Regioselectivity

The term regioselective can be used to describe any process that favors bond formation at a particular atom over other possible atoms. The ‘Regio-’ prefix comes from the Latin for ‘Region’ or an area of something.

A reaction can be said to have high regioselectivity when one major product dominates due to the reaction being highly favored at one atomic position over another.

In the addition reaction example below the preferential formation of a stable tertiary carbocation at the 2-position results in the reaction proceeding to give 2-Chlorobutane as the major product.

Figure 1. Regioselectivity in the hydrochlorination of but-1-ene.