33.2 Esters

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Recall how esters are produced by the reaction of alcohols with acyl chlorides, using the formation of ethyl ethanoate and phenyl benzoate as examples.

Main Content

https://cambridgeassistant.com

What Is an Ester?

An ester is a type of organic compound that contains a –COO– functional group (called the ester linkage). Esters are responsible for many of the sweet, fruity smells you encounter in everyday life — for example, the smell of nail polish remover, certain perfumes, and artificial fruit flavourings.

Esters can be made in more than one way. In this subtopic, you will learn how esters are made using a group of very reactive compounds called acyl chlorides.

What Is an Acyl Chloride?

An acyl chloride is an organic compound that contains a –COCl group. This group is made up of:

  • A carbonyl group (C=O) — a carbon atom double-bonded to an oxygen atom
  • A chlorine atom attached to the same carbon

The general structure looks like this:

R–COCl

where R represents a carbon chain or ring attached to the rest of the molecule.

Two acyl chlorides you need to know are:

  • Ethanoyl chloride — CH₃COCl (a short-chain acyl chloride)
  • Benzoyl chloride — C₆H₅COCl (an acyl chloride containing a benzene ring)

Acyl chlorides are very reactive — they react quickly and vigorously with many substances, including alcohols and phenols.

The Reaction of Alcohols with Acyl Chlorides

When an alcohol (or a phenol — a special type of compound where an –OH group is attached directly to a benzene ring) reacts with an acyl chloride, two products are formed:

  1. An ester
  2. Hydrogen chloride gas (HCl)

The reaction can be written in general terms as:

Acyl chloride + Alcohol → Ester + Hydrogen chloride

R–COCl + HO–R' → R–COO–R' + HCl

In this equation:

  • R and R' are different carbon groups
  • The –OH from the alcohol combines with the –COCl group of the acyl chloride
  • A chlorine atom and a hydrogen atom are released together as HCl gas

This type of reaction — where two molecules join together and a small molecule (HCl) is removed — is called an elimination reaction, specifically a nucleophilic acyl substitution.

Important: You do not need to remember the mechanism name for this syllabus level, but you should understand what the reactants are, what the products are, and why this is a useful reaction.

Key Features of This Reaction

  • No catalyst is needed — acyl chlorides are reactive enough on their own. This is different from making esters using carboxylic acids, which requires an acid catalyst and heat.
  • The reaction is irreversible — it goes to completion, meaning all the reactants are converted into products. This gives a high yield of ester.
  • HCl gas is produced — you would see white, misty fumes forming during the reaction. This is an important observation.
  • The reaction happens rapidly at room temperature.

Sign in to view full notes