Carboxylic Acids and Esters
Carboxylic acids contain the –COOH group and behave as weak acids. React one with an alcohol and you get an ester — the reaction that gives fruits and perfumes their smells.
1. Carboxylic acids
General formula CₙH₂ₙ₊₁COOH, with the functional group –COOH (carboxyl).
| Acid | Formula |
|---|---|
| Methanoic acid | HCOOH |
| Ethanoic acid | CH₃COOH |
| Propanoic acid | C₂H₅COOH |
| Butanoic acid | C₃H₇COOH |
Ethanoic acid is CH₃COOH — the acid in vinegar.
Count the carbons carefully. Confusion over the molecular formula for butanoic acid was recorded — the name gives the total carbons including the one in –COOH, so butanoic acid has four carbons: C₃H₇COOH, or C₄H₈O₂.
The –COOH group must be drawn correctly in a displayed formula: a carbon double-bonded to one oxygen and single-bonded to an –OH.
2. Carboxylic acids are WEAK acids
They are WEAK acids — only PARTIALLY ionised in water, shown with a reversible arrow.
CH₃COOH ⇌ CH₃COO⁻ + H⁺
Carboxylic acids ARE acidic — a recorded error called them neutral. They turn litmus red and have a pH below 7, but a higher pH than a strong acid of the same concentration.
They show all the typical acid reactions:
| With | Products |
|---|---|
| Metal | salt + HYDROGEN |
| Base / alkali | salt + water |
| Carbonate | salt + water + CARBON DIOXIDE |
Examples:
2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂ CH₃COOH + NaOH → CH₃COONa + H₂O 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
The salts are called ETHANOATES — sodium ethanoate, magnesium ethanoate.
The carbonate reaction is a useful TEST: effervescence with a carbonate confirms a carboxylic acid, since alcohols do not react.
3. Making ethanoic acid
Two routes, both by OXIDISING ethanol:
1. Microbial oxidation — bacteria in the air oxidise ethanol slowly. This is why wine turns to vinegar if left open. 2. Chemical oxidation — warm ethanol with acidified potassium manganate(VII), which turns PURPLE → COLOURLESS.
ethanol + oxygen → ethanoic acid + water
The colour change is the observation — purple to colourless with manganate(VII), or orange to green if acidified potassium dichromate(VI) is used.
4. Esters
ESTERIFICATION: carboxylic acid + alcohol ⇌ ESTER + WATER
Conditions: warm, with a concentrated sulfuric acid catalyst.
Example:
ethanoic acid + ethanol ⇌ ethyl ethanoate + water CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
The reaction is REVERSIBLE — use ⇌.
Naming esters — the rule that catches people out:
The FIRST part comes from the ALCOHOL (ending -yl). The SECOND part comes from the ACID (ending -oate).
| Alcohol | Acid | Ester |
|---|---|---|
| ethanol | ethanoic acid | ethyl ethanoate |
| methanol | ethanoic acid | methyl ethanoate |
| propanol | methanoic acid | propyl methanoate |
The alcohol comes FIRST in the name, but the acid provides the second word. Getting this the wrong way round is the standard error — remember alcohol → -yl, acid → -oate.
The ester linkage is –COO–, formed between the acid’s –COOH and the alcohol’s –OH, with water removed.
Properties and uses: esters are volatile with sweet, fruity smells — used in perfumes, food flavourings and as solvents.
5. Link to polymers
Polyesters are made by condensation polymerisation between a dicarboxylic acid and a diol, joined by the same ester linkage — with water lost at each link.
This is the same chemistry as making a single ester, repeated many times.
6. Mistakes that cost marks
Calling carboxylic acids neutral.
Using a single arrow for a weak acid or for esterification.
Miscounting carbons in the acid’s formula.
Drawing the –COOH group wrongly.
Naming the ester the wrong way round.
Forgetting water as a product of esterification.
Omitting the concentrated sulfuric acid catalyst.
Forgetting carbon dioxide in the carbonate reaction.
Frequently asked questions
What is the functional group in carboxylic acids? –COOH (carboxyl).
What is the formula of ethanoic acid? CH₃COOH.
Are carboxylic acids strong or weak? Weak — only partially ionised, shown with ⇌.
What do they produce with a carbonate? Salt + water + carbon dioxide — the effervescence is a useful test.
What are their salts called? Ethanoates (from ethanoic acid).
How is ethanoic acid made from ethanol? By oxidation — either by bacteria in air, or with acidified potassium manganate(VII).
What is esterification? Carboxylic acid + alcohol ⇌ ester + water, with a concentrated sulfuric acid catalyst.
How do I name an ester? Alcohol part first (ending -yl), then the acid part (ending -oate).
What is ethanol + ethanoic acid? Ethyl ethanoate.
What are esters used for? Perfumes, flavourings and solvents — they smell sweet and fruity.
Quick revision checklist
- I know the –COOH group and general formula
- I can write the first four carboxylic acids
- I can draw –COOH correctly
- I know they are weak acids and use ⇌
- I know all three acid reactions and their products
- I know the salts are ethanoates
- I know both routes to ethanoic acid and the colour changes
- I can write the esterification equation with the catalyst
- I can name an ester the right way round
- I know the ester linkage and that water is lost
- I know the uses of esters
- I can link esters to polyesters
These notes cover carboxylic acids and esters in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus for your own exam series.
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