Illustrated Glossary of Organic Chemistry

Stereoselective: A reaction in which only one stereoisomer among a mixture of stereoisomers reacts. Also a reaction that can in principle produce two or more stereoisomers but in fact produces one (or almost only one) stereoisomer product.

This E2 elimination reaction is stereoselective because only the cis diastereomer reacts.

		+
	Trans-1,2-dibromocyclohexane Observed product		Cis-1,2-dibromocyclohexane None formed

The addition of molecular bromine to cyclohexene is stereoselective because only the trans dibromide is produced.