Esters and Their Hydrolysis
A concise revision guide to esters: naming and drawing them, making them, and hydrolysis in acidic and alkaline solution.
- 18.2.1(a)
- 18.2.2-a
- 18.2.2-b
What these spec points say
- 18.2.1(a) 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
- 18.2.2-a describe the hydrolysis of esters by dilute acid and heat
- 18.2.2-b describe the hydrolysis of esters by dilute alkali and heat
Hydrolysing an Ester: Acid or Alkali
Follow every atom as ethyl ethanoate is split by water with an acid catalyst, which reaches an equilibrium, and by sodium hydroxide, which goes to completion; then turn the salt back into the acid.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
Before You Start
Esters are made from carboxylic acids (see Reactions of Carboxylic Acids) and from acyl chlorides (see Acyl Chlorides). This page covers naming them and breaking them down again.
GCSE Recap: Where Esters Come From
Three quick questions on the GCSE ideas this page builds on.
What an Ester Is and How to Name It
An ester contains the ester link, –COO–: a C=O with a second oxygen joining the same carbon to another carbon chain. The general formula is RCOOR′.
Naming
- Find the chain joined to the single-bonded oxygen. It came from the alcohol and gives the first word, ending in -yl.
- Find the chain that contains the C=O carbon. It came from the acid and gives the second word, ending in -oate.
- Count the C=O carbon as part of the acid chain.
| Ester | Structural formula | Acid | Alcohol |
|---|---|---|---|
| methyl ethanoate | CH₃COOCH₃ | ethanoic acid | methanol |
| ethyl ethanoate | CH₃COOCH₂CH₃ | ethanoic acid | ethanol |
| ethyl propanoate | CH₃CH₂COOCH₂CH₃ | propanoic acid | ethanol |
| propyl methanoate | HCOOCH₂CH₂CH₃ | methanoic acid | propan-1-ol |
Skeletal formulae of four esters. In each, the part containing C=O comes from the carboxylic acid and the part joined to the single-bonded oxygen comes from the alcohol.
Naming an ester in three steps: the alkyl group from the alcohol is written first, then the -oate word from the acid, with four examples.
Properties
- Ester molecules have no O–H group, so they cannot hydrogen bond with each other.
- Their boiling points are lower than those of the isomeric carboxylic acids: they are volatile liquids.
- Many have sweet, fruity smells.
- Esters and carboxylic acids are isomers: C₄H₈O₂ can be ethyl ethanoate or butanoic acid.
Remember: The name runs opposite to the formula. In CH₃COOCH₂CH₃ the acid part, CH₃COO, is written first, but the name starts with the alcohol part: ethyl ethanoate.
Check: Naming and Drawing Esters
Name esters from their formulae, draw esters from their names and identify the acid and alcohol each is made from, using esters not used on this page.
Making Esters
| Reagents | Conditions | Equation | Comment |
|---|---|---|---|
| carboxylic acid + alcohol | concentrated H₂SO₄ catalyst, heat | CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O | reversible, slow |
| acyl chloride + alcohol | room temperature | CH₃COCl + CH₃CH₂OH → CH₃COOCH₂CH₃ + HCl | fast, goes to completion |
| acyl chloride + phenol | room temperature | C₆H₅COCl + C₆H₅OH → C₆H₅COOC₆H₅ + HCl | gives a phenyl ester |
Every route joins an acyl group, RCO–, to the oxygen atom of an alcohol or phenol. The first is a condensation: two molecules join and water is lost.
Hydrolysis of Esters
Hydrolysis is the breaking of a bond by reaction with water. In an ester the bond between the C=O carbon and the single-bonded oxygen breaks, giving back the acid part and the alcohol.
Acid hydrolysis
Heat the ester under reflux with a dilute acid, such as dilute H₂SO₄ or HCl. The acid is a catalyst.
CH₃COOCH₂CH₃ + H₂O ⇌ CH₃COOH + CH₃CH₂OH
- This is the reverse of esterification, so it is reversible and does not go to completion.
- A large excess of water, from the dilute acid, pushes the equilibrium towards the products.
Alkaline hydrolysis
Heat the ester under reflux with aqueous sodium hydroxide. The products are the carboxylate salt and the alcohol.
CH₃COOCH₂CH₃ + NaOH → CH₃COONa + CH₃CH₂OH
- The reaction goes to completion: the carboxylate ion does not react with the alcohol, so there is no reverse reaction.
- Adding a dilute strong acid afterwards gives the carboxylic acid: CH₃COONa + HCl → CH₃COOH + NaCl.
| Acid hydrolysis | Alkaline hydrolysis | |
|---|---|---|
| Reagent | dilute H₂SO₄ or HCl (catalyst) | NaOH(aq) (a reactant) |
| Conditions | heat under reflux | heat under reflux |
| Products | carboxylic acid + alcohol | carboxylate salt + alcohol |
| Reversible? | yes | no: goes to completion |
Acid and alkaline hydrolysis of ethyl ethanoate compared: reagents, conditions, products and whether the reaction is reversible.
Worked example
Methyl propanoate is heated under reflux with aqueous potassium hydroxide. Give the equation and name the products.
Step 1. Split the ester at the ester link: CH₃CH₂CO– from the acid and –OCH₃ from the alcohol.
Answer. CH₃CH₂COOCH₃ + KOH → CH₃CH₂COOK + CH₃OH
Exam focus: Acid hydrolysis: reversible, gives the acid. Alkaline hydrolysis: complete, gives the salt. Write a reversible arrow for the first and a single arrow for the second.
Check: Hydrolysis of Esters
Predict the products of acid and alkaline hydrolysis, write the equations and compare the two methods, for esters not used on this page.
Quick Check: Following a Hydrolysis with Numbers
Work out each step on paper, then flip the card to check.
Common Exam Points
Say
- “Heat under reflux with dilute acid” or “heat under reflux with aqueous sodium hydroxide”.
- “Alkaline hydrolysis goes to completion and gives the carboxylate salt.”
- “Acid hydrolysis is reversible; it is the reverse of esterification.”
Do not say
- “Alkaline hydrolysis gives the carboxylic acid” (it gives the salt; acidify to obtain the acid).
- “Ethyl ethanoate is made from ethanol and ethanol” (ethyl from ethanol, ethanoate from ethanoic acid).
- “Esters hydrogen bond to each other” (there is no O–H group).
Watch for
- Reversed formulae: CH₃CH₂OOCCH₃ is still ethyl ethanoate. Find the C=O to find the acid part.
- Methanoates: HCOO– has a hydrogen, not a carbon chain, on the C=O carbon.
Check: Esters From Start to Finish
Link the formation, naming and hydrolysis of esters: identify unknown esters from their hydrolysis products and plan how to make and break them, with compounds not used on this page.
FAQs
Use these quick answers to check esters, their hydrolysis and how they are made.
How do I name an ester?
The first word comes from the alcohol and the second from the acid, ending in -oate. CH₃COOCH₂CH₃, made from ethanoic acid and ethanol, is ethyl ethanoate. Take care: the formula is often written with the acid part first.
What is the difference between acid and alkaline hydrolysis of an ester?
Heating under reflux with dilute acid is reversible and gives the carboxylic acid and the alcohol. Heating under reflux with dilute alkali, such as NaOH(aq), goes to completion and gives the carboxylate salt and the alcohol.
Why does alkaline hydrolysis go to completion?
The carboxylic acid is converted into its carboxylate ion, such as CH₃COO⁻, which does not react with the alcohol. The reverse reaction cannot happen, so no equilibrium is set up. Adding a strong acid afterwards releases the carboxylic acid.
What are the products when ethyl ethanoate is hydrolysed?
With dilute acid and heat: CH₃COOCH₂CH₃ + H₂O ⇌ CH₃COOH + CH₃CH₂OH. With dilute NaOH and heat: CH₃COOCH₂CH₃ + NaOH → CH₃COONa + CH₃CH₂OH, giving sodium ethanoate and ethanol.
Which two routes make an ester?
A carboxylic acid and an alcohol, heated with concentrated H₂SO₄ as catalyst, give an ester and water in a reversible reaction. An acyl chloride and an alcohol react at room temperature, giving the ester and HCl, and go to completion.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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