α-Hydrogen reactions and uses

Hydrogens on the carbon next to C=O (α-hydrogens) are acidic: the carbonyl group withdraws electrons strongly, and the conjugate base is resonance-stabilised.

Aldol reaction: an aldehyde or ketone with at least one α-hydrogen, with dilute alkali as catalyst, gives a β-hydroxy aldehyde (aldol) or β-hydroxy ketone (ketol). Ethanal gives 3-hydroxybutanal.

The aldol or ketol loses water easily to give an α,β-unsaturated carbonyl compound, the aldol condensation product: 3-hydroxybutanal → but-2-enal. Ketones are included under the same name.

Cross aldol condensation uses two different aldehydes and/or ketones. If both have α-hydrogens, four products form: ethanal with propanal gives but-2-enal, 2-methylpent-2-enal, pent-2-enal and 2-methylbut-2-enal.

Cannizzaro reaction: an aldehyde with no α-hydrogen, heated with concentrated alkali, disproportionates. One molecule is reduced to the alcohol and another oxidised to the carboxylic acid salt: 2HCHO + conc. KOH → CH₃OH + HCOOK.

In aromatic aldehydes and ketones the carbonyl group deactivates the ring and directs electrophiles to the meta position.

Formalin, a 40% solution of formaldehyde, preserves biological specimens; formaldehyde also makes bakelite (a phenol-formaldehyde resin) and urea-formaldehyde glues.

Acetaldehyde is the raw material for drugs, polymers, vinyl acetate, ethyl acetate and, mainly, acetic acid. Perfume and dye makers use benzaldehyde; acetone and ethyl methyl ketone are industrial solvents; camphor, acetophenone, vanillin and butyraldehyde are prized for their smell and taste.