Which of the following compounds is not alicyclic?
Cyclopropane
Cyclohexane
Cyclohexene
Isobutane
An alicyclic compound contains one or more all-carbon rings which may be either saturated or unsaturated, but do not have aromatic character. Alicyclic compounds may have one or more aliphatic side chains attached.
1) Cyclopropane has a carbon ring and it is saturated.
2) Cyclohexane has a carbon ring and it is saturated.
3) Cyclohexene has a carbon ring and it is saturated.
4) Isobutane does not have a carbon ring.
Therefore, option (4) is correct.