Cyclopentane is the first ring-shaped, naturally occurring hydrocarbon. It is present in very small quantities in crude oil, the most common source being naphtha from crackers. The chemical formula for this cycloalkane, or naphthene, is C₅H₁₀. 

iso-Pentane is the first branched, liquid hydrocarbon or paraffin. This group of substances is also called alkanes. The iso-Pentane has a chemical formula of C₅H₁₂. 

iso-Octane is a branched hydrocarbon from the alkane group with the chemical formula C8H18. It is the most important of the eighteen octane isomers. Since it is an important component of motor gasoline, it is produced synthetically, but is also found to a small extent in petroleum.

n-Octane is a chain-like hydrocarbon from the alkane group with the chemical formula C₈H₁₈. It is the unbranched representative and of the eighteen octane isomers.