Reactions of Carbonyl Compounds

Carbonyl compounds are a group of organic compounds that feature a carbon-oxygen double bond. This class includes ketones and aldehydes. Carbonyl compounds exhibit a variety of typical reactions due to their unique structure.

Addition Reactions

Addition reactions are a central feature of carbonyl chemistry. These reactions involve the addition of a reagent to the carbonyl group, forming a single product.
The most common addition reaction for carbonyl compounds is their reaction with nucleophiles. The carbonyl carbon atom is electron deficient and has a partial positive charge, making it a target for nucleophiles such as water, alcohols, amines, and hydride donors.
Addition of water to carbonyl compounds will lead to the formation of hydrates. This process is reversible.
Addition of alcohol leads to the formation of hemiacetals and hemiketals, which can further react to form acetals and ketals with the loss of water.
Aldehydes and ketones also undergo condensation reactions such as the Aldol condensation, which involves the reaction of two carbonyl compounds to form a β-hydroxy carbonyl compound.

Oxidation and Reduction Reactions

Carbonyl compounds can be both oxidised and reduced. Aldehydes can be oxidised to carboxylic acids while ketones resist oxidation.
The reduction of carbonyl compounds using hydride donors such as lithium aluminium hydride (LiAlH4) or sodium borohydride (NaBH4) leads to the formation of alcohols.
Further reduction of aldehydes and ketones using hydrogen gas in the presence of a metal catalyst will lead to the formation of primary and secondary alcohols respectively.

Miscellaneous Reactions

Other common reactions include the formation of cyanohydrins through reaction with hydrogen cyanide (HCN) and the Baeyer-Villiger oxidation. This latter reaction involves the oxidation of a ketone to an ester or an aldehyde to a carboxylic acid.
Importantly, don’t forget the Cannizzaro reaction that is specific to aldehydes without alpha-hydrogens. In this reaction, one aldehyde molecule is reduced to the corresponding alcohol, while a second is oxidised to a carboxylic acid.

Mastering the reactions of carbonyl compounds is crucial to understanding organic chemistry as many synthesis reactions involve carbonyl compounds. Knowledge of the reagents, conditions and mechanisms will enable you to predict product formation in complex carbonyl reactions.