Chemistry:Aldol

Short description: Organic compound of the form R–CH(OH)–CHR–C(=O)–R

Generalized structure of the aldol moiety: When R³ is -H, it is a β-hydroxy aldehyde, otherwise it is a β-hydroxy ketone.

In organic chemistry, an aldol describes a structural motif consisting of a 3-hydroxy ketone, R–CH(OH)–CHR'–C(=O)–R", or 3-hydroxyaldehyde, R–CH(OH)–CHR'–CH=O. Both are composed of a hydroxy group (–OH) and either a ketone (>C=O) or an aldehyde (–CH=O, which is merely a ketone with a hydrogen substituent). An aldol consisting of a 3-hydroxy ketone is called a β-hydroxy ketone, and an aldol consisting of a 3-hydroxy aldehyde is called a β-hydroxy aldehyde. The term "aldol" may refer to 3-hydroxybutanal.

Synthesis and reactions

Isomers of generalized aldol.

Aldols are usually the product of aldol addition, i.e. the condensation of two aldehydes. Stereoselective syntheses of aldols is an active area of asymmetric synthesis.

The chemistry of aldols is dominated by one reaction, dehydration:

RC(O)CH₂CH(OH)R' → RC(O)CH=CHR' + H₂O

Applications

When two molecules of aldehydes react to form an aldol (β-hydroxy aldehyde), the aldol usually produces secondary compounds since it is unstable. Secondary compounds can be diols, unsaturated aldehydes, or alcohols. The aldol 3-hydroxybutanal is a precursor to quinaldine, a precursor to the dye quinoline Yellow SS.

Aldols are also used as intermediates in the synthesis of natural products and drugs. The synthesis of Oseltamivir, an antiviral medicine used to treat the flu, involves the aldol reaction.

The structural motif of aldols is often found in polyketide natural products, which can be used to manufacture antibiotics.

Hydroxypivaldehyde is a rare example of a relatively robust aldol.

See also

• Mukaiyama aldol addition