IDdian Journal of CIlemktryVol. 19A, October 1980, pp. 980-983

KineticsAnilines

& Mechanism of Oxidation ofby TI(III)-A Change from Ionic

Presence of Ru(III)

SubstitutedPathway in

Aniline &to Radical

P. s. RADHAKRISHNAMURn* & s. N. PAnDepartment of Chemistry. Berhampur University. Berhampur 760 007

Received 21 March 1979; revised 14 September 1979; accepted 22- March 1980

The title reactioos have been investigated in aq. acetic acid medium in the presence of acids like sulphuricand perchloric. In the absence of Ru(1II) cbloride the reaction proceeds. through a cationic intermediate leadingto an ionic process, wbereas in the presence of Ru(ID) chloride the reactions take place by a radical mechanism.The reactions in the presence or absence of Ru(m) are first order each in [substrate) and [oxidant). The catalysedreaction bas first-order dependence on [Ru(lII)). Tbere is unit dependence in percbloric and sulpburic acids in tbeabsence of Ru(lII) but in the presence of tbe catalyst the reaction is independent of acid. These observations havebeen rationalised and suitable mechanisms postulated.

THE kinetics of oxidation of aromatic primaryamines by various oxidants++ have been studiedby different workers. Yatsimirskii et al.6

reported Ru(III)-catalysed oxidation of anilines byMn(III). The present communication deals with thekinetic aspects of uncatalysed and Ru(III)-catalysedoxidation of aniline and substituted anilines by thal-lium triacetate in aq. acetic acid medium in thepresence of perchloric and sulphuric acids underhomogeneous conditions. and to our knowledge thisis the first kinetic report.

Materials and MethodsAniline and substituted anilines were purified by

redistillation or recrystallisation before use. Thalliumtriacetate was prepared and purified as reportedearlier". Estimation of oxidant was done iodometri-cally", Solvent decomposition has been routinelychecked and found to be negligible in Ru(III)-cata-lysed oxidations, whereas under uncatalysed experi-mental conditions Tl(III) has been found to be quitestable. The solution of ruthenium trichloride wasprepared as per the procedure adopted by Singhet al.8 The values of Ho for acetic acid-watermixtures in the presence of sulphuric acid were cal-culated by our previous method'; Wiberg has deter-mined Ho for acetic acid-water mixtures in thepresence of perchloric acid'",

The solutions of the substrate and oxidant in themixed solvent were equilibrated at the desired tem-perature and then mixed in a dark-coloured reactionvessel. Aliquots (5 ml) of the reaction mixturewere quenched by adding potassium iodide solutionat different time intervals and the liberated iodinetitrated against standard thiosulphate.

Results and DiscussionUncatalysed oxidation of anilines - The reactions

are first order each in [substrate] and [oxidant].

980

The reaction is dependent on concentrations of per-chloric and sulphuric acids and the order with respectto Ho is unity. The plots of log k2 versus Ho arelinear suggesting that Ho is a more satisfactorymeasure of acidity of the medium than is the stoi-chiometric concentration of the acid'" (Table 1).The acid dependence is traced to the hydrolysisequilibria of the type :

TI(OAch + H+ ~ TI(OAc)2+ + HOAcTI+(OAc)2CIO(- + H+ ¢ TI(OAc)2+ + HCIO(

In view of the above, the reaction of acid with subs-trates is negligible as reactions between inorganicspecies are instantaneous as compared to its reactionwith weak bases. The data in Table 2 show first-order dependence on [aniline].

At a fixed [substrate] the [Tl(III) acetate] wasvaried. The constancy of the first-order rate constantsclearly indicates a first order dependence on [TI(III)](Table 3).

TABLE 1- DEPENDENCE OF RATE CONSTANTS ON ACIDITY{Solvent: HOAc (20%. v/v); [Substrate] = 0.05M; [Tl(III»)

= 0.0025M; temp. = 60°}

k, X 10' (minr")[HCIO.l

M Aniline m-Toluidine p-Chloroaniline

1.0 0.37 0.65 0.141.5 1.00 1.15 0.182.0 1.51 1.70 0.253.0 0.38

[HaSOJM1.0 1.01 2.011.5 1.54 2.812.0 2.04 5.60

RADHAKRISHNAMURTI & PATI : TI(IlI)-OXIDATION OF ANILINES

TABLE 2 - DEPENDENCE OF [SUBSTRATE] ON REACTION RATE

{Solvent : HOAc (20% v/v); [HCIO.] = 1.0 M; [Tl(III)]= 0.0025 M; temp. = 60°}

TABLE 3 - EFFECT OF [OXIDANT] ON THE REACTION RATE OFUNCATALYSED OXIDATION OF ANILINE

{Solvent: HOAc (20%, vlv), [HCIO.] = 1.0 M; [Substrate]= 0.05 M; temp. = 60°}

[Aniline] 103 X k, [m- Toluidine] 103xk,J02[Tl(JII) ] 10'xk, 10"[TI(III)] 103x k,M (min-I) M (min-')

M (mln=') M (min-I)0.0130 0.088 0.0181 0.1800.0252 0.181 0.0363 0.340 0.106 0.372 0.426 0.375

0.250 0.371 0.538 0.3860.0506 0.371 0.0476 0.475 0.301 0.3800.1400 1.097 0.1055 1.111

On the basis of these results the following rateexpression is proposed.

- d[Tl(III)] = k [s] [Ho] [TI(III)]dt

The order of reactivity is p-toluidine > o-toluidine> m-toluidine > aniline> o-chloroaniline >p-bro-moaniline > p-chloroaniline > m-chloroaniline >m-nitroaniline. This indicates that electron releasinggroups enhance the reaction rate and electron with-drawing groups retard the rate (Table 4).

pcr-relationship - A plot of log k2 versus a islinear and p value is found to be -3.0 (correlationcoefficient = 0.95). This observation is similar tothe one reported earlier in the hexacyanoferrate(III)oxidation of amines. The reaction occurs througha two-electron transfer and gives a cationic inter-mediate in the rate-determining step. Such pvalues for reactions involving two-electron transferhave been reported earlier by Hull et al." in theoxidation of aliphatic tertiary amines by CIOz' Itmay be pertinent to remark that the mechanismswhich involve electron deficient centres usually havep values between -2 to -5 (ref. 11).

Nucleophilicity - Swain and Scott-" have shownthat log kjko = Sn where 'S' is the susceptibilityfactor and 'n' is the nucleophilicity constant. Then-values computed by USI3 were plotted againstlog kz, a linear plot showing the relationshipbetween nucleophilicity and reactivity was obtained.The plot of log k2 versus pKa was also found to belinear.

Application of Edwards equation - The Swain-Scott linear free energy relationship has been improv-ed by Edwards by a four-parameter equation. It hasbeen noted that the electrode potentials for manyreactions, parallel the nucleophilicity of many ions.According to Edwards

log kj ko = ex. En + (3 H

where ex. and (3 are the constants. The other para-meters are defined as :

En = Eo + 2.6and H = pKa. + 1.74

Using the kinetic data the values of H, En, ex. and13for this oxidation in 20 % HOAc, at 600 arecomputed for the first time with the help of partialmultiple correlation theory. The accuracy of Enand H values have been checked by plotting log k2

TABLE 4 - EFFECT OF SUBSTITUENT ON SECOND ORDER RATECONSTANTS IN Tl(IlI)-OXIDATION OF ANILINES

{Solvent: HOAc (20%. vlv): [HCIO.] = 1.0 M; [Substrate]= 0.05 M; [TI(III)] = 0.0025 M; temp. =600

}

Substrate 10'xk. Substrate 102xk.(litre mol=min ") (litre mo[-lmin-1)

Anilinep-Toluidinem-Toluidineo-Toluidinep-Chloroaniline

m-Chloroanilineo-Chloroanilinep-Bromoanilinem-Nitroaniline

0.2030.7090.3590.193

0.7534.3111.0023.8010.278

TABLE 5 - DEPENDENCE OF RATE ON [OXIDANT] IN THE Ru3+-CATALYSED OXIDATION OF ANILINES.BY TI(III)

{Solvent: HOAc (20%, vlv); [HCIO.] = 1.0 M; [Substrate]=0.05 M; [Ru(III)] = 0.185 x 10-8 M; temp. = 60°}

[Tl(III)] x 10' kl x 102(min-') [TI(I1I)] x 102 k, x 10'(min-')M M m-Toluidine

Aniline o-Chloroaniline

0.1280.2500.4090.578

2.0932.1232.0062.046

1.9171.8461.8491.860

0.1140.2560.4660.743

2.2432.3082.2562.353

versus En and log k~ versus H respectively. Theplots are linear indicating the general applicabilityof these values.

Mechanism - Based on these observations amechanism depicted in Scheme 1 is suggested.

Ru(III)-catalysed oxidation - Ruthenium(III)chloride catalysed oxidation of aniline and substi-tuted anilines by thallium triacetate in the presenceof perchloride acid in aq. acetic acid medium indi-cates the reaction to be of first-order each with res-pect to substrate (Table 6), catalyst and oxidant(Table 5) but independent of perchloric acid(Table 7).

Ho

Ar - N:

H

Tl(OAC)3+ H';==:: H(OAC)2++HOAc

H

+ Tt(OAC)2+ ~ Ar-~: +Tl (I) + H++

Ho

Ar- N:t

Ho

+ :N -ArIH

H HI I

Ar-N-N+-ArI

H

H H~ Ar-~ - ~ -Ar.!Q.!!.. Ar-N = N -Ar

SCHEME I

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INDIAN J. CHEM., VOL. 19A, OCTOBER 1980

TABLE 6 - DEPENDENCE OF FIRST ORDER RATE CONSTANT ON[SUBSTRATE}IN Ru(III)-CATALYSED OXIDATION OF ANI LINES BY

Tl(III)

{Solvent: HOAc (20%. v/v); [HCIO.] = 1.0 M; [TI(lII)] =0.0025 M; [Ru(IlI)] = 0.185 x 10-1 M; temp.=600}

10"[substrate] 10" x k, lO"[substrate] 10. x k,M (min-') M (min<)

ANILINE m-ToLUIDINE

1.56 0.91 1.18 0.932.82 1.58 2.59 1.673.64 2.12 3.52 2.30

10.77 5.10 10.55 5.36

p- BROMOANILINE p-CHLOROANILINE

1.40 0.59 0.98 0.382.65 1.27 1.85 0.755.02 1.92 5.05 1.709.89 4.02 9.93 3.82

TABLE 7 - DEPENDENCE OF RATE CONSTANTS ON ACIDITY INTHE RU3+-CATALYSED OXIDATION OF ANILINES BY Tl(III)

{Solvent: HOAc (20%. v/v); [TI(III)] = 0.0025 M; [Subs-trate] = 0.05M; [Ru(I1I)] = 0.185 x 10-6 M; temp.r=Su"}

10" X k , (mirr-')[RClO.]

M Aniline m-Tolui- p-Chloro- p-Bromo-dine aniline aniline

2.12 2.30 1.70 1.92

2.02 2.54 1.30 1.63

2.03 2.63 1.29 1.69

2.03 2.63 1.29 1.69

1.01.5

2.03.0

It appears that the hydrolysis equilibria whichleads to the dependence on acid is not important inthe presence of Ru(IH) as the reaction betweenTl(HI) and Ru(HI) is fast leading to formation ofactive Ru(V). Due to the intervention of the inor-ganic oxidation step hydrolysis equilibria are of nosignificance and hence independence to acidity isobserved.

Reactivity order in substituted anilines - Amongstthe various substituted anilines studied the observedorder of reactivity is, p-toludine > o-toluidine >m-toluidine > aniline> p-bromoaniline > o-chloro-aniline > p-chloroaniline > m-chloroaniline >m-nitroaniline. It clearly indicates that electronreleasing groups accelerate the reaction and electronwithdrawing groups retard the reaction rate. Thereaction rates for the catalysed reaction arealmost 80 to 90 fold more than that for uncatalysedreaction showing thereby the specificcatalytic natureof Ru(IH). The second order rate constants ofdifferent substituted anilines are recorded in Table 8.

pa-relationship - A plot of log k2 versus a islinear (correlation coefficient = 0.98) and the pvalue has been found to be -0.76 which establishesthat oxidation of amines by T1(HI) in Ru(HI)-catalysed reaction proceeds through a radical pro-cess, whereas in uncatalysed reaction it proceedsthrough a cationic centre. This is the novelty in the

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TABLE 8 - EFFECT OF SUBSTITUENT ON SECOND ORDER RATECONSTANTS IN Ru(III)-CATALYSED OXIDATION OF ANILINES

BY Tl(III)

{Solvent: HOAc (20%. v/v); [Substrate] = 0.05 M; [Tl(III)}.=0.0025 M; [Ru(IIl)] = 0.185 X 10-6 M; temp. =600

}

Substrate k.(litre mol=rnirr-')

Substrate k2/(litremol=tmirr'")

Anilinep-ToJuidineo-Toluidinem-Toluidinep-Chloroaniline

0.5811.4200.6730.6530.356

m-ChloroaniJineo-Chloroanilinep-Bromoanilinem-Nitroaniline

0_2610.3030.3730_191

present study, where Ru(Hl) participates in thereaction sequence and changes the mechanism fromionic to radical mechanism.

Nature of Ru(III) species - It has been confirmedby electronic spectral studies-! that Ru(IH) chlorideexists in the hydrated form as [Ru(H20p+. Theplot log kl versus log [Ru3+] is linear with a unitslope.

Solvent effect - An increase in the percentage ofacetic acid decreases the reaction rate. For aniline,the values of kl at 20. 40, and 60% HOAc are2.12, 1.87, and 1.50 x 10-2 mirr-' respectively.

Mechanism - It has been stated earlier that thereaction involves a radical pathway. Arguments infavour of the radical process are :

(i) In the NBS bromination of toluenes Pearsonand Martin-" have reported the reactions ofa radical type. The plot of log k2. (NBSbromination of toluenes) versus log k2 [Tl(III)oxidation of anilines] is linear indicating theoperation of identical mechanisms in boththe reactions. Hence, the present reactionalso occurs by a radical process.

(ii) The plot oflog k2 [V(V) oxidation of anilines']versus log k2 [T1(IH) oxidation of anilines]is fairly linear confirming the reactionsproceeding through a similar radical mecha-nisms.

(iii) The effect of acid is reversed from one ofdependence to independence in the presenceof Ru(III) indicating a change of mechanism.

(iv) Drummonds and Waters!" tests for radicalprocess are positive especially the test ofacrylonitrile.

As stated earlier, in Ru(IH)-catalysed oxidationelectron transfer takes place from Ru(HI) to Tl(III)to give intermediate Ru(V) and T1(I). The Ru(V)functions as an active oxiding species. Such parti-cipation of intermediate Ru(V) species in the oxi-dation processes has been invoked by Yatsimirskii!?in the Ru(IH)-catalysed oxidations by Ce(IV),Ru(V) reacts with aniline to give ArNH, Ru(IV).and a proton. The intermediate Ru(IV) then reactswith another molecule of aniline to give ArNH.These two free radicals dimerise to give hydrazo-benzene which is further oxidised by another mole-cule of Tl(III)-Ru(III) couple to give azobenzene.

RADHAKRISHNAMURTI & PATI: TI(III)·OXIDATION OF ANIUNES

n (In) + Ru (1lI) fast. Ru (v) + run

Ru (V). Ar-~: slow. Ar N H • H+ • Ru (IV)H .Ru(IV)+ ArNHz fast. ArNH + H+" Ru(lll)

ArNH+ArNH fast, Ar-NH-NH-Ar

~ Ar- N = N - Ar

SCHEME 2

A sequence of reaction is shown in Scheme 2 whichexplains the observed experimental results.

Product analysis - In the present TI(III) oxida-tion of ani lines the products azobenzenes obtainedwere isolated and characterized by the literaturemethod18'1' •

AcknowledgementOne of the authors (S. N. P.) is gtateful to the

'CSIR, New Delhi for the award of a senior researchfellowship.

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