# How to determine number of alpha hydrogens in a cyclic compound?

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

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.

• The discussion with alpha hydrogens would have worked given structures like hex-3-ene and 2,5-dimethylhex-3-ene. – 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.