Synthesis of anthranilic acid from phthalic anhydride

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To prepare anthranilic acid from phthalic anhydride.


Nuclophilic attack of amino group of urea towards one of the carbonyl group of phthalic anhydride forms intermediate substituted pthalamide. It gets cyclized on heating to form phthalimide. Amides get a molecular rearrangement (Hoffmann rearrangement) with solutions of sodium or potassium hypobromide to give primary amines.

In this rearrangement carbonyl carbon atom of the amide is detached and the R group of amide gets attached to the nitrogen atom of amine directly.1

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Synthesis of anthranilic acid from phthalic anhydride


Step 1:

Synthesis of anthranilic acid from phthalic anhydride

Step 2:

Synthesis of anthranilic acid from phthalic anhydride

Anthranilic acid is used in the manufacture of dyes and drugs.



  • Phthalic anhydride
  • Urea
  • NaOH
  • Br2 solution
  • 10% KOH solution
  • Glacial acetic acid


  • Beaker
  • Pipette
  • Round bottom flask – 250 ml
  • Condenser
  • Glass rod
  • Buchner funnel


Step 1: Preparation of phthalimide.

Mix phthalic anhyde (10 g) and urea (2 g) in a 250 ml RBF. Heat the flask on an oil bath/sand bath/heating mantle at 130-135°C till the contents melts, froth up and become solid. Remove the flame and allow cooling. Add water (about 50 ml) to disintegrate the solid. Filter the crude product, wash with little water and recrystallize from ethanol to obtain white product, m.p. 230°C, yield about 92%.2,3

Note: Concentrated ammonia solution can be used in place of urea.

Step 2: Preparation of anthranilic acid.

Dissolve 7.5 g NaOH in 40 ml water and cool in ice bath to about 0°C temperature and then add 2.1 ml Br2 solution to it. To this solution add 6 g phthalamide and 20 ml 10% KOH solution, then heat the solution for 5-10 minutes till phthalamide dissolves. Filter the solution and neutralize the solution with glacial acetic acid. Filter the solid crystals of anthranilic acid, wash with water and recrystallize from hot water with the addition of a little decolourising carbon; collect the acid on a Buchner funnel and dry at 100 °C. The pure anthranilic acid, m.p. 145 °C.


Here limiting reagent is phthalic anhydride; hence yield should be calculated from its amount.

C8H4O3 = Molecular formula of phthalic anhydride

C7H7NO2 = Molecular formula of anthranilic acid

Molecular weight of phthalic anhydride = 148 g/mole

Molecular weight of anthranilic acid = 137 g/mole

Theoretical yield:

148 g phthalic anhydride forms 137 g anthranilic acid

Therefore, 10 g phthalic anhydride will form …….? (X) g anthranilic acid

X =(137 ×10)/148 = 9.26 g

Theoretical yield = 9.26 g

Practical yield = ————- g

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


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


  1. Experimental Pharmaceutical Chemistry by Anees Ahmad Siddiqui and Seemi Siddiqui, publication year-2005, page no 202,
  2. Practical in organic chemistry, by Hitesh G. Raval, Sunil L. Baldania and Dimal A. Shah, Nirav Prakashan, Page no: 270, 298.
  3. 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. 898, 1065.

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