Illustrated Glossary of Organic Chemistry

Regioselective: Any process that favors bond formation at a particular atom over other possible atoms. The description of a reaction's regioselectivity (or the absence of regioselectivity) is called the reaction's regiochemistry.


Major product
From tertiary carbocation

Minor product
From secondary carbocation
Addition of HBr to 1-methylcyclohexene is a regioselective reaction because it favors formation of a bond between the alkene tertiary carbon and the bromine atom instead of the alkene secondary carbon and the bromine atom. This regioselectivity is the basis for Markovnikov's rule.