Allylic strain

Allylic strain or 1,3-allylic strain in organic chemistry and its associated strain energy results from an unfavorable molecular conformation for the allyl group. The simplest representative of the allyls, propene, in its ground state has the methyl C-H bond eclipsed with the carbon carbon double bond. When this bond is pointing away the resulting unfavorable conformation is called bisected.

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The activation energy for rotating around the allyl CC bond is 1.98 kcal/mol^[1].

This type of strain also extends to carbonyls.

References