Question

The total number of cyclic isomers for the hydrocarbon having the formula $C_4{H}_6$ is ___

Answer 1

First calculate the degree of unsaturation / DBE:

$DBE=C+1-\frac{H}{2}+\frac{N}{2}=4+1-3=2$

Plus we want only the cyclic hydrocarbons, so there should be at least one ring:

The required isomers can have two rings or a ring + double bond. Let us list down the structures:

There are five structures.