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Thursday, July 23, 2020

To prepare benzimidazole from o- phenylenediamine


Aim: To prepare benzimidazole from o- phenylenediamine.

REFERENCES   1. S. Furniss Brian, J.Hannaford Antony etal Vogel’s Textbook of Practical Organic Chemistry published by Pearson books Fifth Edition.

to-prepare-benzamidazole-from o-phenylenediamine.


Chemicals: o-phenylenediamine, Formic acid (90%), NaOH (10%)

Apparatus:  Round bottomed flask (250 ml) Beaker, Buchner funnel, Measuring cylinder, filter paper

Principle: The two Carbon-nitrogen(C-N) bonds in benzimidazole when disconnected give o-phenylenediamine and formic acid. Therefore, the synthesis of benzimidazole is affected by simply heating the 0-phenylenediamine and formic acid together (condensation type of reaction).

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Chemical reaction



1.    Place 27 g. (0.25 mol) of O- phenylenediamine in a 250 ml of round bottom flask  

2.    And add 17.5 g (16 ml, 0.34 mol) of 90% formic acid. 

3.    Heat the mixture on a water bath at 100 °C for 2 h. 

4.    Cool and add 10% sodium hydroxide slowly, with constant rotation of the flask, until the mixture is just alkaline to litmus. 

5.    Filter off the synthesized crude benzimidazole by using the pump, wash with ice cold water, drain well and wash again with 25 ml of cold water.

6.    Recrystallization :- Dissolve the synthesized product in 400 boiling water, add 2 g of decolourising carbon and digest for 15 min. 

7.    Filter rapidly through a preheated Buchner funnel and flask at the pump. 

8.    The Cool the filtrate to about 10 °C, filter off the benzimidazole, wash with 25 ml of cold water and dry at 100 °C. 

9.    The yield of pure benzimidazole, m.p. 171-172 °C is 25 g (85%)


Molecular formula of O-phenylenediamine C6H8N2

Molecular formula of benzimidazole = C7H6N2

Molecular weight of o-phenylenediamine 108 g/mole 

Molecular weight of benzimidazole 118 g/mole

Theoretical yield

108 g o-phenylenediamine forms 118 g benzimidazole


27 g o-phenylenediamine will form. .? (X) g benzimidazole

X = ( 118 x 27)/108 = 29.5 g.

Theoretical yield -- 29.5 g.

Practical yield_______?

% yield =   (Practical yield/Theoretical yield)×100

Result: - Benzamidazole was synthesized and the percentage was found to be………..%.


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