Aldehyde: Encyclopedia II - Aldehyde - Chemistry

Aldehyde - Chemistry

Aldehyde - Preparation

There are three notable methods for preparing aldehydes:

• Reacting a primary alcohol with an oxidizing agent,
• Reacting an alkene (if there is a vinylic hydrogen) with ozone will cause the C=C bond to break yielding an aldehyde upon workup, in a process called ozonolysis.
• Reacting an ester with DIBAL-H can cause reduction, yielding an aldehyde.
• Reduction of an acid chloride using the Rosenmund reduction.

The primary means of synthesis is the oxidation of a primary alcohol. In the laboratory this may be achieved by heating the alcohol with a chromium(VI) reagent an acidified solution of potassium dichromate, which is reduced to green Cr³⁺ during the reaction. Excess dichromate will further oxidise the aldehyde to form a carboxylic acid, so either the aldehyde is distilled out as it forms (if volatile), or milder methods such as PCC oxidation, IBX acid, Dess-Martin periodinane or Swern oxidation are used. The equation is shown below with propan-1-ol being oxidised to form propanal.

A similar process reacting pentan-1-ol to form pentanal is illustrated below.

Aldehyde - Common reactions

• aldehydes react with alcohols and an acid or base to a hemiacetal, the hemiacetal and alcohol and more acid catalyst further react to an acetal and water

• Treating aldehydes with oxidizing agents such as potassium permanganate, nitric acid, chromium(VI) oxide or acidified potassium dichromate), will yield a carboxylic acid.
• Treating aldehydes with Tollens' reagent (which is prepared by adding a drop of sodium hydroxide solution into silver nitrate solution to give a precipitate of silver(I) oxide, and then adding just enough dilute ammonia solution to redissolve the precipitate in aqueous ammonia to produce [Ag(NH₃)₂]⁺ complex) will convert aldehydes to carboxylic acids without attacking carbon-carbon double-bonds. This reaction is also known as the silver mirror test. See also oxidation of aldehyde
• Aldehydes can react with water (under acidic or basic conditions) to form hydrates, R-C(H)(OH)(OH), although these are only stable when strong electron withdrawing groups are present, as in chloral hydrate. The mechanism is identical to hemiacetal formation.
• Aldehydes can react with HCN to form cyanohydrins, R-C(H)(OH)(CN).
• Treating an aldehyde with a Grignard reagent can yield an alcohol with a substituted group from the Grignard reagent.
• Treating aldehydes with NH₃ derivatives (like NH₂OH, H₂NNH₂, 2,4-dinitrophenylhydrazine) will result in the formation of an oxime or hydrazone. This is classed as an addition-elimination reaction or addition-condensation reaction. If the simple hydrazone (RCH=NHNH₂) of an aldehyde is heated with a base such as KOH, the terminal carbon is fully reduced via the Wolff-Kishner reaction to a methyl group. The Wolff-Kishner reaction may be performed as a one-pot reaction, giving the overall conversion RCH=O → RCH₃.
• Reaction of aldehydes with reducing agents such as magnesium gives diols in a Pinacol coupling reaction
• The Wittig reaction takes aldehydes to alkenes and the Corey-Fuchs reaction takes aldehydes to alkynes both with a triphenylphosphine reagent. The Corey-Chaykovsky reagent is a sulfonium ylide which convert aldehydes to epoxides.

Aldehyde - Nucleophilic addition

1. aldehyde + nucleophile —→ tetrahedral carbonyl addition compound
   • aldehyde + ammonia or primary amine —→ tetrahedral carbonyl addition compound
     • tetrahedral carbonyl addition compound + acid (catalyst) —→ imine + water

Aldehyde - Keto-enol tautomerism

Equilibration of keto and enol tautomers is catalyzed by acid.

Aldehyde - Oxidation & Reduction

• Aldehydes are oxidized to carboxylic acids.
• Aldehydes are reduced to primary alcohols.

Adapted from the Wikipedia article "Chemistry", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki