A typical organic reaction proceeds in a special mechanism. There may be many proposed mechanisms for a typical organic reaction. Experimental methods have many instrumental limitations such as trapping the intermediates or transition states (TSs) in confirming the mechanism that reactions proceed from it. Computational methods can make confirming the mechanism easer, cheaper and exacter. We solved such an example in order to confirm and detailed study of the rearrangement mechanism of allyl p-tolyl ether (Scheme 1). The rearrangement of allyl p-tolyl ether may proceed in two different mechanisms: (i) a concerted mechanism in which the transition state forms in one-step (Claisen rearrangement) and (ii) a stepwise mechanism, involving forming a radical as the intermediate. The first one is one of the pericyclic mechanisms. Pericyclic reactions have relatively nonpolar TSs with considerable cyclic delocalization referred to as aromatic character.
