# Alcohols

> Edexcel International GCSE Chemistry · 4CH1 2017
> Source: https://www.owlsprep.com/study/edexcel-igcse-chemistry-s4-alcohols/

This guide covers all Edexcel IGCSE Chemistry (4CH1) Paper 2C content for alcohols, including functional group identification, structure drawing, ethanol oxidation pathways, and manufacture methods.

**Prerequisites:** [Knowledge of alkene functional groups and structure drawing](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s4-alkenes/); [Understanding of basic oxidation and reduction reactions](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s3-redox-reactions/)

## Learning objectives

- Identify the -OH hydroxyl functional group in alcohols
- Draw displayed and structural formulae for methanol, ethanol, propan-1-ol and butan-1-ol
- Describe the three oxidation pathways for ethanol
- Recall exact conditions for both ethanol manufacture methods
- Explain fermentation requirements for no air and optimum temperature

## Alcohol Functional Group & Nomenclature

**Alcohol** — A homologous series of organic compounds with the general formula $C_nH_{2n+1}OH$, 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. 2. Attach the -OH functional group to the first (end) carbon atom, as specified by the -1-ol suffix.
3. 3. Fill all remaining bonding slots on each carbon with hydrogen atoms, to ensure each carbon has 4 total bonds.
4. $$C_4H_9OH$$
5. 4. Acceptable shorthand name: butanol.

**Check your understanding**

1. What is the defining functional group of alcohols?

   - -COOH
   - -OH
   - -C=C-
   - -COO-

   *Why:* The hydroxyl (-OH) group is the unique functional group for all alcohols.

2. What is the molecular formula of methanol?

   - $CH_3OH$
   - $C_2H_5OH$
   - $C_3H_7OH$
   - $C_4H_9OH$

   *Why:* Methanol has 1 carbon atom, following the $C_nH_{2n+1}OH$ general formula.

*Calculator:* allowed

## Oxidation Reactions of Ethanol

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. 2. Visible change: Orange dichromate ions ($Cr_2O_7^{2-}$) turn green as they form chromium(III) ions ($Cr^{3+}$).
3. 3. Oxidation product: Ethanol is fully oxidised to ethanoic acid, a carboxylic acid.

> **tip**
>
> Complete combustion of any alcohol produces only carbon dioxide and water, just like alkanes and alkenes.

> **Exam tip:** Always specify that potassium dichromate(VI) is acidified with dilute sulfuric acid to score full marks.

## Ethanol Manufacture: Hydration of Ethene

**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 = $C_2H_4$, steam = $H_2O$
2. 2. Write product formula: Ethanol = $C_2H_5OH$
3. $$C_2H_4 + H_2O \rightarrow C_2H_5OH$$
4. 3. Advantage: This is a fast continuous process that produces very pure ethanol with no waste products.

## Ethanol Manufacture: Fermentation of Glucose

**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. 2. ~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.

## Comparison of Ethanol Manufacture Methods

| 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, $H_3PO_4$ catalyst | ~30 °C, 1 atm, no air, yeast |
| Product purity | Very pure ethanol | Dilute ethanol (requires distillation) |
| Reaction rate | Fast | Slow (takes days) |

**Check your understanding**

1. Which method produces a renewable ethanol product?

   - Hydration of ethene
   - Fermentation
   - Both
   - Neither

   *Why:* Fermentation uses glucose from regrowable crops, making it a renewable process.

2. What catalyst is used for ethene hydration?

   - Yeast
   - Iron
   - Phosphoric acid
   - Nickel

   *Why:* Phosphoric acid is the required catalyst for the industrial hydration reaction.

## Common pitfalls

- **Wrong:** Drawing the -OH group bonded to hydrogen instead of a carbon atom in the alkyl chain
  - Why it fails: The hydroxyl group is covalently attached to the carbon chain, not a free ion, so this structure is chemically incorrect.
  - Correct: Always attach the oxygen atom of the -OH group directly to a carbon atom in the alkyl chain.
- **Wrong:** Stating potassium dichromate(VI) oxidises ethanol without mentioning it is acidified with dilute sulfuric acid
  - Why it fails: Potassium dichromate only acts as an oxidising agent in acidic conditions, so this missing detail costs you marks.
  - Correct: Always refer to 'acidified potassium dichromate(VI)' or explicitly mention the dilute sulfuric acid.
- **Wrong:** Stating fermentation is done at 100 °C to speed up the reaction
  - Why it fails: Temperatures above ~40 °C denature yeast enzymes, stopping the reaction entirely.
  - Correct: Recall fermentation uses an optimum temperature of ~30 °C to maximise enzyme activity without denaturing.
- **Wrong:** Forgetting to specify that fermentation must be carried out in the absence of air
  - Why it fails: Oxygen in air oxidises ethanol to unwanted ethanoic acid, ruining the desired product.
  - Correct: Always state fermentation uses anaerobic (no air) conditions.
- **Wrong:** Drawing the -OH group on the second carbon for propanol or butanol
  - Why it fails: Only propan-1-ol and butan-1-ol are required for this specification; 2-ol isomers are out of scope.
  - Correct: Attach the -OH group to the first (end) carbon in the chain for propanol and butanol.

## Cheatsheet

| Concept | Key Recall Point |
| --- | --- |
| Functional group | -OH (hydroxyl) |
| Methanol formula | $CH_3OH$ |
| Ethanol formula | $C_2H_5OH$ |
| Propanol formula | $C_3H_7OH$ |
| Butanol formula | $C_4H_9OH$ |
| Ethanol combustion products | $CO_2 + H_2O$ |
| Ethanol oxidation product | Ethanoic acid |
| Dichromate colour change | Orange → Green |
| Hydration conditions | ~300°C, 60-70 atm, $H_3PO_4$ 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 |

## 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](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s4-alkenes/)

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