Alcohols

This page covers content standard 10.6 of the Form 5 Carbon Compounds chapter: alcohols. After the hydrocarbons (alkanes and alkenes), alcohols are the first family whose functional group contains oxygen, and the reactions of ethanol appear again and again in the exam. Learn the functional group, how ethanol is made, and its four key reactions, and this standard becomes very predictable.

What are alcohols?

Alcohols are a homologous series whose functional group is the hydroxyl group, –OH. Their general formula is CₙH₂ₙ₊₁OH. Because every member has the same –OH group, the whole family shares similar chemical properties, while physical properties change gradually down the series.

The members. methanol (CH₃OH), ethanol (C₂H₅OH), propanol (C₃H₇OH) and butanol (C₄H₉OH). The first two are the ones you must know in detail; ethanol is by far the most examined.

Name	Molecular formula
Methanol	CH₃OH
Ethanol	C₂H₅OH
Propanol	C₃H₇OH
Butanol	C₄H₉OH

Alcohols are liquids at room temperature (for the first members), mix well with water because the –OH group forms hydrogen bonds with water, and their boiling points rise down the series as the molecules get larger.

Making ethanol

There are two routes you must know:

• Fermentation. Glucose from starchy or sugary material is broken down by enzymes in yeast, in the absence of air, at about 37 °C, to give ethanol and carbon dioxide: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. This is a slow, renewable, batch process.
• Hydration of ethene. Ethene from cracking petroleum is reacted with steam using a phosphoric acid catalyst: C₂H₄ + H₂O → C₂H₅OH. This is a fast, continuous, industrial process.

The reactions of ethanol

There are four reactions of ethanol you should be able to write and explain.

• Combustion. Ethanol burns completely in plenty of air to give carbon dioxide and water, releasing heat: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O. This is why ethanol is used as a fuel.
• Oxidation to a carboxylic acid. Warming ethanol with an oxidising agent, acidified potassium dichromate(VI) (orange) or acidified potassium manganate(VII) (purple), oxidises it to ethanoic acid. The dichromate turns from orange to green; the manganate is decolourised from purple.
• Dehydration to an alkene. Passing ethanol vapour over hot aluminium oxide, or heating it with excess concentrated sulfuric acid, removes water and forms ethene: C₂H₅OH → C₂H₄ + H₂O. Dehydration is the reverse of hydration.
• Esterification. Ethanol reacts with a carboxylic acid (with a concentrated sulfuric acid catalyst) to form a sweet-smelling ester, covered fully in the esters standard.

Uses of alcohols

Ethanol is used as a solvent (in inks, perfumes and medicines), as a fuel and fuel additive, as an antiseptic, and in alcoholic beverages. Methanol is an industrial solvent and feedstock. You should be able to link each use to a property, for example, ethanol is a good solvent because it dissolves both water-soluble and many non-polar substances.

Worked example

Question. Glucose, C₆H₁₂O₆, undergoes complete fermentation. Calculate the mass of ethanol produced from 90 g of glucose. [Relative atomic masses: H = 1, C = 12, O = 16]

Step 1, Write the balanced equation.

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Step 2, Find the molar masses. Glucose: (6 × 12) + (12 × 1) + (6 × 16) = 72 + 12 + 96 = 180 g mol⁻¹. Ethanol: (2 × 12) + (6 × 1) + 16 = 24 + 6 + 16 = 46 g mol⁻¹.

Step 3, Find the moles of glucose.

moles = mass ÷ molar mass = 90 g ÷ 180 g mol⁻¹ = 0.5 mol

Step 4, Use the mole ratio. From the equation, 1 mol glucose → 2 mol ethanol, so 0.5 mol glucose → 1.0 mol ethanol.

Step 5, Find the mass of ethanol.

mass = moles × molar mass = 1.0 mol × 46 g mol⁻¹ = 46 g

Answer. 90 g of glucose produces 46 g of ethanol.

Practice question

Question. Ethanol is warmed with acidified potassium dichromate(VI) solution. State the colour change you would observe and name the organic product formed.

Answer. The orange acidified potassium dichromate(VI) turns green. Ethanol is oxidised to ethanoic acid (a carboxylic acid), CH₃COOH. The colour change from orange to green is the observation that shows oxidation has taken place.

Exam tip

Keep ethanol’s four reactions separate and label them correctly: combustion (with oxygen → CO₂ + H₂O), oxidation (with an oxidising agent → ethanoic acid), dehydration (removing water → ethene), and esterification (with an acid → ester). The two colour changes are reliable marks: acidified dichromate goes orange → green, acidified manganate(VII) goes purple → colourless. For fermentation, always give the three conditions, yeast, no air, about 37 °C, and remember that a temperature too high would kill the yeast. State symbol and balancing errors in the combustion equation are the most common way to drop an easy mark, so always check the oxygen count.

Telling combustion apart from oxidation

Many students confuse “combustion” with “oxidation”, even though both add oxygen. In combustion, ethanol is burned in air and the whole molecule is broken down to carbon dioxide and water, releasing a large amount of heat, this is the reaction used when ethanol is a fuel. In oxidation by an oxidising agent such as acidified potassium dichromate(VI), only the –OH group is changed: ethanol is converted to ethanoic acid, and the carbon skeleton stays intact. The key to telling them apart is the product: combustion gives CO₂ + H₂O, while oxidation gives ethanoic acid. Writing the correct product for each is the safest way to score full marks when a question tests these two reactions side by side.

Where this fits

This is content standard 10.6 of the Carbon Compounds chapter. It links back to alkenes (hydration of ethene makes ethanol) and forward to carboxylic acids (oxidation of ethanol makes ethanoic acid) and esters. Practise the combustion and fermentation calculations in the chapter worked examples, and check the colour changes on the common mistakes page. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers make sure you can name and write every reaction of ethanol without hesitation. The reactions of ethanol appear so often across SPM Chemistry that securing them now rewards you throughout the paper.

Quick recap

• Alcohols: homologous series, functional group –OH, general formula CₙH₂ₙ₊₁OH.
• Members: methanol, ethanol, propanol, butanol.
• Ethanol is made by fermentation (yeast, no air, ~37 °C) or hydration of ethene (steam + H₃PO₄).
• Reactions of ethanol: combustion, oxidation → ethanoic acid, dehydration → ethene, esterification.
• Colour tests: dichromate orange → green; manganate(VII) purple → colourless.