Indian Journal of ChemistryVol. 23A, March 1984, pp. 237-238

Bromination of p-Acetotoluididein Aqueous Medium:Substituent

Effect

V T DANGAP, S L BONDE & G Y ROHOKALEDepartment of Chemistry, Nowrosjee Wadia College, Pune 411 001

Received 5 September 1983; revised and accepted 31October 1983

The kinetics of bromination of p-aeetotoluidide in aqueoussolution has been studied using a rotating platinum electrode. Thespecific reaction rate and the energy of activation for the reaction at25.4C are 319 drn ' mol \ s -\ and 40.2 kJ mol -\, respectively.These results have been compared with those reported earlier for thebrominations of acetanilide, o-acetotoluidide and m-acetotoluidideunder identical conditions to provide quantitative verification forthe reactivities of these substrates.

The kinetics of bromination of acetanilide, 0-

acetotoluidide and m-acetotoluidide have beenreported by Rao and Mali) . Presently we have studiedthe kinetics of bromination of p-acetotoluidide underparallel conditions, using the rotating platinumelectrode (RPE») --3, in order to elucidate the influenceof the relative positions of the - NHCOCH 3 and- CH 3 groups upon the reactivity of the ring substrateundergoing the electrophilic attack.

The main bromo product has been identified as 3-bromo-4-acetaminotoluene while the other isomersare fermed in traces". Among the reactants and theproducts, only bromine is reducible at the RPE. Hencethe reaction was followed by measuring the diffusioncurrent due to bromine at various intervals of time,using a moving coil mirror galvanometer with asensitivity of 5.0 x 10--9 amp mm':' with lamp andscale arrangement and a shunt. The galvanometerreadings were calibrated by recording the deflectionsfor various bromine solutions in the concentrationrange 0.1 x 10 -4 mol dm -3 to 0.8 x 10 -4 mol dm --3in8 x 10 - 3 mol dm - 3 potassium chloride.

Analytical grade chemicals were used to preparestock solutions of 1.6 x 10-4 mol dm 3 bromine (A)and 1.6 x 104 mol dm -3 p-acetotoluidide (B), bothcontaining 8 x 10 3 mol dm -3 potassium chloride asthe supporting electrolyte.

Solutions (50 ml each) of (A) and (B) were quicklypoured into a beaker containing the RPE and thesaturated calomel electrode (SCE). A stop-clock wassimultaneously started and the galvanometer readingswere recorded at 10s intervals upto 80s. Thecalibration plot of deflection versus concentration waslinear from which the concentrations of unreacted

bromine at various instants in the reaction wereevaluated.

The diffusion current measurements were repro-ducible within ± 0.3 em of deflection of the light spotand hence the specific reaction rates have less than2.5% error.

The experiment was carried out at five differenttemperatures and the energy of activation, entropy ofactivation and frequency factor for the reaction wereevaluated as 40.2 kJ mol -I, - 30.6 J mol -1 K -I and3.41 x 109 dm ' mol -I s -I at 25.4°C respectively.

The reaction follows the generally, acceptedmechanism for the bromination of aromaticcompounds suggested by Berliners. The rate of theelectrophilic substitution reaction is given by Eq.(l).

Rate = k [Br2] [p-acetotoluidide] -.. (1)

This implies that the reaction is of the second order.This is indeed found to be the case. The plot of [Br 2] -Iversus time was linear (Fig.l) and the slope of this plotgave a specific rate = 319 dm ' mol :' s -1 at 25.4°C.

The above mechanism is envisaged in spite of theHOBr present in aqueous bromine solutions due to theequilibrium,

Br2 + H20 ~ HOBr + H + + Braq aq

The forward and reverse specific rates of thishydrolysis being I.l x 102 S -I and 1011 dm" mol :" S-I

4.

0

ME 0

"r, )-00

E~0<.\,-, 0C'mL..,

20

Time, Sf'C

Fig. I-Variation of [Br2] -\ with time for the bromination ofp-acetotoluidide at 25.4°C

237

INDIAN J. CHEM., VOL 23A, MARCH 1984

respectively" and since aromatic substrates are knownto react very slowly with hypohalous acids ascompared to molecular halogens", all the HOBr atequilibrium is converted into molecular bromine andBr 2 has been assumed to be the sole brominatingspecies.

The specific reaction rate obtained for thebromination of p-acetotoluidide in this study may becompared with those obtained earlier for thebromination of acetanilide, o-acetotoluidide and m-acetotoluidide under identical conditions. The specificreaction rate (at 25.0C) for the bromination of m-acetotoluidide (1510 dm 3 mol - 1 S I) is the highestfollowed by acetanilide (440 drn ' mol :" SI) and p-acetotoluidide (315 drn ' molls I) while that for 0-

acetotoluidide is the lowest (85 dm ' mol-l s -I).

In the event of an electrophilic attack. - CH 3 isknown to predominantly activate the ortho positionsrelative to it while the - NHCOCH3 group stronglyactivates the para and weakly the ortho positions -. Inm-acetotoluidide the strong para directing effect of the- NHCOCH 3 group and the ortho directing effect ofthe -CH3 group are operative in unison and hence itsbromination has the highest specific rate, the - Brentering the position para to the - NHCOCH3. In thecase of acetanilide the absence of the - CH.1 groupreduces the degree of activation of the position para tothe - NHCOCH.1 group and a lower specific rate ofbromination IS observed.

The para position with respect to the - NHCOCH.1group is occupied by the - CH 3 group in P:acetotoluidide and the 3-bromo-4-acetaminotolueneformed in the brornination reaction at the specific rateof 315 dm' mol -: S I confirms the weaker orthoorienting influence of the - NHCOCH,1 as comparedto its para directing influence. The ottho orientinginfluence of - CH.1 group is insignificant 111this case as

238

this position being meta to - NHCOCH3, thecorresponding carbonium ion formed would not bestable 7.

In o-acetotoluidide, the spatial proximity of the-CH3 group to the bulky - NHCOCH3 group resultsin the lack of coplanarity of the latter with the benzenering. Consequently, the stabilization of the positivecharge on the nitrogen is considerably hindered andthe formation of the carbonium ion, which is the rate-determining step, is the slowest resulting in therelatively lowest ,specific rate of bromination of 0-

acetotoluidide. The role of the -CH3 group as anortho orienting group is 'again insignificant as in thecase of p-acetotoluidide.

The reactivities of the different aromatic substratesconsidered here have been qualitatively speculated onthe basis of substituent group characteristics andstereochemical principles. but direct kinetic measure-ments' for a quantitative assessment of the speculatedreactivities in aqueous solution were lacking. Thepresent studies have thus provided a directquantitative verification for the reactivities of varioussubstrates.

We are thankful to Dr T S Rao and Dr M S Wadiafor helpful discussions.

ReferencesI Rao T S & Mali S I. / Nuturtorsch. 31a (1<)76) li35.

2 Rao T S. Mali S 1& Dangat V T . i t IIi; Poona. s., It'eh. 52( 1979)Ill.

3 Rao T S. Mali S I & Dangat V T. Tetrahedron. 34 (1971') 205.

4 De La Mare P B D & Ridd J H. Aromatic substitution(Butterworths. London) 1959. 131.

5 Berliner E . .I (II< III l.du. 43 I I<)66) 124.

(, Eigcn \1 & Kusun K . .I ,.1111 '/1<'111 Soc. H2 (1962) 1355.

Morrison R T & Boyd R N. (}rgaIJ/( Chl'III1.llJT IPrentice Hall ofIndia I Ilj71. 35X-365