Alcohols

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 4.29 — Alcohols contain the –OH functional group (bold C statement — Paper 2 only)
• 4.30 — Structural and displayed formulae for methanol, ethanol, propan-1-ol and butan-1-ol (bold C statement — Paper 2 only)
• 4.31 — The three ways ethanol can be oxidised (bold C statement — Paper 2 only)

Specification section 4(e) Alcohols and (f) carboxylic acids — also taught in Alkenes, Alkene Reactions And The Bromine Test, Making Ethanol, Carboxylic Acids.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

The ALCOHOLS are the first family on this course that are not hydrocarbons. They contain an oxygen atom, in a group written –OH, and that single group is what defines them. Take an alkane chain, remove one hydrogen and put an –OH in its place, and you have the alcohol.

The –OH group is the functional group, so it is the seat of everything alcohols do. The rest of the molecule is an alkane chain and behaves like one: it burns, it gets less volatile as it lengthens, and it is otherwise unremarkable. Change the length of the chain and you change the physical properties; change the group on the end and you change the chemistry.

Building the first four alcohols

1. METHANOL: one carbon. CH₃OH — three hydrogens and an –OH on the single carbon.
2. ETHANOL: two carbons. CH₃CH₂OH, often written C₂H₅OH. The –OH sits on the end carbon.
3. PROPAN-1-OL: three carbons, with the –OH on carbon 1. CH₃CH₂CH₂OH.
4. BUTAN-1-OL: four carbons, with the –OH on carbon 1. CH₃CH₂CH₂CH₂OH.
5. The pattern is an alkyl chain with –OH on the end, which is why the general formula is written CₙH₂ₙ₊₁OH.
6. The number in the name states which carbon the –OH is attached to, exactly as the number in an alkene name states where the double bond begins.

One point of care with the formula. Ethanol can be written C₂H₆O, which is the molecular formula and is correct, but it is almost never useful, because it hides the –OH group. Written as CH₃CH₂OH or C₂H₅OH the functional group is visible, and the whole point of a structural formula is to show the part that reacts. Examiners want the form that shows the group.

The rest of this page is statement 4.31, which asks for three ways ethanol can be OXIDISED. They look completely unrelated — one is setting fire to it, one is leaving it out, one is boiling it with an orange solution — and it is worth seeing why they belong together before learning them separately.

Think of it like three ways of getting to the same floor

A lift, a staircase and a fire escape look nothing alike. One is fast and dramatic, one is slow and unremarkable, and one you only use deliberately. But all three take you to the same floor, and if you only knew about the lift you would think the building had one route through it. Complete combustion, souring in air and heating with an oxidising agent are three routes to the same destination: ethanol with more oxygen in it. What differs is the speed, the conditions and how far the journey goes.

The three oxidations of ethanol — statement 4.31

1. ONE: COMBUSTION. Burn ethanol in a good supply of air and it undergoes complete combustion to carbon dioxide and water. C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O. This is oxidation taken as far as it can go, and it releases a great deal of heat.
2. TWO: MICROBIAL OXIDATION. Leave ethanol exposed to the air and bacteria present in the air oxidise it to ETHANOIC ACID. This is slow, needs no equipment, and is exactly what happens to a bottle of wine left open — it turns to vinegar.
3. THREE: OXIDISING AGENT. Heat ethanol with potassium dichromate(VI) in dilute sulfuric acid, and it is oxidised to ethanoic acid deliberately and quickly.
4. In that third route the colour change is the observation to remember: the solution goes from ORANGE to GREEN as the dichromate is reduced.
5. Routes two and three give the SAME product, ethanoic acid. Route one goes further, all the way to carbon dioxide.

It is worth being precise about the word oxidation here. In each of these three the ethanol gains oxygen — that is the definition being used, and it is the one this course works with. Combustion adds oxygen until nothing carbon-containing is left but carbon dioxide. The other two stop at ethanoic acid, which has one oxygen more than ethanol did.

Route two matters more than it looks. It is the reason wine goes sour, the reason vinegar exists as a product at all, and the reason the fermentation described on the next page has to be carried out in the absence of air. If you let air at the ethanol you have just made, something will oxidise it for you.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Alcohol

A member of the homologous series whose functional group is –OH, with the general formula CₙH₂ₙ₊₁OH.

CₙH₂ₙ₊₁OH

n is the number of carbon atoms in the chain carrying the –OH group

Units: No units — a general formula relating atom counts, not a measured quantity

The four alcohols the specification names

The three oxidations of ethanol — statement 4.31

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

Conditions: Complete combustion of ethanol in a good supply of oxygen. Oxidation taken as far as it can go.

C₂H₅OH + [O] → CH₃COOH + H₂O

Conditions: Oxidation to ethanoic acid, either by bacteria in the air over time, or deliberately by heating with potassium dichromate(VI) in dilute sulfuric acid.

The words this page is marked on

–OH

The functional group of the alcohols, sometimes called the hydroxyl group.

Oxidation

Gain of oxygen. Ethanol gains oxygen in all three routes on this page.

Microbial oxidation

Oxidation carried out by bacteria present in the air, turning ethanol into ethanoic acid over time.

Potassium dichromate(VI)

The oxidising agent used with dilute sulfuric acid. Orange when it starts, green when it has been reduced.

Model answer [5 marks]

Describe the three ways in which ethanol can be oxidised, giving the product of each. [5]

First, by combustion: burning ethanol in a good supply of air oxidises it completely to carbon dioxide and water. Second, by microbial oxidation: bacteria present in the air slowly oxidise ethanol left exposed to the air, converting it to ethanoic acid, which is why wine left open turns to vinegar. Third, by an oxidising agent: heating ethanol with potassium dichromate(VI) in dilute sulfuric acid oxidises it to ethanoic acid deliberately, and the solution changes from orange to green as the dichromate is reduced. The second and third routes give the same product; the first goes further.

Model answer [3 marks]

State what is observed when ethanol is heated with potassium dichromate(VI) in dilute sulfuric acid, and name the organic product. [3]

The solution changes colour from orange to green, because the potassium dichromate(VI) is reduced as it oxidises the ethanol. The organic product formed is ethanoic acid, CH₃COOH. This is the same product that bacteria in the air would eventually give, but the oxidising agent achieves it quickly and deliberately rather than over days of exposure.

Things about alcohols that are frequently examined

• An alcohol is NOT a hydrocarbon — it contains oxygen
• Write ethanol as CH₃CH₂OH or C₂H₅OH, so the –OH group is visible
• The number in the name says which carbon carries the –OH group
• All three oxidation routes add oxygen; two of them stop at ethanoic acid
• Orange to green is the colour change for the dichromate route

Not this: Ethanol is a hydrocarbon, since it is made of carbon and hydrogen chains.

This: It contains an oxygen atom in its –OH group, and a hydrocarbon contains hydrogen and carbon ONLY. Ethanol is an organic compound but not a hydrocarbon, and writing C₂H₆O rather than C₂H₅OH is what hides this.

Mark-losing trap. Orange to GREEN for the dichromate oxidation. Orange to colourless is the bromine-water test, and belongs to alkenes.

Mark-losing trap. Two of the three oxidations give ethanoic acid. Only combustion goes all the way to carbon dioxide.

Mark-losing trap. Write C₂H₅OH, not C₂H₆O. The molecular formula is correct but hides the group that reacts.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · Identify [1 mark] — What is the functional group of the alcohols?
2. Grade 7 · State [2 marks] — What colour change is seen when ethanol is oxidised by heating with potassium dichromate(VI) in dilute sulfuric acid?
3. Grade 8 · Explain [3 marks] — Explain why a bottle of wine left open for several days develops a sour taste.
4. Grade 9 · Compare [4 marks] — Select every statement that belongs in a full comparison of the three ways ethanol can be oxidised.
5. 9+ · Determine [6 marks] — An alcohol with the –OH group on the end carbon has a relative molecular mass of 74. Determine the number of carbon atoms in one molecule. (Aᵣ: H = 1, C = 12, O = 16)

The people behind this science

Two ways into the same idea — the one who showed a group of atoms can travel through reactions as a unit, and the one who defined the word that makes three unlike processes into one. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Friedrich Wöhler — the one who showed a group of atoms can travel through reactions as a unit

The whole idea of a functional group — that one part of a molecule can be picked out as the part that reacts, while the rest merely comes along — had to be established, and it was established by Wöhler and Liebig. In 1832 they showed that a particular group of atoms, which they called the benzoyl radical, passed unchanged through a whole series of different reactions, appearing intact in every product. Berzelius called it the beginning of a new day in chemistry. That is the direct ancestor of saying that –OH is what makes an alcohol an alcohol, whatever chain it is attached to.

• “Can a group of atoms really pass through a reaction unchanged?”
• “How did you show the same group appeared in every product?”
• “Why does it matter which part of a molecule reacts?”
• “What made Berzelius so excited by that result?”
• “How do you decide where one part of a molecule ends?”

Antoine Lavoisier — the one who defined the word that makes three unlike processes into one

This page claims that setting ethanol alight, leaving it open to the air for a week, and boiling it with an orange solution are three versions of the same thing. That claim only makes sense because of Lavoisier, who identified oxygen, named it, and established that combining with it is a single kind of change whether it happens in a flame or imperceptibly over time. Before him, burning and slow tarnishing had no reason to be classed together at all. He is the right figure to ask why speed makes no difference to what a reaction IS.

• “Is a slow change and a fierce fire really the same reaction?”
• “How did you show that burning takes something from the air?”
• “What made you sure it was one substance in air and not the whole of it?”
• “Why should a chemical name describe what a substance does?”
• “Can you tell oxidation is happening if you see nothing at all?”

Carry on through the course

• Previous lesson: Alkene Reactions And The Bromine Test
• Next lesson: Making Ethanol
• What Organic Chemistry Is — Two facts about carbon generate several million compounds — and three words keep them in order.
• Naming, Drawing And Classifying — A name that tells you the structure, a method that finds every isomer, and three words that cover every organic reaction here.
• Crude Oil And Its Fractions — One mixture, separated by boiling point into six useful materials — and one causal chain that explains every trend down the column.
• Burning Fuels — Two ways a hydrocarbon can burn, and why one of them fills a room with a gas you cannot smell.
• Pollutants And Acid Rain — One pollutant comes out of the fuel and one comes out of the air — and telling them apart decides which fix works.
• All of Chemistry · Chemistry key terms · required practicals · equations · Incandio Science · The 4CH1 specification