September 30, 2023

Synthesis of benzimidazole from o-phenylenediamine

Synthesis of benzimidazole from o-phenylenediamine

BP406P Medicinal Chemistry I Practical

AIM: To prepare and submit benzimidazole from o-phenylenediamine.


1.  Medicinal chemistry – I, Mrs Sheethal V.Patil, Mrs.Swati G.Patil, Dr.Sunila T.Patil, Dr.Md.Rageeb, Md.Usman page no:233-235.


Round bottom flask, Beaker, Measuring cylinder,Waterbath, Buchner funnel, O-phenylene diamine, Formic acid(90%),Sodium hydroxide (10%)


The preparation of benzimidazole can be done by reaction between O-phenylene diamine with formic acid in presence of base i.e sodium hydroxide.It is a condensation type of reaction in which o-phenylene diamine condensed with formic acid to give benzimidazole with removal of two molecules of water.


Placed 27g of O-phenylenediamine in a round bottomed flask of 250ml and added 17.5g (16ml) of 90% formic acid. Heated the mixture on a water bath at 1000C for 2 hour. Cooled and added 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.

Recyrstallisation: Dissolved the synthesized product in 400ml of boiling water, added 2g of decolorizing carbon and digest for 15minutes. Filter rapidly through Buchner funnel and a flask at the pump. Cool the filtrate to about 100C, filter off the benzimidazole, wash with 25ml of cold water and dry at 1000C. The yield of pure benzimidazole is 25g (85%), m.p 171- 1720C.


Here limiting reagent is O-phenylene diamine; hence yield should be calculated from its amount taken.

Molecular formula of O-phenylene diamine = C6H8N2 Molecular formula of benzimidazole = C7H6N2 Molecular weight of O-phenylene diamine  = 108g/mole Molecular weight of benzilidazole = 118g/mole

108g of O-phenylene diamine  forms 118g benzimidazole

Therefore, 27g O-phenylene diamine will form………(X) g benzimidazole

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

Theoretical yield = 29.5g

Practical yield = ……..g

% yield = (practical yield)  X  100

(theoretical yield)


Benzimidazole was synthesized from O-phenylene diamine and submitted.

Second Year B Pharm Notes, Syllabus, Books, PDF Subjectwise/Topicwise

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