C6H4

C6H4 is the molecular formula that represents a hydrocarbon containing six carbon atoms and four hydrogen atoms. As an empirical formula it corresponds to several distinct isomers and reactive intermediates derived from benzene, whose parent formula is C6H6. The removal of two hydrogen atoms from benzene generates a highly unsaturated framework that can exist as either a benzyne species or as a phenylene diradical, each possessing a unique bonding pattern and reactivity.

Benzyne, often described as 1,2‑didehydrobenzene, features a triple bond between two adjacent carbon atoms of the six‑membered ring, resulting in a strained, highly electrophilic intermediate that participates in nucleophilic addition reactions and cycloaddition processes. Phenylene, the 1,4‑diradical form, contains two unpaired electrons at opposite positions of the ring and serves as a building block in polymer chemistry, most notably in the synthesis of polyphenylene and related conductive materials.

The C6H4 skeleton is also the core of numerous polycyclic aromatic compounds, such as naphthalene derivatives, where the formula represents a fused benzene ring system after substituent removal. Because of its unsaturation and aromatic character, C6H4 derivatives display characteristic UV‑visible absorption spectra and undergo characteristic chemical transformations, including Birch reductions and Diels‑Alder reactions. Despite being transient in isolation, C6H4 intermediates are central to many synthetic routes in organic chemistry and materials science.