carbocycles

Carbocycles are chemical compounds in which the rings are composed entirely of carbon atoms. The term covers monocyclic carbocycles, such as cyclopropane, cyclobutane, cyclopentane, cyclohexane, and cycloheptane; cycloalkenes such as cyclohexene; and aromatic carbocycles such as benzene. It also includes polycyclic carbocycles, where several rings are fused, bridged, or spiro-connected, such as naphthalene, anthracene, phenanthrene, chrysene, and other polycyclic aromatic hydrocarbons.

Carbocycles are distinguished from heterocycles, which contain one or more non-carbon atoms in the ring. They are also distinct from carbon allotropes that do not feature defined ring systems, such as graphite or fullerenes, though those materials contain extensive carbocyclic character.

Ring size and fusion pattern determine properties. Small rings (three to four members) suffer from ring strain, while six-membered rings (cyclohexane, benzene) tend to be stable and common in nature and synthesis. Aromatic carbocycles exhibit delocalized π-electron systems, imparting unusual stability and characteristic reactivity. Polycyclic carbocycles show extended conjugation, influencing color, electronic properties, and reactivity.

Synthesis and occurrence: Carbocycles arise in natural products, petrochemicals, and as scaffolds in pharmaceuticals and materials science. They are built by cyclization, aryl–aryl coupling, and dehydrogenation sequences, among other methods. Carbocyclic compounds underpin a wide range of chemical disciplines and applications, from basic organic chemistry to advanced materials research.