Free Aldehydes and Ketones MCQs with Answers

11. Aldehydes and ketones have lower boiling points than alcohols of similar molar mass because

• A. they have smaller molecules
• B. they are non polar
• C. they are ionic
• D. they cannot hydrogen bond to one another, having no hydrogen attached to oxygen

Explanation: Carbonyl compounds are polar and attract one another by dipole to dipole forces, which lifts their boiling points above those of alkanes, but with no O-H bond they cannot form hydrogen bonds between their own molecules and so boil below the corresponding alcohols. They can still accept hydrogen bonds from water, which is why the smaller members are water soluble. Propanone dissolving in water while boiling at only 56 degrees illustrates both points.

12. The reaction of a carbonyl compound with sodium hydrogen sulphite gives a crystalline adduct that is useful for

• A. measuring the pH of the compound
• B. separating and purifying the carbonyl compound, since the adduct can be decomposed to regenerate it
• C. converting the compound into an alkane
• D. detecting the presence of a benzene ring

Explanation: The bisulphite adduct crystallises out and can be filtered off, and treating it with dilute acid or alkali releases the original aldehyde or ketone in a pure state. Only aldehydes, methyl ketones and a few cyclic ketones form the adduct, because larger groups hinder the approach of the bulky nucleophile. It is therefore a purification method and, incidentally, a structural test.

13. In nucleophilic addition to a carbonyl group, the carbon atom changes hybridisation from

• A. sp3 to sp2
• B. sp to sp2
• C. sp2 to sp3
• D. sp3 to sp

Explanation: The carbonyl carbon starts as trigonal planar sp2 with a bond angle of 120 degrees, and when the nucleophile adds it acquires a fourth group and becomes tetrahedral sp3 with angles near 109.5 degrees. This geometrical change is why bulky groups around the carbonyl slow the reaction down so much. Elimination reactions run the same change in reverse.

14. 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.

15. Which of the following compounds is a ketone?

• A. CH3CHO
• B. CH3COCH3
• C. CH3COOH
• D. CH3CH2OH

Explanation: In propanone the carbonyl group lies between two carbon atoms, which is the defining feature of a ketone, while CH3CHO is the aldehyde ethanal with the carbonyl at the end of the chain. CH3COOH is a carboxylic acid, whose carbonyl carbon also carries a hydroxyl group, and CH3CH2OH is an alcohol. Reading which atoms flank the carbonyl carbon is the whole test.

16. Oxidation of an aldehyde with acidified potassium permanganate gives

• A. a primary alcohol
• B. a ketone
• C. a carboxylic acid with the same number of carbon atoms
• D. carbon dioxide only

Explanation: The hydrogen on the carbonyl carbon is replaced by a hydroxyl group, so ethanal becomes ethanoic acid with no change in the length of the chain. This ready oxidation is why aldehydes act as reducing agents and give positive results with Tollens and Fehling's reagents. Ketones cannot be oxidised in this way without breaking the carbon skeleton.

17. The compound formed when ethanal reacts with a Grignard reagent, followed by acid hydrolysis, is

• A. a primary alcohol
• B. a secondary alcohol
• C. a tertiary alcohol
• D. a carboxylic acid

Explanation: The alkyl group of the Grignard reagent adds to the carbonyl carbon, which then carries that new group, the original methyl group, a hydrogen and a hydroxyl, making the product a secondary alcohol. Methanal would give a primary alcohol and a ketone would give a tertiary one. This makes Grignard reagents a general method for building alcohols of any class.

18. Benzaldehyde gives a positive test with Tollens reagent but a negative one with Fehling's solution. This shows that

• A. benzaldehyde is a ketone
• B. benzaldehyde contains no carbonyl group
• C. Fehling's solution reacts only with aromatic compounds
• D. aromatic aldehydes are oxidised less readily than aliphatic aldehydes

Explanation: Fehling's solution is the milder oxidising agent of the two, so it fails with aromatic aldehydes whose carbonyl group is stabilised by conjugation with the ring, while the stronger Tollens reagent still succeeds. Benzaldehyde is definitely an aldehyde, as the silver mirror confirms. This exception is a favourite examination point precisely because it looks like a contradiction.

19. Aldehydes and ketones both react with hydroxylamine to form

• A. oximes
• B. esters
• C. alcohols
• D. amines

Explanation: The nitrogen of hydroxylamine attacks the carbonyl carbon and water is then eliminated, so the product contains a carbon to nitrogen double bond and is called an oxime. Hydrazine and its derivatives react in the same condensation pattern to give hydrazones. These crystalline derivatives were once the standard way to identify a carbonyl compound from its melting point.

20. 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.