A DFT investigation on aromatic nucleophilic substitution (SNAr) reaction between 4-fluoro-1-naphthaldehyde/4-fluoro-2-naphthaldehyde/1-fluoro-2-naphthaldehyde/1-fluoronaphthalene and methylthiolate ion in gas phase and in protic/aprotic solvents

Abstract

We have investigated the mechanism of aromatic nucleophilic substitution (SNAr) reaction between 4-fluoro-1-naphthaldehyde/4-fluoro-2-naphthaldehyde/1-fluoro-2-naphthaldehyde/1-fluoronaphthalene with methylthiolate by DFT calculations at B3LYP/6–31+G (d,p) level of theory in gas phase and also in polar protic/aprotic solvents. The results show that aromatic nucleophilic substitution reaction proceeds by concerted mechanism via formation of single transition state as it has lower activation energy barrier as compared with stepwise mechanism. The introduction of solvent increases the activation energy barrier indicating significant effect of solvents on transition state. The activation energy barrier was found to be a minimum in gas phase then in DMSO which is followed by protic polar solvents methanol and water. Thus, the polar aprotic solvent DMSO works best for aromatic nucleophilic substitution of fluorine. The presence of a formyl group was found to be essential for reaction to take place, and in the gas phase as well as in solution phase, the order of reactivity was found to be 4-fluoro-1-naphthaldehyde > 1-fluoro-2-naphthaldehyde > 4-fluoro-2-naphthaldehyde > 1-fluoronaphthalene.