CONCURRENT STEPWISE AND CONCERTED SUBSTITUTION-REACTIONS OF 4-METHOXYBENZYL DERIVATIVES AND THE LIFETIME OF THE 4-METHOXYBENZYL CARBOCATION

The rates of reaction of 4-methoxybenzyl chloride, pentafluorobenzoate, and 3,5-dinitrobenzoate in 50:50 (v/v) trifluoroethanol/water are zero order in the concentration of azide ion. These reactions give good yields of the azide adduct from trapping of the 4-methoxybenzyl carbocation intermediate, and there is also strong common chloride ion inhibition of the reaction of 4-MeOArCH2Cl. The yields of the azide and solvent adducts give the rate constant ratio k(az)/k(s) = 25 M-1, and analysis of the chloride common ion effect gives k(Cl)/k(s) = 9 M-1, for partitioning of the carbocation intermediate. The reaction of azide ion with the carbocation intermediate serves as a diffusion-controlled ''clock'' (k(az) = 5 X 10(9) M-1 s-1) for the reaction of solvent (k(s) = 2 X 10(8) s-1) and chloride ion (k(Cl) = 2 X 10(9) M-1 s-1). There is a solvent-induced change in mechanism and in the less ionizing solvents of 80:20 (v/v) and 70:30 (v/v) acetone/water; the reaction of azide ion with 1 produces the azide adduct by both trapping of the carbocation intermediate and by a concerted bimolecular substitution reaction.