Synthesis of benzilic acid from benzoin

BACKGROUND

Principle:

In the first step alcohol group of benzoin is oxidized to ketone group forming benzil in presence of concentrated nitric acid. Nitration of aromatic ring is not occurring as sulphuric acid is totally absent in the whole process. In the second step on treatment with hydroxide ion molecular rearrangement occurs to 1,2-diketone forming potassium salt of α-hydroxy acid which on acidification yield α-hydroxy acid (benzilic acid).¹

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Aim: To prepare benzilic acid from benzoin.

Reaction:

Step 1:

Step 2:

Mechanism:

Step 1:

Step 2:

Use:

It is used in the manufacture of glycollate pharmaceuticals including clidinium, dilantin, and flutropium.

REQUIREMENTS

Chemicals:     

Benzoin

conc. HNO₃

Benzil,

KOH

Ethanol,

conc, HCl

Benzene

Apparatus:     

Conical flask

Round bottom flask

Condenser

Beaker

Pipette

Glass rod

Buchner funnel

PROCEDURE

Step 1: Preparation of Benzil from Benzoin.

Heat a mixture of benzoin (20 g) and conc. nitric acid (100 ml) in a test tube on boiling water bath for about 1.5 hour until the evolution of oxides of nitrogen ceases. Then pour the contents into ice cold water (300 – 400 ml) with continuous shaking. Filter the product, wash with cold water, and recrystallize from ethanol, m.p. 94-96 ^oC.

Step 2: Preparation of Benzilic acid from Benzil.

Dissolve KOH (35 g) in water (70 ml) by heating on hot plate and the cool to room temperature. Add 90 ml rectified spirit and 35 g benzil and reflux the content on a water bath till blue colour disappears (10-15 min). Pour the contents of the flask into a porcelain dish and cool in ice, collect the colorless needles of potassium benzilate. Dissolve the solid in 350 ml hot water and acidify with HCl slowly. Collect the solid benzillic acid and recrystallize from hot benzene (6 ml per g), m.p. 150 ^oC.

Calculation:

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

C₁₄H₁₂O₂ =Molecular formula of benzion

C₁₄H₁₂O₃ =Molecular formula of benzilic acid‎

Molecular weight of benzion = 212 g/mole

Molecular weight of benzilic acid = 228 g/mole

Theoretical yield:

212 g benzion forms 228 g benzilic acid

Therefore, 20 g benzion will form …….? (X) g benzilic acid

X =(228 ×20)/212 = 21.5 g

Theoretical yield = 21.5 g

Practical yield = ————- g

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

CONCLUSION

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

REFERENCES

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.- 1045-1046
2. Practical in organic chemistry, by Hitesh G. Raval, Sunil L. Baldania and Dimal A. Shah, Nirav Prakashan, Page no: 273, 300.

Also read:

• Synthesis of acetanilide from aniline
• Synthesis of aspirin from salicylic acid using acetic anhydride
• Synthesis of aspirin from salicylic acid using acetyl chloride and pyridine
• Synthesis of 7-hydroxy-4-methylcoumarin
• Determination of saponification value of the given oil/fat
• Determination of saponification value of fat (coconut oil)
• Protein denaturation by egg albumin method (in vitro)

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