text: Polarity also has an effect on allylic strain. In terms of stereoselectivity, polar groups act like large, bulky groups.  Even though two groups may have approximately the same A values the polar group will act as though it were much bulkier. This is due to the donor character of the polar group. Polar groups increase the HOMO energy of the σ-system in the transition state.  This causes the transition state to be in a much more favorable position when the polar group is not interacting in a 1,3 allylic strain.