Ch 2 · Esters · AS & A Level Chemistry 9701

Topic 2 of 2 in Carboxylic acids and derivatives — AS & A Level Chemistry 9701 — 47 lessons.

Introduction

Esters, RCOOR', are carboxylic acid derivatives formed by condensation with an alcohol. Their formation is reversible, and the two directions of hydrolysis, acidic and alkaline, define their chemistry alongside their familiar role as flavourings and fragrances.

Key ideas — Formation of esters

An ester forms by heating a carboxylic acid with an alcohol under reflux, with a few drops of concentrated sulfuric acid as catalyst. Water is eliminated as the ester bond forms, so this is a condensation reaction, and it is reversible: CH_3COOH + CH_3OH leftharpoons CH_3COOCH_3 + H_2O

🧑‍🏫 Teacher's explanation

Think of making an ester like a marriage between two molecules. A carboxylic acid and an alcohol want to join together, but to do so, they must both give up a small piece of themselves. The acid loses an -OH group and the alcohol loses an -H atom. These two pieces combine to form water, which is why we call this a 'condensation' reaction.

📖 Definitions

🔍 Worked example — Easy

Name the ester formed when ethanoic acid reacts with methanol.

🔍 Worked example — Standard

Calculate the molar mass of ethyl ethanoate, ceCH_3COOCH_2CH_3, given C=12, H=1, O=16.

🔍 Worked example — Exam-style

A student reacts 0.2 mol of propanoic acid with 0.3 mol of ethanol to form an ester. Calculate the maximum moles of ester produced.

⚠️ Common errors

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🎯 Exam question

(a) Identify the structure of X if it is a carboxylic acid. [1] (b) Write the balanced equation for the reaction between X and methanol to form Y. [2] (c) Describe how the ester Y could be separated from the reaction mixture. [1] [4 marks]

Key ideas — Hydrolysis of esters

Heating an ester under reflux with dilute acid (e.g. dilute H_2SO_4) is the reverse of esterification, slowly reforming the carboxylic acid and alcohol; because the reaction is an equilibrium, it is generally incomplete, and a large excess of water is needed to push the position of equilibrium toward hydrolysis. CH_3COOCH_3 + H_2O leftharpoons CH_3COOH + CH_3OH Heating under reflux with dilute alkali (e.g. NaOH) instead is irreversible: the carboxylate salt formed does not react back with the alcohol, so hydrolysis goes to completion, giving a much higher yield. CH_3COOCH_3 + NaOH arrow CH_3COONa + CH_3OH Naturally occurring fats and oils are esters of the triol glycerol (propane-1,2,3-triol) with long-chain carboxylic acids; alkaline hydrolysis of these esters, producing glycerol and the sodium salts of the acids, is the basis of soap manufacture.

🧑‍🏫 Teacher's explanation

Think of an ester like a LEGO structure built from two specific bricks: a carboxylic acid and an alcohol. Esterification is the process of snapping them together. Hydrolysis is the process of using water to 'un-snap' them. It is like a chemical tug-of-war where water is trying to pull the molecule apart.

📖 Definitions

🔍 Worked example — Standard

Write the balanced chemical equation for the acid-catalyzed hydrolysis of methyl ethanoate, CH_3 extCOOCH_3, using water.

🔍 Worked example — Exam-style

A 0.500 mol sample of ethyl propanoate is reacted with 2.00 mol of NaOH via alkaline hydrolysis. Calculate the moles of sodium propanoate produced, assuming the reaction goes to completion.

⚠️ Common errors

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🎯 Exam question

Key ideas — Properties and uses of esters

Esters are volatile, generally water-insoluble molecular compounds with a characteristic sweet, fruity smell, since their molecules cannot hydrogen-bond to one another (only to water) in the way carboxylic acids and alcohols can. This is why esters occur naturally in ripe fruit and are manufactured as flavourings in the food industry and as fragrances in perfumes and cosmetics; their volatility and low water-solubility also make small esters, such as ethyl ethanoate, useful as solvents, for example in nail varnish and its remover.

🧑‍🏫 Teacher's explanation

Imagine you are trying to hold hands with a group of people. Alcohols and carboxylic acids have a special 'sticky' feature called hydrogen bonding, which allows them to cling to each other tightly. Esters, however, have lost that specific 'sticky' feature. Because they cannot hydrogen-bond to themselves, they don't stick together well; they are 'volatile', meaning they evaporate easily into the air. This is why you can smell them so easily!

📖 Definitions

🔍 Worked example — Easy

Name the ester formed from the reaction between methanol and propanoic acid.

🔍 Worked example — Standard

Calculate the molar mass of ethyl ethanoate, C_4H_8O_2. (Given C=12, H=1, O=16)

🔍 Worked example — Exam-style

A student reacts 0.50 moles of ethanoic acid with excess ethanol to form ethyl ethanoate. If the yield is 85\%, calculate the mass of ester produced. (Molar mass of ethyl ethanoate C_4H_8O_2 = 88)

⚠️ Common errors

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🎯 Exam question

Key ideas — Naming esters

An ester is named from its two "parent" compounds: the alcohol part is named first, ending in "-yl", and the acid part is named second, ending in "-oate". The ester of ethanol and propanoic acid is ethyl propanoate, CH_3CH_2COOC_2H_5; the ester of methanol and ethanoic acid is methyl ethanoate, CH_3COOCH_3.

🧑‍🏫 Teacher's explanation

Think of an ester like a hybrid car. It is a single entity, but it is built from two distinct parts: an alcohol part and a carboxylic acid part. To name it, you simply name the parts separately and glue them together. The alcohol part always comes first and gets a new suffix, '-yl', while the acid part comes second and changes its '-ic acid' ending to '-oate'.

📖 Definitions

🔍 Worked example — Easy

Name the ester formed from the reaction between methanol and propanoic acid.

🔍 Worked example — Standard

Write the structural formula for ethyl ethanoate.

🔍 Worked example — Exam-style

An ester has the formula CH_3 extCH_2 extCH_2 extCOOCH_3. Name it.

⚠️ Common errors

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🎯 Exam question

Worked example

Question. Ester X has the structural formula CH_3CH_2COOCH_3. (a) Name ester X. [1] (b) Identify the alcohol and carboxylic acid from which X could be prepared. [2] (c) X is heated under reflux with an excess of dilute sodium hydroxide. Name the two organic products and explain why this hydrolysis goes to completion. [3]

Ch 2 · Esters · AS & A Level Chemistry 9701

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