Describe the reaction of a carboxylic acid with an alcohol using an acid catalyst to form an ester

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Organic Chemistry – Esterification (Cambridge IGCSE/A‑Level Syllabus 11.7)

Learning Objective

Write a balanced equation for the acid‑catalysed reaction of a carboxylic acid with an alcohol, identify all functional groups, draw the displayed (structural) formulae of the reactants and product, explain the mechanism (including every proton transfer and the leaving group), and relate the reaction to practical laboratory conditions and yield optimisation (AO1, AO2, AO3).

Key Concepts

  • Reversibility – esterification is an equilibrium process. Removing water (or using excess alcohol) shifts the equilibrium toward the ester (Le Chatelier’s principle).
  • Functional groups involved
    • Carboxylic acid: –COOH
    • Alcohol: –OH
    • Ester: –COOR
  • Acid catalyst – concentrated sulphuric acid (H₂SO₄) supplies H⁺ and, by its dehydrating nature, helps remove water.

Overall (Balanced) Equation

\[ \text{R–COOH} + \text{R'}\text{–OH} \;\xrightarrow[\text{heat}]{\text{H}^{+}} \text{R–C(=O)–O–R'} + \text{H}_2\text{O} \]

The catalyst does not appear in the net equation.

Displayed (structural) formulae – example set

Carboxylic acid (type)Alcohol (type)Ester (type)
Acetic acid, CH₃–C(=O)–OH (carboxylic acid) Methanol, CH₃–OH (alcohol) Methyl acetate, CH₃–C(=O)–O–CH₃ (ester)
Butyric acid, CH₃CH₂CH₂–C(=O)–OH Ethanol, CH₃CH₂–OH Ethyl butyrate, CH₃CH₂CH₂–C(=O)–O–CH₂CH₃
Formic acid, H–C(=O)–OH Isopropanol, (CH₃)₂CH–OH Isopropyl formate, H–C(=O)–O–CH(CH₃)₂

Mechanism (Acid‑Catalysed Esterification)

All steps are shown with curved‑arrow notation in the diagram below. Key points are highlighted in bold.

Step‑wise arrow‑pushing mechanism: (1) carbonyl protonation, (2) nucleophilic attack by alcohol, (3) intramolecular proton transfer, (4) loss of water, (5) de‑protonation of the ester
Acid‑catalysed esterification mechanism (arrow‑pushing). The leaving group is water (H₂O).
  1. Protonation of the carbonyl oxygen – H⁺ adds to the carbonyl O, making the carbonyl carbon more electrophilic.
  2. Nucleophilic attack – the lone pair on the alcohol O attacks the activated carbonyl carbon, giving a tetrahedral intermediate.
  3. Intramolecular proton transfer – a proton shifts from the protonated alcohol oxygen to the original –OH of the acid, converting it into a good leaving group (water).
  4. Elimination of water – the –OH₂ group departs, forming a protonated ester.
  5. De‑protonation – the catalyst is regenerated by removal of a proton from the ester, giving the neutral ester product.

Experimental Plan (AO3 – Practical Skills)

Objective: Prepare methyl acetate by esterifying acetic acid with methanol.

Reagents (quantities for a small‑scale preparation)
  • Acetic acid: 5.0 g (0.083 mol)
  • Methanol: 6.0 g (0.150 mol) – 1.8 × molar excess
  • Concentrated H₂SO₄: 2 mL (≈2 % v/v of total reaction volume)
  • Distilled water (for work‑up)
Apparatus
  • 250 mL round‑bottom flask
  • Reflux condenser
  • Dean‑Stark trap (or alternatively a simple azeotropic distillation set‑up)
  • Heating mantle with temperature controller
  • Thermometer, magnetic stirrer, retort stand
Procedure (summary)
  1. Add acetic acid and methanol to the flask, then carefully add the sulphuric acid (acid to water rule).
  2. Attach the condenser and Dean‑Stark trap, start stirring, and heat to reflux (≈70 °C).
  3. Reflux for 45 min, allowing water to collect in the Dean‑Stark trap. Record the volume of water removed.
  4. Cool the mixture, transfer to a separatory funnel, wash the organic layer with cold water, then with a saturated NaHCO₃ solution to neutralise any residual acid.
  5. Dry the organic layer over anhydrous Na₂SO₄, filter, and distil the product (boiling point 57 °C) to obtain pure methyl acetate.
Evaluation of water‑removal methods
  • Dean‑Stark trap – continuously removes water as a separate phase; excellent for reactions where the ester is less dense than water (as in most cases). Requires a condenser and a graduated trap.
  • Azeotropic distillation (toluene or benzene) – forms an azeotrope with water that distils off. Useful when a Dean‑Stark apparatus is unavailable, but adds an extra solvent that must later be removed.
Yield calculation (AO2 example)

Theoretical yield of methyl acetate:

Molar mass CH₃COOH = 60.05 g mol⁻¹
Moles of acid = 5.0 g / 60.05 = 0.083 mol
1 : 1 stoichiometry → 0.083 mol methyl acetate
Molar mass CH₃COOCH₃ = 74.08 g mol⁻¹
Theoretical mass = 0.083 mol × 74.08 g mol⁻¹ = 6.15 g
If 5.0 g of product is isolated:
%Yield = (5.0 g / 6.15 g) × 100 = 81 %

Using excess methanol and removing 5 mL of water (≈0.28 mol) drives the equilibrium strongly toward product, explaining the high experimental yield.

Factors Affecting Yield (AO2 – Optimisation)

  • Acid strength and concentration – Strong acids increase the rate of carbonyl protonation but can also cause dehydration of the alcohol. 2–5 % v/v H₂SO₄ is optimal for IGCSE‑level experiments.
  • Excess alcohol – Using 1.2–1.5 × the stoichiometric amount of the alcohol shifts the equilibrium toward ester formation.
  • Water removal – Continuous removal (Dean‑Stark or azeotropic) reduces the concentration of the product of the reverse reaction, increasing overall conversion.
  • Temperature – Reflux (≈60–80 °C) provides enough energy for the reaction without causing side‑reactions. Temperatures >100 °C may lead to ether formation or ester decomposition.
  • Reaction time – 30–60 min is usually sufficient; longer times give diminishing returns and may increase impurity formation.

Real‑World Applications (Supplementary)

  • Solvents – Ethyl acetate is a common, low‑toxicity solvent used in nail‑polish removers and in the extraction of plant pigments.
  • Flavours & fragrances – Many esters (e.g., isoamyl acetate) give fruity aromas and are employed in the food and perfume industries.

Safety & Environmental Considerations

  • Concentrated H₂SO₄ – Highly corrosive; always add acid to water, wear gloves, goggles, lab coat, and work in a fume hood.
  • Reflux set‑up – Secure all joints, use a retort stand, and never leave a heated reflux unattended.
  • Ester vapour – Many esters are volatile and flammable; keep away from open flames and handle under a fume hood.
  • Waste disposal – Collect acidic aqueous waste separately, and dispose of organic waste according to institutional solvent‑waste protocols.

Quick Revision Checklist

  1. Write the balanced overall equation and draw the displayed structural formulae for the reactants and product.
  2. Identify and label the functional groups –COOH, –OH, and –COOR in the structures.
  3. Explain why the reaction is reversible and how removal of water or excess alcohol shifts the equilibrium.
  4. Outline the five mechanistic steps, showing the direction of each proton transfer and naming the leaving group (water).
  5. List the apparatus needed for a reflux set‑up and for water removal; state why each piece is required.
  6. Recall typical quantitative conditions (acid % v/v, alcohol excess, temperature, time) and be able to perform a simple %‑yield calculation.
  7. Give one real‑world use of an ester and mention at least one safety or environmental precaution.

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