= Allylic strain
<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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