..indian Journal of ChemistryVol. 17A, May 1979, pp. 495-497

•Kinetics of Oxidation of Aniline & Substituted Anilines

by Bromate Ion tVIJAYALAKSHMI & E. V. SUNDARAM·

Department of Chemistry, University C'ollege, Kakatiya University, Warangal 506009

Received 24 February 1978; accepted 13 November 1978

The oxidation of aniline and substituted anilines by potassium bromate has been studiedin acid medium in the presence of mercuric acetate in binary solvent mixtures of acetic acid andwater. The order with respect to the substrate and oxidant is one. First order dependence isobserved with respect to hydrogen ions. A decrease in the dielectric constant of the mediumincreases the rate. The reaction is of an ion-ion type, the reactive species being anilinium ionand bromate ion. A mechanism involving the ~ecomposition of aniline-bromate complex inthe rate-determining step is proposed. The electron withdrawing groups are found to acceleratethe reaction and p is found to be +1,74.

DIFFERENT oxidants have been employedin the study of oxidation kinetics of aromaticaminest+. In this note We report our results

-on the kinetics of oxidation of aniline and substi-tuted anilines by bromate. This is in continuationof our earlier study" on the oxidation of phenolsby bromate.

Materials and MethodsAll the compounds used were of AR grade. The

anilines were distilled or recrystallized just beforeuse. Distilled acetic acid was used.

The reaction was followed by estimating theunreacted bromate iodometrically. The productBe was complexed with Hg2+ as HgBr~- to avoidthe production of Br2 and its reaction with thesubstrate. The substrate concentration was kept.ten-fold in excess over the oxidant and first order'fate constants were calculated from the slopes oflog (a-x) versus time plots, which were linear up to70% completion of the reaction. Azobenzene Wasidentified as the product of oxidation. The reaction

did not promote polymerization of acrylonitrileindicating absence of free radicals.

Results and DiscussionThe reaction Was first order in oxidant. The

order with respect to substrate Was fractionalfor all the anilines except m-nitroaniline where therate Was independent of substrate concentration(Table 1). The plot of Ijk versus Ij[ArNH2J waslinear with an intercept indicating the formationof a complex between the substrate and the oxidant.The reacti~n ~as .first order w~th respe~t to H2S04(Table 2) indicating protonation of either anilineor bromate ion. It is known that aniline formsanilinium ion in acid medium suggesting thatthe reaction is between two oppositely chargedions and the reactive species are anilinium ion andbromate ion.

The effect of dielectric constant on the reactionrate Was studied by varying the percentage com-positio!l of a~etic acid from 10% t~ 50% (Table 3).The dielectric constants for vanous acetic acid

TABLE 1- EFFECT OF VARYING [SUBSTRATE] ON THE REACTION RATE

{[BrOii] = 0'005M; [H2SO,] = 0·4M; acetic acid = 40% (v/v); temp.= 30°C; [mercuric acetate] = 0'005M}

[Substrate] k X 102 (min-I) for [Substrate] k X 102 (min-I) forM M _

0·050·100·15'0·20'0·25

2'885·126·168·228'22

m-Cl p-Cl m-CH3 p-CH3

12·12 7·20 2'38 2·00 0·0219·19 12·45 4·14 3·60 0·0327·91 15'88 5·48 4·61 0·0434·12 20·93 6'47 5·64 0·0540·05 22,47 7'38 6'73 0·06

*[H2SO.] = 0·05M-.

3·29'HI4'804-874·90

Aniline P-NO:

2·973·544·194'705·12

m-NO;

tPresented in the seminar on oxidation-reduction reactions held at University of Madras, January 1978.

495

INDIAN J. CHEM., VOL. 17A, MAY 1979

TABLE 2 - EFFECT OF VARYING H2SO. CONCENTRATIONON THE RATE OF REACTION OF ANILINES

WITH BROMATE

{[BrOs) = 0'005M; [substrate) = '0'05M; acetic acid = 40%. (v/v) [mercuric acetate] = 0'005M; temp.= 30°}

Substrate k X 10' (min-I) at [H2SO.] (M)

0·1 0'2 0·4 0'6 0'8 1·0

Aniline 0'95 1'71 2'88 4·27 5·50 7·20P-N02 11·52 26·17 65'80m-NO. 9·60 17·72 35·43m-Cl 3-84 6·14 12·12 15'36 21'42 25·03p-CI 2-68 4·04 7·20 10'23 n55 15'88m-CH3 0'81 H2 2'38 3-68 4'85 6·40p-CHs 0'75 1·23 2·00 2-88 3-88 4'73

TABLE 3 - EFFECT OF VARYING SOLVENTCOMPOSITION ON THE RATE OF REACTION

{[BrOs] = 0'005M; [H2S04] = 0'4M; [sub.] = 0'05M; temp.. = 30°; [mercuric acetate) = 0'005M}

[Substrate) h X 102 min-I in AcOH (%, v/v)

10 20 30 40 50

Aniline 0'98 1·36 1-88 2·88 5·01*P-N02 3·29 4'70 11'81*m-N02 1·06 1'54 2·64 4·43 8'55m-CI 2'46 3'69 . 5'85 12·12 18'79p-CI 1'79 2'71 4·19 7'20 15'36ni-CH3 0'82 1·12 1-55 2·38 3-84p-CHs 0'68 0·92 1·28 2·00 3'29

*[H2SO.] = 0·05M.

water mixtures were calculated using an approxi-mate validity method? T?~ plot of log. k v~rsl;lsliD Was linear with a pO~Itlve sl~pe, which IS .lD

agreement with the equation for IOns of oppositecharge

ZAZBe2

log kD = log kD=<1J - DKTr

From the slopes of log k versus liD, the r valueswere calculated, which were in the range 2 to 4 A.

The effect of change of ionic strength from0·1 to 0'6M was studied using sodium sulphateas well as NaCI04 at low concent~ati~m of sul-phuric acid (O·OSM) so that the IOnIC strengthcan be kept low. The r<;tte.of the rea~ti~n ~e-creased on increasing the IOnIC strength indicatingthat the reaction is between ions of opposite sign.Plot of log k versus VtL was linear .with a slopeof 1·0 which is in good agreement WIth Bronsted-Bjerrum equation. All the above facts suggestthat the reaction is between anilinium ion and thebromate -ion. .

The rate measurements were made at differenttemperatures so as to evaluate the thermodynamicparameters (Table 4). The activation ene::gy forthe oxidation of aniline was also determined atconstant composition of acetic acid (40%) and atconstant dielectric constant (48'5) and the valueswere 18·3 and 13·40 kcal mole-1 respectively.Using these values the r value was calculated from

496

,.

I

TABLE 4- THERMODYNAMIC PARAMETERS

{[BrO,) = 0'005M; [H2S04) = 0'05M; [sub.] = 0'05M; aceticacid = 40% (v/v); temp. = 40°; [mercuric acetate) = 0'005M}

Substrate k X 10~ t.Et t.m t.Stmin-I heal heal . e.u.

mole-I mole-I

~niline 1'44. 18'31 17'69 -10'55(1-06) (13:40) (12'78) (26'82),

P-NO. 9·40 n07 12·45 -23'56m-N02 9'38 13-92 13'30 -20'83m-Cl 4·70 16·14 15'52 -15'11p-CI 4'24 21'97 21·35 +3·31m-CH3 1'18 18·94 18·32 -8,94P-CH3 1·10 17·60 16'98 -13'32

Values in parentheses are at fixed dielectric constant.

---------------------------------------------~the following equations:

_ _ ZAZBNTe2 [!_ 30 2nN!1.] dDEA ED- D2] r 10 10DKT dT

where EA is the activation energy at constant.composition of acetic acid and ED is the activationenergy at constant dielectric constant, !1. is theionic strength of the medium and] is the mecha-nical equivalent of heat. The r value calculatedfrom this equation is 4·16 A which is in goodagreement with the r value (4'43 A) obtainedfrom the slope of log k versus 1JD plot.

The oxidation of several substituted anilinesWas studied in detail and the order of reactivityshows that the electron releasing groups retardthe reaction and electron attracting groups,accelerate the reaction as expected. The plotof log k versus cr was linear with positive slope.The P value calculated was +1'74. For a betterunderstanding of the effect of substituent, a com--parison of equilibrium constants of complex for-mation (step-Z of Scheme 1) and rate constants forthe decomposition of the complex (step-S, Scheme 1)-has also been made. The respective rate constantsare given as k2 and k3 in TableS. These values.also show the expected trend based on the inductiveeffect of the substituent, i.e. the electron releasing-groups retard the reaction and electron attracting:groups accelerate the reaction. From the mecha-

k,ArNH2 + H+ <= ArNH~

H+ k, I

Ar NH3+BrOii <= Ar-N-Br02 + H20HI x, +

Ar-N-Br02 -~ Ar NH + BrO;;slow

+ Dimerization2-Ar NH -----+ Ar-N = N-Ar + 2H+-

ORH H

+- I IAr NH + ArNH2->- ArN-N-Ar + H+

H HI I oxidn

Ar-N-N-Ar--->- Ar-N = N-Ar:

Scheme 1

rVIJAYALAKSHMI & SUNDARAM: OXIDATION OF SUBSTITUTED ANlLINES

TABLE 5 - EQUILIBRIUM CONSTAI'\TS OFCOMPLEX FORMATION (k.) AND RATE CONSTANTS (k3)

FOR THE DECOMPOSITION OF THE COMPLEX

[Substrate] »; X 103

(litre mole-I)k, X 10-3

(litre mole-")

0·0970·9800·2600·0470·0330·28900·2100

0·201·05O·~0·160·140·090·07

Anilinem-Clp-Clm-CH.P-CH.p-NO.m-NO.

43·23·0

15·058·9

123·00·010·29

nism shown in Scheme 1 the rate expression wasderived in terms of [BrOiJtotal. The intermediateBrOi formed further oxidises the substrate in faststeps, finally giving Br as the product.[Br0:iJtotal = [BrO:iJ+klk2[ArNH2][BrO;J[H+] ... (1)the rate expression obtained is:

d[BrO;] _ k1k2k3[ArNH2J[H]+[[BrO;Jtotaldt - 1+k1k2[ArNH2JlH+J

The plot of Ijkobs against Ij[ArNH2J should belinear with an intercept = l/k3 and slope = 1/klk2k3[H+J when the slopes of plots of l/k versusIj[ArNH2J at different acid strengths are plottedagainst IjlH+], a linear curve passing throughthe origin should be obtained. Such a plot isgiven in Fig. 1 for aniline, thereby confirming thereaction to be between two oppositely charged ions.

If protonation of BrO:; is taken as the first stepinstead of protonation of aniline we will arrive atanother rate expression given by:

d[BrO:il klk2k31 ArNH2] [H+][BrO;Jtota!--at = 1+k1[H+] +k1k2[ArNH2]

According to this expression, a linear plot will beobtained when 1jk versus 1/[ArNH2] is plotted atconstant [H+] , with an intercept = 1/k3 and :slope = 1/k2k3+1/k3klk2[H+]. When the slopes atdifferent (H+J are plotted against 1/lH+] a linearplot with definite intercept equal to 1/k2k3 will beobtained.

From the intercept and slopes of plots of 1jkversus 1j[ArNH2] one can calculate k2 and k3values if kl values for the anilines are available.The kl is nothing but the equilibrium constantfor the protonation of anilines. Using the klvalues=P, we have calculated the k2 and k3

,.

(

~ .'''[r"r~ I

0·005

o o-e11 [H+]

Fig. 1- Plot drawn between the values of the slopes ofthe plot of l/k versus 1f[ArNH2] and l/EH+]

1·2

values for all anilines and the results are given illTable 5.

One of the authors (V.L.) wishes to thank theCSIR, New Delhi for the award of a junior research.fellowship.

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