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Saturday, 25 February 2017

Aldehydes, Ketones and Carboxylic acid - III


Reactions of aldehydes and ketones:
  • Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons (or inductive effect).
  • Electronic Effect: Relative reactivities of aldehydes and ketones in nucleophilic addition reactions is due the positive charge on carbonyl carbon. Greater positive charge means greater reactivity. Electron releasing power of two alkyl groups in ketones is more than one in aldehyde. Therefore positive charge is reduced in ketones as compared to aldehydes. Thus ketones are less reactive than aldehydes.
  • Stearic Effect: As the number and size of alkyl group increase, the hindrance to the attack of nucleophile also increases and reactivity decreases. In aldehydes there is one alkyl group and one hydrogen atom, whereas in ketones there are two alkyl groups (same or different).
 Nucleophilic addition reactions of aldehydes and ketones:

(a) Addition of hydrogen cyanide (HCN) to form cyanohydrins

(b) Addition of sodium hydrogensulphite (NaHSO3) to form bisulphate addition compound

(c) Addition of Grignard reagent (RMgX) to form alcohol

(d) Addition of alcohol:
(i) Aldehydes on addition of monohydric alcohol in presence of dry HCl forms hemiacetal and acetal.

(ii) Ketones do not react with monohydric alcohols. Ketones react with ethylene glycol under similar conditions to form cyclic products known as ethylene glycol ketals.

(e) Addition of ammonia and its derivatives:

Reduction of aldehydes and ketones:

(a) Reduction to alcohols:
Aldehydes and ketones on catalytic hydrogenation in presence of Ni, Pt or Pd by using lithium aluminium hydride (LiAlH4) or sodium borohydride (NaBH4 ) forms primary and secondary alcohols respectively.

(b) Reduction to hydrocarbons:

(i) Clemmensen reduction: Carbonyl group of aldehydes and ketones is reduced to CH2 group on treatment with zinc amalgam and concentrated hydrochloric acid.

(ii) Wolff-Kishner reduction: Carbonyl group of aldehydes and ketones is reduced to CH2 group on treatment with hydrazine followed by heating with sodium or potassium hydroxide in high boiling solvent such as ethylene glycol.

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