Preparation of Aldehydes and Ketones

A few of the many laboratory methods of preparation of aldehydes and ketones are outlined below; (most of these are already familiar to us) Industrial preparation is generally patterned after these laboratory methods, but with use of cheaper reagents alcohols are oxidized catalytically with air, or by dehydrogenation over hot copper.

• Preparation of Aldehydes

?1. Oxidation of primary alcohols

2. Oxidation of methyl benzenes

Example.

3. Reduction of acid chlorides

4. Reimer-Tiemann Reaction-Phenolic Aldehydes

5. By heating a mixture of the calcium salts of formic acid and any one of its homologues.

(RCO₂)₂Ca + (HCO₂)₂ Ca → 2RCHO + CaCO₃

6. Stephen’s Method

H₂O (Hydrolysis of intermediate) RCH = NH to RCHO and NH₃

• Preparation of Ketones

1. ?Oxidation of Secondary alcohols

2. Friedel – Crafts acylation

3. Acylation of Alkenes

4. With Organometallics

a)   Reaction of acid chlorides with organocadmium compounds

5. By heating the calcium salt of any monocarboxylic acid other than formic acid

6. Acetoacetic ester synthesis of ketones

7. Ring Ketones from Dicarboxylic acids and their Derivaties

• Preparation of Aldehydes by Oxidation Methods

?Aldehydes are easily oxidised to carboxylic acids by the same reagent acidic dichromate, in their syntheses. Hence, removal of aldehyde as fast as it is formed is to be accomplished. So it is best to use P.C.C. in CH₃Cl a selective reagent that does not further oxidise the aldehydes to the acids. In the case of toluene , oxidation of the side chain can be interrupted by trapping the aldehyde in the form a non-oxidisable derivative, the gem-diacetate, which can be isolated and then hydrolysed.

• Synthesis of Ketones and Aldehydes Using 1,3-Dithianes 

?1,3-Dithiane is a weak proton acid (pK­_a  = 32) that can be deprotonated by strong bases such as n-butyllithium. The resulting carbanion is stabilized by the electron withdrawing effect of two highly polarizable suffer atoms.

Alkylation of the dithiane anion by a primary alkyl halide or tosylate gives a thioacetal (sulphur acetal) that can be hydrolysed using an acidic solution of mercuric chloride. The product is an aldehyde bearing the alkyl group that was added by the alkylating agent. This is a useful synthesis of aldehydes.

Alternatively, the thioacetal can be alkylated once more to give a thioketal. Hydrolysis of the thioketal gives a ketone

For example, 1-phenyl-2-pentanone may be synthesized as shown below:

In each of these sequence, dithiane is alkylated once or twice, then hydrolyzed to give a carbonyl group bearing the alkyl group(s) used in the alkylation. We often consider dithiane to be a synthetic equivalent of a carbonyl group that can be made nucleophilic and alkylated.

• Gatterman-Koch Aldehyde Synthesis

Benzaldehyde may be synthesized by bubbling a mixture of carbon-monoxide and hydrogen chloride through a solution of either nitrobenzene or solution containing benzene and a catalyst of AlCl₃ and small amount of cuprous chloride.C₆H₆  + CO  +  HClC₆H₅CHO  +  HCl

The mechanism of this reaction is uncertain, but it appears likely that the formyl cation is the active species. CO  + HCl  +  AlCl_{3 →} [CH=O H-CO⁺] + AlCl₄^–

It also appears likely that the cuprous chloride forms a complex with the CO, thereby increasing its local concentration.

The Gatterman - Koch aldehyde synthesis is not applicable to phenols and their ethers, or when the substituent is strongly deactivating eg. nitrobenzene.

Gattermann Aldehyde Synthesis:

When benzene is treated with a mixture of HCN and HCl in the presence of AlCl₃, and the complex so produced decomposed with water, benzaldehyde is produced. C₆H₆ + HCN + HClC₆H­₅CH = NH×HCl C₆H₅CHO + NH₃

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