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.
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Allylic strain refers to the steric and electronic interactions that occur in allylic systems, where substituents are located on adjacent carbon atoms in a double bond configuration or where there is a single carbon-carbon bond adjacent to a double bond. This strain arises mainly from the positioning of substituents at the allylic positions, which can lead to increased steric hindrance and torsional strain.