Alcohols

Edexcel International GCSE Chemistry· 4.29C–4.33C· 12 min read

1. Alcohol Functional Group & Nomenclature★★☆☆☆⏱ 2 min

✓ Calculator OK

📘 Definition

Alcohol

A homologous series of organic compounds with the general formula $C_{n} H_{2 n + 1} O H$ , defined by the presence of the hydroxyl (-OH) functional group attached to an alkyl chain.

For this specification, you only need to recall straight-chain 1-ol isomers, where the -OH group is attached to the end carbon of the alkyl chain. You can use the unqualified names 'propanol' and 'butanol' instead of propan-1-ol and butan-1-ol in your answers.

📐 Worked Example

Draw the displayed formula for butan-1-ol, write its molecular formula, and state its acceptable shorthand name.

1
1. Draw a straight chain of 4 carbon atoms connected by single covalent bonds, as 'butan-' indicates 4 carbons.
2
1. Attach the -OH functional group to the first (end) carbon atom, as specified by the -1-ol suffix.
3
1. Fill all remaining bonding slots on each carbon with hydrogen atoms, to ensure each carbon has 4 total bonds.
4
$C_4H_9OH$
5
1. Acceptable shorthand name: butanol.

✓ Quick check

What is the defining functional group of alcohols?
- -COOH
- -OH
- -C=C-
- -COO-
Reveal answer
-OH —
The hydroxyl (-OH) group is the unique functional group for all alcohols.
What is the molecular formula of methanol?
- $C H_{3} O H$
- $C_{2} H_{5} O H$
- $C_{3} H_{7} O H$
- $C_{4} H_{9} O H$
Reveal answer
$CH_3OH$ —
Methanol has 1 carbon atom, following the $C_{n} H_{2 n + 1} O H$ general formula.

2. Oxidation Reactions of Ethanol★★★☆☆⏱ 3 min

Ethanol undergoes three key oxidation reactions you must recall for exams. All reactions (except incomplete combustion) produce ethanoic acid as the end product.

Complete combustion (burning) in air/oxygen to form carbon dioxide and water
Microbial oxidation by oxygen in air to form ethanoic acid (vinegar)
Heating with acidified potassium dichromate(VI) to form ethanoic acid

📐 Worked Example

Describe the visible change and product formed when ethanol is heated with dilute sulfuric acid and potassium dichromate(VI).

1
1. Identify the oxidising agent: Acidified potassium dichromate(VI) is reduced during the reaction.
2
1. Visible change: Orange dichromate ions ( $C r_{2} O_{7}^{2 -}$ ) turn green as they form chromium(III) ions ( $C r^{3 +}$ ).
3
1. Oxidation product: Ethanol is fully oxidised to ethanoic acid, a carboxylic acid.

Exam tip:

Always specify that potassium dichromate(VI) is acidified with dilute sulfuric acid to score full marks.

3. Ethanol Manufacture: Hydration of Ethene★★★☆☆⏱ 2 min

📘 Definition

Hydration of Ethene

Continuous industrial addition reaction where ethene reacts with steam to form pure ethanol, using high temperature, pressure and a catalyst.

You must recall the exact conditions for this reaction: phosphoric acid catalyst, ~300 °C temperature, 60–70 atm pressure. This process uses non-renewable ethene sourced from crude oil cracking as its raw material.

📐 Worked Example

Write the balanced symbol equation for the hydration of ethene to ethanol, and state one advantage of this method over fermentation.

1
1. Write reactant formulae: Ethene = $C_{2} H_{4}$ , steam = $H_{2} O$
2
1. Write product formula: Ethanol = $C_{2} H_{5} O H$
3
$C_2H_4 + H_2O \rightarrow C_2H_5OH$
4
1. Advantage: This is a fast continuous process that produces very pure ethanol with no waste products.

4. Ethanol Manufacture: Fermentation of Glucose★★★☆☆⏱ 2 min

📘 Definition

Fermentation

Anaerobic batch reaction where glucose from plant sources is broken down by yeast enzymes to form ethanol and carbon dioxide.

Required conditions for fermentation: ~30 °C optimum temperature, absence of air (anaerobic conditions), yeast enzymes as a biological catalyst. This process uses renewable glucose from crops like sugar cane or corn as its raw material.

📐 Worked Example

Explain why fermentation is carried out in the absence of air and at ~30 °C, instead of 100 °C.

1
1. No air: Oxygen in air would cause microbial oxidation of ethanol to unwanted ethanoic acid (vinegar), ruining the product.
2
1. ~30 °C is the optimum temperature for yeast enzymes: Lower temperatures slow the reaction, while temperatures above ~40 °C (like 100 °C) denature enzymes so they can no longer catalyse the reaction.

5. Comparison of Ethanol Manufacture Methods★★★★☆⏱ 3 min

Feature	Hydration of Ethene	Fermentation of Glucose
Raw material	Ethene (crude oil, non-renewable)	Glucose (crops, renewable)
Process type	Continuous 24/7 operation	Batch (stopped when product forms)
Conditions	~300 °C, 60-70 atm, $H_{3} P O_{4}$ catalyst	~30 °C, 1 atm, no air, yeast
Product purity	Very pure ethanol	Dilute ethanol (requires distillation)
Reaction rate	Fast	Slow (takes days)

✓ Quick check

Which method produces a renewable ethanol product?
- Hydration of ethene
- Fermentation
- Both
- Neither
Reveal answer
Fermentation —
Fermentation uses glucose from regrowable crops, making it a renewable process.
What catalyst is used for ethene hydration?
- Yeast
- Iron
- Phosphoric acid
- Nickel
Reveal answer
Phosphoric acid —
Phosphoric acid is the required catalyst for the industrial hydration reaction.

6. Common Pitfalls

Wrong move:

Drawing the -OH group bonded to hydrogen instead of a carbon atom in the alkyl chain

Why:

The hydroxyl group is covalently attached to the carbon chain, not a free ion, so this structure is chemically incorrect.

Correct move:

Always attach the oxygen atom of the -OH group directly to a carbon atom in the alkyl chain.

Wrong move:

Stating potassium dichromate(VI) oxidises ethanol without mentioning it is acidified with dilute sulfuric acid

Why:

Potassium dichromate only acts as an oxidising agent in acidic conditions, so this missing detail costs you marks.

Correct move:

Always refer to 'acidified potassium dichromate(VI)' or explicitly mention the dilute sulfuric acid.

Wrong move:

Stating fermentation is done at 100 °C to speed up the reaction

Why:

Temperatures above ~40 °C denature yeast enzymes, stopping the reaction entirely.

Correct move:

Recall fermentation uses an optimum temperature of ~30 °C to maximise enzyme activity without denaturing.

Wrong move:

Forgetting to specify that fermentation must be carried out in the absence of air

Why:

Oxygen in air oxidises ethanol to unwanted ethanoic acid, ruining the desired product.

Correct move:

Always state fermentation uses anaerobic (no air) conditions.

Wrong move:

Drawing the -OH group on the second carbon for propanol or butanol

Why:

Only propan-1-ol and butan-1-ol are required for this specification; 2-ol isomers are out of scope.

Correct move:

Attach the -OH group to the first (end) carbon in the chain for propanol and butanol.

7. Quick Reference Cheatsheet

Concept	Key Recall Point
Functional group	-OH (hydroxyl)
Methanol formula	$C H_{3} O H$
Ethanol formula	$C_{2} H_{5} O H$
Propanol formula	$C_{3} H_{7} O H$
Butanol formula	$C_{4} H_{9} O H$
Ethanol combustion products	$C O_{2} + H_{2} O$
Ethanol oxidation product	Ethanoic acid
Dichromate colour change	Orange → Green
Hydration conditions	~300°C, 60-70 atm, $H_{3} P O_{4}$ catalyst
Fermentation conditions	~30°C, no air, yeast catalyst
Fermentation no air reason	Prevents ethanol → ethanoic acid
Fermentation temp reason	Max enzyme activity, no denaturing

8. Frequently Asked

Do I need to learn propan-2-ol or butan-2-ol for this exam?

No, only propan-1-ol and butan-1-ol are required. The unqualified names 'propanol' and 'butanol' are acceptable answers referring to these 1-ol isomers.

What colour change occurs when ethanol is oxidised with acidified potassium dichromate(VI)?

The orange solution of dichromate ions turns green as they are reduced to chromium(III) ions during the oxidation of ethanol to ethanoic acid.

Going deeper

sub_topicAlkenes
sub_topicCarboxylic Acids
sub_topicEsters

What's Next

Now you have mastered alcohol content for Edexcel IGCSE Chemistry, move on to related organic chemistry topics that build directly on this knowledge. First, study carboxylic acids, the product of ethanol oxidation, including their functional group, structure and reactions. Next, learn about esters, which form when alcohols react with carboxylic acids, another high-frequency Paper 2C topic. You can also revisit alkenes to reinforce your understanding of the hydration reaction raw material, or practice organic synthesis questions that combine multiple functional group reactions to prepare for longer 4-6 mark exam questions.

→Alkenes