The outcome of the reaction of acidified methanol with 1-methoxy-1,3-cyclohexadiene, (1) in the scheme below, has been ruled as an “unexpected anti-Markovnikov” addition [Emery et al., J. Org. Chem. 1999, 64, 3788] that can be explained only through “specific interaction between solvent and the substrate”.

We use first principles electronic structure calculations (the G3MP3 method) to show that, first, there is nothing “unexpected” here as the reaction course follows the minimum energy path, second, that the reaction is neither “anti-Markovnikov” nor “Markovnikov”, and third, that no solvent molecule – or other “assisting group”— needs to be invoked in order to explain the reaction mechanism. Using two mechanistic criteria, core/valence charge and reaction intermediate energy, we further show that polar addition reactions to alkenes can be either Markovnikov–type or they follow another route; apparently there are no “anti-Markovnikov” polar additions.

Scheme: