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Benzimidazole: Reaction Type & Mechanism

Reaction Type: Cyclocondensation.

◼ Cyclocondensation reactions require an acidic medium (formic acid) to proceed

because the acidic conditions help to activate the carbonyl group for nucleophilic

attack and facilitate water removal during the condensation step.

1. By the nucleophilic attack, the amino group (nucleophile) displaces OH in formic

acid to form an amide losing a water molecule.

2. The second amino group attacks the amide carbonyl.

3. Condensation of the adduct occurs forming a double bond in the imidazole ring.

Main points in benzimidazole synthesis

Role of formic acid Role of 10% NaOH Role of washing with

water

◼ Activates the carbonyl gp ◼ Neutralizes the excess ◼ Removes the excess

for nucleophilic attack. unreacted formic acid. unreacted formic acid.

◼ Facilitates water removal ◼ Alkalinizes the reaction

during the condensation mixture to precipitate

step (dehydrating agent). benzimidazole.

◼ A source of 2-C of

imidazole ring.

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