Esters

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Recall the reaction (reagents and conditions) by which esters can be produced: the condensation reaction between an alcohol and a carboxylic acid with concentrated H₂SO₄ as catalyst

  2. Describe the hydrolysis of esters by dilute acid and by dilute alkali and heat

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What Are Esters?

Esters are organic compounds that contain a specific arrangement of atoms called the -COOR functional group. A functional group is simply a group of atoms that gives a molecule its characteristic chemical properties.

Esters are known for their pleasant, sweet, and fruity smells. Because of this, they are commonly used in:

  • Artificial fruit flavourings
  • Perfumes and fragrances
  • Organic solvents (liquids that can dissolve other substances)

Formation of Esters (Esterification)

The Condensation Reaction

Esters are made through a type of reaction called esterification. This is a condensation reaction, which means two molecules join together and, in the process, a small molecule (in this case, water) is removed or "eliminated."

The reaction involves:

  • An alcohol (a compound containing an -OH group attached to a carbon chain)
  • A carboxylic acid (a compound containing a -COOH group)

Reagent and catalyst:

  • Concentrated sulfuric acid (conc. H₂SO₄) acts as the catalyst. A catalyst is a substance that speeds up a reaction without being used up itself.

Conditions:

  • The mixture is heated to make the reaction happen at a reasonable speed.

General Equation

Alcohol + Carboxylic Acid ⇌ Ester + Water

(The ⇌ symbol means the reaction is reversible—it can go both forwards and backwards.)

With concentrated H₂SO₄ as catalyst and heat:

Carboxylic Acid + Alcohol ⇌ Ester + H₂O

Understanding the Mechanism

When an alcohol and a carboxylic acid react:

  1. The -OH group from the carboxylic acid and the -H atom from the alcohol's -OH group are removed.
  2. These combine to form water (H₂O).
  3. The remaining parts of the alcohol and carboxylic acid molecules join together to form the ester.

Simple tip to work out the ester product:

  • Place the alcohol and carboxylic acid structures next to each other so their -OH groups are close.
  • Remove the -H from the alcohol and the -OH from the carboxylic acid (this makes water).
  • Join what's left together—you now have your ester!

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