Carbonyl compounds - Aldehydes and Ketones

Carbon-oxygen double bond is the active group of carbonyl compounds. There are two kinds of carbonyl compounds as aldehydes and ketones. Reactions of carbonyl group depends on nature and polarity of double bond.

Aldehyde general structure

general aldehyde group

R - alkyl or aryl groups

Ketone general structure

general ketone group

R1, R2 - alkyl or aryl groups

Aldehydes and ketones examples

aldehydes  ketones examples

Aldehyde and ketone reactions

Aldol condensation of aldehydes and ketoness

Aldol condensation is occurred only in carbonyl compounds which have a α hydrogen. These α H is acidic, therefore they are removed easily with bases and give carbanion.

What is alfa hydrogen?

All H atoms existing on α carbon are defined as α H. All carbon atoms which are adjacent to carbonyl carbon are defined as α carbon.

alfa carbon alfa hydrogen

Aldehyde or ketone which have αH react with any strong bases such as NaOH, KOH and Ba(OH)2 and give aldol as the product. This reaction doubles the number of carbon atoms of initial aldehyde or ketone. To dehydrate the aldol compound, it is heated alone or with I2.

Ethanal and Acetone aldol condensation

  • There are two carbon atoms in ethanal molecule. But in the product, there are four carbon atoms in the carbon chain.
  • Acetone (propanone) contains three carbon atoms. After reaction with NaOH(aq) or KOH(aq), there are six carbon atoms in the carbon chain.
aldol condensation acetone,propanal

Dilute NaOH or KOH

Reaction is stared by OH- ions provided from NaOH or KOH. At the end, OH- is regenerated.

Therefore OH- ions are behaved as a catalyst.

Acetaldehyde aldol condensation mechanism

acetaldehyde aldol condensation mechanism

Acetone aldol condensation mechanism

acetone aldol condensation mechanism

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