Classifying and oxidising alcohols

Classify by the number of carbon atoms attached to the carbon carrying the OH: primary (0 or 1), secondary (2), tertiary (3).

The oxidising agent is acidified potassium dichromate(VI), K₂Cr₂O₇/H₂SO₄ — orange Cr₂O₇²⁻ is reduced to green Cr³⁺. We write the oxidising agent as [O].

• Primary alcohol → aldehyde → carboxylic acid. The apparatus decides which you get:
  • Distil the product off as it forms → you isolate the aldehyde (it has a lower boiling point than the alcohol, because it cannot hydrogen bond to itself).
  • Heat under reflux with excess oxidising agent → the aldehyde stays in the flask and is oxidised again → the carboxylic acid.
• Secondary alcohol → ketone (and no further — there is no H on the carbonyl carbon to remove).
• Tertiary alcohol → no reaction. There is no hydrogen atom on the C–OH carbon to be removed, so it cannot be oxidised without breaking a C–C bond.

Reflux means "boil without losing anything": the vertical condenser returns the vapour to the flask, so volatile products cannot escape and the mixture can be heated for a long time. Distillation does the opposite — it deliberately removes the product the moment it forms.

Classify each alcohol

Tap an alcohol, then tap whether it is primary, secondary or tertiary.

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Aldehydes, ketones and nucleophilic addition

The C=O bond is strongly polar: oxygen is far more electronegative, so the carbon is δ+. That makes carbonyls a target for nucleophiles — the opposite of alkenes, which attract electrophiles.

Reduction with NaBH₄ (in aqueous methanol) — the nucleophile is the hydride ion, H⁻:

• Aldehyde + 2[H] → primary alcohol (e.g. CH₃CHO + 2[H] → CH₃CH₂OH)
• Ketone + 2[H] → secondary alcohol

Addition of HCN (using KCN followed by dilute acid) — the nucleophile is the cyanide ion, CN⁻. This produces a hydroxynitrile and, crucially, adds a carbon atom to the chain:

The mechanism: CN⁻ attacks the δ+ carbon (curly arrow from the lone pair on carbon); the C=O π electrons move onto the oxygen, giving an intermediate with a negatively charged oxygen; that oxygen then takes a proton from H⁺ or water.

Why HCN addition gives a racemic mixture. The carbonyl group is planar, so the cyanide ion can attack from either face with equal probability. If the product has a chiral centre, both enantiomers form in equal amounts — an optically inactive racemate.

Hazard: KCN and HCN are extremely toxic, so this reaction is done in a fume cupboard.

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Carboxylic acids and esterification

Carboxylic acids are weak acids (only partially dissociated), but they are strong enough for one decisive test:

Esterification — a carboxylic acid plus an alcohol, with a concentrated sulfuric acid catalyst. It is reversible, so the yield is limited:

Esters are named alcohol part first, then the acid part: ethyl ethanoate. They have sweet, fruity smells and are used as solvents, plasticisers, perfumes and flavourings.

Ester hydrolysis:

• Acid hydrolysis (dilute acid, reflux) is reversible and gives the carboxylic acid + alcohol.
• Base hydrolysis (dilute NaOH, reflux) goes to completion and gives the carboxylate salt + alcohol. Applied to a fat, this is saponification — it makes soap plus glycerol (propane-1,2,3-triol).

Biodiesel is a mixture of methyl esters of long-chain carboxylic acids, made by reacting vegetable oil with methanol using a KOH catalyst.

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Acyl chlorides and acid anhydrides

Acyl chlorides (RCOCl) are the most reactive carboxylic acid derivatives. Every reaction follows the same pattern — a nucleophile replaces the Cl, and HCl gas is given off (steamy white fumes):

• + water → carboxylic acid: CH₃COCl + H₂O → CH₃COOH + HCl (violent, at room temperature)
• + alcohol → ester: CH₃COCl + CH₃OH → CH₃COOCH₃ + HCl (fast, irreversible, and gives a much better yield than esterification)
• + ammonia → amide: CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl
• + primary amine → N-substituted amide: CH₃COCl + 2CH₃NH₂ → CH₃CONHCH₃ + CH₃NH₃Cl

Acid anhydrides ((RCO)₂O) do exactly the same chemistry but more gently, releasing the carboxylic acid instead of corrosive HCl gas:

Why aspirin is made with ethanoic anhydride, not ethanoyl chloride. The anhydride is cheaper, less corrosive, less vulnerable to hydrolysis by moisture and does not produce dangerous HCl fumes — a genuine industrial-safety judgement, and a favourite exam question.

Match the reagent to its result

Tap a reagent on the left, then its result on the right.

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Apparatus, tests and the reactions that go nowhere

• The apparatus decides the product. Distil → aldehyde. Reflux → carboxylic acid. If the question shows a diagram, read it before you answer.
• Tollens’ and Fehling’s are positive for ALDEHYDES only. They are gentle oxidising agents, and a ketone has no H on the carbonyl carbon to remove.
• A tertiary alcohol cannot be oxidised — no hydrogen on the C–OH carbon.
• Only a carboxylic acid gives CO₂ with a carbonate. Alcohols and phenols do not — this is the definitive test.

Nucleophilic addition, in one sentence: "The C=O carbon is δ+ because oxygen is more electronegative, so it is attacked by a nucleophile; the π electrons shift onto the oxygen, giving a negatively charged intermediate, which is then protonated." That sentence, plus correct arrows, is full marks.

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The big ideas to know

That is AQA 3.3.5, 3.3.8 and 3.3.9. Press Finish.