Free Alcohols 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. 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.

9. 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.

10. 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.