Alcohols

Alcohol structures from C1 to C4, ethanol production, combustion, and oxidation to ethanoic acid at the required K324 depth.

SEC G3 Pure Chemistry 2027

On this page

Learning objectives

describe the alcohols as a homologous series containing the –OH group
draw the structures of branched and unbranched alcohols, C1 to C4, and name the unbranched alcohols methanol to butanol
describe the reactions of alcohols in terms of combustion and oxidation to carboxylic acids
describe the formation of ethanol by the catalysed addition of steam to ethene and by fermentation of glucose
describe the formation of ethanoic acid by the oxidation of ethanol by atmospheric oxygen or acidified potassium manganate(VII).

Start by recognising the -OH group and drawing alcohol structures. Then use ethanol to learn production, combustion and oxidation reactions.

1. Definition

A. Alcohol

Alcohols are a homologous series containing the hydroxyl functional group, -OH. In an alcohol, this group is bonded to a carbon atom.

B. Hydroxyl group vs hydroxide ion (common trap)

The hydroxyl group, -OH, is covalently bonded in an alcohol molecule. It is not the hydroxide ion, OH⁻. So alcohols are not alkalis.

2. Key Ideas

General formula: CₙH₂ₙ₊₁OH.
Draw branched and unbranched alcohol structures containing one to four carbon atoms; learn the names of the unbranched series from methanol to butanol.
Alcohols are neutral (not alkalis) because -OH is covalent, not OH⁻ ions.
Ethanol can be made by:
- fermentation (renewable, slower, dilute ethanol), or
- hydration of ethene (fast, continuous, non-renewable ethene).
Complete combustion of ethanol gives CO₂ and H₂O.
Ethanol can be oxidised to ethanoic acid by atmospheric oxygen or acidified potassium manganate(VII).

Alkenes (hydration source): Alkenes
Fuels + fermentation context: Fuels and crude oil
Separating ethanol (distillation): Separation techniques

3. Detailed Explanations

Alcohols contain the hydroxyl group, -OH; general formula CₙH₂ₙ₊₁OH.
-OH (hydroxyl group) is not OH⁻ (hydroxide ion), so alcohols are neutral (not alkalis).
Fermentation: glucose + yeast under warm, oxygen-free conditions → ethanol + CO₂.
Hydration: ethene + steam at high temperature and pressure with a phosphoric(V) acid catalyst → ethanol.
Oxidation: ethanol → ethanoic acid using atmospheric oxygen or acidified KMnO₄ (purple → colourless).

A. General formula and examples

Alcohols have the general formula CₙH₂ₙ₊₁OH.

Name	Molecular formula	One-line structural formula
methanol	CH₃OH	CH₃-OH
ethanol	C₂H₅OH	CH₃-CH₂-OH
propanol	C₃H₇OH	CH₃-CH₂-CH₂-OH
butanol (butan-1-ol)	C₄H₉OH	CH₃-CH₂-CH₂-CH₂-OH

Only the unbranched names above are required. You must also be able to draw and recognise alternative structures without assuming that -OH is always on an end carbon.

Molecular formula	Alternative structural formula	What changed?
C₃H₇OH	CH₃-CH(OH)-CH₃	the -OH group is on the middle carbon
C₄H₉OH	CH₃-CH(OH)-CH₂-CH₃	the -OH group is on carbon 2
C₄H₉OH	(CH₃)₂CH-CH₂-OH	the carbon chain is branched
C₄H₉OH	(CH₃)₃C-OH	branched chain with -OH on the central carbon

Count every carbon and hydrogen, include one -OH group, and make sure each carbon has four bonds. Different page orientations do not create different structures.

B. Manufacture of ethanol: fermentation vs hydration

Method	Raw material	Conditions	Renewable?	Typical product
fermentation	glucose solution	yeast; warm; no oxygen	yes	dilute ethanol + CO₂
hydration	ethene + steam	high temperature and pressure; H₃PO₄ catalyst	no (ethene from crude oil)	ethanol (continuous process)

Fermentation equation

C₆H₁₂O₆(aq) → [yeast] 2C₂H₅OH(aq) + 2CO₂(g)

Use warm conditions. Excessive heat denatures yeast enzymes, while low temperature makes fermentation slow.
No oxygen: otherwise yeast respire aerobically to form CO₂ and H₂O (less ethanol).

Hydration equation

C₂H₄(g) + H₂O(g) → [high\ temperature,\ high\ pressure][H₃PO₄] C₂H₅OH(l)

C. Combustion (ethanol as a fuel)

Complete combustion of ethanol: C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l)

D. Oxidation (ethanol → ethanoic acid)

Ethanol can be oxidised to ethanoic acid in two required ways.

Atmospheric oxygen: C₂H₅OH(l) + O₂(g) → CH₃COOH(aq) + H₂O(l)

Acidified potassium manganate(VII): the purple solution becomes colourless as it oxidises ethanol.

Exam-safe equation using [O]: C₂H₅OH(l) + 2[O] → CH₃COOH(aq) + H₂O(l)

Acidified KMnO₄: purple → colourless.

4. Common Mistakes

Calling alcohols “alkalis” because they contain -OH (wrong: that is a covalent group, not OH⁻ ions).
Writing fermentation “needs oxygen” (wrong: oxygen reduces ethanol yield).
Using boiling conditions for fermentation; excessive heat denatures yeast enzymes.
Memorising exact hydration temperatures and pressures instead of the required ideas: high temperature, high pressure and catalyst.
Forgetting that atmospheric oxygen is also a route from ethanol to ethanoic acid.

5. Exam Tips

Write: “yeast”, “warm”, “anaerobic/no oxygen”, “glucose → ethanol + CO₂”.

Write: “ethene + steam”, “high temperature and pressure”, and “H₃PO₄ catalyst”. Exact numerical values are not required.

6. Worked Examples

Problem

Why must air be excluded from a vessel used to ferment glucose into ethanol?

Study the worked solution

Identify the condition being controlled
Method
Keep the vessel anaerobic by excluding oxygen in air.
Reason
Yeast produces ethanol from glucose under anaerobic fermentation conditions.
Working
No oxygen → conditions favour ethanol fermentation rather than aerobic respiration.
Link the condition to yield
Method
Explain that oxygen can reduce the recovered ethanol yield.
Reason
With oxygen, yeast can respire aerobically to form CO₂ and H₂O; exposure also permits bacterial oxidation of ethanol to ethanoic acid.
Working
Exclude air to maintain anaerobic fermentation and limit loss of ethanol.

Problem

A factory needs a fast, continuous supply of ethanol. Choose fermentation or hydration and justify the choice by comparison.

Match the process to the factory priorities

Chosen process

Hydration of etheneFermentation of glucose

Relevant comparison

Continuous; less separation neededRenewable; batch process

Hints

Hint 1: priority words

Underline “fast” and “continuous”; renewable feedstock is not the stated priority.

Hint 2: product stream

Fermentation is slower and produces dilute ethanol that needs separation.

View solution step by step

Choose hydration
Method
Select hydration of ethene.
Reason
Hydration can operate continuously and produces ethanol quickly.
Working
Factory requirement → hydration.
Justify by comparison
Method
Contrast hydration with fermentation.
Reason
Fermentation is slower, runs as a batch process and gives dilute ethanol requiring further separation.
Working
Choose hydration for fast, continuous production; fermentation does not match those stated priorities.

Learner equation

A learner writes

C₂H₅OH + 4O₂ → 2CO₂ + 3H₂O for complete combustion of ethanol. Correct the oxygen coefficient and explain the counting error.

View solution step by step

Balance carbon and hydrogen
Method
Use two carbon dioxide and three water molecules.
Reason
Ethanol contains two C atoms and six H atoms.
Working
C₂H₅OH + O₂ → 2CO₂ + 3H₂O.
Count every oxygen source
Method
Use three O₂ molecules.
Reason
The products contain seven O atoms; ethanol already supplies one, so O₂ must supply the remaining six.
Working
C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l).

Examination question

Draw two different alcohol structures with molecular formula

C₄H₉OH: one with an unbranched carbon chain and one with a branched carbon chain. [4 marks]

Draw and check both structures

Draw both structures and show your atom-count check

View solution step by step

Draw an unbranched structure
2 marks
Method
Use a continuous four-carbon chain and attach one -OH group.
Reason
The structure must contain four carbon atoms and one hydroxyl group.
Working
CH₃-CH₂-CH₂-CH₂-OH.
Draw a branched structure
2 marks
Method
Use a three-carbon chain with a one-carbon branch and attach one -OH group.
Reason
This changes the carbon skeleton without changing the molecular formula.
Working
(CH₃)₂CH-CH₂-OH.

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 the carbon skeleton, hydroxyl group and atom count in each structure.

Mark points

1 mark: Draws a valid unbranched four-carbon skeleton.
1 mark: Includes one correctly bonded hydroxyl group in the unbranched structure.
1 mark: Draws a valid branched four-carbon skeleton.
1 mark: Includes one correctly bonded hydroxyl group in the branched structure.

Reaction transfer

State two ways to oxidise ethanol to ethanoic acid. For the reagent method, give the expected colour change.

Connect both routes to the same product

Hints

Hint 1: two oxidants

One route uses oxygen in air; the other uses the prescribed purple oxidising agent.

Hint 2: organic change

Both routes follow ethanol

→ ethanoic acid.

View solution step by step

State the atmospheric route
Method
Use atmospheric oxygen to oxidise ethanol.
Reason
Oxygen from air converts ethanol into ethanoic acid.
Working
C₂H₅OH + O₂ → CH₃COOH + H₂O.
State the reagent route
Method
Use acidified potassium manganate(VII) and record purple to colourless.
Reason
The reagent is reduced while oxidising ethanol to the same organic product.
Working
C₂H₅OH + 2[O] → CH₃COOH + H₂O; product: ethanoic acid.

Next step

Try Fix the alkali mistake independently.

7. Mind Stretchers

Mind stretcher 1: Fix the alkali mistakeExtension

Question: A student says: “Ethanol is an alkali because it contains -OH.” Explain why this is wrong.

Show Answer

In ethanol, the -OH is a covalently bonded hydroxyl group, not hydroxide ions, OH⁻.

Alkalis produce OH⁻ ions in aqueous solution; ethanol does not.

Final: Ethanol is not an alkali because it does not dissociate to give OH⁻ ions.

Mind stretcher 2: Fermentation failure analysisExtension

Question: A student ferments glucose at 60 °C and gets very little ethanol. Give the most likely reason.

Show Answer

60 °C is too hot for yeast enzymes, so they denature and the fermentation slows/stops.

Final: Temperature too high; yeast enzymes denature.

8. Quiz

Test definitions, fermentation vs hydration, combustion products, and oxidation observations.

Practice next

Lesson quizCheck this lesson quickly with targeted MCQs.Structured practiceSwitch to exam-style written responses after the quiz.

Alcohols

Continue where you stopped

Learning objectives

1. Definition

A. Alcohol

B. Hydroxyl group vs hydroxide ion (common trap)

2. Key Ideas

3. Detailed Explanations

A. General formula and examples

B. Manufacture of ethanol: fermentation vs hydration

Fermentation equation

Hydration equation

C. Combustion (ethanol as a fuel)

D. Oxidation (ethanol → ethanoic acid)

4. Common Mistakes

5. Exam Tips

6. Worked Examples

Oxygen exclusion (fermentation)

Identify the condition being controlled

Link the condition to yield

Compare production methods

Choose hydration

Justify by comparison

Combustion equation

Balance carbon and hydrogen

Count every oxygen source

Draw unbranched and branched alcohols

Draw an unbranched structure

Draw a branched structure

Two routes to ethanoic acid

State the atmospheric route

State the reagent route

7. Mind Stretchers

Mind stretcher 1: Fix the alkali mistakeExtension

Mind stretcher 2: Fermentation failure analysisExtension

8. Quiz