CxHy

CxHy is a general chemical formula used to describe hydrocarbons—compounds composed solely of carbon and hydrogen. The symbol x denotes the number of carbon atoms and y the number of hydrogen atoms in a given molecule or empirical unit. Because many different structures can share the same CxHy composition, the notation identifies a broad class rather than a single compound.

For open-chain hydrocarbons, simple saturation patterns reflect carbon valence. Saturated acyclic hydrocarbons, or alkanes, have y = 2x + 2 (for x ≥ 1). Acyclic alkenes, which contain at least one carbon–carbon double bond, have y = 2x. Acyclic alkynes, with at least one triple bond, have y = 2x − 2. Cycloalkanes with one ring and no other unsaturation follow y = 2x, while cycloalkenes and other unsaturated variants deviate from these patterns. In general, CnH2n is the typical formula for many saturated monocyclic alkanes such as cyclohexane (C6H12).

The degree of unsaturation (or double bond equivalents) can be estimated for hydrocarbons with DBE = x − y/2 + 1 (assuming no nitrogen present). DBE counts rings and pi bonds, providing a sense of how many multiple bonds or rings a molecule possesses.

Examples illustrate the concept: methane is CH4 (C1H4); ethane is C2H6; ethene is C2H4; acetylene is C2H2; benzene is C6H6. The CxHy formula is a useful shorthand for classifying hydrocarbons, but it does not uniquely identify a specific structure; many isomers can share the same empirical or molecular formula.