Question

Explain aldol and cross aldol condensation.Answer the question in detail.

Answer 1

Aldol Condensation: It is a type of reaction that involves the molecular rearrangement in the compound like aldehyde or ketones.

When two molecules of aldehydes  or Ketones containing α -hydrogen atom (i.e. a hydrogen atom on the next carbon than the aldehyde or ketones or other functional group) is treated with dilute base like dil. NaOH, dil.Ba(OH)2 etc., they undergo self condensation or rearrangement to produce β-hydroxy aldehydes/ketones.

This type of organic reaction gives a product having an aldehyde/keto group and an alcohol group so named as aldol (ald= aldehyde ,and ol= alcohol). 

The Mechanism of reaction involves three steps: 

1. Formation of enolate ion or carbanion: In this step, the base, i.e, OH- ion attracts one of the α -hydrogens to form a carbanion or an enolate ion which is stabilized by resonance.


2. Nucleophilic attack: The carbanion, being a strong nucleophile, attacks on the carbonyl group of second molecule of aldehyde to form the anion (A)



3. The anion (A) further abstracts hydrogen from water to give an aldol as;


Cross aldol condensation is the variation of aldol condensation where, the two different aldehydes or two different ketones or one aldehyde or one ketone having an alpha hydrogen atom are treated with dilute alkali(NaOH or KOH) to form β-hydroxy aldehyde/ketones which further undergo elimination to form the corresponding alkenes. The recation can produce a mixture of four products. However, the mechanism is same.