Part 2: Oxidation, reflux and identifying products

Choose conditions that stop a primary alcohol at an aldehyde or oxidise it to an acid, and distinguish those products from ketones.

The OH-bearing carbon decides the pathway

Warm an appropriate alcohol with potassium dichromate(VI) acidified using dilute sulfuric acid. When oxidation occurs, orange dichromate(VI) is reduced to green chromium(III). The alcohol is oxidised while the oxidising agent is reduced.

Primary alcohols can form aldehydes and then carboxylic acids. Secondary alcohols form ketones. Tertiary alcohols have no C–H bond on the OH-bearing carbon, so they resist oxidation by acidified dichromate under these normal conditions. This is not a claim that tertiary alcohols can never be destroyed by any oxidising conditions.

Use [O] as oxidant bookkeeping

[O] is a convenient representation of oxygen supplied by an oxidising agent; it is not the reagent bottle label. You still need acidified potassium dichromate(VI) when asked for the reagent. Write CHO for a condensed aldehyde group, so the C=O connectivity is clear.

The examples below use butan-1-ol and butan-2-ol. Compare the hydrogen counts: conversion of an alcohol to a carbonyl removes two hydrogens, while conversion of the aldehyde to an acid adds oxygen.

Distil an aldehyde; reflux for an acid

To favour an aldehyde, use limited oxidant, warm gently and distil the volatile aldehyde away as it forms. Removing it reduces its contact with oxidant and therefore further oxidation. A cooled receiver can reduce loss of a volatile product.

To obtain a carboxylic acid, use excess oxidant and heat under reflux. A vertical condenser returns condensed vapour to the flask, allowing prolonged heating with reduced loss of volatile material. Reflux does not collect a separate distillate or by itself guarantee completion.

In distillation, position the thermometer bulb beside the side-arm opening to measure the vapour entering the condenser. In both arrangements, cooling water enters the jacket at the bottom and exits at the top. Add anti-bumping granules before heating; use suitable electrical heating and leave the apparatus open to the atmosphere.

Distinguish an aldehyde from a ketone

Tollens’ reagent contains the diamminesilver(I) complex in alkaline solution. Warm gently: a typical aldehyde gives a silver mirror or silver deposit; a simple ketone gives no visible change. The aldehyde is oxidised and silver(I) is reduced to silver metal.

Fehling’s solution is an alkaline copper(II) reagent. On warming with a typical aliphatic aldehyde, the blue solution produces a brick-red precipitate of Cu₂O. A simple ketone leaves the blue solution without that precipitate. The organic oxidation product in these alkaline mixtures is a carboxylate ion, not predominantly the free acid.

Use freshly prepared Tollens’ reagent only in a supervised test and dispose of it promptly under the laboratory procedure; do not store it or let residues dry. Do not use acidified dichromate alone to distinguish an aldehyde from a primary or secondary alcohol, because both can reduce it.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.

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