S(N)1-S(N)2 and S(N)2-S(N)3 mechanistic changes revealed by transition states of the hydrolyses of benzyl chlorides and benzenesulfonyl chlorides.

Hydrolysis reactions of benzyl chlorides and benzenesulfonyl chlorides were theoretically investigated with the density functional theory method, where the water molecules are explicitly considered. For the hydrolysis of benzyl chlorides (para-Z-C6H4-CH2-Cl), the number of water molecules (n) slightly influences the transition-state (TS) structure. However, the para-substituent (Z) of the phenyl group significantly changes the reaction process from the stepwise (S(N)1) to the concerted (S(N)2) pathway when it changes from the typical electron-donating group (EDG) to the typical electron-withdrawing one (EWG). The EDG stabilizes the carbocation (MeO-C6H4-CH2(+)), which in turn makes the S(N)1 mechanism more favorable and vice versa. For the hydrolysis of benzenesulfonyl chlorides (para-Z-C6H4-SO2-Cl), both the Z group and n influence the TS structure. For the combination of the large n value (n > 9) and EDG, the S(N)2 mechanism was preferred. Conversely, for the combination of the small n value and EWG, the S(N)3 one was more favorable.