Tuesday, February 5, 2008

Aldehydes - Ketones aldol condensation

Aldehydes and ketones containing α-hydrogen (H-atoms attached to the C-atom adjacent to the carbonyl group)undergo condensation in the presence of dilute alkali.

In the resulting compound both aldehyde group and alcohol group are present.

acetaldehyde and acetone undergo aldol condensation.
Formaldehyde, banzaldehyde do not undergo aldol condensation.

Reagents : commonly a base such as NaOH or KOH is added to the aldehyde.

The reaction involves an enolate reacting with another molecule of the aldehyde.

Remember enolates are good nucleophiles and carbonyl C are electrophiles.

Since the pKa of an aldehyde is close to that of NaOH, both enolate and aldehyde are present.

The products of these reactions are β-hydroxyaldehydes or aldehyde-alcohols = aldols.

The simplest aldol reaction is the condensation of ethanal.

Step 1:
First, an acid-base reaction. Hydroxide functions as a base and removes the acidic α-hydrogen giving the reactive enolate.
Step 2:
The nucleophilic enolate attacks the aldehyde at the electrophilic carbonyl C in a nucleophilic addition type process giving an intermediate alkoxide.
Step 3:
An acid-base reaction. The alkoxide deprotonates a water molecule creating hydroxide and the β-hydroxyaldehydes or aldol product.

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