A MECHANISTIC STUDY ON NUCLEOPHILIC-SUBSTITUTION REACTIONS OF ARYL SUBSTITUTED BENZENESULFONATES WITH ANIONIC NUCLEOPHILES

Second-order rate constants have been measured spectrophotometrically for the nucleophilic substitution reactions of aryl substituted benzen esulfonates (1, X-C6H4SO2-OC6H4-Y) with aryloxides (Z-C6H4O-) and etho xide (EtO-) in absolute ethanol at 25-degrees-C. The nucleophilicity o f aryloxides increases with increasing electron donating ability of th e substituent (Z) on aryloxides, and results in a good Hammett correla tion with sigma- constant. The reactivity of 1 toward aryloxides and e thoxide shows also significant dependence on the electronic nature of the substituent X and Y Large positive rho(acyl) values have been obta ined for the reaction of 1 with phenoxide and ethoxide, indicating tha t the leaving group departure is little advanced at the transition-sta te of the rate-determining step. This has been further supported from the fact that sigma- constant gives extremely poor Hammett correlation , while sigma0 does reasonably good correlation for the reaction of 1 with ethoxide. Thus, the present sulfonyl-transfer reaction is propose d to proceed via a rate-determining attack of nucleophile to the sulfu r atom of 1 followed by a fast leaving group departure.