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21 cze 2022 · Aldehydes and ketones that have a proton on the alpha carbon can participate in keto-enol tautomerism, where an equilibrium exists between two constitutional isomers – the keto and enol forms. Constitutional isomers that are in equilibrium are called “tautomers”.
Aldehydes and symmetrical ketones typically only have one possible enol tautomer while asymmetrical ketones can have two or more. The preferred enol tautomer formed can be often be predicted by considering effects which can stabilize alkenes, such as conjugation and alkyl group substitution.
In a broader definition, keto-enol tautomerization pertains not only to the equilibrium between a ketone and its tautomer enol but also to any carbonyl compound such as aldehydes, esters, amides and etc. and its enol form.
Keto–enol tautomerism of carbonyl compounds is catalyzed by both acids and bases. Acid catalysis occurs by protonation of the carbonyl oxygen atom to give an intermediate cation that loses H + from its α carbon to yield a neutral enol (Figure 22.2a).
16 sie 2022 · Base-catalyzed keto-enol tautomerism of aldehydes and ketones goes through an enolate. Enolates will react with halogens (e.g. Br 2, Cl 2, I 2) aldehydes/ketones, and alkyl halides. Enolates with two electron withdrawing groups attached to the alpha carbon are particularly easy to form.
22 maj 2020 · The following examples show how an enol whose hydroxyl group is at the end of a carbon chain tautomerizes into an aldehyde, whereas an enol whose hydroxyl group is in the middle of a carbon chain tautomerizes into a ketone.
Under acidic conditions, the enol tautomer forms. Under basic conditions, the enolate tautomer forms. Both the enol and enolate are nucleophiles that can undergo subsequent reactions.
