Enolate ion

Enols, or more formally, alkenols, are a type of reactive structure or intermediate in organic chemistry that is represented as an alkene (olefin) with a hydroxyl group attached to one end of the alkene double bond. The terms enol and alkenol are portmanteaus deriving from "-ene"/"alkene" and the "-ol" suffix indicating the hydroxyl group of alcohols, dropping the terminal "-e" of the first term. Generation of enols often involves removal of a hydrogen adjacent (α-) to the carbonyl group—i.e., deprotonation, its removal as a proton, H⁺. When this proton is not returned at the end of the stepwise process, the result is an anion termed an enolate (see images at right). The enolate structures shown are schematic; a more modern representation considers the molecular orbitals that are formed and occupied by electrons in the enolate. Similarly, generation of the enol often is accompanied by "trapping" or masking of the hydroxy group as an ether, such as a silyl enol ether.

Critically, the terminus of the double bond in enols, enolates, and silyl enol ethers that lacks the hydroxyl group is nucleophilic, and so can act as an electron donor in reactions with electrophilic organic compounds; this reactivity underlies the tremendous importance of enol-based intermediates in a wide array of important life processes (i.e., in biochemistry, as intermediates in enzyme-catalysed reactions), as well as being central to modern synthetic organic chemistry (e.g., in applications of aldol and related reactions). The importance of enols in accomplishing nature and humankind's chemical transformations makes them irreplaceable.

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