Aldehydes, Ketones And Carboxylic Acids

- Learn important concepts of the chapter.

Distinguish between aldehydic and ketonic group

The difference between an aldehyde and a ketone is that in an aldehyde, the carbonyl group is attached to one hydrogen atom and one alkyl group. In a ketone, on the other hand, the carbonyl group is attached to two alkyl groups.

where and are alkyl groups.

Nature and structure of carbonyl group

Carbonyl group is polar in nature. That's because of the development of partial positive charge on carbon atom and partial negative charge on oxygen atom as a result of electronegativity difference between them. 

Important nucleophilic addition reactions of aldehydes and ketones

  1. Addition of hydrogen cyanide (HCN)
  2. Addition of sodium hydrogensulphite
  3. Addition of Grignard reagents
  4. Addition of alcohols
  5. Addition of ammonia and its derivatives

Reasons for greater reactivity of aldehydes than ketones

Aldehydes are more reactive than ketones on account of the following reasons:
  1. Steric reasons: The presence of two relatively large substituents in ketones hinders the approach of nucleophile to carbonyl carbon as compared to aldehydes which have only one such substituent.
  2. Electronic reasons: Ketones are less reactive than aldehydes because the two alkyl groups reduce the electrophilicity of the carbonyl carbon of ketones more effectively than in aldehydes.

Nucleophilic addition in aldehydes and ketones

Aldehydes and ketones undergo nucleophilic addition reactions. Mechanism of nucleophilic addition is:
  1. A nucleophile attacks the electrophilic carbon atom of the polar carbonyl group from a direction approximately perpendicular to the plane of sp hybridised orbitals of carbonyl carbon.
  2. The hybridisation of carbon changes from sp to sp and a tetrahedral alkoxide intermediate is produced.
  3. This intermediate captures a proton from the reaction medium to give the electrically neutral product.
  4. The net result is addition of Nu and H across the carbon oxygen double bond.

Tests to differentiate between aldehydes and ketones

1. Tollen's Test: Aldehydes give positive Tollen's test (silver mirror) while ketones do not give any reaction.
2. Fehling's test: Aliphatic aldehydes on treatment with Fehling's solution give a reddish brown precipitate (positive result) while aromatic aldehydes and ketones do not.
3. Benedict's Test: Aliphatic aldehydes on treatment with Benedict's solution give a colored precipitate while aromatic aldehydes and ketones (except -hydroxy methyl ketones) do not react with Benedict's solution.
4. Oxidation with Potassium Dichromate: Aldehydes are oxidized by acidified potassium dichromate solution, turning the orange solution to green, whereas ketones show no effect.

Causes of acidic nature of carboxylic acids

1. The reason for the enhanced acidity of carboxylic group is the stability of the carboxylate ion formed on loss of a proton.
2. The conjugate base of carboxylic acid, a carboxylate ion, is stabilised by two equivalent resonance structures in which the negative charge is at the more electronegative oxygen atom.