How do we determine the number of alpha hydrogens in a compound? Where should we start the naming from? For example, bicyclo[4.2.0]oct-7-ene and bicyclo[4.2.0]oct-3-ene:

Compare the stabilities of the following alkenes

Structures a and b

In both these structures, I need to determine their stabilities based on their hyperconjugating structures. I need to determine the number of alpha hydrogens it has, to determine the number of hyperconjugating structures it makes. Where do I start?

I am aware that stability is directly proportional to the number of hyperconjugating structures. More the number of alpha hydrogens, greater is the number of hyperconjugating structures, and hence greater the stability.

  • $\begingroup$ The discussion with alpha hydrogens would have worked given structures like hex-3-ene and 2,5-dimethylhex-3-ene. $\endgroup$
    – Jan
    Nov 15 '16 at 15:07

The relative stability of your two compounds is not determined by arguments based on hyperconjugation. Rather the relative stabilities have to do with ring strain. Placing a double bond in smaller and smaller rings increases the angle strain in the ring system. A double bond, being roughly $\ce{sp^2}$ hybridized, would prefer to have bond angles around 120°. This becomes difficult in 3- and 4-membered rings where the internuclear angle is 60° and 90° respectively. Therefore molecules containing double bonds in 3- and 4-membered rings will be more strained than molecules with the double bond in a larger ring.

Edit: Response to OP's comment

No, the given solution is not the correct answer to the problem. It is correct that structure B has more alpha (alpha to the double bond) hydrogens (4) than structure A (only 2 alpha hydrogens). It is also true that hyperconjugation can be used to explain why alkyl groups attached to a double bond act to stabilize a double bond (see this earlier answer for an explanation), and that the double bond in B is more stabilized by hyperconjugation than the double bond in A. However, hyperconjugation has a much smaller effect on the stability of these molecules than the ring strain effect.


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