Alcohols — classification and oxidation

Alcohols are primary, secondary or tertiary, according to how many carbons are attached to the carbon bearing the –OH.

Oxidation with acidified potassium dichromate(VI): the solution goes orange → green as Cr₂O₇²⁻ is reduced to Cr³⁺. The oxidising agent is written [O].

• Primary + distil: the aldehyde is removed as it forms (it has the lowest boiling point — no hydrogen bonding).
• Primary + reflux with excess oxidant: straight through to the carboxylic acid.
• Secondary + reflux: the ketone, which resists further oxidation.
• Tertiary: no reaction — there is no hydrogen on the C–OH carbon to remove.

Also: dehydration with hot concentrated H₂SO₄ or Al₂O₃ gives an alkene; and alcohols with a CH₃CH(OH)– group give the iodoform (triiodomethane) reaction — a yellow crystalline precipitate with I₂ and NaOH.

Oxidise the secondary alcohol

Phenols — why they are acidic

In phenol, C₆H₅OH, a lone pair on the oxygen is delocalised into the ring. That has two big consequences.

• Phenol is a weak acid, but far more acidic than an alcohol. Losing H⁺ gives the phenoxide ion, in which the negative charge is spread around the ring — so the anion is stabilised and the equilibrium lies further right. Phenol reacts with NaOH to give sodium phenoxide, but is too weak to react with Na₂CO₃ — it will not fizz. Carboxylic acids will.
• The ring is activated. The extra electron density makes phenol much more reactive than benzene towards electrophiles. With bromine water (no halogen carrier, no heat) it gives an immediate white precipitate of 2,4,6-tribromophenol — and the bromine water is decolourised.

Two tests worth memorising: phenol gives a violet/purple colour with neutral iron(III) chloride solution; and a carboxylic acid fizzes with sodium carbonate while a phenol does not. Together they separate the two.

Phenol and bromine water

Aldehydes and ketones — nucleophilic addition

The C=O bond is strongly polar (Cδ+=Oδ−) and planar. The δ+ carbon attracts nucleophiles, and the π bond can open — so carbonyls undergo nucleophilic addition.

With HCN (in the presence of KCN):

Mechanism: the CN⁻ nucleophile attacks the δ+ carbon; the π electrons shift onto the oxygen, giving an alkoxide intermediate; that oxygen then takes an H⁺ (from HCN or water). Since the C=O is planar, attack occurs equally from both faces — giving a racemate.

Reduction with NaBH₄ (a source of the hydride nucleophile, H⁻) reverses the oxidation: an aldehyde → a primary alcohol; a ketone → a secondary alcohol.

Aldehyde or ketone?

Name the nucleophile

Your turn — reduction yield

Which family?

Tap a compound, then the family it belongs to.

Carboxylic acids

–COOH is a carbonyl and a hydroxyl on the same carbon. Carboxylic acids are weak acids — they only partially dissociate — but they are strong enough to displace CO₂ from carbonates:

Why are they acidic at all? The carboxylate ion RCOO⁻ is stabilised by delocalisation of the negative charge over both oxygens — both C–O bonds become equal in length. An alkoxide ion (from an alcohol) has no such stabilisation, which is why alcohols are essentially neutral.

They also form hydrogen-bonded dimers, giving unexpectedly high boiling points, and the short-chain ones are very soluble in water.

Tell them apart

Acyl chlorides and esters

Acyl chlorides (RCOCl) are the most reactive derivative — the C is bonded to two electronegative atoms, so it is very strongly δ+. They react violently and irreversibly with any nucleophile, giving steamy HCl fumes:

Esters are made by esterification — a carboxylic acid plus an alcohol, with a concentrated H₂SO₄ catalyst. This route is reversible and slow, so the yield is limited:

Hydrolysis of an ester:

• Acid hydrolysis (dilute H₂SO₄, reflux) is reversible and gives back the carboxylic acid and the alcohol.
• Alkaline hydrolysis (NaOH(aq), reflux) — saponification — goes to completion, because the product is the carboxylate salt, which cannot re-esterify. This is how soap is made from fats.

Esters have fruity smells and are used as flavourings, perfumes, solvents and plasticisers. Vegetable oils are esters of glycerol (propane-1,2,3-triol).

Your turn — Mr of an ester

Your turn — esterification yield

Your turn — hydrolysing an acyl chloride

Alkaline hydrolysis

Reagents and conditions

Tap a reagent and condition on the left, then its product on the right.

The big ideas to know

That is the whole of OA2. Press Finish to see your score.