Organic chemistry · GCSE Chemistry
Alcohols and carboxylic acids
GCSE Chemistry (often Triple) revision on alcohols and carboxylic acids: homologous series, ethanol by fermentation and hydration, combustion, weak-acid behaviour, carbonates, and esters from alcohol plus acid.
Alcohols have –OH (CnH2n+1OH). Carboxylic acids have –COOH and are weak acids. Ethanol: fermentation of sugar or steam plus ethene. Alcohol + carboxylic acid ⇌ ester + water, acid catalyst, fruity smell.
The important bits
What you need to know
- 1
Alcohols are a homologous series with the –OH functional group and general formula CnH2n+1OH. The first four: methanol CH₃OH, ethanol C₂H₅OH, propanol C₃H₇OH, butanol C₄H₉OH.
- 2
Alcohols burn completely to carbon dioxide and water and are used as fuels. They react with sodium to give hydrogen and a sodium alkoxide. They are liquids with higher boiling points than matching alkanes because of hydrogen bonding (mention only if your spec wants it).
- 3
Ethanol is made by fermentation: glucose from sugar or starch, yeast, about 30–40 °C, anaerobic conditions. C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Air must be kept out or the yeast respires aerobically and you get CO₂ and water instead of alcohol.
- 4
Ethanol is also made by hydrating ethene from crude oil: C₂H₄ + H₂O → C₂H₅OH, phosphoric acid catalyst, high temperature and pressure. Continuous process, high purity, but uses a finite feedstock.
- 5
Carboxylic acids have the –COOH functional group: methanoic HCOOH, ethanoic CH₃COOH (vinegar). They are weak acids: they ionise partially in water, so the pH is higher than a strong acid of the same concentration, but they still turn universal indicator orange/red and react with metals, bases and carbonates.
- 6
Carboxylic acid + metal carbonate → salt + water + carbon dioxide. Limewater going cloudy confirms CO₂. The salt name ends in -anoate: ethanoic acid plus sodium carbonate gives sodium ethanoate.
- 7
Alcohols can be oxidised to carboxylic acids (acidified potassium dichromate, orange to green, is the usual school oxidising mixture on Triple papers). Ethanol oxidises to ethanoic acid, which is why wine goes sour in air.
- 8
Esters form when a carboxylic acid reacts with an alcohol in the presence of a strong acid catalyst (often concentrated sulfuric acid): acid + alcohol ⇌ ester + water. Esters are sweet- or fruit-smelling and are used in flavourings and solvents. Name pattern: alcohol part becomes -yl, acid part becomes -anoate (ethanol + ethanoic acid → ethyl ethanoate).
Quotations worth analysing
Short evidence. Real method.
“C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ yeast, anaerobic, ~30–40 °C”
Name yeast, no oxygen, and a moderate temperature. Too hot denatures the enzymes. This is not hydration of ethene.
“Carboxylic acids are weak acids: they ionise only partially in water.”
Partial ionisation explains a higher pH than hydrochloric acid of the same concentration. They still react with carbonates to give CO₂.
“carboxylic acid + alcohol ⇌ ester + water (acid catalyst)”
Reversible, concentrated sulfuric acid as catalyst and dehydrating agent. Ethyl ethanoate from ethanol and ethanoic acid is the named example to learn.
Go deeper
Two routes to ethanol, two evaluation tables
Fermentation uses a renewable sugar source, low temperature, and simple equipment, but it is a batch process, needs to be distilled to raise the alcohol concentration, and is slow. Yeast enzymes stop when the mixture becomes too alcoholic or when the temperature is wrong. Hydration of ethene is continuous, gives a high yield of pure ethanol, and is fast once the plant is running, but ethene comes from cracking crude oil, a finite resource, and the conditions are high temperature and pressure with a phosphoric acid catalyst. A compare question wants feedstock (sugar versus oil), conditions, type of process (batch versus continuous), and purity. Do not say fermentation is “natural so better” without a scientific point. Carbon dioxide from fermentation can be collected; burning either ethanol still produces CO₂.
Go deeper
Weak is not dilute, and –COOH is not an alcohol
Students circle the OH inside –COOH and call ethanoic acid an alcohol. The functional group is the whole carboxyl group, –COOH. Alcohols have –OH attached to a carbon chain that is not part of a carboxyl group. Weak acid means a small proportion of molecules donate H⁺ at any moment: CH₃COOH ⇌ CH₃COO⁻ + H⁺. Concentrated ethanoic acid can still be dangerous; concentrated describes moles per dm³, not ionisation. Reaction with a carbonate is the practical discriminator from an alcohol: acids fizz (CO₂), alcohols do not. Sodium will fizz with both (hydrogen), so that test does not separate them. Smell and indicator help, but the carbonate test is the one to write first.
Go deeper
Esters are named from both halves
The alcohol supplies the alkyl name (methyl, ethyl, propyl). The acid supplies the alkanoate name (methanoate, ethanoate, propanoate). Ethanol plus ethanoic acid, concentrated sulfuric acid, warm: ethyl ethanoate and water. The sulfuric acid is a catalyst and also absorbs water, pulling the equilibrium towards the ester. The product smells sweet, unlike vinegar or ethanol. Reverse the reaction (hydrolysis) and you get the acid and alcohol back. You do not need mechanism at GCSE. You do need the reversible equation, the catalyst, a named example, and a use (flavourings, perfumes, solvents). If displayed formulae are asked, keep –COOH on the acid and –OH on the alcohol, then show the ester as –COO– between the two carbon chains.
See the idea in action
A student warms ethanol with ethanoic acid and a few drops of concentrated sulfuric acid, then pours the mixture into water. A sweet-smelling oily layer of ethyl ethanoate forms. Equation: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O. The sulfuric acid is a catalyst. Separately, the same ethanoic acid fizzes with sodium carbonate: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂, and the gas turns limewater cloudy. Ethanol with carbonate does not fizz. Ethanol can instead be made from glucose and yeast without air: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.
Exam technique
Turn knowledge into marks
Name the functional group (–OH or –COOH) and give the first four members. For ethanol, match the method to the feedstock: sugar and yeast versus ethene and steam. For acids, say weak / partial ionisation and write the carbonate reaction. For esters, give both names, water, and the acid catalyst.
Common mistakes
Do not give these marks away
- 01
Calling –COOH an alcohol group, or treating weak acid as the same as dilute acid.
- 02
Writing fermentation conditions for hydration of ethene, or claiming hydration uses yeast.
- 03
Forgetting water as a product of esterification, or naming the ester from only one of the reactants.
Which conditions are used to make ethanol by fermentation?
ASteam, phosphoric acid catalyst, high pressure
BYeast, about 30–40 °C, no oxygen
CIron catalyst, 450 °C, 200 atmospheres
DNickel catalyst and hydrogen
Show the answer
Yeast, about 30–40 °C, no oxygen. Fermentation is anaerobic respiration of sugar by yeast at a moderate temperature. Steam and phosphoric acid are the conditions for hydrating ethene instead.
Quick questions
If this is the bit you searched
How is ethanol made in GCSE Chemistry?
By fermentation of glucose with yeast at about 30–40 °C without oxygen, or by hydration of ethene with steam and a phosphoric acid catalyst at high temperature and pressure.
Why are carboxylic acids described as weak acids?
They ionise only partially in water, so they produce a lower concentration of H⁺ than a strong acid of the same concentration. They still react with carbonates to give carbon dioxide.
How do you make an ester from ethanoic acid and ethanol?
Warm them with a concentrated sulfuric acid catalyst. The products are ethyl ethanoate and water. The ester has a sweet, fruity smell.
What is the functional group of an alcohol and of a carboxylic acid?
Alcohols have –OH. Carboxylic acids have –COOH. Do not confuse the OH inside the carboxyl group with an alcohol.