Selectivity in the electrophilic addition reaction

With asymmetric alkenes it’s possible to get two different products during some addition reactions. Symmetrical alkenes have the same R groups on either side of the double bond, whereas an asymmetric alkene has two different R groups attached to either side of the double bond.

Figure 1: Symmetric and asymmetric alkene examples

During the addition of a molecule H-X to asymmetric alkene, propene, you could in theory arrive at two products depending on which way around the H-X adds to the double bond.

Figure 2: Possible addition products from reaction of H-X with an asymmetric alkene

In most cases it’s the first reaction that proceeds due to the synthesis of a more stable tertiary carbocation under a rule known as Markovnikov’s Rule which can be used to predict the favored regioselectivity of addition reaction products.

Electrophilic addition to symmetric alkenes can sometimes also be stereospecific because the geometry of the starting alkene (either cis or trans) determines which diastereoisomer product is obtained.