Alkyl

An alkyl is a hydrocarbon substituent derived from an alkane by removing one hydrogen atom. In chemical notation, alkyl groups are often represented as R-, with the general formula CnH2n+1 for an alkyl group containing n carbon atoms. Alkyl groups are saturated, composed of carbon and hydrogen with sp3-hybridized carbons, and they are typically nonpolar and hydrophobic. They form carbon–carbon bonds to attach to a parent molecule and are common features in organic compounds.

Common alkyl groups include methyl (CH3-), ethyl (C2H5-), propyl (C3H7-), and butyl (C4H9-). Branched forms such as isopropyl (propan-2-yl) and tert-butyl (2-methylprop-2-yl) are widely used. Linear, unbranched chains are referred to as n-alkyl groups (e.g., n-propyl, n-butyl). In IUPAC naming, substituents are described as alkyl groups, while branched substituents are named using systematic forms such as propan-2-yl for isopropyl and, for tert-butyl, 2-methylprop-2-yl, though common names are still widely used.

In chemical reactions, alkyl groups are transferred or modified through alkylation, oxidation, or substitution processes. They influence physical properties such as boiling point and solubility, typically increasing hydrophobic character. Alkyl substituents are central to the structure and reactivity of countless organic molecules, including alkanes, alcohols, ethers, esters, and polymers.

Note that alkyl should be distinguished from aryl, which denotes substituents derived from aromatic rings. Alkyl denotes saturated hydrocarbon substituents, whereas aryl refers to aromatic ring systems.