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The Chemical Bond

Functional Groups in Organic Chemistry


Chemical Thermodynamics and the Equilibrium State

Reactions of Aldehydes and Ketones

The enol form is not a resonance form of an aldehyde or ketone. An aldehyde or ketone and its enol are constitutional isomers that exist in equilibrium. Equilibrium generally favors the carbonyl form, but some enols are especially stable due to a lowering of the free energy of the enol form by resonance, which occurs with phenol and β-diketones. In order for an aldehyde or ketone to have an enol tautomer, it must possess an α-proton, which migrates, accompanied by a switch of a single and an adjacent double bond.

Keto-enol tautomerism opens aldehydes and ketones up to an entire class of reactions involving electrophilic attack at the α carbon.

A tricky curveball with keto-enol tautomerism might arise from the fact that if the α carbon is a chiral center, enolization leads to racemization of the aldehyde or ketone.


The Chemical Bond


Chemical Thermodynamics and the Equilibrium State

Acids and Bases

Organic Acids and Bases

Reactions of Aldehydes and Ketones

Enol formation can be either acid or base catalyzed. In the first step of acid-catalyzed enolization, the carbonyl oxygen acts as a Brønsted base and abstracts a proton from hydronium in the solvent environment. The arrival of a proton increases the already strong pull oxygen exerts on the bonding electrons it shares with carbon in the carbonyl group, so that oxygen pulls the π bonding pair it shares with carbon completely into its own orbit where they become a nonbonding pair while carbon simultaneously compensates by forming a new π bond with the α carbon, which loses a proton in the process.

Alternatively to acid catalysis, enolization might also be promoted by basic conditions. A proton bonded to the α carbon of an aldehyde or ketone is substantially more acidic (Ka around 10-16 to 10-20) than is typical with C-H bonds. This is because, subsequent to deprotonation, an enolate anion is formed in which delocalization of negative charge occurs by resonance with the carbonyl oxygen (decrease in internal energy, enthalpy, and free energy compared to typical carbanions, explaining the elevated Ka). Base catalyzed enol formation begins with abstraction of a proton by hydroxide ion from the α carbon of the aldehyde or ketone, leading to the enolate anion. Proton transfer from water to the enolate ion oxygen forms the neutral enol.

Reactions of Aldehydes and Ketones


Bioenergetics and Cellular Respiration

Keto-enol tautomerism is a very significant process in biochemistry. The means in biochemistry of interconversion of an aldose, such as glucose, to a ketose, such as fructose, occurs via their common enolate isomer. For example, the isomerization of the 6-phosphate ester of D-glucose to D-fructose by way of keto-enol tautomerism occurs as one of the first steps of glycolysis. Additionally, a similar isomerization occurs a few steps later to convert the ketose dihydroxyacetone phosphate to glyceraldehyde 3-phosphate.

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