10.1 Preparation of Esters

Introduction

Esters can be prepared by several methods. The FBISE syllabus (C-12-E-38, C-12-E-40) focuses on the reaction of alcohols and phenols with acyl chlorides as the primary route, using ethyl ethanoate and phenyl benzoate as key examples.

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Method: Reaction of Acyl Chlorides with Alcohols

Acyl chlorides (acid chlorides) react with alcohols at room temperature to produce esters and hydrogen chloride gas. No catalyst or heating is required.

General equation: $\text{RCOCI}+\text{R’OH}\to \text{RCOOR’}+\text{HCl}$

Example 1: Formation of Ethyl Ethanoate

Ethanoyl chloride reacts with ethanol at room temperature:

${\text{CH}}_3\text{COCl}+{\text{C}}_2{\text{H}}_5\text{OH}\to {\text{CH}}_3{\text{COOC}}_2{\text{H}}_5+\text{HCl}$

• Reactants: Ethanoyl chloride + Ethanol
• Products: Ethyl ethanoate (an ester) + Hydrogen chloride
• Conditions: Room temperature, no catalyst needed
• Observation: Misty fumes of HCl are produced

Example 2: Formation of Phenyl Benzoate

Benzoyl chloride reacts with phenol at room temperature:

${\text{C}}_6{\text{H}}_5\text{COCl}+{\text{C}}_6{\text{H}}_5\text{OH}\to {\text{C}}_6{\text{H}}_5{\text{COOC}}_6{\text{H}}_5+\text{HCl}$

• Reactants: Benzoyl chloride + Phenol
• Products: Phenyl benzoate (an ester) + Hydrogen chloride
• Conditions: Room temperature

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Why Use Acyl Chlorides Instead of Carboxylic Acids?

Acyl chlorides are more reactive than carboxylic acids because the $\text{C-Cl}$ bond is more polarised and chloride is a better leaving group than hydroxide.

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Reactions of Acyl Chlorides (C-12-E-40)

Acyl chlorides undergo several reactions at room temperature:

a. Hydrolysis (with water)

$\text{RCOCI}+{\text{H}}_2\text{O}\to \text{RCOOH}+\text{HCl}$ Produces the carboxylic acid and HCl. Misty fumes observed.

b. Reaction with an Alcohol → Ester

$\text{RCOCI}+\text{R’OH}\to \text{RCOOR’}+\text{HCl}$ Produces an ester and HCl.

c. Reaction with Phenol → Ester

$\text{RCOCI}+\text{ArOH}\to \text{RCOOAr}+\text{HCl}$ Produces an aryl ester and HCl. Example: phenyl benzoate from benzoyl chloride and phenol.

d. Reaction with Ammonia → Amide

$\text{RCOCI}+2{\text{NH}}_3\to {\text{RCONH}}_2+{\text{NH}}_4\text{Cl}$ Produces a primary amide and ammonium chloride.

e. Reaction with a Primary or Secondary Amine → N-substituted Amide

$\text{RCOCI}+{\text{RNH}}_2\to \text{RCONHR’}+\text{HCl}$ Produces an N-substituted amide (secondary amide) and HCl.

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Key Observations

• All reactions of acyl chlorides occur at room temperature.
• Misty white fumes of HCl are a characteristic observation in all these reactions.
• The reactions are vigorous and exothermic.