Alcohols

Classifying alcohols & their properties

Before any reaction, sort the alcohol. Look at the carbon carrying the –OH group and count how many other carbon atoms are attached to it:

This one label decides how — and whether — the alcohol oxidises later, so get quick at spotting it.

Alcohols also behave differently from hydrocarbons physically, because the polar O–H group lets them form hydrogen bonds. That raises their boiling points well above alkanes and alkenes of similar size, and makes short-chain alcohols soluble in water.

📌 Recap structural isomerism and hydrogen bonding — both come up in alcohol exam questions.

Making ethanol: two routes

Ethanol is made industrially two completely different ways, and AQA loves to make you compare them. One grows it; one builds it from crude-oil feedstock.

Hydration of ethene

Ethene from cracking is reacted with steam over a phosphoric acid (H₃PO₄) catalyst at about 300 °C and high pressure. The mechanism is electrophilic addition of water across the C=C — the same three arrows as the concentrated-sulfuric-acid route on the alkenes page, except that water itself is the nucleophile attacking the carbocation, so a final H⁺ is lost from the –OH₂⁺ intermediate. That is exactly the dehydration mechanism further down this page, run in reverse.

CH₂=CH₂(g) + H₂O(g) ⇌ C₂H₅OH(g)

It is continuous, fast, gives pure ethanol and has 100% atom economy — but the feedstock is non-renewable (crude oil) and it needs a lot of energy.

Fermentation of glucose

Sugars from plants are fermented by the enzymes in yeast, at about 35 °C, in the absence of air (anaerobic).

C₆H₁₂O₆(aq) → 2C₂H₅OH(aq) + 2CO₂(g)

The conditions are worth justifying: ~35 °C because the enzymes work too slowly when cold and denature when hot; no air because oxygen lets bacteria oxidise the ethanol to ethanoic acid (vinegar). It uses a renewable feedstock and little energy, but it is slow, runs in batches, and gives a dilute, impure mixture that must be concentrated by fractional distillation.

Bioethanol & carbon neutrality

A biofuel is a fuel made from recently living material (biomass). Ethanol from fermentation is a biofuel, and is often called carbon-neutral — meaning no net carbon dioxide is added to the atmosphere, because the CO₂ released when it burns was taken in by the plant during photosynthesis.

Follow the carbon through three equations — in one, out in two:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

2C₂H₅OH + 6O₂ → 4CO₂ + 6H₂O

Six CO₂ in; two out in fermentation plus four out in combustion — six out. On paper, neutral. In practice it is not truly carbon-neutral: the machinery, fertilisers, transport and the distillation itself are usually powered by fossil fuels, and land that could grow food is cleared to grow the fuel crop instead.

Oxidation of alcohols & Required Practical 5

This is the section the 1°/2°/3° label was for. The oxidising agent is acidified potassium dichromate(VI) (K₂Cr₂O₇ with dilute H₂SO₄). When it does its job it changes orange solution to green solution (the dichromate(VI) ion Cr₂O₇²⁻ is reduced to green Cr³⁺). In equations you may use [O] to represent oxygen from the oxidising agent.

Primary alcohols — and the distil-vs-reflux choice

A primary alcohol oxidises in two stages: first to an aldehyde, then to a carboxylic acid. You choose which you get by how you run it:

• Want the aldehyde? Distil it off as it forms, using a limited amount of oxidant — the aldehyde has the lowest boiling point (no hydrogen bonding), so it leaves before it can be oxidised further.
• Want the carboxylic acid? Reflux with excess oxidant — heating without losing anything keeps the product in the flask to be oxidised the whole way.

Secondary & tertiary alcohols

A secondary alcohol oxidises to a ketone, and stops there (there is no easy further oxidation).

A tertiary alcohol is not oxidised by acidified dichromate — there is no hydrogen on the carbon bearing the –OH to remove — so the mixture stays an orange solution.

Telling an aldehyde from a ketone

Both come from oxidising alcohols, and both contain C=O — but only the aldehyde is easily oxidised further, and two gentle reagents exploit that:

The aldehyde reduces the reagent as it is itself oxidised to a carboxylic acid. A ketone cannot be oxidised this way, so nothing happens.

Elimination: dehydrating alcohols to alkenes

Run the hydration reaction backwards and you dehydrate the alcohol: remove a molecule of water and form a C=C double bond. The reagent is a hot concentrated acid catalyst — concentrated H₂SO₄ or H₃PO₄, around 170 °C.

C₂H₅OH → CH₂=CH₂ + H₂O

It is an acid-catalysed elimination, and the mechanism is three moves:

When the alcohol is unsymmetrical, the H⁺ can be lost from carbons on either side of the carbocation, so you get a mixture of alkenes (including E/Z isomers). Pentan-2-ol, for example, gives pent-1-ene and pent-2-ene.

Why bother? These alkenes can be turned into addition polymers without using monomers made from crude oil — a renewable route into plastics.

🔗 Ethanolic KOH gives the other elimination route to an alkene — compare it on the halogenoalkanes page.

Same alcohol, different reactions

The exam trap on this topic is mixing up the reactions. Oxidation and dehydration start from the same alcohol but need different reagents and give different products — and “tertiary” is not the answer to every “why no reaction?”. Read the verb: oxidise, dehydrate, reflux, distil.

🔗 The aldehydes, ketones and carboxylic acids you just made are studied fully at A2 in Aldehydes & Ketones (3.3.8) and Carboxylic Acids (3.3.9). Next, learn the tests that identify them all in Organic Analysis (3.3.6).