Mechanism of micellar effects in imidazole catalysis. Acylation of benzimidazole and its N-methyl derivative by p-nitrophenyl carboxylates

Abstract

The effect of sodium dodecylsulphate (SDS) and cetyltrimethylammonium bromide (CTAB) on the rate of acylation of benzimidazole and its N-Me derivative by p-nitrophenyl carboxylates over a wide pH range (5-11·5) has been studied. (i) Both cationic and anionic micelles produce but a weak effect on the acylation of both N-methylbenzimidazole and the electroneutral form of benzimidazole. The fact that no micellar effects seem to be present is accounted for by that the favourable contribution due to increasing the reagent concentration in the micellar "phase" (∼ 100-fold acceleration in the case of p-nitrophenyl heptanoate) is almost completely compensated by the unfavourable effect of the micellar environment on the true second-order rate-constant. (ii) CTAB micelles are extremely effective catalysts of acylation of benzimidazole anion. The mechanism of acceleration (∼ 105 times in the case of p-nitrophenyl heptanoate) is due to the reagents being concentrated in the micelles (∼ 100 times), to the apparent pKa shift of the nucleophile under the action of the surface micelle charge (∼ 100 times) and to the favourable effect of the micellar environment on the true second-order rate-constant (∼ 10 times). (iii) The inhibiting effect of salts (F- < Cl- < BrO3- < Br- < NO3-) on micellar catalysis (a 100-fold inhibition in the prese 0·12 M KNO3) has only one cause-a lower solubility of the anionic reagent by the cationic micelles. (iv) Comparison is made of the true reactivity of electroneutral (AH) and anionic (A-) forms of benzimidazole in the surface layer of the micelle. If in water A- exceeds AH by approx 1013 times with respect to nucleophility, in the micellar environment the ratio of the true second order rate-constants is as high as 106. A mechanism of this phenomenon is suggested, which may also help understand certain polymer and protein effects on the imidazole catalysis. © 1975.