Free Alcohols and Phenols MCQs with Answers

11. 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.

12. 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.

13. 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.

14. 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.

15. 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.

16. 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.

17. 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.

18. The oxidation of propan-2-ol with acidified potassium dichromate gives

• A. propanal
• B. propanoic acid
• C. propene
• D. propanone

Explanation: Propan-2-ol is a secondary alcohol, so oxidation removes the single hydrogen on the carbinol carbon and leaves a ketone, which resists further oxidation because no hydrogen remains on that carbon. A primary alcohol would give an aldehyde and then an acid. This difference in oxidation product is the standard way of identifying the class of an unknown alcohol.

19. Alcohols with short carbon chains are soluble in water, but solubility falls sharply as the chain lengthens because

• A. the hydroxyl group disappears in longer molecules
• B. long chain alcohols are gases
• C. the growing non polar hydrocarbon chain cannot form hydrogen bonds with water and dominates the molecule
• D. longer alcohols react with water

Explanation: Methanol, ethanol and propan-1-ol mix with water in all proportions because the hydroxyl group hydrogen bonds effectively, but as the chain grows the hydrophobic portion outweighs it and solubility falls until the higher alcohols are essentially insoluble. Branching increases solubility slightly by making the molecule more compact. The same principle explains why soaps need a hydrophilic head on a long tail.

20. 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.