Question

which of the following is not a constitutional (structural) isomer of pentane? 2 - methylbutane 2,2 - dimethylpropane cyclopentane all of the above are constitutional isomers of pentane

Explanation:

Constitutional (structural) isomers have the same molecular formula but different connectivity of atoms. Pentane has the molecular formula $C_5H_{12}$. 2 - methylbutane and 2,2 - dimethylpropane are chain - isomers of pentane with the same $C_5H_{12}$ formula. Cyclopentane has the molecular formula $C_5H_{10}$, which is different from that of pentane. So it is not a constitutional isomer of pentane.

Answer:

cyclopentane