Parent chain Structural Isomers: You may have noticed that the structures of some hydrocarbons differ only in the positions of substituent groups or of multiple bonds in their molecules. Look at the structural formulas for butane and 2 methylpropane shown below. Even though both compounds have the formula C4H10 their boiling points and other properties differ. Because their structures are different, they are different substances. Compounds that have the same molecular formula but different molecular structures are called isomers, Occurrence of two or more compounds having the same molecular formula, but different structural formulae called isomerism. Isomerism is possible only in hydrocarbons containing four or more carbon atoms. Thus, methane, ethane and propane do not show isomerism. Butane, pentane, hexane and heptane(and so on) show isomerism.