Distinguish between the structures and chemical properties of a primary alcohol and a carboxylic acid, with reference to their functional groups.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A primary alcohol has a hydroxyl group (–OH) attached to a carbon that is bonded to at most one other carbon.
A carboxylic acid has a carboxyl group (–COOH), consisting of a carbonyl (C=O) and a hydroxyl (–OH) on the same carbon.
Carboxylic acids are acidic and can donate a proton from the –COOH to give a carboxylate ion, whereas primary alcohols are not acidic under normal conditions and do not readily donate a proton.
Primary alcohols can be oxidised (e.g. by acidified potassium dichromate) first to an aldehyde and then to a carboxylic acid, whereas carboxylic acids resist further oxidation under normal laboratory conditions.
Carboxylic acids undergo esterification with alcohols to give esters and water; this condensation does not occur between two alcohol molecules under the same conditions.
A carboxylic acid has a carboxyl group (–COOH), consisting of a carbonyl (C=O) and a hydroxyl (–OH) on the same carbon.
Carboxylic acids are acidic and can donate a proton from the –COOH to give a carboxylate ion, whereas primary alcohols are not acidic under normal conditions and do not readily donate a proton.
Primary alcohols can be oxidised (e.g. by acidified potassium dichromate) first to an aldehyde and then to a carboxylic acid, whereas carboxylic acids resist further oxidation under normal laboratory conditions.
Carboxylic acids undergo esterification with alcohols to give esters and water; this condensation does not occur between two alcohol molecules under the same conditions.
Examiner tips
- Use the exact functional group names (hydroxyl, carboxyl).
- Show the acid–base difference clearly.
- Mention oxidation and esterification as key property contrasts.
- Keep the answer concise and point‑wise.
Common mistakes
- Confusing the structure of the carboxyl group (missing the C=O).
- Claiming alcohols are acidic or that acids can be further oxidised.
Mark scheme (4 marks)
- A primary alcohol contains the hydroxyl functional group (–OH) attached to a carbon bonded to at most one other carbon, whereas a carboxylic acid contains the carboxyl functional group (–COOH), which consists of a carbonyl (C=O) and a hydroxyl (–OH) on the same carbon.
- Carboxylic acids are acidic and can donate a proton (H⁺) from the –COOH group to form a carboxylate ion, whereas primary alcohols are not acidic under normal conditions and do not readily donate a proton.
- Primary alcohols can be oxidised (e.g. by acidified potassium dichromate) first to an aldehyde and then to a carboxylic acid, whereas carboxylic acids resist further oxidation under normal laboratory conditions.
- Carboxylic acids undergo condensation reactions with alcohols (esterification) to form esters and water, a reaction not possible between two alcohol molecules under the same conditions.
Key terms in this question
primary alcohol · carboxylic acid
Related
- All IB DP Chemistry Standard Level (2023 syllabus) revision notes →
- How to answer a "Distinguish" question →
- Decode the mark scheme abbreviations →
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