Free Alcohols and Phenols MCQs with Answers

1. Alcohols have much higher boiling points than alkanes of similar molar mass because

• A. their molecules are hydrogen bonded to one another
• B. they are ionic compounds
• C. their covalent bonds are stronger
• D. they have larger molecules

Explanation: The hydroxyl group allows hydrogen bonding between molecules, and a great deal of extra energy is needed to break those bonds before the liquid can boil, so ethanol boils at 78 degrees Celsius while propane, of similar mass, boils at minus 42. The same hydrogen bonding makes the shorter alcohols completely miscible with water. Boiling point does not depend on the strength of bonds inside the molecule.

2. Oxidation of a primary alcohol with acidified potassium dichromate, distilling the product as it forms, gives

• A. a ketone
• B. an aldehyde
• C. a carboxylic acid
• D. an alkene

Explanation: Distilling removes the aldehyde as soon as it is produced, before it can be oxidised further, whereas heating under reflux keeps it in the flask and the reaction continues to the carboxylic acid. The orange dichromate turns green as chromium is reduced from plus 6 to plus 3. Secondary alcohols stop at the ketone, since no hydrogen remains on the carbonyl carbon.

3. A tertiary alcohol is not readily oxidised because the carbon carrying the hydroxyl group

• A. carries a positive charge
• B. is attached to a halogen
• C. has no hydrogen atom attached to it
• D. is part of a benzene ring

Explanation: Oxidation of an alcohol involves removing the hydroxyl hydrogen together with a hydrogen from the same carbon, and in a tertiary alcohol that carbon holds three alkyl groups and no hydrogen at all. It can only be oxidised under conditions vigorous enough to break carbon to carbon bonds. This difference is the basis of using acidified dichromate to distinguish the three classes of alcohol.

4. Dehydration of ethanol with concentrated sulphuric acid at about 170 degrees Celsius gives

• A. ethane
• B. ethanoic acid
• C. ethoxyethane
• D. ethene

Explanation: At the higher temperature water is eliminated from one molecule to give the alkene, while at about 140 degrees two molecules condense instead and the product is the ether ethoxyethane. Aluminium oxide at 400 degrees achieves the same dehydration catalytically. Controlling temperature to select between two products is a recurring theme in this topic.

5. Ethanol reacts with sodium metal to give

• A. sodium ethoxide and hydrogen
• B. sodium ethanoate and water
• C. ethene and sodium hydroxide
• D. no reaction at all

Explanation: The hydroxyl hydrogen is weakly acidic, so sodium displaces it to give an ionic alkoxide and hydrogen gas, though the reaction is far gentler than that of sodium with water because an alcohol is a weaker acid. The steady effervescence without ignition is a useful test for a hydroxyl group in a dry sample. Alkoxides are strong bases and are hydrolysed by water back to the alcohol.

6. The reaction of ethanol with ethanoic acid in the presence of a little concentrated sulphuric acid produces

• A. an aldehyde
• B. an ester with a fruity smell
• C. an alkene
• D. a soap

Explanation: Esterification joins the acid and the alcohol with the loss of water, giving ethyl ethanoate, and the sulphuric acid acts both as catalyst and as a dehydrating agent to shift the equilibrium to the right. The reaction is reversible, so the yield is never complete. Esters of this kind are used as flavourings and solvents.

7. 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.

8. 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.

9. 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.

10. 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.