Synthesis of Benzimidazole from o-phenylenediamine

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The two Carbon-nitrogen bonds in benzimidazole when disconnected give o-phenylenediamine and formic acid. Therefore, synthesis of benziemidazole is affected by simply heating the o-phenylenediamine and formic acid together (condensation type of reaction).1

Aim: To prepare benzimidazole from o-phenylenediamine.


Synthesis of Benzimidazole from o-phenylenediamine


Synthesis of Benzimidazole from o-phenylenediamine


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  • o-phenylenediamine
  • Formic acid (90%)
  • NaOH (10%)


  • Round bottomed flask (250 ml)
  • Beaker
  • Buchner funnel
  • Measuring cylinder
  • Filter paper


Place 27 g (0.25 mol) of o-phenylenediamine in a round bottomed flask of 250 ml and add 17.5 g (16 ml, 0.34 mol) of 90% formic acid. Heat the mixture on a water bath at 100 °C for 2 h. Cool and add 10% sodium hydroxide solution slowly, with constant rotation of the flask, until the mixture is just alkaline to litmus.

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.


Dissolve the synthesized product in 400 ml of boiling water, add 2 g of decolourising carbon and digest for 15 min. Filter rapidly through a preheated Buchner funnel and a flask at the pump. Cool the filtrate to about 10 °C, filter off the benzimidazole, wash with 25 ml of cold water and dry at 100 °C. The yield of pure benzimidazole, m.p. 171-172 °C, is 25 g (85%).


Here limiting reagent is o-phenylenediamine; hence yield should be calculated from its amount taken.

C6H8N2 = Molecular formula of o-phenylenediamine

C7H6N2 = Molecular formula of benzimidazole

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

Therefore, 27 g o-phenylenediamine will form ………? (X) g benzimidazole

X =( 118 × 27)/108 = 29.5 g

Theoretical yield = 29.5 g

Practical yield =

————- g

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


Benzimidazole was synthesized and the percentage yield was found to be………..%.


  1. Vogel’s Textbook of Practical Organic Chemistry by Brian S. Furniss, Antony J. Hannaford, Peter W. G. Smith & Austin R. Tatchell; Fifth Edition; Page No.- 1162
  2. Practical in organic chemistry, by Hitesh G. Raval, Sunil L. Baldania and Dimal A. Shah, Nirav Prakashan, Page No.- 301.

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