Connect the OH group to physical properties, compare two ethanol production routes and evaluate the carbon balance behind biofuel claims.
Identify the carbon carrying OH
Alcohols have an OH group bonded to a saturated carbon. Number the parent chain to give OH the lowest appropriate locant: CH₃CH₂CH(OH)CH₃ is butan-2-ol. Saturated acyclic alcohols with one OH group have formula CₙH₂ₙ₊₁OH. This formula does not cover every compound containing OH.
A primary alcohol has one carbon neighbour on its OH-bearing carbon; a secondary has two and a tertiary three. The number of OH groups is a separate question. Methanol has no carbon neighbours but follows the primary-alcohol oxidation pattern. Carbon atoms with four single bonds are approximately tetrahedral; oxygen has two bonds and two lone pairs. Actual C–O–H angles are not universally identical to the angle in water.
| Structure | Name | Class |
|---|---|---|
| CH₃CH₂CH₂CH₂OH | butan-1-ol | Primary |
| CH₃CH(OH)CH₂CH₃ | butan-2-ol | Secondary |
| (CH₃)₃COH | 2-methylpropan-2-ol | Tertiary |
| HOCH₂CH₂OH | ethane-1,2-diol | Two OH groups, each on a primary carbon |
Hydrogen bonding is between molecules
The δ+ hydrogen of an O–H group is attracted to a lone pair on the oxygen of another molecule. This intermolecular hydrogen bond is different from the covalent O–H bond inside a molecule. Alcohols also have London attractions.
Compared with a hydrocarbon of similar size, an alcohol usually boils at a higher temperature because more energy is required to overcome its intermolecular attractions. Short-chain alcohols mix well with water through hydrogen bonding. As the hydrocarbon portion becomes larger, water solubility generally falls.
Do not say boiling breaks O–H covalent bonds. The alcohol molecules remain intact when they vaporise.
Fermentation makes a dilute ethanol solution
Yeast enzymes convert glucose into ethanol and carbon dioxide under anaerobic conditions, typically around 30–40 °C. A lower temperature slows enzyme-catalysed reactions; excessive heating denatures enzymes. Excluding oxygen favours fermentation rather than aerobic respiration and helps avoid oxidation of the ethanol.
The product is an aqueous mixture containing other material, so separate solids as appropriate and use fractional distillation to enrich the ethanol. Ordinary ethanol–water distillation does not produce completely water-free ethanol because the mixture forms an azeotrope. Further drying is needed if anhydrous ethanol is required.
Hydration of ethene
Ethene reacts reversibly with steam over a phosphoric-acid catalyst, typically around 300 °C and 60–70 atm. These are industrial operating examples, not exact values for every plant. Unreacted gases are recycled and ethanol is condensed from the product stream.
Hydration is exothermic and changes two moles of gas into one mole of gaseous product at reactor conditions. Lower temperature favours equilibrium conversion but slows reaction; higher pressure favours conversion but increases equipment and compression costs. A catalyst increases the rate of reaching equilibrium without changing its position.
Diagram placeholder
Acid-catalysed hydration: electron-pair sequence
Labels to include:
- C=C pair attacks H⁺
- CH₃CH₂⁺ intermediate
- water oxygen lone pair
- CH₃CH₂OH₂⁺
- O–H pair returns to O
- H⁺ regenerated
In the simplified AQA mechanism, protonate ethene using an arrow from C=C to H⁺. Draw the ethyl carbocation, then an arrow from a water oxygen lone pair to its positively charged carbon. The resulting oxygen has three bonds and charge +1. Show the O–H bond pair returning to O as H⁺ is lost, producing ethanol and regenerating the acid. Do not omit the charged oxonium intermediate.
Compare the route, not just the reaction
| Feature | Fermentation | Hydration |
|---|---|---|
| Feedstock | Sugars from biomass can be renewed | Ethene commonly comes from fossil feedstocks |
| Operation | Often batch; slower; low temperature | Continuous; faster; elevated temperature and pressure |
| Separation | Dilute product makes concentration energy-intensive | Product still needs separation and unreacted gases are recycled |
| Wider costs | Land, fertiliser, irrigation and transport | Pressure equipment, energy and feedstock supply |
Write all stages of the ideal carbon balance
Quick checks
Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.
Q1. Classify CH₃CH₂C(OH)(CH₃)₂.Show answer
Tertiary: its OH-bearing carbon is bonded to three other carbons. There is only one OH group, which does not make it primary.
Q2. Why does ethanol hydrogen-bond to water?Show answer
Both molecules have an O–H hydrogen with partial positive charge and oxygen lone pairs. A hydrogen bond forms between an O lone pair on one molecule and an O–H hydrogen on the other.
Q3. Find the theoretical ethanol mass from 54.0 g glucose, using Mᵣ values 180.0 and 46.0.Show answer
54.0/180.0 = 0.300 mol glucose gives 0.600 mol ethanol. Mass = 0.600 × 46.0 = 27.6 g.
Q4. Why is a very low temperature not chosen for industrial ethene hydration despite its exothermic nature?Show answer
It would favour equilibrium yield but make the reaction too slow. The operating temperature is a compromise between rate and equilibrium conversion.
Q5. Give two reasons why fermentation ethanol may not be carbon-neutral in practice.Show answer
Examples include fossil energy used for distillation and CO₂ emissions from farming or transport. State the emitting process and connect it to extra atmospheric CO₂ beyond the ideal photosynthesis–fermentation–combustion balance.
Sources
Sources and examiner guidance (reviewed 2 October 2026)
- Chemrevise: Alcohols — Coverage checklist, pp1–7; original lessons and practice.
- AQA 7405 specification — 3.3.5.1–3.3.5.3 and required practical 5.
- AQA June 2022 AS Paper 2 mark scheme — Q07.1–07.3 p25: biofuel balance and comparisons.
- AQA June 2022 AS Paper 2 examiner report — Q07 pp4–5: equations and lifecycle reasoning.
Finesse Tuition is not endorsed by AQA or Chemrevise. All explanations and examples here are our own.