Free Phenols MCQs with Answers

1. Phenol is more acidic than ethanol because

• A. phenol contains more oxygen
• B. phenol is a solid at room temperature
• C. the hydroxyl group in phenol is attached to an sp3 carbon
• D. the phenoxide ion is stabilised by delocalisation of the negative charge into the benzene ring

Explanation: Losing the hydroxyl hydrogen gives an ion whose negative charge is spread over the aromatic ring rather than concentrated on one oxygen, so the ion is far more stable and its formation is much more favourable. An ethoxide ion has no such delocalisation available. This is why phenol dissolves in sodium hydroxide solution while ethanol does not.

2. Phenol reacts with sodium hydroxide solution but not with sodium carbonate solution, which shows that phenol is

• A. a stronger acid than carbonic acid
• B. a weaker acid than carbonic acid but strong enough to react with a strong base
• C. not acidic at all
• D. a base

Explanation: Phenol is acidic enough to be neutralised by a strong alkali, giving sodium phenoxide, but too weak to displace carbon dioxide from a carbonate, which a carboxylic acid can do. This pair of tests therefore distinguishes a phenol from a carboxylic acid in the laboratory. Ethanol reacts with neither reagent, so the three classes can be told apart in turn.

3. Phenol reacts with bromine water at room temperature to give an immediate white precipitate of

• A. bromobenzene
• B. phenyl bromide
• C. 2,4,6-tribromophenol
• D. benzene hexabromide

Explanation: The hydroxyl group is strongly activating and directs substitution to the two and four positions, so all three are brominated at once without any catalyst, whereas benzene itself requires a halogen carrier and gives only monosubstitution. The white precipitate is a standard test for phenol. Activation arises because a lone pair on oxygen is donated into the ring, raising its electron density.

4. The reaction of phenol with neutral iron three chloride solution gives

• A. a violet coloured complex
• B. a white precipitate
• C. carbon dioxide gas
• D. no visible change

Explanation: Phenols form a coloured complex with iron three ions, usually violet, and this is a quick confirmatory test for the phenolic hydroxyl group. Alcohols give no colour at all. The colour varies with the particular phenol, which is why the general term coloured complex is safer than naming one shade.

5. Phenol is obtained industrially in large quantity by the

• A. hydration of benzene
• B. oxidation of toluene
• C. cumene process, which also yields propanone
• D. fermentation of glucose

Explanation: Benzene is alkylated with propene to give cumene, which is oxidised and then cleaved with acid into phenol and propanone, so two useful products come from one process and it dominates world production. Benzene cannot simply be hydrated, since the ring resists addition. Phenol is the starting material for resins, aspirin and polycarbonates.

6. Which reagent distinguishes ethanol from phenol?

• A. Sodium metal
• B. Bromine water
• C. Water
• D. A lighted splint

Explanation: Phenol decolourises bromine water immediately and gives a white precipitate of the tribromo compound, whereas ethanol produces no visible change. Sodium metal is not suitable, since both compounds contain a hydroxyl group and both release hydrogen with it. Neutral iron three chloride is the other reliable distinguishing test.

7. Warming a phenol with a carboxylic acid gives an ester only with difficulty, whereas an alcohol reacts readily. This is because

• A. phenols contain no hydroxyl group
• B. the phenolic oxygen donates its lone pair into the ring, making it a poorer nucleophile
• C. phenols are stronger bases than alcohols
• D. esters cannot contain a benzene ring

Explanation: Delocalisation of the oxygen lone pair into the aromatic system, the very effect that makes phenol acidic, leaves that oxygen less available to attack the carbonyl carbon of an acid. Phenyl esters are therefore prepared using more reactive acyl chlorides or anhydrides instead. The same delocalisation also shortens and strengthens the carbon to oxygen bond in phenol.

8. Which of the following best explains why phenol reacts with aqueous NaOH, but alcohols do not?

• A. Phenol has a lower boiling point than alcohol
• B. Alcohols contain a strong O-H bond that cannot be broken by weak bases
• C. Phenol form hydrogen bonding that facilitates ionization
• D. Phenol is weakly acidic due to resonance stabilization of phenoxide ion

Explanation: Losing the hydroxyl hydrogen gives phenol an ion whose negative charge is delocalised into the benzene ring, so the phenoxide ion is far more stable than an alkoxide and the acid dissociates readily enough to be neutralised by sodium hydroxide. An alkoxide has no such delocalisation, so ethanol is not acidic enough to react. Both compounds hydrogen bond, so that cannot be the deciding factor.