Questions: Aldehyde and Ketone Structure and Nomenclature

Question 1

Why are aldehydes generally more reactive than ketones in nucleophilic addition reactions?

• A. Aldehydes have higher molecular weight, giving them more kinetic energy
• B. Ketones contain a hydrogen on the carbonyl, making them more sterically hindered
• C. Aldehydes have only one alkyl group on the carbonyl carbon — less steric crowding and less electron donation than ketones with two alkyl groups
• D. Aldehydes are less oxidized, so the carbonyl carbon carries less electrophilic character

Answer: (C)

Two factors make the aldehyde carbonyl carbon more electrophilic and more accessible to nucleophiles. First, less steric bulk: only one alkyl group flanks the carbonyl (versus two in ketones), so incoming nucleophiles face less obstruction. Second, less electron donation: alkyl groups push electron density toward the carbonyl carbon through induction, partially neutralizing its positive character. Two alkyl groups in a ketone donate more than one in an aldehyde, making the ketone less electrophilic. Option B reverses the structure — it is aldehydes, not ketones, that have a hydrogen on the carbonyl carbon.

Question 2

A student needs to name the compound CH₃CH₂CH₂CHO using IUPAC rules. Which name is correct?

• A. 1-butanone
• B. butanal
• C. butan-1-one
• D. 4-oxobutane

Answer: (B)

The compound has a four-carbon chain with a carbonyl at one end bearing a hydrogen — an aldehyde. The IUPAC suffix -al replaces the terminal -e of the parent alkane name (butane → butanal). No position number is needed because -al always places the carbonyl at carbon 1; there is no other position possible. '1-butanone' and 'butan-1-one' would name a ketone, which requires the carbonyl to be bonded to two carbon groups — not a hydrogen. '4-oxobutane' is not standard IUPAC nomenclature for this compound.

Question 3

Aldehydes can be oxidized to carboxylic acids under mild conditions, but ketones cannot be oxidized the same way.

• T. True
• F. False

Answer: True

An aldehyde has a hydrogen on the carbonyl carbon; oxidation removes this hydrogen and adds a hydroxyl group, converting R-CHO to R-COOH (a carboxylic acid). A ketone has two alkyl groups on the carbonyl and no hydrogen to remove — oxidation to a higher state would require breaking a carbon-carbon bond, which demands harsh conditions and cleaves the molecule. This difference is exploited analytically: Tollens' reagent and Fehling's test distinguish aldehydes from ketones precisely because aldehydes are oxidized and ketones are not.

Question 4

The compound propan-2-one (acetone) is a ketone because its carbonyl carbon is bonded to two hydrogen atoms.

• T. True
• F. False

Answer: False

Acetone (propan-2-one) has the structure CH₃-CO-CH₃: the carbonyl carbon is bonded to two methyl groups, not two hydrogens. That is precisely what makes it a ketone — two alkyl (or aryl) groups on the carbonyl, with no hydrogen directly attached. If a hydrogen were on the carbonyl carbon, the compound would be an aldehyde. The confusion may arise from associating 'two groups' with hydrogens, but the defining groups for ketones are carbon-containing substituents.

Question 5

Explain in your own words why the structural difference between aldehydes and ketones — one versus two alkyl groups on the carbonyl carbon — produces real differences in chemical reactivity.

Model answer: The aldehyde has a hydrogen on the carbonyl carbon; the ketone has two alkyl groups. This matters for two reasons: (1) Reactivity in nucleophilic addition — the aldehyde carbonyl has less steric shielding and less electron donation from substituents, making the carbonyl carbon a better electrophile and more accessible to nucleophilic attack. (2) Oxidation — the aldehyde hydrogen can be removed in oxidation, converting R-CHO to a carboxylic acid. Ketones lack this hydrogen and resist oxidation under ordinary conditions.

A seemingly small structural difference — one vs. two alkyl groups, presence vs. absence of a carbonyl hydrogen — propagates into two key chemical distinctions: susceptibility to nucleophilic addition and ability to be oxidized. Understanding these consequences is more important than memorizing the structural difference alone, because it predicts an entire network of reactions in carbonyl chemistry.