INTRODUCTION:

Various inorganic salts of Cr (VI) are well known oxidizing reagents used in synthetic organic chemistry for the oxidation of organic compounds. However, these salts are rather drastic and nonselective oxidants. Further, they are insoluble in most of the organic solvents. Therefore, miscibility is a one of the problem. To overcome these limitations, a large number of organic derivatives of Cr (VI) have been prepared and used in organic synthesis as mild and selective oxidizing reagents in non-aqueous solvents¹, tetrakis (pyridine) silver dichromate (TPSD) is also one of such compounds reported by Firouzabadi et al.² used as mild and selective oxidizing agent in synthetic organic chemistry. Only a few reports on the oxidation aspects of TPSD are available in the literature^3-6.

We have been interested in the kinetic and mechanistic aspects of the oxidations by complex salts of Cr (VI) and have already published a few reports on oxidation by halochromates^9-11. In continuation of our earlier work, we report in the present article the kinetics of oxidation of some monosubstituted benzaldehydes by TPSD in DMSO as solvent. The major objective of this investigation was to study the structure-reactivity correlation for the substrate undergoing oxidation.

MATERIALS AND METHODS:

Materials:

TPSD was prepared by reported method² and its purity was checked by iodometric method. The aldehydes were commercial products. The liquid aldehydes were purified through their bisulfite addition compounds and distilling them, under nitrogen, just before use¹². The solid aldehydes were recrystallized from ethanol. Deuteriated benzaldehyde (PhCDO) was also prepared by the literature method¹³. Its isotopic purity, as ascertained by its NMR spectrum, was 97±5%. Due to non-aqueous nature of the solvent, toluene-p‑sulphonic acid (TsOH) was used as a source of hydrogen ions. Solvents were purified by the usual literature methods.

Product analysis:

The product analysis was carried out under kinetic conditions. In a typical experiment, benzaldehyde (5.25 g, 0.05 mol) and TPSD (7.48 g, 0.01 mol) were made up to 50 cm³ in DMSO and kept in the dark for ca. 15 h to ensure completion of the reaction. The solution was then treated with an excess (200cm³) of a saturated solution of 2,4‑dinitrophenylhydrazine in 2mol dm^‑3HCl and kept overnight in a refrigerator. The precipitated 2,4‑dinitrophenylhydrazone (DNP) was filtered off, dried, weighed, recrystallized from ethanol, and weighed again. The yields of DNP before and after recrystallization were 2.46 g (86%) and 2.28 g (80%) respectively. The DNP was found identical (m.p. and mixed m.p.) with the DNP of benzaldehyde. Similar experiments were performed with other aldehydes also. The oxidation state of chromium in completely reduced reaction mixtures, determined by an iodometric method was 3.92 ± 0.10.

Kinetic Measurements:

The pseudo‑first order conditions were attained by maintaining a large excess (´ 15 or more) of the aldehydeover TPSD. The solvent was DMSO, unless specified otherwise. The reactions were followed,at constant temperatures (±0.1 K), by monitoring the decrease in [TPSD] spectrophotometricall at 370 nm. No other reactant or product has any significant absorption at this wavelength. The pseudo‑first order rateconstants, k_obs, were evaluated from the linear (r = 0.990 ‑ 0.999) plots of log [TPSD] againsttime for up to 80% reaction. Duplicate kinetic runs showed that the rate constants were reproducible to within ±3%. The second order rate constant, k₂, was obtained from the relation: k₂ = k_obs / [aldehyde].All experiments, other than those for studying the effect of hydrogen ions, were carried out in the absence of TsOH.

RESULTS AND DISCUSSION:

The rates and other experimental data were obtained for all the aldehydes. Since the results are similar, only representative data are reproduced here.

Stoichiometry:

Oxidation of benzaldehydes by TPSD results in the formation of corresponding benzoic acids. Analysis of products and the stoichiometric determinations indicate the following overall reaction as equation (1).

3ArCHO+Cr₂O₇^-2+8H⁺

3ArCOOH + 4H₂O+2Cr³⁺ (1)

Thus, TPSD undergoes a three-electron change. This is in accord with the earlier observations with TPSD⁷. The other halochromates^8-11as well as both pyridinium fluorochromate (PFC)¹⁴ and pyridinium chlorochromate (PCC)¹⁵ act as two electron oxidants and are reduced to chromium (IV) species.

Test for free radicals:

The oxidation of benzaldehyde by TPSD, in an atmosphere of nitrogen failed to induce the polymerisation of acrylonitrile. Further, an addition of a radical scavenger, acrylonitrile, had no effect on the rate (Table 1). To further confirm the absence of free radicals in the reaction pathway, the reaction was carried out in the presence of 0.05 mol dm-3 of 2,6-di-t-butyl-4-methylphenol (butylated hydroxytoluene or BHT). It was observed that BHT was recovered unchanged, almost quantitatively.

Rate laws:

The reactions were found to be first order with respect to TPSD. Figure-1 depicts a typical kinetic run. Further, the pseudo first order rate constant, kobs, was independent of initial concentration of TPSD. The reaction rate increases linearly with increase in the concentration of the aldehydes (Table 1).

Figure 1.Oxidation of para-methyl benzaldehyde by TPSD: A typical kinetic run

A double reciprocal plot of 1/kobs against 1/[Aldehyde] is linear (r²> 0.995) as shown in Figure 2.

The dependence of reaction rate on the reductant concentration was studied at different temperatures and the values of K and k₂ were evaluated from the double reciprocal plots. The activation parameters of the decomposition of the complexes were calculated from the values of k₂ at different temperatures (Tables 2).

Effect of acidity:

The reaction is catalysed by hydrogen ions (Table 3). The hydrogen-ion dependence taking the form as equation-2.

kobs = a + b [H⁺] (2)

The values fora and b for benzaldehyde are 19.98 ± 0.22 ´ 10^-⁴ s^-¹and 34.2 ± 0.36 ´ 10^-⁴ mol^-¹ dm³ s^-¹ respectively (r² = 0.999).This suggests that, TPSD is protonated is a fast pre-equilibrium and both the protonated and unprotonated forms are reactive oxidizing species.

Figure 2. Oxidation of para-methyl benzaldehyde by TPSD: A double reciprocal plot.

Table 1. Rate constants for the oxidation of para-methyl benzaldehyde by TPSD at 298 K.

10³[TPSD]	[Aldehyde]	[TsOH]	10⁴kobs
(mol dm^-3)	(mol dm^-3)	(mol dm^-3)	(s^-1)
1.00	0.10	0.00	3.98
1.00	0.20	0.00	8.07
1.00	0.40	0.00	16.2
1.00	0.60	0.00	25.1
1.00	0.80	0.00	31.9
1.00	1.00	0.00	39.6
1.00	2.00	0.00	80.3
2.00	0.40	0.00	17.4
4.00	0.40	0.00	16.5
6.00	0.40	0.00	17.1
8.00	0.40	0.00	16.8
1.00	0.40	0.00	16.9*

*Contained 0.001 mol dm^-3acrylonitrile.

Table 2. Rate constants and activation parameters for the decomposition of TPSD-Aldehyde complexes.

Substituent		10⁴k₂, dm³ mol^-1 s^-1			∆H^*	∆S^*	∆G^*
	288 K	298 K	308 K	318 K	kJ mol^-1	J mol^-1 K^-1	kJ mol^-1
H	7.83	19.8	48.6	117	66.1±0.6	-75 ± 2	88.4±0.5
p-Me	16.2	39.6	93.6	216	63.2±0.4	-79 ± 1	86.7±0.3
p-Ome	37.8	90.0	207	468	61.3±0.5	-79 ± 2	84.7±0.4
p-F	9.36	24.3	60.3	144	66.8±0.3	-71 ± 1	87.9±0.2
p-Cl	5.76	15.3	38.5	96.3	68.8±0.5	-68 ± 2	89.1±0.4
p-NO₂	0.48	1.44	4.14	11.7	78.4±0.7	-56 ± 2	94.9±0.5
p-CF₃	1.26	3.51	9.72	26.1	74.4±0.8	-62 ± 3	92.7±0.6
p-COOMe	1.66	4.50	12.2	31.5	72.3±0.7	-67 ± 2	92.0±0.6
p-Br	5.63	14.4	37.7	93.6	69.0±0.8	-68 ± 3	89.1±0.6
p-NHAc	18.0	44.1	108	243	63.7±0.4	-77 ± 1	86.4±0.3
p-CN	0.81	2.38	6.66	18.0	76.1±0.4	-59 ± 1	93.7±0.4
p-Sme	22.7	53.1	126	288	62.1±0.7	-81 ± 2	85.9±0.6
p-NMe₂	153	342	720	1500	55.3±0.3	-88 ± 1	81.4±0.2
m-Me	14.1	34.2	82.8	189	63.5±0.5	-80 ± 1	87.0±0.4
m-Ome	14.4	34.8	80.1	180	61.5±0.3	-86 ± 1	87.0±0.2
m-Cl	2.40	6.30	16.2	40.0	68.9±0.5	-75 ± 2	91.2±0.4
m-Br	2.34	6.21	15.8	39.6	69.2±0.6	-75 ± 2	91.3±0.4
m-F	2.97	7.56	19.0	46.8	67.4±0.7	-79 ± 2	90.8±0.6
m-NO₂	0.25	0.72	2.16	6.03	78.5±0.9	-61 ± 3	96.5±0.8
m-CO₂Me	1.26	3.56	9.54	24.3	72.6±0.2	-68 ± 1	92.7±0.2
m-CF₃	0.88	2.48	6.75	18.0	74.0±0.6	-66 ± 2	93.5±0.5
m-CN	0.45	1.31	3.67	9.81	75.7±0.4	-66 ± 1	95.1±0.3
m-Sme	9.99	24.3	56.7	126	61.8±0.2	-88 ± 1	87.9±0.2
m-NHAc	8.82	21.6	51.3	117	63.1±0.3	-85 ± 1	88.2±0.3
PhCDO	1.28	3.44	8.71	22.0	69.5±0.5	-79 ± 2	92.8±0.2
k_H/k_D	6.12	5.76	5.58	5.32

Table 3. Dependence of the reaction rate on hydrogen ion concentration.

[Benzaldehyde] = 0.10 mol dm^-3 ; [TPSD] = 0.001 mol dm^-3 ; Temp. 298 K.
[TsOH],moldm³	0.10	0.20	0.40	0.60	0.80	1.00
10⁴k_obs/s^-1	23.4	27.0	33.3	40.5	47.7	54.0

Effect of temperature:

The rates of the oxidation of benzaldehydes by TPSD were determined at different temperatures and the activation parameters were calculated (Table 2).

Kinetic isotope effect:

To ascertain the importance of the cleavage of the aldehydic C-H bond in the rate-determining step, the oxidation of α,α-dideuterio-benzaldehyde (PhCDO) was studied. The results (k_H/k_D = 6.12 at 288 K) showed the presence of a substantial primary kinetic isotope effect (Table 2).

Effect of solvents:

The oxidation of benzaldehyde was studied in 19 different organic solvents. The choice of solvents was limited by the solubility of TPSD and its reaction with primary and secondary alcohols. There was no reaction with the solvents chosen. Kinetics is similar in all the solvents. The values of k₂ are recorded in Table 4.

Table 4. Solvent effect on the oxidation of benzaldehyde by TPSD at 298 K

Solvents	10⁴k₂(s^-1)
Chloroform	38.0
1,2-Dichloroethane	45.7
Dichloromethane	39.8
DMSO	117
Acetone	41.7
DMF	57.5
Butanone	30.2
Nitrobenzene	43.7
Benzene	15.8
Cyclohexane	1.51
Toluene	12.0
Acetophenone	51.3
THF	22.4
t-Butylalcohol	14.5
1,4-Dioxane	21.4
1,2-Dimethoxyethane	10.5
Carbon disulfide	17.0
Acetic acid	5.62
Ethyl acetate	7.94

The correlation between activation enthalpies and entropies of the oxidation of the twenty-four banzaldehydes is just satisfactory (r²= 0.8900), indicating the operation of a weak compensation effect¹⁶. The value of the isokinetic temperature is 624± 50 K. However, according to Exner¹⁷, an isokinetic relationship between the calculated values of activation enthalpies and entropies is often vitiated by random experimental errors. Exner suggested an alternative method for establishing the isokinetic relationship. Exner's plot between log k₂ at 288 K and at 318 K (Figure 3) was linear (r² = 0.998). The value of isokinetic temperature evaluated from the Exner's plot is 774 K. The linear isokinetic correlation implies that all the aldehydes are oxidized by the same mechanism and the changes in the rate are governed by changes in both the enthalpy and entropy of activation.

Figure 3. Exner’s Isokinetic Relationship in the oxidation of benzaldehydes by TPSD.

The rate constants k₂, in eighteen solvents (CS₂ was not considered as the complete range of solvent parameters was not available) were correlated in terms of linear solvation energy relationship (Equation 3) of Kamlet and Taft¹⁸.

log k₂ = A0 + pp* + bb + a a (3)

In this equation, p^* represents the solvent polarity, b the hydrogen bond acceptor basicities and a is the hydrogen bond donor acidity. A₀ is the intercept term. It may be mentioned here that out of the 18 solvents, 13 has a value of zero fora. The results of correlation analyses terms of biparametric equation involving p^* and b, and separately with p^* and b are given below by Equations (4) - (7).

log k₂=- 3.72+ 1.56 (±0.20) p^*+0.26 (±0.15) b+0.12 (±0.15) a (4)

R²= 0.8421; sd = 0.19; n = 18; y = 0.40

log k₂ =- 3.75 + 1.58 (±0.19) p^* + 0.27 (±0.15) b (5)

R²= 0.8351; sd = 0.19; n = 18; y = 0.41

log k₂ = - 3.71 + 1.65 (±0.20) p^* (6)

r² = 0.8010; sd = 0.20; n = 18; y = 0.45

log k₂ = -2.81 + 0.55 (±0.33) b (7)

r² = 0.1430; sd = 0.42; n = 18; y = 0.96

Here n is the number of data points and y is Exner's statistical parameter¹⁹.

Kamlet's¹⁸triparametric equation explain ca. 84 % of the effect of solvent on the oxidation. However, by Exner's criterion the correlation isnot even satisfactory [cf. (4)].The major contribution is of solvent polarity.It alone accounted for ca. 80 % of the data. Both b and a play relatively minor roles.The data on the solvent effect were also analysed in terms of Swain's²⁰ equation of cation‑ and anion‑solvating concept of the solvents (Eq. 8).

log k₂ =aA + bB + C (8)

Here A represents the anion‑solvating power of the solvent and B is the cation‑solvating power.C is the intercept term. (A + B) is postulated to represent the solvent polarity.The rates in different solvents were analysed in terms of Swain equation (Eq. 8), separately with A and B and with (A + B).

log k₂= 0.64 (±0.07)A+1.74 (±0.05)B-3.95 (9)

R² = 0.9860; sd = 0.05; n = 19; y = 0.11

log k₂ = 0.40 (±0.56) A - 2.77 (10)

r²= 0.0290; sd = 0.45; n = 19; y = 1.01

log k₂ = 1.69 (±0.12) B - 3.74 (11)

r² = 0.9140; sd = 0.13; n = 19; y = 0.29

log k₂=1.37 ± 0.15 (A + B) - 3.92 (12)

r² = 0.8423; sd = 0.18; n = 19; y = 0.40

The rates of oxidation of benzaldehyde in different solvents showed an excellent correlation in Swain's equation (Eq. 9) with the cation‑solvating power playing the major role. In fact, the cation‑solvation alone account for ca. 91% of the data. The correlation with the anion‑solvating power was very poor.

The solvent polarity, represented by (A + B), also accounted for ca. 84% of the data. In view of the fact that solvent polarity is able to account for ca. 84% of the data, an attempt was made to correlate the rate with the relative permittivity of the solvent.However, a plot of log k₂ against the inverse of the relative permittivity is not linear (r² = 0.503; sd = 0.33; y = 0.76).

Correlation analysis of reactivity:

The effect of structure on reactivity has long been correlated in terms of the Hammett equation²¹ or with dual substituent‑parameter equations^{22, 23}. In the late 1980s, Charton²⁴ introduced a triparametric LDR equation for the quantitative description of structural effects on chemical reactivities. This triparametric equation results from the fact that substituent types differ in their mode of electron delocalization.

log k₂ = L s_l + D s_d + R s_e + h (13)

Here, s_l is a localized (field and/or inductive) effect parameter, s_d is the intrinsic delocalizedelectrical effect parameter whenactivesite electronic demand is minimal and s_e represents the sensitivity of the substituent to changes in electronic demand by the active site.The latter two substituent parameters are related by Eq. (14).

s_D = hs_e +s_d (14)

Here h represents the electronic demand of the reaction site and is given by h = R/D, and s_D represents the delocalized electrical parameter of the diparametric LD equation.

The rates ofoxidation of meta‑ and para‑substituted benzaldehydes show an excellent correlation in terms of the LDR equations (Table 5).We have used the standard deviation (sd), the coefficient of multiple determination (R²), and Exner's¹⁹ parameter, y, as the measures of goodness of fit.

The comparison of the L and D values for the substituted benzaldehydes showed that the oxidation of para‑substituted benzaldehydes is more susceptible to the delocalization effect than to the localized effect. However, the oxidation of meta‑substituted compounds exhibited a greater dependence on the field effect. In all cases, the magnitude of the reaction constants decreases with an increase in the temperature, pointing to a decrease in selectivity with an increase in temperature.

Table 5. Temperature dependence for the reaction constants for the oxidation of substituted benzaldehydes by TPSD.

T/K	*- L*		*- D*		*- R*	S	h	R²	Sd	y	P_D	P_S
						Para	substituted
288		-1.44		-1.89	-1.23	-	0.65	0.9999	0.007	0.01	56.8	-
298		-1.35		-1.80	-1.16	-	0.64	0.9998	0.005	0.02	57.1	-
308		-1.26		-1.71	-1.08	-	0.63	0.9989	0.003	0.01	57.6	-
318		-1.18		-1.61	-1.07	-	0.63	0.9999	0.005	0.01	57.7	-
					Meta substituted
288		-1.98	-1.44		-1.14	-	0.79	0.9998	0.005	0.02	42.1	-
298		-1.89	-1.36		-1.08	-	0.79	0.9999	0.004	0.01	41.8	-
308		-1.80	-1.26		-0.98	-	0.78	0.9989	0.003	0.04	41.2	-
318		-1.71	-1.17		-0.90	-	0.77	0.9999	0.005	0.01	40.6	-

All three regression coefficients, L, D and R, are negative indicating an electron deficient carbon centre in the activated complex for the rate determining step. The positive value of h adds a negative increment to s_d, reflecting the electron donating power of the substituent and its capacity to stabilize a cationic species.

In the cases of the oxidation of para- and meta-subsituted benzaldehydes, multiple regression analyses indicated that both localization and delocalization effects are significant. There is no significant colinearity between the various substituents constants for the two series. The percent contribution²⁵ of the delocalized effect, P_D, is given by following equation (15).

P_D = (½D½´ 100) / ( ½L½ + ½D½) (15)

The values of P_Dare also recorded in Table 5.The value of P_D for the oxidation of para‑substituted benzaldehydes is ca. 57% whereas the corresponding values for the meta‑substituted aldehydes isca. 41%. This shows that the balance of localization and delocalization effects is differentfor differently substituted benzaldehydes.

The benzoyl cation is reported have a considerable ketene character²⁶ and is thus linear. The linear structure of acylium cation has been confirmed by X-ray crystallography also²⁷. The change from sp²to sp results in steric relief.

MECHANISM:

A hydrogen abstraction mechanism leading to the formation of the free radicals is unlikely in view of the failure to induce polymerization of acrylonitrile and no effect of the radical scavenger on the reaction rate. The presence of a substantial kinetic isotope effect confirms the cleavage of an aldehydic‑C‑H bond in the rate‑determining step. The negative values of the localization and delocalization electrical effects i.e. of L, D and R points to an electron deficient reaction centre in the rate‑determining step. It is further supported by the positive value ofh, which indicates that the substituent is better able to stabilise a cationic or electron‑ deficient reactive site. Therefore, a hydride‑ion transfer in the rate‑determining step is suggested (Scheme 1 and 2). The hydride-ion transfer mechanism is also supported by the major role of cation-solvating power of the solvents.

The observed negative value of entropy of activation also supports the proposed mechanism. As the charge separation takes place in the transition state, the charged ends become highly solvated. This results in an immobilization of a large number of solvent molecules, reflected in the loss of entropy²⁸.

Initially Cr(VI) is reduced to Cr(IV). It is likely to react with another Cr(VI) to generate Cr(V) which is then reduced in a fast step to the ultimate product Cr(III). Such a sequence of reactions in Cr(VI) oxidations is well known²⁹.

Scheme 1: Acid-independent path

Scheme 2: Acid-dependent path

APPLICATIONS:

By this study a suitable mechanism can be suggested.

CONCLUSION:

The reaction is proposed to proceed through a hydride-ion transfer from aldehyde to the oxidant in the rate-determining step. Both de-protonated and protonated forms of TPSD are the reactive oxidising species. An aldehydic C-H bond is cleaved in the rate-determining step.

ACKNOWLEDGEMENTS:

We are thankful to the authorities of R.D. and S.H. National College and S.W.A. Science College, Bandra(W), Mumbai, for providing us necessary facilities to carry out our research work and We are also thankful to Professor P. K. Sharma, Head, Department of Chemistry, J. N. V. University, Jodhpur and Professor Kalyan K. Banerji, Dean, Sciences, National Law University, Mandore, Jodhpur, for their critical suggestions.

CONFLICT OF INTEREST:

The authors declare no conflict of interest.

REFERENCES:

1. Mahanti MKandBanerji KK. J. Indian Chem. Soc., 2002; 79:31.

2. Firouzabadi H, Sardarian Aand Giaribi H. Synth. Communn., 1984; 14: 89.

3. Purohit P, Kumbhani S, Shastri I, Banerji KK and Sharma PK. Indian J. Chem., Sect. A, 2008; 47:1671.

4. Choudhary A, Yajurvedi D, Kumbhani S, Shastri I and Sharma V. J. Indian Chem. Soc., 2009; 86:832.

5. Meena AK, Daiya A, Banerji J, Sajo I, Kotai L and Sharma V. J. Indian Chem. Soc., 2011; 88: 1887.

6. Meena AK, Daiya A, Sharma A, Choudhary A, Banerji J, Kotai L, Sajo I and Sharma V. Int. J. Chem., 2012; 1:55.

7. Patel M, Poonam, Jha K, Baghmar M, Shastri I and Sharma PK. J. Indian Chem. Soc.,2012; 89:1149.

8. Pohani P, Anjana, Sharma PK. Indian J. Chem., 2006; 45A:2218.

9. Yajurvedi D, Kumbhani S, ShastriI,BaghmarMand Sharma PK. J. Indian Chem. Soc., 2008; 85: 459.

10. Soni N, Tiwari V, Kumbhani S, Shastri I, Sharma V.J. Indian Chem. Soc., 2008; 85:857.

11. Tiwari V, Kumbhani S, Shastri I, Sharma V.Indian J. Chem., 2008; 47A:1520.

12. Kalpan. J. Am. Chem. Soc., 1958; 80:2639.

13. Wiberg KB. J. Am. Chem. Soc., 1954; 76:5871.

14. Brown HC, Rao GC, Kulkarni SU. J. Org. Chem., 1979; 44:2809.

15. Bhattacharjee MN, Choudhuri MK, Purakayastha S. Tetrahedron, 1987; 43:5389.

16. Liu, L., Q-X Guo, Chem. Rev., 2001; 101:673.

17. Exner O, Collect. Chem. Czech. Commun., 1977;38:411.

18. Kamlet MJ, Abboud JLM, Abraham MH, Taft RW. J. Org. Chem., 1983; 48:2877.

19. Exner O, Collect. Chem. Czech. Commun., 1966;31:3222.

20. Swain CG, Swain MS, Powel AL, Alunni S. J. Am. Chem. Soc., 1983; 105: 502.

21. Johnson CD, The Hammett equation, University Press, Cambridge. 1973, 78.

22. Dayal SK, Ehrenson S, Brownlee RTC, Taft RW. J. Am. Chem. Soc., 1974; 96:9113.

23. Swain CG, Unger SH, Rosenquest NR, Swain MS. J. Am. Chem. Soc., 1983; 105: 492.

24. Charton M, Charton B. Bull. Soc. Chim. Fr., 1988, 199 and references cited therein.

25. Charton M.J. Org. Chem., 1975; 40: 407.

26. Olah GA, Westermann PW. J. Am. Chem. Soc., 1973; 95:3706.

27. Boer FP. J. Am. Chem. Soc., 1968; 90:6706.

28. Gould ES.Mechanism and structure in organic chemistry, Holt, Rinehart and Winston Inc., New York. 1964.

29. Wiberg KB, Richardson WH. Oxidation in organic chemistry, Vol.5A, Academic Press, New York.1965.

Received on 04.09.2017 Modified on 05.12.2017

Asian J. Research Chem. 2017; 10(6): 803-809.

DOI: 10.5958/0974-4150.2017.00134.1