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Organic chemistry

Paracetamol synthesis (acetylation of 4-aminophenol)

A classic first-year acetylation: weigh out reagents by equivalents, find the limiting reagent, crystallise the product and compare actual with theoretical yield.

Learning objective

Convert 4-aminophenol into paracetamol with acetic anhydride, and practise stoichiometry, limiting-reagent reasoning, crystallisation and percent yield.

Safety and hazards

  • Acetic anhydride is corrosive and lachrymatory (H314, H332). Handle in a fume hood with gloves and eye protection.
  • 4-Aminophenol is harmful if swallowed and a skin sensitiser (H302, H317, H341). Avoid dust formation.
  • Hot water baths and hot glassware cause burns; use tongs or heat-resistant gloves.
  • Quench excess acetic anhydride with water in the fume hood before disposal.

Reagents and materials

  • 4-Aminophenol — 3.00 gM = 109.13 g/mol, limiting reagent
  • Acetic anhydride — 3.5 mLM = 102.09 g/mol, ρ = 1.08 g/mL
  • Water (deionised) — 9 mL
  • Ice — ≈100 gfor the ice bath and cold wash water
  • Büchner funnel, filter paper, 100 mL conical flask, water bath, watch glass

Procedure

1
Add reagent

Weigh 3.00 g of 4-aminophenol into a 100 mL conical flask and add 9 mL of deionised water. Swirl to give a suspension.

  • 3.00 g 4-Aminophenol (1.00 equiv)Reagent
  • 9 mL Water (deionised)Solvent

Stoichiometry calculator

27.5 mmol 4-aminophenol (1.00 eq, limiting)

Acetic anhydride at 1.35 eq = 37.1 mmol = 3.79 g = 3.5 mL

n = m / M

Mass of 4-aminophenol
3.00 g
Molecular weight
109.13 g/mol
Acetic anhydride equivalents
1.35 eq

Equipment: 100 mL conical flask, analytical balance

2
Heat

Warm the suspension to 80 °C in a water bath until the solid has mostly dissolved.

Temperature:
80 °C
Duration:
10 min

Equipment: Water bath

3
Reaction

Add 3.5 mL of acetic anhydride (1.35 eq) in one portion with swirling, and keep the mixture at 80 °C for 20 minutes.

  • 3.5 mL Acetic anhydride (1.35 equiv)Reagent
Temperature:
80 °C
Duration:
20 min

Acetic anhydride is used in slight excess so 4-aminophenol stays the limiting reagent.

4
Cool

Cool the flask to room temperature, then stand it in an ice bath for 15 minutes to crystallise the paracetamol.

Temperature:
0–4 °C
Duration:
15 min
Cooling:
ice bath
5
Filter

Collect the crystals by vacuum filtration on a Büchner funnel and wash with 2 × 5 mL of ice-cold water.

  • 2 × 5 mL Ice-cold waterWashing solvent

Equipment: Büchner funnel, filter flask, water aspirator

6
Mass

Dry the crystals on a watch glass in a 60 °C oven to constant mass, then weigh the product.

Temperature:
60 °C
Duration:
45 min or to constant mass
7
Custom measurement

Calculate the theoretical yield from the limiting reagent and the percent yield from the dry product mass.

Reaction yield calculator

Theoretical yield 4.16 g — 85.0 % for a 3.54 g product

1 mol 4-aminophenol gives 1 mol paracetamol (M = 151.16 g/mol)

yield % = (actual mass / theoretical mass) × 100

Limiting reagent
27.5 mmol 4-aminophenol
Product molecular weight
151.16 g/mol
Theoretical mass
4.16 g
Example actual mass
3.54 g

Expected result

  • White to off-white crystalline solid.
  • Theoretical yield 4.16 g of paracetamol from 3.00 g of 4-aminophenol.
  • Typical teaching-lab yield 70–90 % (2.9–3.7 g).
  • Melting point 168–172 °C for well-dried, pure material.

Calculators used in this protocol

Stoichiometry

n = m / M, equivalents = n / n(limiting)

Equivalents, limiting reagent and required amounts across a reaction table.

Molecular weight

M = Σ (nᵢ × Aᵢ)

Formula mass from a chemical formula, with an element breakdown.

Reaction yield

% yield = actual / theoretical × 100

Theoretical yield and percentage yield from the limiting reagent.

Solution preparation

n = c × V, m = n × M

Mass of solid needed for a target concentration and final volume.

Notes

Percent yield below 70 % usually means product was lost in the filtrate or the wash water was not cold enough.

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