This section will cover aspects of the electrophilic addition mechanism in more detail.

The first point to note is that obviously the intermediate in this reaction is a carbocation - either the cyclic bromonium style cation, or the more simple cation in the HBr reaction. In all cases, the reaction rate will be substantially improved if there are electron donating substituents (abbreviated EDGs) on the alkene - examples of these are atoms with lone pairs that can conjugate into the π system (mesomeric electron donation) - e.g.;

Alkyl groups are also EDGs - by inductive electron donation, and electron rich aryl groups (including phenyl itself) - e.g. para-toluene, para-phenylamine;

In both cases, the presence of resonance canonical forms shows the stabilisation of the cation.

On the flip-side of course, EWGs will reduce the rate considerably - see nucleophilic addition.

Now to focus more upon the reaction of HBr with the alkene, shown on the previous page. Because the species being added to the alkene is unsymmetrical, if the alkene is not symmetrical itself, different products could arise depending upon where the H⁺ adds in the first step.

However, this need not be complicated as clearly the hydrogen will add to give the most stable carbocation - and the most stable carbocation will be that which is most stabilised. If the alkene is a purely hydrocarbon molecule there occurs what is known as Markownikov Addition - whereby the hydrogen will always add to the least substituted carbon, and the halogen (or whatever else might be adding) adds to the most substituted carbon.

Exceptions to this do occur, and when they do they are pointed out as being anti-Markownikov.

If there are other groups present on the alkene (EDGs or/and EWGs), then the H⁺ will add to give whatever is the most stable carbocation.