Free 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. The addition of hydrogen cyanide to ethanal gives

• A. ethanoic acid
• B. 2-hydroxypropanenitrile
• C. ethanol
• D. propanone

Explanation: The cyanide ion attacks the carbonyl carbon and the resulting alkoxide picks up a proton, giving a hydroxynitrile with one more carbon than the starting aldehyde. The reaction is catalysed by a trace of base, which generates the cyanide nucleophile. The product can be hydrolysed to a hydroxy acid, making this a useful chain lengthening step.

6. Reduction of a ketone with sodium borohydride or lithium aluminium hydride gives

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

Explanation: The hydride ion adds to the carbonyl carbon, which then carries two alkyl groups and a hydroxyl, so the product is a secondary alcohol. An aldehyde reduced in the same way gives a primary alcohol. Reduction is simply the reverse of the oxidation that produced the carbonyl compound in the first place.

7. 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.

8. 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.

9. 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.

10. 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.