4eAlcohols

Separate Chemistry only.

The functional group

The hydroxyl group, −OH.

That single group is what makes an alcohol an alcohol, and every alcohol's name ends in -ol.

One caution: this −OH is covalently bonded, not a hydroxide ion. Alcohols are not alkaline — an aqueous solution of ethanol is approximately neutral.

(pH is defined for aqueous solutions, so it is the solution that has a pH, not the neat liquid.)

The first four alcohols

The usual stems apply, with -ol replacing -ane. The −OH is written at the end of the chain.

Writing CH₄O for methanol gives the molecular formula but hides the functional group, so it does not answer a question asking for the structural formula.

Counting the carbons

C₃H₇OH is propanol, with three carbons — the ones in the C₃H₇ part. The OH adds no carbon.

A displayed formula

For ethanol, every bond is drawn: the C–C, all five C–H bonds, the C–O, and — importantly — the O–H.

Showing the OH as a single unit is not a displayed formula. The bond between the oxygen and its hydrogen must be drawn.

The oxygen forms two bonds: one to a carbon and one to a hydrogen. Each carbon still forms four.

Same group, same chemistry

Methanol, ethanol and propanol all contain −OH, so they all react in similar ways.

Their physical properties differ steadily, though — boiling point rises along the series as the molecules get longer and the intermolecular forces strengthen. Chemical similarity, physical trend: the pattern of every homologous series.

Separate Chemistry only.

Two ways to oxidise ethanol

Complete combustion — burning it in plenty of oxygen:

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

Only three O₂ molecules are needed, not four, because ethanol brings one oxygen atom of its own. Forgetting to count it makes the equation impossible to balance.

Microbial oxidation — a much gentler reaction with oxygen in the air, brought about by microorganisms. It stops at ethanoic acid.

That is why a bottle of wine left open turns sour: the ethanol becomes ethanoic acid, the acid in vinegar.

Two oxidations, two different products. Read which one a question is about.

Making ethanol from ethene

ethene + steam → ethanol

C₂H₄ + H₂O → C₂H₅OH

Conditions:

• Phosphoric acid catalyst
• About 300 °C
• High pressure, around 60–70 atmospheres

This is an addition reaction — water adds across the double bond, and there is a single product with no atoms left over.

Making ethanol by fermentation

Yeast converts sugar into ethanol and carbon dioxide, in the absence of air at about 30 °C.

Why exclude air? So the yeast respires anaerobically. Anaerobic respiration produces ethanol; with oxygen present the yeast would respire aerobically and give carbon dioxide and water instead. Excluding air also stops the ethanol being oxidised to ethanoic acid afterwards.

Why about 30 °C? Fermentation depends on enzymes in the yeast, which work best around that temperature. Much hotter and the enzymes are denatured and fermentation stops permanently; much colder and it is simply too slow.

That upper limit is biological, not chemical — which is why fermentation runs so much cooler than the industrial route.

Comparing the two methods

A comparison question wants advantages of both. And there is a real drawback to fermentation worth knowing: land used to grow the crop cannot be used to grow food.