Acid-Base Properties

The α position of a C=O group is much more acidic than expected.

α Hydrogens of ketones (pKa ≈ 20) and aldehydes (pKa ≈ 17)

Compared with an alkane, the C=O group increases the acidity of the neighboring CH by more than 30 orders of magnitude!

It is worth remembering some significant pK_a values.

Bases that can deprotonate an alcohol can also deprotonate the α position of an aldehyde or ketone.

Table of important pKa values: alkanes 51, amines 38, hydrogen 35, alcohols and water 15-16, ammonium 10-11, thiols 10-11, carboxylic acids 4-5, hydrochloric acid −7, with their conjugate bases and roles

But notice that aldehydes and ketones already carry a base within themselves: the oxygen of the C=O group! The oxygen of the CH–C=O group can pull off the α hydrogen of its own molecule, giving an enol.

Animation of a molecular model of acetone turning into its enol — Animation of the keto-enol equilibrium

Equilibrium between the keto and enol forms of acetone — In "normal" aldehydes and ketones, the keto-enol equilibrium is generally strongly shifted toward the keto form.

It is instructive to analyze the amount of enol measured experimentally at equilibrium for a series of carbonyl compounds. Can you explain the variations observed as a function of the structure and, in one case, of the solvent? Look at the problems and exercises for an answer.

Table of the percentage of enol at equilibrium: acetaldehyde below 10⁻⁷, acetone 1.5·10⁻⁴, butanone 1.2·10⁻¹, methyl acetoacetate 0.4 in water and 19.8 in toluene, 2,4-pentanedione 76.4, benzoylacetone 89.2 and phenol 100

The enolization of aldehydes and ketones has important chemical consequences

Keto and enol forms in equilibrium, the enolate ion with its two resonance forms, reaction at the α carbon with an electrophile and trapping of the enolate at oxygen

Notice that the keto and enol forms are different chemical species in equilibrium (blue equilibrium arrows: an atom changes places), whereas there is only one enolate ion, which is described by two resonance forms (green double-headed arrow: no atom changes places).

The most common reaction of an enolate ion takes place at the α carbon and, importantly, forms a new C–C bond. Some reactions, however, take place at the oxygen and allow the enolate to be trapped. Remember this later on.