Alcohol productionAQA A-Level Chemistry: Revision notes
Section 1
Hydration of alkenes
Alcohols are produced industrially by the hydration of alkenes. Ethene reacts with steam in the presence of an acid catalyst:
CH₂=CH₂ + H₂O → CH₃CH₂OH
The reaction is an electrophilic addition, and the acid is regenerated, so it acts as a catalyst.
Section 2
Mechanism of hydration
- The pair of electrons in C=C attacks an H⁺ ion (the electrophile), forming a C–H bond and a carbocation CH₃CH₂⁺.
- A lone pair on the oxygen of a water molecule attacks the carbocation, forming a C–O bond and the ion CH₃CH₂OH₂⁺.
- The O–H bond breaks, releasing H⁺ and forming ethanol.
Because H⁺ is released in the last step, the acid is not used up.
Water attacks the carbocation as a nucleophile through the lone pair on oxygen. Show the arrow starting on the lone pair.
Section 3
Fermentation of glucose
Ethanol is also made by fermentation of glucose:
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
Conditions: yeast (its enzymes act as the catalyst), a temperature of about 35 °C (30–40 °C), in aqueous solution, in the absence of air (anaerobic).
- Too cold, and the reaction is too slow.
- Too hot, and the enzymes denature.
- Air would oxidise the ethanol to ethanoic acid.
The ethanol is separated from the dilute mixture by fractional distillation.
Say that higher temperatures denature the enzymes. Do not say that they are killed or burnt.
Section 4
Ethanol as a biofuel
A biofuel is a fuel made from biological material, such as plants. Ethanol made by fermentation of sugar cane or other crops can be burned in place of petrol, and it is made from a renewable resource.
It is described as carbon neutral because the CO₂ absorbed by the growing crop in photosynthesis (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂) equals the CO₂ released by fermentation and by combustion (C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O).
Section 5
Is it really carbon neutral?
The claim is not valid in practice, since fossil fuels are used to run farm machinery, make fertiliser, transport the crop and heat the distillation. These release extra CO₂.
There are also environmental and ethical issues: land used for fuel crops cannot grow food, so food prices may rise; forests may be cleared, destroying habitats; fertilisers can pollute water. Any discussion should reach a balanced conclusion.
In 'discuss' questions give at least one benefit, at least one problem, and then a reasoned judgement.
Must Know
- Hydration: ethene + steam, acid catalyst, electrophilic addition
- Fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
- Conditions: yeast, ~35 °C, anaerobic, aqueous
- Separation: fractional distillation
- Biofuel: made from biological material, renewable
- Carbon neutral in theory only: energy inputs release CO₂
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Alcohol production
- Ethanol is manufactured industrially in two ways: by the hydration of ethene using steam and an acid catalyst, and by the fermentation of glucose using yeast. The ethanol from fermentation is present in a dilute mixture with water and other substances.Write an equation for the fermentation of glucose to ethanol.2 marks
- Bioethanol made from sugar cane is blended with petrol in some countries. Supporters of this fuel describe it as carbon neutral, so a student investigates the processes involved.Write an equation for the photosynthesis that forms glucose and an equation for the complete combustion of ethanol.2 marks
- A plant manager is comparing two routes to ethanol: fermentation of glucose by yeast, and the acid-catalysed hydration of ethene by steam. The staff must understand the conditions for the first route and the mechanism of the second.Justify the conditions used for the fermentation of glucose: yeast, a temperature of about 35 °C, and the absence of air.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).