medium difficulty7 min

Preparation and Acidity of Carboxylic Acids

Carboxylic acids are organic compounds containing the -COOH group. They can be prepared by oxidation of alcohols or aldehydes, hydrolysis of nitriles, and related routes, and they are acidic due to resonance stabilisation of the carboxylate ion.

In one line: Acid strength follows conjugate-base comfort: the more stable RCOO- is, the stronger the acid.

Practice This Concept

Understand it

The acidity of carboxylic acids depends on the stability of the conjugate base, RCOO-.
Electron-withdrawing groups such as chlorine stabilise the carboxylate ion by the inductive effect and increase acidity.
Electron-donating alkyl groups reduce acidity by destabilising the carboxylate ion.
Lower pKa means stronger acid.

Example

Chloroacetic acid is stronger than acetic acid because the chlorine atom withdraws electron density and stabilises the carboxylate ion.

Rule to remember

Acidity rule: stronger acid has more stable conjugate base and lower pKa.

Preparation patterns: RCH2OH or RCHO + oxidation → RCOOH; RCN + hydrolysis → RCOOH; acid derivative hydrolysis → carboxylic acid.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

Arrange in acidity order: ClCH₂COOH and CH₃COOH.

1The chloro group has a -I effect, so ClCH₂COO^- is more stable than CH₃COO^-.
2Therefore ClCH₂COOH is more acidic than CH₃COOH.

Method you can reuse

1For preparation, identify whether the starting compound can be oxidised or hydrolysed to -COOH.
2For acidity, form the conjugate base mentally, then check resonance, inductive effect, distance, and pKa trend.

Visual tip

A resonance structure diagram of carboxylate ion is useful, but a substituent-effect comparison table also supports exam preparation.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Which is more acidic, CH₃COOH or ClCH₂COOH? Give the reason.

Think first, then show answer

ClCH₂COOH is more acidic because chlorine withdraws electron density and stabilises the carboxylate ion.

Common wrong answer

Saying higher pKa means stronger acid; actually lower pKa means stronger acid.

Don't mix this up

Students often compare acidity using only molecular size and forget that substituent effect and carboxylate ion stability are the main reasons.

Exam tip

For acidity order, compare electron-withdrawing strength, distance of substituent from -COOH, and number of electron-withdrawing groups.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Asked as acidity order, pKa interpretation, reason for stronger acid, preparation from nitriles or aldehydes, and conversion-based questions.

1
Enough for 1 mark
Carboxylic acids are organic compounds containing the -COOH group. They can be prepared by oxidation of alcohols or aldehydes, hydrolysis of nitriles, and related routes, and they are acidic due to resonance stabilisation of the carboxylate ion.
2
Add for 2 marks
Example: Chloroacetic acid is stronger than acetic acid because the chlorine atom withdraws electron density and stabilises the carboxylate ion.
3
Add for 3 marks
The acidity of carboxylic acids depends on the stability of the conjugate base, RCOO-.

MCQ Quiz

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