All Free Chemistry MCQs with Answers

271. In the iodoform test, a yellow precipitate is given by

• A. any primary alcohol
• B. methanol
• C. ethanol and other alcohols containing the CH3CH(OH) group
• D. all tertiary alcohols

Explanation: The test needs a methyl group attached to the carbon bearing the hydroxyl, so ethanol and propan-2-ol respond while methanol and propan-1-ol do not. Iodine in alkali oxidises and then cleaves the molecule, leaving the yellow crystalline triiodomethane. Methyl ketones give the same positive result for the same structural reason.

272. 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.

273. 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.

274. 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.

275. 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.

276. Which of the following is a secondary alcohol?

• A. CH3CH2CH2OH
• B. CH3CH(OH)CH3
• C. (CH3)3COH
• D. CH3OH

Explanation: In propan-2-ol the carbon carrying the hydroxyl group is attached to two other carbons, which is the definition of secondary. Propan-1-ol is primary with one attached carbon, the tertiary butyl compound has three, and methanol has none at all. Classifying the alcohol first is essential, since it determines the oxidation product.

277. Industrial ethanol is manufactured from ethene by

• A. fermentation with yeast
• B. oxidation with acidified dichromate
• C. reduction with hydrogen
• D. direct hydration with steam over a phosphoric acid catalyst

Explanation: Steam and ethene are passed over phosphoric acid at about 300 degrees Celsius and 60 atmospheres, giving a pure product continuously from a petroleum feedstock. Fermentation is the alternative route, renewable and much older, but slower and giving a dilute product that must be distilled. The choice between them is essentially economic.

278. Methanol is far more toxic than ethanol because in the body it is oxidised to

• A. carbon dioxide and water
• B. methanoic acid and methanal, which damage the optic nerve
• C. ethanol
• D. glucose

Explanation: The same enzyme that handles ethanol converts methanol into methanal and then methanoic acid, and these attack the optic nerve and cause acidosis, so blindness or death can follow a small dose. Treatment works by giving ethanol, which competes for the enzyme and lets methanol be excreted unchanged. This is why methylated spirit is deliberately made undrinkable.

279. Ethylene glycol, used as antifreeze, is classified as

• A. a monohydric alcohol
• B. a dihydric alcohol
• C. a trihydric alcohol
• D. a phenol

Explanation: Ethylene glycol contains two hydroxyl groups on adjacent carbons, so it is dihydric, while glycerol with three hydroxyls is trihydric and ethanol with one is monohydric. Extra hydroxyl groups mean more hydrogen bonding, which is why glycol is viscous, high boiling and completely miscible with water. Those same properties make it effective as an antifreeze and coolant.

280. 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.