The role of the α-effect in determining nucleophilic reactivities: aromatic substitutions

Abstract

Chlorine is substituted quantitatively from chloro-2,4-dinitrobenzene by a variety of amines, including ‘α-nucleophiles’ such as methoxylamine or hydrazine, in 3 : 2 dioxan–water. Second-order kinetics, first-order with respect to both reagents, are obtained in all cases. Primary n-alkylamines, spanning an interval of nearly five pK_a′ units, nicely fit the extended Brønsted equation log k= 0·63pK_a′– 8·45, where k is the second-order rate coefficient and pK_a′ values have been determined in the kinetic medium. Hydrazine and methoxylamine show a positive deviation of only ca. 1·3 log k units from the straight line for the primary n-alkylamines. It is argued that the origin of such small deviations is not clear while according to current ideas a large α-effect should have been predicted. These results imply that a high β Brønsted value for nucleophilic substitutions at unsaturated centres may not be a sufficient condition for the presence of a clear α-effect. Therefore we suggest that the occurrence of a clear α-effect requires also a particular structure of the transition state such as that possessed by nucleophilic displacements at the carbonyl carbon where a clear α-effect is present.