The classification of alcohols as primary, secondary and tertiary, their production by hydration of alkenes and by fermentation, and their reactions including combustion, oxidation, dehydration and ester formation.

What this dot point is asking

CCEA wants you to classify alcohols as primary, secondary or tertiary, describe their production by hydration of alkenes and by fermentation, and describe their reactions, including combustion, oxidation, dehydration to alkenes and ester formation.

Classification

For example, ethanol ($\text{CH}_3\text{CH}_2\text{OH}$) is primary, propan-2-ol ($\text{CH}_3\text{CH(OH)CH}_3$) is secondary, and 2-methylpropan-2-ol ($(\text{CH}_3)_3\text{COH}$) is tertiary. Recognising the class quickly is the single most useful skill for this topic, because it predicts whether oxidation gives an aldehyde and acid, a ketone, or no reaction at all.

Production

Alcohols are produced in two main ways:

• Hydration of alkenes: steam adds across the double bond using a phosphoric acid catalyst at about $300\ \text{C}$ and $60$ to $70\ \text{atm}$, a fast continuous industrial process giving a pure product. The reaction is $\text{CH}_2{=}\text{CH}_2 + \text{H}_2\text{O} \rightarrow \text{CH}_3\text{CH}_2\text{OH}$.
• Fermentation: yeast converts sugars to ethanol and carbon dioxide at about $35\ \text{C}$ without air, a renewable but slower batch process giving an impure product. The reaction is $\text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2$.

The two routes weigh up the same factors candidates meet across the specification: hydration is fast, continuous and pure but depends on crude oil, while fermentation is renewable and low-energy but slow, impure and a batch process. The ethanol from fermentation is also a carbon-neutral biofuel in principle, because the carbon dioxide released on burning was first taken from the air by the plant that made the sugar.

Oxidation

Dehydration and ester formation

Alcohols are dehydrated to alkenes by heating with concentrated sulfuric or phosphoric acid (or by passing the vapour over hot aluminium oxide), which removes water. For example, ethanol gives ethene: $\text{CH}_3\text{CH}_2\text{OH} \rightarrow \text{CH}_2{=}\text{CH}_2 + \text{H}_2\text{O}$. This is the reverse of hydration and is a useful way of making an alkene in the laboratory from a renewable alcohol.

Alcohols react with carboxylic acids (with a concentrated sulfuric acid catalyst, under reflux) to form esters and water in an esterification reaction: $\text{CH}_3\text{COOH} + \text{CH}_3\text{CH}_2\text{OH} \rightleftharpoons \text{CH}_3\text{COOCH}_2\text{CH}_3 + \text{H}_2\text{O}$. The reaction is reversible and reaches equilibrium, so the ester is often distilled off or water removed to improve the yield. Esters have characteristic sweet, fruity smells, which is the simplest way to confirm the reaction has worked.

Combustion

All alcohols burn in a plentiful supply of oxygen to give carbon dioxide and water, releasing energy: $\text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}$. This is why ethanol is used as a fuel, blended into petrol or burned neat in some countries. In a limited supply of oxygen, incomplete combustion gives carbon monoxide or soot, exactly as for any hydrocarbon fuel.

Examples in context

Example 1. Bioethanol as a transport fuel. Brazil and the United States blend ethanol made by fermentation into petrol (E10 contains $10\%$ ethanol). The chemistry draws on this whole dot point: the ethanol is made by fermentation of sugar cane or maize, purified by fractional distillation, and burned by complete combustion in the engine. Promoters argue it is close to carbon-neutral because the carbon dioxide released was recently absorbed by the crop, an argument CCEA expects candidates to evaluate alongside the land and energy costs of growing the crop.

Example 2. Distinguishing alcohols in forensic and quality testing. Acidified dichromate is the basis of the classic colour test used to confirm an oxidisable alcohol. In a teaching laboratory, the same reagent separates a primary or secondary alcohol (orange to green) from a tertiary alcohol (stays orange), and a follow-up Tollens' or Fehling's test then separates the aldehyde from a primary alcohol from the ketone of a secondary alcohol. This sequence of tests is a standard CCEA practical for identifying an unknown alcohol from its reactions.

Try this

Q1. State the oxidation product of a secondary alcohol. [1 mark]

• Cue. A ketone.

Q2. State the colour change of acidified potassium dichromate when it oxidises a primary alcohol. [1 mark]

• Cue. Orange to green.