To Synthesise Phenytoin from Benzil and Urea - Practical Medicinal Chemistry

AIM: TO SYNTHESISE PHENYTOIN FROM BENZIL AND UREA.

Reaction Mechanism

REQUIREMENTS: 

Chemicals: Benzil, Urea, Sodium hydroxide, Ethanol, Concentrated hydrochloric acid.

Apparatus: Round bottom flask (100ml), Reflux condenser, Crystallizing dish (500ml), Heating mantle, Stirrer, Beaker (400ml), Filtering flask with Büchner funnel, Graduated cylinders (100ml and 50ml), Petri dish etc.

BACKGROUND:

Principle: Base catalysed reaction is used between benzyl and urea, for the synthesis of phenytoin. The reaction proceeds via intramolecular cyclization to form an intermediate heterocyclic pinacol, which on acidification yield hydantoin (phenytoin) as a result of 1,2-diphenyl shift in pinacol rearrangement reaction.

PROCEDURE:

• Place 5.3g (0.025 mol) of benzil, 3.0g (0.05 mol) of urea, 15 ml of aqueous sodium hydroxide solution (30%) and 75 ml of ethanol in a round bottomed flask of 100 ml capacity. 
• Set up a reflux condenser with the flask and boil using an electric heating mantle for at least 2hrs.
• Cool to room temperature, pour the reaction mixture into 125ml of water and mix carefully.
• Allow the reaction mixture to stand for 15min. and then filter the product under suction to remove an insoluble by-product.
• Render the filtrate strongly acidic with concentrated hydrochloric acid, cool in ice-water and immediately filter off the precipitated product under suction.
• Recrystallise at least once from industrial spirit to obtain about 2.8g (44%) of pure 5,5-diphenylhydantoin, M.P. 297-298°C.

CALCULATIONS:

Here limiting reagent is benzil; hence yield should be calculated from its amount taken.

Molecular formula of benzil - C₁₄H₁₀O₂

Molecular formula of phenytoin = C₁₅H₁₂N₂O₂

Molecular weight of benzil = 210 g/mole

Molecular weight of phenytoin = 252 g/mole

Theoretical yield:

210g benzil forms 252g phenytoin.

Therefore, 5.3g benzil will form __X__ g phenytoin.

X = (252 x 5.3) / 210 - 6.36 g

Theoretical yield = 6.36g 

Practical yield = ____ g

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

RESULT:

Phenytoin was synthesized and the percentage yield was found to be ___.

REFERENCE:

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. 1153.
2. Practical in organic chemistry, by Hitesh G. Raval, Sunil L. Baldania and Dimal A. Shah, Nirav Prakashan, Page No. 313.