Which of the following compounds is a structural isomer of butane?
a. 2-methylbutane
c. 2-methylpropane
b. 2,2-dimethylbutane
d. 2,2-diethylpropane

The compound that is a structural isomer of butane is 2 – methylpropane. Note that butane has four carbon atoms. In 2 – methylpropane, propane has three carbon atoms and methyl is attached at the second carbon. Methyl has only one carbon atom and together, they have a total of four carbon atoms. An isomer is a substance that has the same number of atoms but they differ in their structure. The answer is letter C.