One of the structural isomers has a geometric isomer.
#"1. cyclobutane"#; #"2. methylcyclopropane"#; #"3. 1-butylene"#; #"4. 2-butylene"#; #"5. 2-methylpropene"#.
#"2-butylene"# can of course generate 2 geometric isomers, which are?
Just to add a bit more method to the analysis. It is a fact that an alkane has general formula #C_nH_(2n+2)#. In this analysis, an halide counts for #H#, oxygens are ignored (they can be carbonyls or alcohols or ethers), and if there is a #N# in the compound, we subtract #NH# from the given formula. Each 2 hydrogens less than this formula corresponds to a degree of unsaturation. A degree of unsaturation corresponds to an olefinic bond, OR a ring junction. Thus I knew that the possible isomers of #C_4H_8# were olefins, OR #"3- or 4-membered rings"#.
