butyl

Butyl is a term used in organic chemistry to denote a substituent derived from butane, a four-carbon hydrocarbon. A butyl group has the formula C4H9 and is written as C4H9– when attached to another molecule. The term encompasses several structural isomers, which differ in the arrangement of carbon atoms and thus in steric properties and reactivity. The common forms are n-butyl (linear), sec-butyl (secondary), isobutyl (1-methylpropyl), and tert-butyl (1,1-dimethylethyl).

In naming and synthesis, butyl groups appear in a wide range of compounds, including esters, ethers, and alcohols. Examples include butyl acetate and n-butyl alcohol (1-butanol), both used as solvents in various applications. The tert-butyl group is prominent in 2-methyl-2-propanol (tert-butyl alcohol) and in tert-butyl carbamate protecting groups (Boc) used in organic synthesis. In polymer chemistry, butyl-related materials include butyl rubber, a copolymer of isobutylene with small amounts of isoprene, valued for low gas permeability and used in inner tubes, seals, and caulking.

Safety and handling of butyl-containing compounds vary with the specific chemical. Many butyl derivatives are flammable and require appropriate storage, ventilation, and handling procedures. The term derives from butane and is used broadly to designate any four-carbon alkyl substituent in chemical nomenclature.