Synthesis of aspirin from salicylic acid using acetyl chloride and pyridine

BACKGROUND

Principle: Synthesis of aspirin from salicylic acid occurs by acetylation process in acidic medium. Salicylic acid interacts with acetic anhydride in presence of few drops of concentrated sulphuric acid to produce aspirin and a molecule of acetic acid.

The purpose of adding sulphuric acid (catalyst) is to aid and augent the process of detaching the acetate ion (CH₃COO^–) from acetic anhydride which ultimately gets associated with H⁺ ion from phenolic hydroxy group in salicylic acid to be eliminated as a mole of acetic acid.¹

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Aim: To prepare aspirin from salicylic acid.

Reaction:

Mechanism:

Use: Used for the relief of minor pains/aches and also mild to moderate pain. It is recommended for arthritis and related arthritic conditions, myocardial infarction prophylaxis. It reduces the risk of transient ischemic attacks in men.

REQUIREMENTS

Chemicals:     

Salicylic acid – 10 g

    Pyridine – 7 ml

    Acetyl chloride – 7.5 ml

Reaction:       

CH₃COCl + HOC₆H₄COOH = CH₃COOC₆H₄COOH + HCl

Acetyl chloride Salicylic Acid Aspirin

PROCEDURE

Dissolve 10 g of salicylic acid (o-hydroxybenzoic acid) in dry pyridine (7 ml) in a conical flask of 100 ml. Then without delay (since this solution if allowed to stand tends to become a semi-solid mass) run in 7.5 ml (8.3 g) of acetyl chloride, adding the chloride (1 ml) at a time, and shake the mixture constantly during the addition.

The heat of the reaction causes the temperature of the mixture to rise rapidly: therefore maintain the latter between 50° and 60° throughout the addition, cool the flask in cold water occasionally if necessary. Finally heat the mixture on a steaming water-bath for 5 min and after that cool in cold water, pour it in a thin stream into about 300 ml of cold water, stirring the mixture vigorously meanwhile.

The crude acetylsalicylic acid either solidifies at once, or separates as oil which rapidly crystallises as the stirring proceeds. Filter the solid product by using the pump, wash thoroughly with water and drain. Recrystallise the product from a mixture of equal volumes of water and acetic acid.

Calculation

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

C₇H₆O₃ =Molecular formula of salicylic acid

C₉H₈O₄ =Molecular formula of acetylsalicylic acid (aspirin)

Molecular weight of salicylic acid = 138 g/mole

Molecular weight of acetyl salicylic acid (aspirin) = 180 g/mole

Theoretical yield:

138 g of salicylic acid gives 180 g of aspirin

Therefore, 6 g of salicylic acid will give ? (X) g of aspirin

X =( 180 × 6)/138 = 7.82 g

Theoretical yield = 7.82 g

Practical yield = ————- g

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

CONCLUSION

Aspirin was synthesized and the yield was found to be ——— %. (The product is obtained as colourless crystals, yield 11 g and m.p. 136-137 °C.)

REFERENCES

1. Vogel’s text book of Practical Organic Chemistry, Page No. 1290 no 522.
2. Practical Organic Chemistry by Frederick George Mann and Bernard Charles Saunders Published by Longan Inc., Fourth Edition; Page No. 110.
   

Also read:

• Synthesis of 7-hydroxy-4-methylcoumarin
• Synthesis of phenytoin from benzil and urea
• Synthesis of sulphanilamide from acetanilide
• Synthesis of p-nitroaniline from acetanilide
• 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)
• Analgesic activity study of drugs by hot plate (Eddy’s hot plate) method

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