Carboxylic Acids and Esters
Carboxylic acids and esters: C1–C4 structures, weak acidity, acid reactions, ester formation, naming and formula deduction.
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The core idea
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Learning objectives
- describe the carboxylic acids as a homologous series containing the –CO2H group
- draw the structures of carboxylic acids, C1 to C4, and name the unbranched acids methanoic acid to butanoic acid
- describe the carboxylic acids as weak acids, reacting with carbonates, bases and some metals
- describe the reaction of a carboxylic acid with an alcohol to form an ester, e.g. ethyl ethanoate
- deduce the name and formula of an ester from the unbranched carboxylic acid, C1 to C4, and alcohol, C1 to C4, and vice versa.
Start with the -COOH group and the familiar reactions of acids. Then learn to build an ester formula from an alcohol and a carboxylic acid—and to work backwards from an ester.
1. Definition
A. Carboxylic acid
A carboxylic acid is an organic compound containing the carboxyl functional group, -COOH.
B. Ester
An ester is an organic compound formed when a carboxylic acid reacts with an alcohol (esterification).
C. Esterification
Esterification is the reaction between a carboxylic acid and an alcohol to form an ester and water (a reversible reaction).
2. Key Ideas
- Carboxylic acids are weak acids (partially ionise in water).
- They undergo standard acid reactions with some metals, bases and carbonates.
- Esterification is reversible and uses concentrated H₂SO₄ + heat.
- Ester naming: alkyl (from alcohol) + carboxylate (from acid).
Acids + salts (core acid reactions): Introduction to acids
Making salts (acid + alkali): Preparation of salts
Alcohols (esterification reactant): Alcohols
3. Detailed Explanations
- Carboxylic acids contain -COOH and are weak acids (partial ionisation).
- Acid reactions: some metals → salt + H₂; bases → salt + H₂O; carbonates → salt + H₂O + CO₂.
- Esterification: carboxylic acid + alcohol → ester + water; reversible; uses conc. H₂SO₄ + heat.
- Ester naming: alkyl (from the alcohol) + carboxylate (from the acid), e.g. ethyl ethanoate.
- Carbon counting: the carbon in -COOH counts in the acid name (ethanoic acid has 2 carbons total).
A. Structures and names
| Name | Molecular formula | One-line structural formula |
|---|---|---|
| methanoic acid | HCOOH | H-COOH |
| ethanoic acid | CH₃COOH | CH₃-COOH |
| propanoic acid | C₂H₅COOH | CH₃-CH₂-COOH |
| butanoic acid | C₃H₇COOH | CH₃-CH₂-CH₂-COOH |
The carbon in the -COOH group is counted in the name. Example: ethanoic acid has 2 carbons total (1 in CH₃- and 1 in -COOH).
B. Why carboxylic acids are weak acids
A weak acid only partially ionises in water.
For ethanoic acid: CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq)
C. Acid reactions of ethanoic acid (exam-safe equations)
| Reaction type | Products | Example equation (ethanoic acid) |
|---|---|---|
| acid + some metals | salt + hydrogen | 2CH₃COOH(aq) + Mg(s) → (CH₃COO)₂Mg(aq) + H₂(g) |
| acid + base | salt + water | CH₃COOH(aq) + NaOH(aq) → CH₃COONa(aq) + H₂O(l) |
| acid + carbonate | salt + water + CO₂ | 2CH₃COOH(aq) + Na₂CO₃(s) → 2CH₃COONa(aq) + H₂O(l) + CO₂(g) |
D. Esterification (carboxylic acid + alcohol)
Esterification is the reaction of a carboxylic acid with an alcohol to form an ester and water.
Conditions (exam-safe):
- a few drops of concentrated sulphuric acid, H₂SO₄ (catalyst),
- heat.
Example (ethanoic acid + ethanol): CH₃COOH(l) + C₂H₅OH(l) ⇌ [conc. H₂SO₄][heat] CH₃COOC₂H₅(l) + H₂O(l)
Concentrated H₂SO₄ is corrosive and dehydrating. In practical questions, “wear goggles/gloves” is acceptable.
E. Naming esters (the exact rule)
An ester name has two parts:
- alkyl part from the alcohol (ethanol → ethyl),
- carboxylate part from the acid (ethanoic acid → ethanoate).
Example: ethanol + propanoic acid → ethyl propanoate.
F. Deducing ester names and formulae in both directions
An ester contains the linkage -COO⁻. Use that linkage as the dividing line between the two name parts:
- the group attached after the oxygen comes from the alcohol and gives the first, alkyl part;
- the group containing -COO⁻ comes from the carboxylic acid and gives the second, carboxylate part.
From reactants to ester:
Methanol + propanoic acid gives methyl propanoate: CH₃OH + CH₃CH₂COOH ⇌ CH₃CH₂COOCH₃ + H₂O
From ester back to reactants:
For CH₃COOCH₂CH₃, split at -COO⁻:
- -OCH₂CH₃ gives ethyl, so the alcohol was ethanol;
- CH₃COO⁻ gives ethanoate, so the acid was ethanoic acid.
The ester is ethyl ethanoate.
Count all carbon atoms after joining the two parts. Esterification removes H₂O, not a carbon atom.
4. Common Mistakes
- Calling carboxylic acids “strong acids” (they are weak acids; partial ionisation).
- Forgetting the products of acid + carbonate (must include CO₂).
- Writing ester naming backwards (acid first; wrong). Correct order: alcohol-derived alkyl + acid-derived carboxylate.
- Writing “H₂SO₄ is used up” (it is a catalyst).
- Giving an ester formula without the -COO⁻ linkage.
5. Exam Tips
Use the same reaction patterns as mineral acids: metal → H₂, base → water, carbonate → CO₂.
Write: “reversible”, “concentrated H₂SO₄ catalyst”, “heat”, “ester + water”.
6. Worked Examples
Modelled example 1
Identify weak acid behaviour
Problem
Study the worked solution
State the extent of ionisation
Method
State that ethanoic acid ionises only partially.Reason
Only some acid molecules form ions in water; “weak” does not mean dilute.Working
Most CH₃COOH remains as molecules at equilibrium.Represent the equilibrium
Method
Use a reversible arrow between molecules and ions.Reason
Ionisation and recombination occur in both directions.Working
CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq).
Guided practice 2
Acid + carbonate products
Problem
Apply the acid–carbonate pattern
Hints
Hint 1: reaction pattern
Hint 2: salt formula
View solution step by step
Name the products
Method
Form sodium ethanoate, water and carbon dioxide.Reason
The salt combines sodium ions from the carbonate with ethanoate ions from the acid.Working
CH₃COONa, H₂O and CO₂.Balance the equation
Method
Use two ethanoic acid and two sodium ethanoate units.Reason
This conserves both sodium ions and both ethanoate groups; the remaining atoms form water and carbon dioxide.Working
2CH₃COOH(aq) + Na₂CO₃(s) → 2CH₃COONa(aq) + H₂O(l) + CO₂(g).
Common misconception 3
Name the ester
Learner name
Assign the alcohol and acid name parts
View solution step by step
Take the alkyl part from the alcohol
Method
Convert methanol to methyl.Reason
The first part of an ester name is derived from the alcohol.Working
methanol → methyl.Join the formula parts
Method
Convert ethanoic acid to ethanoate and place it second.Reason
The acid supplies CH₃COO⁻ and methanol supplies the methyl group attached after oxygen.Working
ester: methyl ethanoate, CH₃COOCH₃.
Examiner practice 4
Build an ester from its reactants
Examination question
Use both reactant name parts and the ester linkage
View solution step by step
Build the name
2 marksMethod
Take propyl from propanol and butanoate from butanoic acid.Reason
The alcohol supplies the first name part and the acid supplies the second.Working
propyl butanoate.Build the formula
1 markMethod
Join the butanoate side to the propyl group through -COO⁻.Reason
The ester linkage must remain visible in the condensed formula.Working
CH₃CH₂CH₂COOCH₂CH₂CH₃.State the other product
1 markMethod
Name water.Reason
Esterification is a condensation reaction between the acid and alcohol.Working
Other product: H₂O.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Self-mark both name parts, the linked formula and water.
Challenge 5
Identify reactants from ester name
Reverse-name transfer
Work backwards from both name parts
Hints
Hint 1: first part
Hint 2: second part
View solution step by step
Recover the alcohol
Method
Convert ethyl back to ethanol.Reason
The first ester-name part is the alcohol-derived alkyl group.Working
ethyl → ethanol.Recover the acid and formula
Method
Convert propanoate back to propanoic acid.Reason
The second name part gives CH₃CH₂COO⁻; attach the ethyl group after oxygen.Working
Reactants: ethanol + propanoic acid; ester: CH₃CH₂COOCH₂CH₃.
7. Mind Stretchers
Mind stretcher 1: Error analysis (name order)Extension
Question: A student writes “ethanoic methyl ester” for the product of methanol + ethanoic acid. Explain what is wrong and give the correct name.
Show Answer
The naming order is wrong and the exam expects the ester naming pattern: alkyl (from alcohol) + carboxylate (from acid).
Correct name: methyl ethanoate.
Final: Use “methyl ethanoate” (alkyl first).
Mind stretcher 2: Read an ester formula backwardsExtension
Question: The ester HCOOCH₂CH₂CH₃ was formed from an alcohol and a carboxylic acid. Name both reactants.
Show Answer
Split the formula at -COO⁻. The group after oxygen is propyl, so the alcohol is propanol. The HCOO⁻ part is methanoate, so the acid is methanoic acid.
Answer: propanol and methanoic acid.
8. Quiz
Test weak-acid ionisation, acid reactions, ester naming, and esterification conditions.
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