Question 1

Which hydrogen is acidic in a carboxylic acid?

Accepted Answer

The hydrogen attached to the hydroxyl oxygen in COOH is acidic. It leaves as H+ when the acid ionises, while hydrogens attached directly to carbon remain part of the organic framework.

Question 2

Why is the carboxylate ion resonance-stabilised?

Accepted Answer

The carboxylate ion has two equivalent resonance contributors: O-–C=O and O=C–O-. The negative charge is delocalised over two oxygen atoms, so the two C-O bonds are equivalent rather than one permanently single and one permanently double.

Question 3

How does the -I effect change carboxylic acid acidity?

Accepted Answer

Electron-withdrawing groups pull electron density away from the carboxyl group and stabilise the carboxylate ion. This increases acid strength and lowers pKa. The effect becomes weaker as the withdrawing group moves farther from COOH.

Question 4

Why is trifluoroacetic acid much stronger than acetic acid?

Accepted Answer

CF3COOH contains three fluorines, whose cumulative -I effect strongly stabilises the carboxylate ion. Its pKa is 0.23, compared with 4.76 for acetic acid, so it is much more acidic.

Question 5

Are benzoic acid and phenylacetic acid the same compound?

Accepted Answer

No. Benzoic acid has phenyl directly attached to COOH and has pKa 4.20. Phenylacetic acid has a CH2 group between phenyl and COOH and has pKa 4.31. Their placement of the phenyl group is therefore chemically important.

Carboxylic Acid Acidity: Substituents Tug on the pKa

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