Classification of alcohols, oxidation of primary and secondary alcohols, dehydration and esterification, the acidity and reactions of carboxylic acids, and ester formation and hydrolysis.

What this topic is asking

Eduqas topic C3.4 covers the chemistry of alcohols (classification, oxidation, dehydration and reaction with acids) and carboxylic acids (their acidity, reactions and the formation and hydrolysis of esters). The oxidation products of different alcohol classes and the esterification reaction are the central exam content.

Classifying alcohols

Oxidation of alcohols

Dehydration and substitution

Alcohols are dehydrated to alkenes by passing the vapour over hot aluminium oxide or by heating with concentrated sulfuric or phosphoric acid; water is eliminated. Alcohols also react with the hydrogen halides (or $\text{PCl}_5$, $\text{SOCl}_2$) to form halogenoalkanes, the reverse of the hydrolysis met in topic C3.3.

Carboxylic acids

Carboxylic acids contain the $\text{-COOH}$ group and are weak acids, partially dissociating to give the carboxylate ion and $\text{H}^+$. They react with reactive metals (salt and hydrogen), with carbonates and hydrogencarbonates (salt, water and carbon dioxide, the test for the -COOH group) and with bases (salt and water).

Esters

Esters are sweet-smelling and used as flavourings, solvents and in perfumes; their alkaline hydrolysis is the basis of soap making.

Examples in context

Example 1. Breathalyser chemistry. Older breathalysers used acidified dichromate, which turns from orange to green as ethanol is oxidised; the colour change quantifies the alcohol, a direct use of primary-alcohol oxidation.

Example 2. Biodiesel. Transesterification of vegetable-oil esters with methanol produces methyl ester biodiesel and glycerol, an industrial application of ester interchange closely related to hydrolysis.

Try this

Q1. Name the product of oxidising butan-2-ol with acidified potassium dichromate(VI). [1 mark]

• Cue. Butanone (a secondary alcohol gives a ketone).

Q2. Write the equation for the reaction of ethanoic acid with sodium carbonate. [2 marks]

• Cue. $2\text{CH}_3\text{COOH} + \text{Na}_2\text{CO}_3 \rightarrow 2\text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2$ (effervescence confirms a carboxylic acid).