Aldehydes, Ketones and Carboxylic Acids (Class 12): The Reactions You Must Know

Why aldehydes are more reactive than ketones

Two effects work in the same direction:

• Electronic — a ketone has two alkyl groups pushing electron density toward the carbonyl carbon, reducing its positive character. An aldehyde has only one.
• Steric — two alkyl groups also physically crowd the carbon, hindering nucleophilic attack.

So the order of reactivity toward nucleophilic addition is HCHO > CH₃CHO > CH₃COCH₃.

Distinguishing tests worth memorising

What controls carboxylic acid strength

An acid is stronger when its conjugate base is more stable. Electron-withdrawing groups stabilise the carboxylate anion by dispersing the negative charge; electron-donating groups destabilise it.

• Cl₃C–COOH > Cl₂CH–COOH > ClCH₂–COOH > CH₃COOH — more chlorines, stronger acid.
• The effect weakens with distance: 2-chlorobutanoic acid is stronger than 4-chlorobutanoic acid.
• Benzoic acid is stronger than acetic acid, because the sp² ring carbon withdraws electrons relative to an sp³ methyl group.

Carboxylic acids are also stronger than phenols, because the carboxylate has two equivalent resonance structures spreading the charge over two oxygens, whereas the phenoxide spreads it onto less electronegative ring carbons.

FAQs

Why are aldehydes more reactive than ketones?

Ketones have two electron-donating alkyl groups that reduce the carbonyl carbon's positive charge and sterically crowd it, so nucleophiles attack aldehydes more readily.

Which compounds give a positive iodoform test?

Methyl ketones, ethanal, and alcohols that oxidise to them such as ethanol — anything with a CH₃CO group or which can form one.

Why is trichloroacetic acid stronger than acetic acid?

The three chlorines withdraw electron density and stabilise the carboxylate anion, so the acid ionises more readily.

How do you distinguish a carboxylic acid from a phenol?

Add sodium bicarbonate. The carboxylic acid gives brisk effervescence of carbon dioxide; phenol does not react.