Question

How many isomers exist for `C_5H_10`?

Answer 1

Not counting geometric isomers, I can count at least 9 isomers. There may be more.

Explanation:

A formula of `C_5H_10` has `1^@` of unsaturation. What does this mean? It means that in comparison to the saturated formula, `C_5H_12`, this formula has `2` hydrogens fewer. A degree of unsaturation specifies an unsaturated bond OR a ring.

So I can make at least 4 olefins with this formula (one of which has a geometric isomer); given the formula, the double bond can be either at `C1` or `C2`.

Given a ring junction, there are 5 alicyclic structures possible: a five-membered ring; a four-membered ring; and 3 three-membered rings. One of the three-membered rings can have geometric isomers.