HookThe petrol pump is one-tenth brewery
Since September 2021, standard unleaded petrol at UK forecourts has been E10: up to a tenth of what goes into the tank is ethanol, most of it fermented from wheat and other starch crops at plants such as those on Teesside and in Hull. The very same molecule is also made by the petrochemical industry — ethene and steam pushed over a phosphoric acid catalyst at about 300 °C — for solvents, inks and pharmaceutical manufacture. One product, two completely different factories: a warm, slow, living batch process fed by fields, and a hot, fast, continuous plant fed by crude oil. AQA's favourite 3.3.5 question is simply: which would you build, and why?
That comparison is the first of four jobs in this section. Produce alcohols — fermentation against hydration, including the honest accounting behind calling bioethanol 'carbon-neutral'. Classify them — primary, secondary, tertiary — because classification decides everything that follows. Oxidise them with acidified potassium dichromate(VI), where the choice between distillation and reflux decides whether you stop at the aldehyde or drive on to the carboxylic acid. And eliminate water from them, running addition in reverse to regenerate alkenes — which, chained to 3.3.4, lets industry make poly(ethene) from sugar instead of oil. Required practical 5 threads through all of it, because the skill being assessed is separation: getting a volatile product cleanly out of a reacting mixture.
ModelPrimary, secondary, tertiary — read the carbon, not the OH
Every alcohol carries the same functional group, –OH, so the differences between alcohols cannot live there. They live one atom in: on the carbon bonded to the OH. Count the alkyl groups attached to that carbon. One carbon neighbour (or none, for methanol) makes a primary alcohol — butan-1-ol, ethanol. Two make it secondary — butan-2-ol, propan-2-ol. Three make it tertiary — 2-methylpropan-2-ol, where the OH sits on a carbon already crowded by three methyl-bearing bonds.
The classification looks like bookkeeping until you see what it controls. Oxidation by acidified dichromate needs to remove a hydrogen atom from the OH and a hydrogen from the carbon holding it. A primary alcohol has two such hydrogens — it can be oxidised twice, first to an aldehyde, then onwards to a carboxylic acid. A secondary alcohol has one — a single oxidation to a ketone, then the road ends. A tertiary alcohol has none — the carbon bonded to OH carries no hydrogen at all, so mild oxidation simply cannot start. Three classes, three different fates in the same orange solution: that is the entire logic of 3.3.5.2, and it is why examiners so often begin a question with 'classify this alcohol'.
CaseTwo factories, one molecule — fermentation against hydration
Fermentation is biology doing the chemistry: yeast enzymes convert aqueous glucose to ethanol and carbon dioxide, \(\text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 2\text{CH}_3\text{CH}_2\text{OH} + 2\text{CO}_2\), at around 35 °C — warm enough for a workable rate, cool enough not to denature the enzymes — with air excluded, because oxygen lets the ethanol oxidise onwards towards ethanoic acid. It is a batch process: slow, low-tech, cheap to build, fed by a renewable crop, and it stops itself when the rising ethanol concentration kills the yeast at roughly 15%. The product is a dilute aqueous soup that needs fractional distillation before it is useful as fuel.
Hydration of ethene is the industrial mirror image: \(\text{CH}_2=\text{CH}_2 + \text{H}_2\text{O} \rightleftharpoons \text{CH}_3\text{CH}_2\text{OH}\), steam and ethene over a phosphoric acid catalyst at about 300 °C and 60–70 atmospheres. It is continuous, fast, and delivers essentially pure ethanol; only a few per cent converts on each pass, but the unreacted ethene is recycled until the overall conversion approaches 95%. The costs sit elsewhere: an expensive high-pressure plant, serious energy demand, and a feedstock cracked from finite crude oil.
The green arithmetic favours each route on different lines. Hydration has an atom economy of 100% — one product, nothing wasted — while fermentation's is 51%, because for every glucose two CO₂ molecules leave as by-product. But fermentation's feedstock regrows every summer. AQA wants you to weigh rate, purity, process type, energy and feedstock — and to resist crowning either route the winner without saying for what purpose.
Atom economy of fermentation: useful mass = 2 × 46.0 = 92.0 of ethanol per mole of glucose (Mr = 180.0), so \(92.0 \div 180.0 = 0.511\) — 51.1%. Now the carbon-neutral claim for bioethanol as fuel. Growing the crop fixes six CO₂ by photosynthesis: \(6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2\). Fermentation releases two: \(\text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 2\text{CH}_3\text{CH}_2\text{OH} + 2\text{CO}_2\). Burning both ethanol molecules releases four: \(\text{CH}_3\text{CH}_2\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}\), twice over. Absorbed 6, released 2 + 4 = 6: net zero, so the equations genuinely balance. The evaluation mark is the honest caveat — tractors, fertiliser manufacture, processing and distillation all burn fossil fuels, so real bioethanol is only ever carbon-reduced, not carbon-neutral.
ModelThe oxidation ladder — dichromate, then choose your apparatus
The reagent for the whole ladder is acidified potassium dichromate(VI) — K₂Cr₂O₇ with dilute sulfuric acid — and its signature is the colour change from orange to green as Cr₂O₇²⁻ is reduced to Cr³⁺. In equations, AQA lets you write the oxidant as [O], and the equations must still balance: ethanol to ethanal is \(\text{CH}_3\text{CH}_2\text{OH} + [\text{O}] \rightarrow \text{CH}_3\text{CHO} + \text{H}_2\text{O}\); ethanal onwards is \(\text{CH}_3\text{CHO} + [\text{O}] \rightarrow \text{CH}_3\text{COOH}\); propan-2-ol to propanone is \(\text{CH}_3\text{CH(OH)CH}_3 + [\text{O}] \rightarrow \text{CH}_3\text{COCH}_3 + \text{H}_2\text{O}\). Note which steps make water and which do not — a balancing slip here is the most common lost mark in the topic.
For a primary alcohol the product is a choice, and the apparatus is how you choose. Want the aldehyde? Use dilute dichromate, warm gently, and distil the product out as it forms: ethanal boils at 21 °C against ethanol's 78 °C, so it escapes the hot mixture before the oxidant can touch it again. Want the carboxylic acid? Turn the logic around: excess oxidant and reflux — continuous boiling with a vertical condenser returning every vapour to the flask — so nothing escapes until the oxidation has run to the end. Secondary alcohols need no such care: reflux with dichromate gives the ketone, and mild oxidation stops there. Tertiary alcohols do nothing at all — the solution stays orange — because the carbon bearing the OH has no hydrogen to give up.
You are asked to convert propan-1-ol to each of its two oxidation products. To propanal: add dilute acidified potassium dichromate(VI) dropwise to warm propan-1-ol in distillation apparatus and collect the distillate immediately — propanal boils at 49 °C, well below propan-1-ol's 97 °C, so it leaves before further oxidation: \(\text{CH}_3\text{CH}_2\text{CH}_2\text{OH} + [\text{O}] \rightarrow \text{CH}_3\text{CH}_2\text{CHO} + \text{H}_2\text{O}\). To propanoic acid: reflux with an excess of the same oxidant, then distil off the product afterwards: \(\text{CH}_3\text{CH}_2\text{CH}_2\text{OH} + 2[\text{O}] \rightarrow \text{CH}_3\text{CH}_2\text{COOH} + \text{H}_2\text{O}\). One reagent, two products, and the marks are for the words in bold: dilute versus excess, distil versus reflux, and orange to green in both runs.
MechanismDehydration — running the addition film backwards
Heat an alcohol with concentrated sulfuric acid or concentrated phosphoric acid and it loses water to regenerate an alkene: acid-catalysed elimination. The mechanism is three moves. First, a lone pair on the OH oxygen picks up H⁺ from the acid — protonation turns the poor leaving group OH into the excellent leaving group H₂O. Second, the C–O bond breaks heterolytically and water departs, leaving a carbocation. Third, a base (water does the job) removes a hydrogen from a carbon adjacent to the positive centre, and that C–H bonding pair folds down to become the new double bond. The acid emerges unchanged — catalyst, not reagent.
The subtlety AQA loves: a carbocation may have hydrogens available on both neighbouring carbons, so one alcohol can yield several alkenes. And because the products are alkenes, E/Z stereoisomerism joins in wherever each double-bond carbon carries two different groups. Always enumerate systematically: positional isomers first, then stereoisomers of each.
Why industry cares is the elegant part. Dehydration is the reverse of the hydration reaction in the block above, and chaining the two sections together closes a renewable loop: ferment glucose to ethanol, dehydrate ethanol to ethene, polymerise ethene (3.3.4.3). That is poly(ethene) grown from crops rather than cracked from crude oil — the exact synthesis-route argument that turns up on Paper 2 asking you to 'suggest an advantage of this process'.
Dehydrate butan-2-ol. Protonation and loss of water give the secondary carbocation CH₃CH⁺CH₂CH₃ with the charge on carbon-2. Removing a hydrogen from carbon-1 gives but-1-ene. Removing one from carbon-3 gives but-2-ene — and since each double-bond carbon there carries one H and one methyl/ethyl group, but-2-ene exists as E and Z isomers. Total: three alkenes from one alcohol — but-1-ene, E-but-2-ene and Z-but-2-ene. Listing 'but-1-ene and but-2-ene' scores partial credit; the stereoisomer pair is the top mark, and it is pure 3.3.1.3 knowledge cashed in a 3.3.5 question.
CaseRequired practical 5 — distillation, and the honesty of a 62% yield
RP5's official skill is separating a volatile product from a reacting mixture, and the classic version is making cyclohexene from cyclohexanol. Mix the cyclohexanol with concentrated phosphoric acid (chosen over sulfuric, which chars the organics and generates SO₂), add anti-bumping granules for smooth boiling, and assemble distillation apparatus with the thermometer bulb level with the side arm — it must read the vapour actually leaving, not the liquid. Heat, and collect the fraction boiling around 81–85 °C: cyclohexene boils at 83 °C, far below cyclohexanol's 161 °C, so the product distils out while the reactant stays behind. The condenser's cooling water enters at the bottom and leaves at the top, so the coldest water meets the coolest vapour and the jacket stays full.
The distillate is cloudy — product plus co-distilled water — so purification continues in a separating funnel: shake, let the layers settle, run off the denser aqueous layer. Then dry the organic layer over anhydrous calcium chloride until it runs clear, decant, and redistil, keeping only the fraction at the true boiling point. Each step is also an error story: product left wetting the glassware, cyclohexene evaporating during transfers (it is highly volatile and flammable — hence electric heating, no naked flame), a little product dissolved in the discarded aqueous layer. The same apparatus and logic run the other standard RP5 task, distilling ethanal out of an oxidising ethanol mixture — different chemistry, identical separation thinking.
A student dehydrates 10.0 g of cyclohexanol (Mr = 100.2) and collects 5.10 g of purified cyclohexene (Mr = 82.1). Moles of cyclohexanol: \(10.0 \div 100.2 = 0.0998\ \text{mol}\). The equation is 1:1, so the theoretical yield is \(0.0998 \times 82.1 = 8.19\ \text{g}\). Percentage yield: \(5.10 \div 8.19 = 0.623\), so 62.3%. The follow-up AQA asks: why under 100%? Creditworthy answers name mechanical and chemical losses — product retained on the apparatus and lost in transfers, some cyclohexene evaporated, some dissolved in the aqueous layer removed in the separating funnel, and side reactions of the carbocation intermediate. 'Experimental error' scores nothing; named losses score.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
Conditions are marks, not garnish. For fermentation AQA wants yeast (or its enzymes), about 35 °C, aqueous solution and the absence of oxygen — each a separate marking point; for hydration it wants phosphoric acid catalyst, about 300 °C and high pressure. The six-mark compare-the-routes question is levelled across AO1 and AO3: organise by rate, purity, batch versus continuous, energy demand, feedstock renewability and atom economy, and finish with a judgement tied to a purpose ('for fuel near farmland, fermentation; for high-purity solvent from an existing cracker, hydration'). A list of facts without the concluding judgement sits in the middle band.
Oxidation questions are apparatus questions in disguise. Say dilute oxidant, gentle warming and distil-as-it-forms for the aldehyde; excess oxidant and reflux for the acid; and quote the observation orange to green (staying orange is the tertiary alcohol's fingerprint). Balance every [O] equation — water appears when an alcohol is oxidised, not when an aldehyde is — and practise the practical drawing: a labelled reflux or distillation diagram with the thermometer bulb at the side arm, condenser water in at the bottom, and no sealed apparatus is a recurring 2–3 marks. Practical technique is at least 15% of the whole qualification, and RP5 is a favourite vehicle.
For dehydration, the mechanism marks mirror 3.3.4 in reverse: arrow from the oxygen lone pair to H⁺, arrow from the C–O bond onto oxygen as water leaves, carbocation drawn with the charge on the correct carbon, then an arrow from the adjacent C–H bond into the new double bond. When asked for all possible products, enumerate positional isomers first and then E/Z pairs, and name them fully — 'but-2-ene' without E and Z leaves a mark on the table. In yield calculations show moles, theoretical mass, then the percentage to three significant figures, and when asked why the yield is low, give named, specific losses — transfer losses, evaporation of a volatile product, product in the aqueous layer — never the phrase 'human error'.