Synthesis of hippuric acid (benzoyl glycine) from glycine

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BACKGROUND

Principle:
Benzoyl glycine is an amide and amide contains a molecule of an organic acid linked to a molecule of an amine. A water molecule must be removed to make the new C–N bond. The –OH part comes from the –COOH group, while the –H from the –NH₂ group. This reaction cannot be completed by simple mixing of the acid and the amine compounds because the result of the reaction would be an acid-base neutralization reaction which occur much faster, and the –OH is a very poor leaving group. Thus, the acid must be changed into a derivative first which is more reactive – most often an acid chloride, which has no acidic H and does have an excellent leaving group. Then the glycine (amine) can be mixed and it acts as a nucleophile instead of a base, now attacking the carbonyl carbon of –COCl group of benzoyl chloride.¹

Note: A sacrificial base stronger than glycine’s N must be present to “soak up” protons formed. It is again important, to limit the amount of hydroxide to only what is needed to neutralize the released H⁺.

Aim: To prepare hippuric acid (benzoyl glycine) from glycine.

Reaction:

Mechanism:

Use:
It is used as bio-marker, may have antibacterial effect.

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REQUIREMENTS

Chemicals:     Glycine – 1 g,

10% NaOH – 10 ml

Benzoyl chloride – 1.5 ml

Apparatus:     Conical flask

Beaker

Pipette

Glass rod

Buchner funnel

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PROCEDURE

Dissolve glycine in 10% NaOH solution in a flask and add benzoyl chloride to it. Plug the mouth of flask with cotton plug and shake vigorously until smell of benzoyl chloride can no longer be detected. Then add 1- 2 drops of cone. HCl to make the mixture acidic. Filter the product, wash with water and recrystallize the dried product from boiling water (about 20 ml) with the addition of a little decolourising charcoal. If necessary, filter through a hot-water funnel and allow to crystallise. Collect the benzoylglycine in a Buchner funnel and dry it in an oven, m.p. 192°C.²

Calculation:

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

Molecular formula of glycine = C₂H₅NO₂

Molecular formula of benzoyl glycine = ‎C₉H₉NO₃

Molecular weight of glycine = 75 g/mole

Molecular weight of benzoyl glycine = 179 g/mole

Theoretical yield:

75 g glycine forms 179 g benzoyl glycine

Therefore, 1 g glycine will form …….? (X) g benzoyl glycine

X =(179 ×1)/75 = 2.39 g

Theoretical yield = 2.39 g

Practical yield = ————- g

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

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CONCLUSION

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

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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. 1156.
   
2. Experimental Pharmaceutical Chemistry by Anees Ahmad Siddiqui and Seemi Siddiqui, publication year-2005, page no. 191.