CnHn

CnHn is a shorthand notation used to denote a hydrocarbon in which the numbers of carbon and hydrogen atoms are equal. It is not the name of a single compound or a formal chemical family, but rather a way to express the empirical formula of certain hydrocarbons that satisfy CnHm with m = n.

In general, hydrocarbons have the formula CnHm, where n is the number of carbon atoms and m is the number of hydrogen atoms. The degree of unsaturation, or rings and pi bonds, for a hydrocarbon is DoU = (2n + 2 − m) / 2. When m = n, this becomes DoU = (n + 2) / 2, which is an integer only when n is even. Consequently, stable examples of CnHn appear for even n and are typically associated with rings or multiple bonds.

Representative members and patterns include:

- C2H2 (acetylene), a linear molecule with a triple bond.

- C4H4 (cyclobutadiene), a cyclic, highly reactive compound with significant antiaromatic character.

- C6H6 (benzene), a quintessential aromatic ring that is unusually stable for a compound with CnHn.

- C8H8 (cyclooctatetraene), a cyclic polyene that adopts a nonplanar geometry to relieve antiaromatic strain.

Overall, CnHn serves as a mathematical or conceptual marker rather than a distinct, well-defined class of compounds. Its relevance arises in discussions of formula constraints, degrees of unsaturation, and the structural possibilities of hydrocarbons with equal carbon and hydrogen counts.

See also: hydrocarbon formula, degree of unsaturation, aromatic hydrocarbons, cycloalkenes and alkynes.