Alcohols as products of fermentation and hydration of alkenes. Classification as primary, secondary and tertiary. Oxidation of alcohols with acidified potassium dichromate(VI) to aldehydes, carboxylic acids and ketones. Elimination to form alkenes. The biofuel debate.

What this dot point is asking

AQA wants you to describe how alcohols are made by fermentation and by hydration of alkenes, classify alcohols as primary, secondary or tertiary, give the products and conditions for their oxidation, describe dehydration to alkenes, and discuss ethanol as a biofuel.

Making alcohols

Fermentation: glucose is converted to ethanol by yeast enzymes at around 35 degrees Celsius in anaerobic conditions.

Hydration of ethene: ethene reacts with steam over a phosphoric acid catalyst.

Classification

A primary alcohol has the $-OH$ carbon bonded to one (or zero) other carbon; a secondary alcohol to two; a tertiary alcohol to three. The class matters because it controls the oxidation products: only primary and secondary alcohols can be oxidised by acidified dichromate (they have a hydrogen on the carbon bearing the $-OH$ that can be removed), whereas tertiary alcohols have no such hydrogen and resist oxidation. Alcohols also have higher boiling points than alkanes of similar size because the $-OH$ group allows hydrogen bonding between molecules, and the shorter-chain alcohols are fully miscible with water for the same reason.

Oxidation

The oxidising agent is acidified potassium dichromate(VI), $K_2Cr_2O_7 / H_2SO_4$, which turns from orange to green ($Cr_2O_7^{2-}$ to $Cr^{3+}$) when oxidation occurs. Use $[O]$ to represent the oxidising agent.

• Primary alcohol to aldehyde (distil off as it forms): $CH_3CH_2OH + [O] \rightarrow CH_3CHO + H_2O$
• Primary alcohol to carboxylic acid (reflux with excess): $CH_3CH_2OH + 2[O] \rightarrow CH_3COOH + H_2O$
• Secondary alcohol to ketone: $CH_3CH(OH)CH_3 + [O] \rightarrow CH_3COCH_3 + H_2O$
• Tertiary alcohols are not oxidised (no H on the $-OH$ carbon), so dichromate stays orange.

Elimination (dehydration) to alkenes

Heating an alcohol with hot concentrated sulfuric acid (or passing vapour over hot $Al_2O_3$) removes water to form an alkene.

This is the reverse of hydration and provides alkenes from a renewable source. The mechanism is acid-catalysed: the $-\text{OH}$ is protonated, water leaves to give a carbocation, and a hydrogen is lost from the next carbon to form the double bond.

Ethanol as a biofuel

Ethanol made by fermentation can be described as carbon neutral in principle, because the carbon dioxide released when it burns equals the carbon dioxide the crop absorbed by photosynthesis while growing. In practice the balance is not perfect: energy is used to plant, harvest, ferment, distil and transport the fuel, often from fossil sources, and using farmland for fuel crops can compete with food production and drive deforestation. AQA expects a balanced evaluation rather than a one-sided answer, weighing the renewable, lower-net-carbon advantage against the energy inputs and land-use costs.

Try this

Q1. Give the colour change when a secondary alcohol is oxidised. [1 mark]

• Cue. Orange to green.

Q2. Name the product of dehydrating propan-1-ol. [1 mark]

• Cue. Propene (with water).