The carbon atoms are present  in a ring structure and a single bond exists between carbon atoms. Cycloparaffins or naphthenes have the formula, C_nH_2n and cyclopropane (C₃H₆), cyclobutane (C₄H₈),  cyclohexane (C₆H₁₂) etc.,  are its examples. The cycloparaffins having more than 6 carbon atoms are not common.

Olefins

Olefins or alkenes are open chain hydrocarbons having one or more carbon-carbon double bonds. The compounds having one double bond are called mono-olefins and their empirical formula is C_nH_2n.  The examples are ethylene, propylene, butene, octene etc. Those having two double bonds are called as diolefins or dienes, the chemical formula being C_nH_2n-2. The position of the double bond (s) is indicated by a prefix like 1-octene, 1, 3- butadiene etc. The diolefins are highly unstable during storage and therefore, are undesirable compounds in the engine fuels. Another family of unsaturated hydrocarbons has triple carbon-carbon bond. These compounds are known as acetylenes or alkynes. The empirical chemical formula for alkynes is C_nH_2n-2 and the first member of the series is acetylene (C₂H₂). Higher alkynes are similar to higher alkenes with each double bond replaced by triple bond

Aromatics

Aromatics are double bonded hydrocarbons arranged in a ring structure of carbon atoms. Each ring of aromatics has 6 carbon atoms. Benzene (C₆H₆) is the first member of the family. Benzene structure has three double bonds which alternate in position between carbon atoms. Other aromatics are formed when hydrogen atom(s) attached to carbon in the aromatic ring is substituted by an alkyl radical such as methyl-, ethyl-, propyl- etc. Some examples of aromatics having side chains attached to ring are toluene (methylbenzene), ethyl benzene, xylene (dimethyl benzene) etc.  Many aromatic hydrocarbons have two or more aromatic rings such as naphthalene, anthracene, benzo(a)pyrene. These compounds are known as polycyclic aromatic hydrocarbons (PAH).