If, in a reaction, stereoisomeric (see stereoisomers) reactants gives stereoisomeric products, the reaction is said to be stereospecific.
Reactions 1 and 2 are SN2 reactions of stereoisomeric substrates 1 and 2 with the same nucleophile, leading to stereoisomeric substitution products 3 and 4, respectively. Thus, SN2 reactions are stereospecific.
Reactions 3 and 4 are Diels-Alder reactions of stereoisomeric dienophiles 5 and 6 with the same diene, 1,3-butadiene. The products of the reaction of 5 is 7; the products of the reaction of 6 are 8 and 9. 7 and 8 are stereoisomers, so are 7 and 9. Thus, Diels-Alder reactions are stereospecific.