Why are carboxylic acids classed as weak acids?

They only partially dissociate in water, so the equilibrium releasing hydrogen ions lies well to the left and the solution contains far more undissociated acid than ions.

Give two ways a carboxylic acid can be prepared.

By oxidising a primary alcohol or an aldehyde with acidified dichromate(VI) under reflux, or by hydrolysing a nitrile with dilute acid under reflux.

What is formed when a nitrile is hydrolysed under acidic conditions?

A carboxylic acid and an ammonium salt. Under alkaline conditions the salt of the acid forms instead, and acid must then be added to release the acid itself.

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Carboxylic acids and their reactions

3 things to know
Papers 2 and 3

Overview

The carboxyl group, hydrogen-bonded dimers and solubility, why carboxylic acids are weak, preparing them by oxidation or nitrile hydrolysis, and their reactions with metals, sodium carbonate, sodium hydroxide, LiAlH₄, phosphorus(V) chloride and alcohols.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What is the carboxylic acid functional group?

Key term

Why do carboxylic acids have higher boiling temperatures than alcohols of similar relative molecular mass?

Key term

Why are the shorter carboxylic acids very soluble in water?

Questions from this subtopic

Each one with the model answer underneath.

1
What is the carboxylic acid functional group?
Model answer
The carboxyl group, COOH, in which a hydroxyl group is attached to a carbonyl carbon. The name ends in -oic acid.
2
Why do carboxylic acids have higher boiling temperatures than alcohols of similar relative molecular mass?
Model answer
Each molecule can form two hydrogen bonds through the carboxyl group, so pairs associate as dimers and considerably more energy is needed to separate them.
3
Why are the shorter carboxylic acids very soluble in water?
Model answer
The carboxyl group hydrogen bonds readily to water. Solubility falls as the chain lengthens and the non-polar hydrocarbon part comes to dominate.
4
Why are carboxylic acids classed as weak acids?
Model answer
They only partially dissociate in water, so the equilibrium releasing hydrogen ions lies well to the left and the solution contains far more undissociated acid than ions.
5
Give two ways a carboxylic acid can be prepared.
Model answer
By oxidising a primary alcohol or an aldehyde with acidified dichromate(VI) under reflux, or by hydrolysing a nitrile with dilute acid under reflux.
6
What is formed when a nitrile is hydrolysed under acidic conditions?
Model answer
A carboxylic acid and an ammonium salt. Under alkaline conditions the salt of the acid forms instead, and acid must then be added to release the acid itself.

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