Free Preparation of Aldehydes and Ketones MCQs with Answers

1. Ethanal can be prepared industrially by the oxidation of

• A. ethanol
• B. ethanoic acid
• C. ethene with hydrogen
• D. propanone

Explanation: Oxidising the primary alcohol ethanol with acidified dichromate, distilling off the product as it forms, gives ethanal, and the same conversion is achieved industrially by passing ethanol vapour over hot copper. Oxidation of ethanoic acid is not possible, since it is already at a higher oxidation level. The general rule is that primary alcohols give aldehydes and secondary alcohols give ketones.

2. Propanone is prepared by the oxidation of

• A. propan-1-ol
• B. propane
• C. propan-2-ol
• D. propanoic acid

Explanation: Propan-2-ol is a secondary alcohol, so removing the hydrogen from the carbinol carbon leaves a ketone, and the reaction stops there because no further hydrogen remains on that carbon. Propan-1-ol, being primary, would give propanal and then propanoic acid instead. Most propanone is actually obtained as the by product of the cumene process for phenol.

3. Methanal, HCHO, is used in a 40 per cent aqueous solution known as formalin, mainly for

• A. preserving biological specimens
• B. flavouring food
• C. treating drinking water
• D. use as a fuel

Explanation: Methanal cross links the proteins of tissue, hardening it and killing microorganisms, which is why formalin preserves specimens and is also used to make thermosetting resins such as bakelite. It is a gas at room temperature, so the aqueous solution is the practical form. The compound is toxic and is now treated as a carcinogen, so exposure is controlled.

4. Propanone is widely used in industry and in the laboratory as

• A. a solvent, because it dissolves both polar and non polar substances and evaporates quickly
• B. an oxidising agent
• C. a fuel for vehicles
• D. a food preservative

Explanation: Its polar carbonyl group and small hydrocarbon portion let propanone dissolve a wide range of materials, and its low boiling point of 56 degrees Celsius means it dries rapidly, which is why it is used as nail polish remover and for cleaning glassware. It is also miscible with water in all proportions. Being highly flammable, it must be kept away from flames.