Esters: Esterification and Hydrolysis

Esters are the compounds behind nearly every pleasant smell in the natural world — from pear drops to ripe bananas to rose perfume. They are also the workhorse molecules of industry: polyester fabric, plasticisers, solvents, fragrances and flavours are all built on ester chemistry.

An ester is a condensation product of a carboxylic acid and an alcohol. This lesson covers the OCR A-Level Chemistry A (H432) specification point 6.2.1 (d): formation, nomenclature, hydrolysis and uses of esters.

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1. Structure and Naming of Esters

An ester has the general formula R–COO–R', where R comes from the acid and R' comes from the alcohol. The functional group –COO– contains a C=O and an adjacent C–O–C linkage.

  O
  ||
  R-C-O-R'
  (acid part) (alcohol part)

1.1 Naming rules

Ester names come in two parts: the alcohol part comes first, then the acid part with the suffix -oate.

Rule: [alkyl from alcohol] [acyl from acid with -oate ending]

Acid + alcohol	Ester	Name
Ethanoic acid + methanol	CH₃COOCH₃	Methyl ethanoate
Ethanoic acid + ethanol	CH₃COOCH₂CH₃	Ethyl ethanoate
Methanoic acid + ethanol	HCOOCH₂CH₃	Ethyl methanoate
Propanoic acid + methanol	CH₃CH₂COOCH₃	Methyl propanoate
Butanoic acid + ethanol	CH₃(CH₂)₂COOCH₂CH₃	Ethyl butanoate

Exam Tip: Students often get this backwards. The name reads: "alcohol alkyl group" + "acid minus -oic + -oate". Methyl ethanoate is the ester from methanol and ethanoic acid, not the other way around.

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2. Esterification: Making an Ester

Esterification is the condensation reaction between a carboxylic acid and an alcohol to give an ester and water.

2.1 Overall equation

$CH_3COOH + C_2H_5OH \rightleftharpoons CH_3COOC_2H_5 + H_2O$CH3​COOH+C2​H5​OH⇌CH3​COOC2​H5​+H2​O

Ethanoic acid + ethanol gives ethyl ethanoate, catalysed by concentrated H₂SO₄.

Note the equilibrium arrows — the reaction is reversible, and usually sits around 65–70% conversion before equilibrium is reached. To push it towards the ester, industrial chemists use excess alcohol or remove the water (Le Chatelier's principle).

2.2 Conditions

• Reagents: Carboxylic acid + alcohol
• Catalyst: A few drops of concentrated H₂SO₄ (acts as both catalyst and dehydrating agent — removing water also pushes equilibrium towards ester).
• Temperature: Warm under reflux for 20–30 minutes.

Why reflux? The reaction is slow at room temperature, but both the reactants and the ester are volatile and would otherwise boil away. A reflux condenser allows heating without losing material.

2.3 Mechanism (brief)

You are not required to know the detailed esterification mechanism for OCR H432, but you should understand:

1. H⁺ from H₂SO₄ protonates the C=O of the acid, making the carbonyl carbon even more electrophilic.
2. The alcohol's lone pair attacks the δ+ C.
3. After proton shuffling, water is eliminated, giving the ester.

2.4 Observations and isolation

When you carry out the practical:

• Smell is the most obvious sign — a fruity, sweet smell develops as the ester forms.
• To isolate the ester: distil off the ester (lower bp than the acid), wash with aqueous Na₂CO₃ to remove unreacted acid, then dry with anhydrous MgSO₄ before a final distillation.

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3. Hydrolysis of Esters

Hydrolysis is the reverse of esterification — the ester is broken apart by water to give an alcohol and either a carboxylic acid (acid hydrolysis) or a carboxylate salt (base hydrolysis). OCR requires both.

3.1 Acid hydrolysis

$CH_3COOC_2H_5 + H_2O \rightleftharpoons CH_3COOH + C_2H_5OH$CH3​COOC2​H5​+H2​O⇌CH3​COOH+C2​H5​OH

• Conditions: Dilute H₂SO₄ (or HCl) catalyst; reflux.
• Products: Carboxylic acid + alcohol.
• Reversible: Note the ⇌ arrows — acid hydrolysis is an equilibrium, so yields are typically only 50–70%.

To drive the equilibrium towards hydrolysis, use a large excess of water.