alkoxy

An alkoxy group is a functional substituent in organic chemistry consisting of an alkyl group bonded to an oxygen atom, written as -OR, where R is an alkyl group. It is a neutral substituent that attaches to the rest of a molecule through the oxygen. The term is used in both nomenclature and structural descriptions of ethers, esters, and related compounds.

Common examples include methoxy (-OCH3), ethoxy (-OCH2CH3), and propoxy (-OCH2CH2CH3). When attached to an aromatic ring, these groups appear in compounds such as anisole, which is methoxybenzene. The alkoxy group influences the physical properties of a molecule, including polarity, solubility, and boiling point, and it can donate electron density through the oxygen atom.

In structural terms, alkoxy groups occur in two broad contexts. In ethers, the functional group is R–O–R', with oxygen single-bonded to two carbon atoms. In esters, the alkoxy group forms the OR' portion of the acyl ester moiety R–CO–OR', and OR' can be transferred or cleaved during hydrolysis, transesterification, or related reactions. The alkoxy group also participates in reactions characteristic of alkoxide chemistry, such as the Williamson ether synthesis, where an alkoxide (RO−) reacts with an alkyl halide to form an ether.

Alkoxide and alkoxy are related but distinct: an alkoxide (RO−) is the negatively charged conjugate base of an alcohol, whereas an alkoxy group (RO) is the neutral substituent attached to a molecule.