Wikipedia definition
1. Allylic strainAllylic strain (also known as A strain, 1,3-allylic strain, or A-strain) in organic chemistry is a type of strain energy resulting from the interaction between a substituent on one end of an olefin with an allylic substituent on the other end. If the substituents (R and R') are large enough in size, they can sterically interfere with each other such that one conformer is greatly favored over the other. Allyic strain was first recognized in the literature in 1965 by Johnson and Malhotra.
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