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Allylic strain is an unfavourable steric interaction involving substituents around the region next to a carbon–carbon double bond. The double bond restricts rotation and makes the relative placement of nearby groups particularly important.

A rotation around an adjacent single bond can reduce crowding while changing which face of the alkene is exposed to a reagent. This provides a route from conformational preference to reaction stereoselectivity. The term should specify the interacting groups and conformation; it is not a universal fixed energy penalty for every allylic fragment.

Thomas C. Coombs coauthored 1,3-Allylic Strain as a Strategic Diversification Element for Constructing Libraries of Substituted 2-Arylpiperidines by Thomas C. Coombs and others.

Allylic strain arises from unfavorable spatial contacts among substituents associated with an alkene and a neighboring saturated center. The publication uses 1,3-allylic strain as a conformational design element in constructing substituted piperidines, connecting a local steric interaction with control of a synthetic scaffold.