The toluene methyl group alkylation by propene over a basic catalyst, approximated by the Na-2 molecule, was modeled by the B3LYP/6-311++G** method. The process starts by formation of the PhCH2Na molecule in complex with the NaH molecule. Then, the two reaction paths are considered: Path A at which PhCH2Na center dot center dot center dot NaH complex is dissociated, and Path B at which the whole PhCH2Na center dot center dot center dot NaH complex enters in the next reaction step. Next, at the two paths both insertion of the propene double bond into the C-Na bond and the Na/H exchange, leading to one of the isomers of butylbenzene. are examined. At Path A, the PhCH2Na molecule plays role of catalyst: it is re-formed in the last stage and can return to the propene insertion step. At Path B, the Na-2 molecule is a catalyst: it is re-constructed in the last stage and can return to the beginning step of formation of the PhCH2Na molecule. The energy levels at Path A are always above those of Path B, whereas the barriers at the essential steps of Path A are always significantly lower than the appropriate ones at Path B. Path A seems to be preferred kinetically while Path B thermodynamically. At the two paths isobutylbenzene is favored over n-butylbenzene by both thermodynamic and kinetic factor. This is in good agreement with experimental findings. We interpret Path A as describing the reaction in the bulk gas phase whereas Path B as describing the reaction at the metal surface. Because, the number of molecules is usually greater in the bulk than in the surface Path A, which is practically the same as that described by Pines et al. more than fifty years ago, is the main mechanism describing the toluene side chain alkylation by propene over a basic catalyst. (C) 2008 Elsevier B.V. All rights reserved.