Nomenclature of aldehydes and ketones
They are named by replacing the -ane ending with -al for aldehydes and -one for ketones.
Structure and physical properties of the carbonyl group
The carbonyl group consists of an oxygen with lone pairs, which is protonated in acidic media, and a positively polarized carbon, which is attacked by nucleophiles. Aldehydes are more reactive than ketones. Substituents donate charge to the carbonyl group, decreasing its reactivity.
Synthesis of aldehydes and ketones
The oxidation of alcohols, the oxidative cleavage of alkenes with ozone, and the hydration of alkynes are the most important methods for obtaining carbonyls.
Reaction with alcohols and water
Alcohols react with aldehydes and ketones to form hemiacetals and acetals. Cyclic acetals are used as protecting groups.
Reaction with ammonia and derivatives
Primary amines form imines by reaction with aldehydes and ketones. Secondary ones give rise to enamines. With hydroxylamine, oximes are formed, and with hydrazine, hydrazones.
2,4-Dinitrophenylhydrazine test
Aldehydes and ketones react with phenylhydrazines to form phenylhydrazones, which precipitate as a yellow color. This reaction is used as an analytical test to identify aldehydes and ketones.
Wolff-Kishner reduction
Aldehydes and ketones are reduced to alkanes by heating with hydrazine in a basic medium. This reaction is known as the Wolff-Kishner reduction.
Clemmensen reduction
Reduces carbonyls to alkanes. Clemmensen uses a Zn(Hg) amalgam in an acidic medium as the reagent.
Formation of enamines
Enamines are obtained by the reaction of aldehydes or ketones with secondary amines. They have an important application in the alkylation of carbonyls.
Formation of cyanohydrins
The reaction of hydrocyanic acid with aldehydes or ketones produces cyanohydrins. These compounds can be hydrolyzed to carboxylic acids or reduced to amines.
Wittig reaction
Aldehydes and ketones condense with phosphorus ylides to form alkenes. This reaction is known as the Wittig reaction.
Sulfur ylides
Sulfur ylides are obtained by the reaction of sulfides and haloalkanes followed by deprotonation with a strong base (butyllithium). The reaction of sulfur ylides with aldehydes or ketones generates epoxides.
Baeyer-Villiger oxidation
Peroxyacids transform ketones into esters and aldehydes into carboxylic acids.
Fehling's and Tollens' tests
These are specific analytical tests for aldehydes. Fehling's test uses a basic solution of Cu(II) as the reagent, which precipitates as cuprous oxide, red in color, in the presence of an aldehyde.
Formation of enols and enolates
The hydrogens on the carbon adjacent to the carbonyl are acidic. The loss of these hydrogens in a basic medium generates enolates. When working in an acidic medium, enols are formed.