Enol 1238210 216812712 2008-06-03T10:07:13Z 221.127.210.111 /* Natural occurrences */ '''Enols''' (also known as '''alkenols''') are [[alkene]]s with a [[hydroxyl group]] affixed to one of the carbon atoms composing the [[double bond]]. Enols and [[carbonyl compound]]s (such as [[ketones]] and [[aldehydes]]) are in fact [[isomer]]s; this is called [[keto-enol tautomerism]]: [[Image:enol.png|center]] The enol form is shown on the left. It is usually unstable, does not survive long, and changes into the keto (ketone) form shown on the right. This is because [[oxygen]] is more [[electronegative]] than [[carbon]] and thus forms [[bond energy|stronger]] multiple [[chemical bond|bonds]]. Hence, a carbon-oxygen ([[carbonyl]]) double bond is more than twice as strong as a carbon-oxygen single bond, but a carbon-carbon double bond is weaker than two carbon-carbon single bonds. Only in 1,3-dicarbonyl and 1,3,5-tricarbonyl compounds does the (mono)enol form predominate. This is because the [[resonance (chemistry)|resonance]] and intermolecular [[hydrogen bond]]ing that occurs in the enol form is not possible in the keto form. Thus, at equilibrium, over 99% of propanedial (OHCCH<sub>2</sub>CHO) molecules exist as the monoenol. The percentage is lower for 1,3-aldehyde ketones and diketones. Enols (and enolates) are important intermediates in many organic reactions. The words '''enol''' and '''alkenol''' are [[portmanteau|portmanteaux]] of the words '''alkene''' (or just [[-ene]], the suffix given to [[alkene]]s) and [[alcohol]] (which represents the enol's hydroxyl group). ==Enolate ion== When the hydroxyl group (&minus;OH) in an enol loses a hydrogen ion (H<sup>+</sup>), a negative '''enolate''' ion is formed as shown here: <center>[[Image:Formation of Enolate.PNG]]</center> Enolates can exist in quantitative amounts in strictly Brønsted acid free conditions, since they are generally very basic. 1,3-dicarbonyl and 1,3,5-tricarbonyl compounds are quite acidic because of the strong resonance stabilization created when one of the hydrogens is removed (from either the keto or enol forms). The resonance of the enol is exactly analogous to that used to explain the acidity of [[phenols]] and consists of the delocalisation of the enolate ion's negative charge to the [[alpha carbon]]. These enolate ions are very valuable in synthesis of complicated alcohols and carbonyl compounds (aldol additions). The synthetic value is due to the [[nucleophile|nucleophilicity]] of α-carbon of enolate group. In ketones (a type of carbonyl) with acidic α-hydrogens on either side of the carbonyl carbon, selectivity of deprotonation may be achieved to generate the enolate directly from the ketone. At low temperatures (-78°C, i.e. dry ice bath), in aprotic solvents, and with bulky non-equilibrating bases (e.g. [[lithium diisopropylamide|LDA]]) the "kinetic" proton may be removed. The "kinetic" proton is the one which is [[steric effects|sterically]] most accessible. Under thermodynamic conditions (warmer temperatures, weak base, and protic solvent) equilibrium is established between the ketone and the two possible enolates, the enolate favoured is termed the "thermodynamic" enolate and is favoured because of its lower energy level than the other possible enolate. Thus, by choosing the "correct" conditions to generate an enolate, one can increase the yield of the desired product while minimizing formation of undesired products. ==Natural occurrences== [[Vitamin C]] is a sugar acid containing an enol bond. It can lose a proton as pictured, which makes it an [[acid]]: [[Image:Ascorbic acidity3.png|thumb|center|500px|Movement of electron pairs in deprotonation of [[ascorbic acid]] ([[vitamin C]])]] [[Category:Functional groups]] [[de:Enole]] [[es:Alquenol]] [[fr:Énol]] [[he:אנול]] [[hu:Enol]] [[mk:Енол]] [[ja:エノラート]] [[pl:Enole]] [[pt:Enol]] [[ru:Енолы]] [[fi:Enoli]] [[sv:Enol]] [[zh:烯醇]]