Indian Journal of Chemistry Vol. 43A, January 2004, pp. 79-82

Oxidative conversion of benzoic and o-chlorobenzoic acid hydrazides to their

corresponding acids by thallium(III) : A mechanistic study

A R Tardale, A N Phadkule, J B Patil & G S Gokavi*

Department of Chemi stry, Shivaji Uni versity, Kolhapur 4 I 6004, Maharashtra, India

Received 17 Jallllary 2003; revised 9 Jlllle 2003

The reac tions between thalliull1(III ) and benzoic ac id and 0-

chlorobenzoic acid hydrazides have been studied in a mixture of perchloric and hydrochlori c ac id medium. The reaction in volves formati on of complex between the reactants, which decompose in the subsequent step to give prod ucts. The reaction proceeds by direct two electron transfer without interventi on of free rad icals. Increase in [H+J and [C!"] decreases the rate of the reacti on. The increase in the ionic strength does not affect the rate of the reacti on wh ile dec rease in the relative permiti vi ty increases the same. The effect of temperature on the react ion has been studied between 25 to 45uC and the ac ti vation parameters werc determined fo r the slow step of the reacti on.

Hydraz ides, which are derivatives of both carboxylic ac ids and hydrazi ne have been utili zed as starting materi als in organic sy nthes is I. Th e ox idati ve transformation of hydrazides with most ox idants g ive cOlTcsponding ac ids2 and in some cases3 esters or am ides. Hydrazides have also been converted into N­N-diacylhydrazines wi th various oxidants4

. Formation of ac ids and their deri vati ves in presence of different nucleophiles is the indication of direct two-electron transfer to the ox idant, while cycli zat ion products generally invo lve a single electron transfer with in tervention of a free rad ical. Thallium(IU) salts are well known oxidants5 in organic sy nthes is. Many of its reac tions proceed via formation of organothallium intermediates or via bond formation between thallium and 0 , S or N, which subsequently decompose to give thallium(l) and the ox idised product. In reactions where direct bond formation is no t invo lved thallium(IIT) can act as an electron accepto r with the intervention of unstable TI(l[) as the probable path.

The redox potenti al of TI (III)/TI(I) couple is sensitive to the anion present in the so lution. In perchloric and su lphuric ac id medi aG it has the highest va lue of 1.23 V with e ither free TI 3

+, TIOH2+ or

thall ium(fII) sulphate comp lexes as acti ve species

respectively. Addition of small amounts of halide ions to the thallium(III) containing solution results in catalysis 7 but at higher concentrations of halide ion the rate of the oxidation decreases considerably. The reason for rate retardation is due to the decrease8 in the redox potential (0.68 V in 5 mol dm-3 HCl) as a result of formation of strong thallium(III) halide­complexes . Therefore, thallium(III) can be utili zed both as a strong(in pechloric and sulphuric acid media) and as a mild oxidant (in HCl medium) by changing the reactive species. Although thallium(III) has been utili sed for splitting of carbon-nitrogen bonds9 no mechanistic investigati on has been carried out. The present work deals with the kinetics and mechanism of oxidation of benzoic and o-chloro­benzoic acid hydrazides in a mixture of perch loric and hydrochloric ac id medium.

Experimental Thallium(III) solution was prepared by dissolving

TI 20 }(BDH) in 1.0 mol dm-3 HCl and the concen­tration was ascertained by iodometric titrati on. The benzoic and (BAH) o-chlorobenzoic acid (o-CIBAH) hydrazides were prepared from reported 10 procedure and characterised by determining their me lting po int. Stock solution of hydraz ides were prepared in 50% v/v ethanol. Jonic strength was maintained with sodi um perchlorate prepared from sodium carbonate (Loba) and perchloric acid (Merck).

The reac tions were carried out in 50% v/v ethanol (BDH) under pseudo-first-order conditions keeping concentration of hydrazides in large excess over that of the oxidant. The solutions containing the reactants and all other constituents were thermally equilibriated separately , mixed and the reaction mixture was analysed for unreac ted thallium(IIl) iodometrically by litrat if1 :?; against standard thiosulphate. The pseudo­first-order rate constants were determined from the s lopes of linear log[Tl (IH)] versus time plots. The results were reproducib le upto ±6%. Kinetic runs were followed to about three half-lives of the reaction. Under the experimental conditions oxidation of ethanol did not occur.

The sto ichiometry of the reaction was determined using a known excess of thallium(lJI) over hydrazide and determining the remaining oxidant iodometrically

80 INDIAN J CHEM. SEC A. JANUARY 2004

after 24 hrs . The results consistent with Eq. 1 were obtained. The corresponding carboxylic acids were characterised by determining their mp" (122 and 138 °c for benzoic and o-chlorobenzoic acid respectively) .

RCONHNH2 + 2TI(IIJ) + H20 ~ RCOOH + N2 + 4H+ + 2TI(I)

Results and discussion

... (1)

The reaction occurs rapidly in perchloric acid medium but in the presence of hydrochloric ac id the rate is measurable. Therefore, the reaction was carried out in a mixture of both the acids. The effec t of reactants on the reaction was studied at constant [HCI] and [HCI04] of 0.1 mol dm·3 each and ionic strength of 0.3 mol dm·3. Concentration of the oxidant was varied from 1.0x lO·3 to 1.0x lO·2 mol dm·3

keeping the [hydrazide] constant at 5.0xlO·2 mol dm·3.

Since, the pseudo-first-order rate constants were fairly

constant (7.6 + 0.l x lO·5 S·I for BAH at 25°C and 5.2 + 0.lxI0·5 S·I for o-CIBAH at 35°C), the order i~ [oxidant] is unity . The effect of [hydrazide] was studied between the concentration range from 1.6x 10·2 to 1.6x lO·1 mol dm·3 keeping the [oxidant]

constant at 3.0xI0·3 mol dm·3. The pseudo-first-order rate constants increase with the increase in concentration and the orders in [hydrazide] as determined from log-log plots of pseudo-first-order rate constants versus concentration were found to be fractional(0.9 for benzoic acid hydrazi de and 0.7 for o-chlorobenzoic acid hydrazide) .

To study the effect of [H+] and [Cr], [oxidant], [hydrazide land ionic strength were kept constant at 3.0xlO·3, 3.0xlO·2 and 0.3 mol dm·3 respectively. To vary [H+] and [Cr], HCI04 and NaCI were used. Increase in [H+] from 2.0xl0·2 to 2.7xI0· 1 mol dm·3

decreases 105 kobs (S·l) from 6.4 to 1.7 at 25 °c for BAH and from 5.1 to 1.1 at 35°c for o-CIBAH. Increase in [CI'] from 3.0xl0·2 to 3.0xlO· [ mol dm·3

decrease the values lOS kobs from 25.6 to 0.73 for BAH at 25°c and 15.4 to 0.67 for o-CIBAH at 35°c. The the orders in [H+] and [Cn as determined from log-log piots of pseudo-first-order rate constants versus concentration were found to be about -0.6 and -1 .5 for both the hydrazides.

The ionic strength of the reaction solution was varied between 0.2 to 0.4 mol dm-3 with sodium perchlorate at constant concentrations of oxidant, reductant, perchloric acid and hydrochloric acid at 3.0xI0·3, 3.0xlO·2, 0.1 and 0 .1 mol dm·3 respectively and there was no effect on the rate of the reaction.

The relative permittivity was varied by changing the ethanol content from 30 to 70% v/v. The rate was found to decrease wi th decrease 111 relati ve permittivity.

Added acrylonitrile in the concentration range 0.5 to 2 .5 vol. % by keeping concentrations of oxidant, reductant, perchloric acid, hydrochloric acid and ionic strength fixed did not produce any precipitate due to polymerization of the added acrylonitrile and there was also no effect on the pseudo-first-order rate constants indicating the absence of free radical.

Oxidation of organic compounds by thallium(lII) may involve either one-electron or two-electron transfer reactions. In the present swdy, since there was no effect of added acrylonitrile, the reaction proceeds with two electron transfer step. The order in thallium(IlI) was found to be unity and the orders in both the hydrazides were found to be fractional. Such fractional order in substrate concentration is due to

TIIII+Hydrazide ~ Complex Kc

Complex ~ Til + Intermediate

TIIII+lntermediate ~ Til + Products

Scheme 1

the prior complex formation equilibrium between the reactants. The Michealis-Menten plots of lIkobs versus II[Hydrazide] were linear with an intercept in support of the complex formation . Therefore, in agreement with the results obtained the mechanism of the reaction can be represented as in Scheme 1. The rate according to Scheme 1 is given by Eq 2. Since, total [TIIII] exists in the form of free [TIII I] and the complex (Eq. 3) therefore, the [TI'II] rree is given by Eq. 6. The overall rate law is now expressed by Eq. 7 and the the pseudo-first-order rate constant, kobs, ''Jy Eq . 8.

Rate=kl[Complex]=kIKc [Hydrazide]rree [TIIII]rree

[TIIII]total = [TIIII]rrce + [Complex]

[TIIII] [TIIII] K [H d 'd ][TIIII] total = rree + c y raz, e free

[TIIII]rrec = [TIIII] total/l + Kc [Hydrazide]

Rate = k,Kc [Hydrazide] [TIIII]rree

kobs = klKc [Hydrazide] I (1 + Kc [Hydrazide])

(2)

(3)

(5)

(6)

(7)

(8)

Rate law 8 is verified by plotting lei kobs against II [Hydrazide] at four different temperatures and from the slopes and intercepts of these plots the values of kl and Kc were calculated and are given in Table 1.

NOTES 81

The effect of hydrogen and chloride ion concen­trations on the reaction is due to the protonation of hydrazides ' 2 and different chi oro-complexes 13 of thallium(III) present in the solution . Hydrazides are known to be protonated in acid medium according to Eq. 9. Therefore, total [Hydrazide] can be expressed by Eq. 10 and the by the fact that there was no effect

KH ... (9)

free [Hydrazide] by Eq. 12. Since the rate of the reaction decreases as the [H+] increases, free hydrazide is the active species. This is also supported

[H ydrazide ],o,al=[H ydrazide ]rree +[H ydrazide] prolonaled

[Hydrazide ]'o'al=[H ydrazide ]rree +KH [Hydrazide] rree

[Hydrazide]rree=[Hydrazide],o,al l (1 + KH [H+])

(10)

(11 )

(12)

of ionic strength on the reaction indicating one of the reactant is neutral. Thallium(III) forms strong complexes with chloride ions of the formula TICI/n

where n is the number of chlorides complexed with thallium(TII) as represented in equi libria 13 to 16. The values of respective stabi lity constants l3 are K,=1.38x 108, K2 =3.98xlO13

, K3=6.02x lO ' S and K4=1.0x l018 mor l dm3

. The presence of 3.0x l0-2 mol dm-3 chloride ion concentration (which is the minimum [Cn used in the present study) all the

TI3++Cr ~ TICI2+ KI (13)

TICI2+ +cr ~ TICI2 + K2

TICI2 + +cr ~ TICb K3

TICI3+Cr ~ TICI4- K4

(14)

(15)

(16)

thallium(III) will exists as TICI/ and its concentration can be expressed by Eq. 17. The [TICI/ hree can now be given by Eq. 19 where ~I =K3IK2=lS1 and ~2=KJK3=166. Further, using Eqs. 18 and 19 the con­centrations of [TICI2 +]rrcc ,TICI3 and TICI4- were calcu-

[TIIII],o,al = [TICI/ ],o,al = [TICI 2 +]free + [TICI3] + [TICI4-] (17)

[TICI/]1D1a1 = [TICI/] free (I + ~I [Cn + ~2 [crf) (18)

[TICI/]frec=[TICh +]101"1/(1 + ~I [Cn+~2 [crf) (19)

lated at different chloride ion concentrations and compared with the change in the rate constant as the

chloride ion concentration varied. The concentrations of both [TICI/] free and TICI3 parallel the values of rate constants as [Cn changes but the order [Cn is - 1.5 which makes [TICl/]free as the only active species.

TICI 2+ + Hydrazide ~ Complex Kc

Complex -7 RCONNH+ TICh- +2H+ kl

RCONNH+H20+ TICl; -7 RCOOH+N2+2H+ + TICh-

fast

R = C6Hs for benzoic acid hydrazide and o-CIC6H4 for o-chlorobenzoic acid hydrazide

Scheme 2

The mechanism considering TICI/ of oxidant and free hydrazide of the substrate as the active species can now be represented by Scheme 2 with respecti ve rate law and the expression for the pseudo-first-order rate constant by Eqs. 20 and 21. The rate law 21 was verified by plotting l/kobs against l/[Hydrazide]and II kobs agai nst [H+] which were found to be linear. From the slopes and intercepts of these plots the values of Kc and KH were determined. The values of Kc are

R k, Kc [Hydrazide] ,ola' [TiCI; ]101a1

ate= " (1 + Kc[Hydrazide])(1 +K H [H +])

(20)

(1 + ~I [Cl- ] + ~2 [Cr]2)

k = k,KJHydrazide],o,al obs (I + KJHydrazide])(1 +K

1" [H +])

... (21)

(I + ~ I [CI - ] + ~2 [CI - ]2)

given in Table I and those of KH were fou nd to be 13 and 16 mor l dm3 for benzoic and o-chlorobenzoic acid hydrazides respectively. The electrophilic character of TICI/ among the thallium(III) chlorocomplexes is highest thus making it the reactive species . The detailed mechanism involves electrophi lic substitution on the nitrogen of the

Table I-Values of Kc and kl

[HCI] = 0.1 mol dm-3, [HCI041 = 0.1 mol dm-3,

[TIIII] = 3.0x I0-3 mol dm-3, ) = 0.3 mol dm-3 in 50% v/v ethanol

Temp. Kc (mor l dm3) 104xk l (S-I)

°C BAH a-CIBAH BAH a-CIBAH

25 )0.3 3.2 2.6 1.7

35 10.7 4.5 5.3 2.5

45 11.1 5.0 10 5.0

55 11.6 6.8 20 6.7

82 INDIAN J eHEM, SEC A, JANUARY 2004

+

o H H o H H o H H II I I II I I

-H' ~ 1).1 -.:; R-C-N-N

~~ R-C-N··N-H +TK<h'~ R-C-N-N-H

1 n

/"- /"-

~:,l CI

· H'

CI CI

Complex

o i

H' ·. RCOOH ~ RoC I 0'

/).;. H H

TICI,'

1 TlCIi + 2H' + N,

R • CoH. lor t>.nzok: acJd hydrulde and o-ClCoH. lor crchlorobenzok: acJd hydrulde

Scheme 3

hydrazide with the formation of N-TI bond which decomposes in the subsequent step with direct two electron transfer from hydrazide to thallium to give an intermediate followed by fast steps (Scheme 3). Such N-TI bond formation has been pos tulated during thallium(II1) oxidation of nitrogen 14 contammg compounds. The activation parameters, with respect to slow step, k l , 11Ft (kJ mor l

) , I1C# (kJ mor l) and -

11~ (J KI mor l) were found to be 51.8, 116.8 and

190.6 and 52.9, 111.7 and 190.1 respectively for benzoic and o-chlorobenzoic acid hydrazides. Consi­derable decrease in the entropy of activation is due to formation more ordered transition state as shown in Scheme 3. Chloro-substitution in the ortllO position retards the rate of oxidation of the hydrazide. Although, chloro group can withdraw electron density from saturated carbon chains by induction it is also capable of supplying 7t-electrons to the conjugated system thus deactivating the carbonyl group of the hydrazide which retards the oxidation of o-chloro­benzoic acid hydrazide. The mechani sm involves neutral hydrazide as the active substrate thus the reaction is unaffected by the change in the ionic

strength. The increase in ethanol content in the reaction medium decreases the rate . Such an effect of the solvent is due to the stabilization of the complex formed between l5 reactants in a medium of low relative permittivity.

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