octadecenoic acid octadecenoate [O-]C([*])=O CHEBI:78049 A fatty acid anion containing 18 carbons and one double bond. Formed by deprotonation of the carboxylic acid group. Major species at pH 7.3. -1 281.4534 C18H33O2 281.24806 chebi_ontology monounsaturated fatty acid anion