Synthesis of Aspirin (Acetylsalicylic Acid)

Making one of the world's most common drugs

Organic Synthesis Advanced (University) 90 min ~$15.00

Objective

Synthesize aspirin (acetylsalicylic acid) by acetylation of salicylic acid with acetic anhydride, and assess purity by melting point determination.

Background

Aspirin synthesis is a classic organic chemistry experiment. Salicylic acid reacts with acetic anhydride in the presence of a phosphoric acid catalyst to form acetylsalicylic acid and acetic acid. The product is purified by recrystallization and verified by melting point (135°C for pure aspirin).

Safety Warnings

  • Acetic anhydride is corrosive and has a pungent odor — work in fume hood
  • Phosphoric acid is corrosive
  • Do not consume the synthesized aspirin
  • Wear gloves and goggles

Required PPE

goggles gloves lab_coat

Materials

  • Salicylic acid (2 g)
  • Acetic anhydride (5 mL)
    Corrosive, pungent
  • Phosphoric acid (H₃PO₄) (5 drops)
    85%, catalyst
  • Distilled water (100 mL)
    Ice-cold for crystallization
  • Ethanol (10 mL)
    For recrystallization

Equipment

Erlenmeyer flask (125 mL) Hot water bath (beaker) Buchner funnel Filter flask Vacuum pump Melting point apparatus Thermometer Ice bath

Procedure

1

Weigh 2.0 g of salicylic acid into the Erlenmeyer flask. Record the exact mass.

3 min
2

In a fume hood, add 5 mL of acetic anhydride and 5 drops of phosphoric acid catalyst.

3 min Acetic anhydride is corrosive and pungent
3

Heat the flask in a water bath at 70-80°C for 15 minutes, swirling occasionally.

15 min
4

Remove from heat and carefully add 20 mL of cold water to decompose excess acetic anhydride.

3 min Exothermic — add water slowly
5

Cool the flask in an ice bath until crystals form. Scratch the flask wall with a glass rod to initiate crystallization.

10 min
6

Collect the crude product by vacuum filtration using the Buchner funnel. Wash with cold water.

10 min
7

Recrystallize: dissolve the crude product in 5 mL warm ethanol, add 15 mL warm water, then cool slowly in ice bath.

15 min
8

Collect purified crystals by vacuum filtration. Dry and determine the melting point.

10 min
9

Calculate the percent yield. Pure aspirin melts at 135°C.

5 min

Expected Results

White needle-like crystals with a melting point of 134-136°C (pure aspirin is 135°C). Typical yield is 70-90%. A depressed or broad melting range indicates impurities (likely unreacted salicylic acid).

Cleanup

Dispose of organic waste in designated containers. Do not consume the product. Rinse all glassware.

Details

Category
Organic Synthesis
Difficulty
Advanced (University)
Duration
90 min
Est. Cost
$15.00
Steps
9
Materials
5

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Frequently Asked Questions

What is the objective of Synthesis of Aspirin (Acetylsalicylic Acid)?
Synthesize aspirin (acetylsalicylic acid) by acetylation of salicylic acid with acetic anhydride, and assess purity by melting point determination.
How difficult is Synthesis of Aspirin (Acetylsalicylic Acid)?
This experiment is rated as Advanced (University). It takes approximately 90 minutes to complete.
What safety precautions are needed for Synthesis of Aspirin (Acetylsalicylic Acid)?
Key safety precautions include: Acetic anhydride is corrosive and has a pungent odor — work in fume hood; Phosphoric acid is corrosive; Do not consume the synthesized aspirin.
What materials are needed for Synthesis of Aspirin (Acetylsalicylic Acid)?
The main materials required are: Salicylic acid, Acetic anhydride, Phosphoric acid (H₃PO₄), Distilled water, Ethanol.
What results should I expect from Synthesis of Aspirin (Acetylsalicylic Acid)?
White needle-like crystals with a melting point of 134-136°C (pure aspirin is 135°C). Typical yield is 70-90%. A depressed or broad melting range indicates impurities (likely unreacted salicylic acid).