Free Reactivity of Aldehydes and Ketones MCQs with Answers

1. The carbonyl carbon of an aldehyde or ketone is attacked by nucleophiles because

• A. it carries a partial positive charge, oxygen being more electronegative
• B. it carries a partial negative charge
• C. it has a lone pair of electrons
• D. it is bonded to hydrogen

Explanation: The carbon to oxygen double bond is strongly polarised towards oxygen, leaving the carbon electron deficient and open to attack by a species with a lone pair. This is why the characteristic reaction of the carbonyl group is nucleophilic addition, in contrast with the electrophilic addition of alkenes, whose double bond is non polar. The oxygen carries the lone pairs and the partial negative charge.

2. Aldehydes are more reactive than ketones towards nucleophilic addition because in a ketone

• A. the carbonyl group is absent
• B. two alkyl groups release electron density towards the carbonyl carbon and also hinder attack
• C. the carbonyl carbon carries a full positive charge
• D. the carbon to oxygen bond is much longer

Explanation: Alkyl groups push electron density towards the carbonyl carbon, reducing its positive character, and their bulk physically blocks the approaching nucleophile, so two of them make a ketone markedly less reactive than an aldehyde with only one. Methanal, having none at all, is the most reactive carbonyl compound of the series. Both electronic and steric factors point the same way here.

3. Tollens reagent, ammoniacal silver nitrate, gives a silver mirror with

• A. ketones only
• B. both aldehydes and ketones
• C. aldehydes only
• D. neither aldehydes nor ketones

Explanation: An aldehyde still has a hydrogen on the carbonyl carbon, so it is easily oxidised to a carboxylic acid while the silver ions are reduced to metallic silver, which deposits as a mirror on clean glass. A ketone has no such hydrogen, so it resists oxidation and gives no reaction. This is the standard test for distinguishing the two classes.

4. Fehling's solution is reduced to a red precipitate of copper one oxide by

• A. propanone
• B. benzaldehyde
• C. ethanol
• D. ethanal

Explanation: Aliphatic aldehydes such as ethanal reduce the blue copper two complex to a brick red precipitate, whereas ketones do not react at all. Aromatic aldehydes such as benzaldehyde are the notable exception, since they give a negative result with Fehling's solution although they do respond to Tollens reagent. Ethanol must first be oxidised to an aldehyde before it could react.

5. Aldehydes and ketones both react with 2,4-dinitrophenylhydrazine to give

• A. a silver mirror
• B. a colourless solution
• C. carbon dioxide
• D. an orange crystalline precipitate

Explanation: This reagent, often called Brady's reagent, condenses with any carbonyl group to give a bright orange or yellow solid, so it detects the presence of the group without distinguishing aldehyde from ketone. The precipitate has a sharp melting point characteristic of the parent compound, which allows identification. Tollens or Fehling's is then used to decide which of the two classes it is.

6. Which compound gives a positive iodoform test?

• A. Propanal
• B. Propanone
• C. Benzaldehyde
• D. Methanal

Explanation: The test requires a methyl group attached directly to the carbonyl carbon, and propanone is the simplest methyl ketone, giving the pale yellow precipitate of triiodomethane with iodine in alkali. Ethanal is the only aldehyde that responds, since it too has that methyl group. Propanal, methanal and benzaldehyde all lack the required structure.

7. 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.

8. 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.

9. 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.

10. 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.