433 0.417 0.433 0.410 0.410 0.433 0.390
0.61 0.61 0.58 0.58 0.58 0.64 0.58 0.70 0.58 0.61 0.70 0.47 0.61 0.64
431
I
-0.2.
I
-O.G
I
I
-I.0
POTEIVTIA / / V
=-
Fig.
1. T y p i c a l p o l a r o g r a m s o f a r y l a z o p y r i m i d i n e s at C = 1.0 × 10 - 4 M. ( 1 ) R = H, h = 4 5 c m ; ( 2 ) R = H, h = 6 0 c m ; ( 3 ) R = 3 - N O 2 , h = 4 5 c m ; ( 4 ) R = 4 - C H 3 , h = 45 c m ; ( 5 ) R = 3-CH3, h = 4 5 c m .
tial of these pyrimidines with the Hammett substituent constant [13] to express the effect of substituent quantitatively. The values of o were taken from obenzoic acids. Figure 2 represents a linear plot of --Ez/2 vs. o for arylazopyrimidines in dimethylformamide. The values for o r t h o derivatives, viz., 2-OCHa, 2-NO2, 2-Br, 2-4-di-OCH3, were found to show deviation from the regression line and from the values of meta and para derivatives, only the 4-C1 derivative shows deviation. The value of specific reaction constant (p) was found to be 0.08 V, which is comparable with the reported values for similar systems [15]. The El/2 value of di-substituted compounds (XIII and XIV) indicated that the 2,4-disubstituted derivative does not follow the additivity principle [16]
I ~ = 01.~hoder.iveHves }
O.GO
--
"~'o.~5,e-~°'58 I -O.G
Fig.
~ or -0.3
0.0 O--
+0.2
°z~z~ +O.&
2. P l o t o f - - E l ~ 2 vs. O f o r a r y l a z o p y r i m i d i n e s in d i m e t h y l f o r m a m i d e .
~0.$
432
and shows a smaller shift of 0.02 V, whereas the 3,5-disubstituted derivative shows a reasonable additivity of 3 and 5 shifts. The smaller shift of 0.02 V in the case of the ortho derivative may be attributed to the steric hindrance to coplanarity [ 17 ]. REFERENCES
1 P.J. Elving, W.A. Struck and D.L. Smith, Mises Point Chim., Anal. Org. Pharm. Bromotol., 14 (1965) 141. 2 D.L. Smith and P.J. Elving, J. Amer. Chem. Soc., 84 (1962) 2741. 3 V.K. Mahesh, R.N. Goyal and Om Prakash, J. Electroanal. Chem., 72 (1976) 117. 4 B. Janik and P.J. Elving, J. Amer. Chem. S o c , 92 (1970) 235. 5 V.K. Mahesh, R.N. Goyal and R. Gupta, J.I.C.S., accepted. 6 T. Deries and J.L. Kroon, J. Amer. Chem. Soc., 75 (1953) 2484. 7 V. Nejedly, Collect. Czech. Chem. Commun., 1 (1922) 319. 8 L. Meites, Polarographic Techniques, Interscience, New York, 1967, p. 232. 9 B. Nygard, Arkiv Kemi, 26 (1966) 167. 10 C.R. Castor and H.J. Saylor, J. Amer. Chem. Soc., 75 (1953) 1427. 11 G.S. Hortley, J. Chem. Soc., (1938) 633. 12 P.J. Hilson and P.P. Birubaun, Trans. Faraday Soe., 48 (1952) 478. 13 L.P. H a m m e t t , J. Amer. Chem. Soc., 59 (1937) 96. 14 H.H. Jaffe, Chem. Rev., 53 (1953) 191. 15 P. Zuman, Substituent Effects in Organic Polarography, Plenum Press, New York, 1967. 16 R.W. Brockman and D.E. Pearson, J. Amer. Chem. Soc., 74 (1952) 4128. 17 M. Charton and B.I. Charton, J. Org. Chem., 36 (1971) 260.