Question

Consider the following two isomeric alkenes. The first isomer is a mono substituted alkene, while the second isomer is a disubstituted alkene. We might expect the second isomer to be more stable, yet heats of combination for these two compounds indicate that the first isomer is more stable. Offer an explanation.

Answer 1

Explanation:

Since both the alkenes contain a 4 membered ring, both will contain ring strain.

We know that ring strain decreases the stability of the compound. Hence higher the ring strain, lower will be stability.

First alkene contains only sp3 carbons in the ring. Hence

ring starin = (109.50 - 900) =9.75 0 around each carbon

Similarly in the second alkene, ring contains two sp2 carbons and 2 sp3 carbons

Hence angle strain forsp2 carbons = (120 0 - 90 0) / 2 = 15 0

Therefore in the second alkene, due to higher angle strain around sp2 carbon atoms, the compound is less stable than the first alkene.

Hence first alkene is more stable than the the second one.