Alcohols, Carbonyls and Carboxylic Acids

A-Level Chemistry · Organic Chemistry: Functional Groups and Mechanisms

Alcohols

Alcohols contain the –OH functional group. They are classified as primary (1°), secondary (2°) or tertiary (3°) depending on how many carbon atoms are attached to the carbon bearing the –OH.

Making alcohols

Hydration of alkenes (steam + H₃PO₄ catalyst) — industrial ethanol.
Fermentation of sugars by yeast (renewable, but slow and impure).

Oxidation of alcohols

Using an oxidising agent (acidified potassium dichromate(VI), which turns from orange to green):

Primary alcohol → aldehyde (distil off) → carboxylic acid (reflux with excess oxidant).
Secondary alcohol → ketone (cannot be oxidised further easily).
Tertiary alcohol → not oxidised (no H on the carbon with –OH) — dichromate stays orange (a test to distinguish tertiary alcohols).

Carbonyl compounds — aldehydes and ketones

Both contain the C=O group.

Aldehydes (–CHO) are at the end of the chain; ketones (C=O) are in the middle.
Distinguishing test: aldehydes are easily oxidised, so they give a positive result with Tollens' reagent (silver mirror) and Fehling's/Benedict's (red precipitate); ketones do not react.
Both can be reduced back to alcohols using NaBH₄ (aldehyde → primary alcohol; ketone → secondary alcohol).

Carboxylic acids

Contain –COOH. They are weak acids (partially dissociate), so they:

react with metals → salt + hydrogen;
react with carbonates → salt + water + CO₂ (a test — fizzing distinguishes them from phenols/alcohols);
react with alcohols (acid catalyst) → esters + water (esterification), giving sweet-smelling products used as flavourings and solvents.

Worked example

How can you distinguish between propanal (an aldehyde) and propanone (a ketone)?

Warm each with Tollens' reagent: the aldehyde (propanal) is oxidised and gives a silver mirror; the ketone (propanone) gives no reaction. ✓

Common mistakes

Forgetting tertiary alcohols are not oxidised by dichromate.
Mixing up the oxidation products (1° → aldehyde → acid; 2° → ketone).
Confusing the tests: Tollens'/Fehling's distinguish aldehydes from ketones; carbonate fizz identifies carboxylic acids.

Exam tips

Know the oxidation pathways for 1°/2°/3° alcohols and the dichromate colour change (orange→green).
Learn the aldehyde vs ketone tests and the carboxylic-acid carbonate test.
Recall esterification (acid + alcohol → ester + water).

Key facts to remember

Alcohols (1°/2°/3°): oxidised by acidified dichromate (orange→green) — 1° → aldehyde → carboxylic acid; 2° → ketone; 3° not oxidised.
Aldehydes vs ketones: aldehydes give a silver mirror (Tollens') and red precipitate (Fehling's); ketones don't.
Carboxylic acids are weak acids: react with carbonates to give CO₂; with alcohols to form esters (esterification).