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Introduction to CH-acidic Compounds

The Acidity of Carbonyl Compounds

Aldehydes and ketones have remarkably low pKa values that range between 15 and 20. Thus, they may act as a Brønsted acid in an acid-base reaction with a strong base. The acidic hydrogen is the hydrogen that is bound to the carbon adjacent to the carbonyl carbon. Due to the only minor difference in electronegativity between hydrogen and carbon, C-H bonds in alkanes are hardly polarized at all. Thus, hydrogens of alkanes are in fact not acidic. The pKa values of alkanes amount to only approximately 50.

The acidic hydrogen of carbonyl compounds is known as α hydrogen, as it is connected with the carbonyl compound's α carbon that is directly bound to the carbonyl carbon.
Acidity of alkanes and ketones.

The carbonyl compounds' relatively high acidity may be explained by the resonance stabilization of the conjugate base by the carbonyl group, or, in other words, through the stabilization of the anion formed by deprotonation. This anion is called an enolate anion. The negative charge is mainly distributed among the α carbon and the carbonyl oxygen, by resonance, which leads to the stabilization of the otherwise highly energized carbanion.

Stability of enolate anions

Both the bonding and non-bonding π orbitals of the enolate are occupied, while the antibonding π orbital remains free. The distribution of the negative charge and the nucleophilic qualities of the enolate anion are mostly represented through the occupied non-bonding π orbital, as it is the enolate's HOMO. The illustration of the non-bonding π orbital is indicative of the location of the non-bonding π electrons, which are at the α carbon and the carbonyl oxygen. Thus, the α carbon and the carbonyl oxygen are the nucleophilic positions of enolate anions. The smaller extension of the non-bonding π orbital lobe at the carbonyl oxygen clearly supports the fact that the electron (density) is bound more strongly and closely to the oxygen atom than is the case with the α carbon. Thus, the non-bonding π orbital lobes' extensions substantiate the fact that the α carbon is a soft base, while the carbonyl oxygen is a hard base. As a result, soft Lewis acids (electrophiles), such as alkyl halides and the carbonyl carbon of carbonyl compounds, tend to be nucleophilically attacked by the enolate's α carbon.

π orbitals of an enolate anion.
Bonding π orbital (occupied) Non-bonding π orbital, HOMO (occupied) Antibonding π orbital, LUMO (unoccupied)

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Exercise 1

Exercise 2

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