Esters: Hydrolysis, Fats, Soap and Biodiesel
A concise revision guide to esters: naming and drawing them, making them, and hydrolysis in acidic and alkaline solution, with the uses of esters, fats and oils, soap and biodiesel.
- 3.3.9.1i-b
- 3.3.9.1iv
- 3.3.9.1v
- 3.3.9.1vi-a
- 3.3.9.1vi-b
- 3.3.9.1vii
- 3.3.9.1viii
What these spec points say
- 3.3.9.1i-b know the structures of esters
- 3.3.9.1iv know the common uses of esters in solvents, plasticisers, perfumes and food flavourings
- 3.3.9.1v know that vegetable oils and animal fats are esters of propane-1,2,3-triol (glycerol)
- 3.3.9.1vi-a know that esters can be hydrolysed in acid conditions to form alcohols and carboxylic acids
- 3.3.9.1vi-b know that esters can be hydrolysed in alkaline conditions to form alcohols and salts of carboxylic acids
- 3.3.9.1vii know that vegetable oils and animal fats can be hydrolysed in alkaline conditions to give soap and glycerol
- 3.3.9.1viii know that biodiesel is a mixture of methyl esters of long-chain carboxylic acids produced by reacting vegetable oils with methanol in the presence of a catalyst
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 and Acid Anhydrides). 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 |
| acid anhydride + alcohol | warm | (CH₃CO)₂O + CH₃CH₂OH → CH₃COOCH₂CH₃ + CH₃COOH | goes to completion, no HCl |
Every route joins an acyl group, RCO–, to the oxygen atom of an alcohol. 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.
- Alkaline hydrolysis of an ester is also called saponification, because it is how soap is made.
| 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.
Uses of Esters
| Use | Why esters suit it |
|---|---|
| Solvents | polar enough to dissolve many organic compounds, and volatile, so they evaporate quickly (glues, nail varnish) |
| Plasticisers | the molecules sit between polymer chains and let them slide, making a plastic such as PVC flexible |
| Perfumes | pleasant smells and volatile |
| Food flavourings | sweet, fruity smells and tastes, such as pear, banana and pineapple |
Fats, Oils and Soap
Vegetable oils and animal fats are esters of propane-1,2,3-triol (glycerol), CH₂(OH)CH(OH)CH₂OH, with three long-chain carboxylic acids. Each molecule is a triester.
- The long-chain acids are often called fatty acids, for example octadecanoic acid, C₁₇H₃₅COOH.
- Fats come mostly from saturated acids and are solids. Oils come mostly from unsaturated acids, with C=C bonds, and are liquids.
Making soap
Heating a fat or oil with aqueous sodium hydroxide hydrolyses all three ester links. The products are glycerol and the sodium salts of the long-chain acids, which are soap.
triester + 3NaOH → CH₂(OH)CH(OH)CH₂OH + 3RCOO⁻Na⁺
Alkaline hydrolysis of a fat or oil: the triester reacts with three NaOH to give propane-1,2,3-triol (glycerol) and three carboxylate ions, the soap.
- A soap ion has an ionic –COO⁻ head that mixes with water and a long non-polar tail that mixes with grease.
- Glycerol has three O–H groups, so it forms many hydrogen bonds and is very soluble in water.
Biodiesel
Biodiesel is a mixture of methyl esters of long-chain carboxylic acids. It is made by reacting a vegetable oil with methanol in the presence of a catalyst, such as potassium hydroxide.
triester + 3CH₃OH → CH₂(OH)CH(OH)CH₂OH + 3RCOOCH₃
- Each triester molecule gives three methyl ester molecules and one molecule of glycerol.
- The reaction swaps one alcohol for another in the ester, so it is called transesterification.
- The oil comes from crops such as rapeseed, so biodiesel is a renewable fuel.
- It is not fully carbon neutral: farming, processing and transport all use energy.
Fats and oils as triesters: alkaline hydrolysis to glycerol and soap, reaction with methanol and a catalyst to give biodiesel, and four everyday uses of esters.
Exam wording: “Biodiesel is a mixture of methyl esters of long-chain carboxylic acids, produced by reacting vegetable oils with methanol in the presence of a catalyst.”
Quick Check: Applying Fats, Soap and Biodiesel
Pick the one accurate statement in each set, using a triester that is new to this page.
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.”
- “Soaps are the sodium salts of long-chain carboxylic acids.”
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.
- Balancing with fats and oils: three moles of NaOH or of methanol react with each mole of triester.
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, fats, soap and biodiesel.
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 fats and oils, and how is soap made from them?
They are triesters of propane-1,2,3-triol (glycerol) with long-chain carboxylic acids. Heating with NaOH(aq), called saponification, hydrolyses the three ester links to give glycerol and the sodium salts of the acids, which are soap.
What is biodiesel?
Biodiesel is a mixture of methyl esters of long-chain carboxylic acids. It is made by reacting vegetable oils with methanol and a catalyst, a process called transesterification, which also gives propane-1,2,3-triol.
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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