What is the aldehyde functional group, and where does it sit in a molecule?

A carbonyl group, C=O, with at least one hydrogen atom attached to the carbonyl carbon. It therefore sits at the end of a carbon chain, and the name ends in -al.

What is the ketone functional group, and where does it sit in a molecule?

A carbonyl group, C=O, with a carbon atom attached on each side. It therefore sits within the chain rather than at the end, and the name ends in -one.

How does the absence of hydrogen bonding affect the boiling temperatures of carbonyl compounds?

Their boiling temperatures are lower than those of alcohols of similar relative molecular mass, because only permanent dipole and London forces need be overcome, but higher than those of alkanes, because the polar C=O group gives a permanent dipole.

Why is the carbonyl carbon open to attack by a nucleophile?

Oxygen is considerably more electronegative than carbon, so the C=O bond is strongly polarised and the carbon carries a partial positive charge that attracts an electron-rich species.

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Aldehydes, ketones and their oxidation

3 things to know
Papers 2 and 3

Overview

Recall questions on naming and structure, on physical properties, and on the Tollens', Fehling's and dichromate tests that distinguish an aldehyde from a ketone.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Process

Which tests distinguish an aldehyde from a ketone?

Key term

Why can aldehydes and ketones not form hydrogen bonds with one another?

Key term

Why are the shorter aldehydes and ketones soluble in water?

Questions from this subtopic

Each one with the model answer underneath.

1
What is the aldehyde functional group, and where does it sit in a molecule?
Model answer
A carbonyl group, C=O, with at least one hydrogen atom attached to the carbonyl carbon. It therefore sits at the end of a carbon chain, and the name ends in -al.
2
What is the ketone functional group, and where does it sit in a molecule?
Model answer
A carbonyl group, C=O, with a carbon atom attached on each side. It therefore sits within the chain rather than at the end, and the name ends in -one.
3
Why can aldehydes and ketones not form hydrogen bonds with one another?
Model answer
Hydrogen bonding requires a hydrogen atom bonded directly to oxygen, nitrogen or fluorine. In a carbonyl compound every hydrogen is bonded to carbon, so there is no sufficiently positive hydrogen for a bond to form.
4
How does the absence of hydrogen bonding affect the boiling temperatures of carbonyl compounds?
Model answer
Their boiling temperatures are lower than those of alcohols of similar relative molecular mass, because only permanent dipole and London forces need be overcome, but higher than those of alkanes, because the polar C=O group gives a permanent dipole.
5
Why are the shorter aldehydes and ketones soluble in water?
Model answer
The lone pairs on the carbonyl oxygen can accept a hydrogen bond from a water molecule. As the carbon chain lengthens the non-polar part dominates and solubility falls.
6
Why is the carbonyl carbon open to attack by a nucleophile?
Model answer
Oxygen is considerably more electronegative than carbon, so the C=O bond is strongly polarised and the carbon carries a partial positive charge that attracts an electron-rich species.

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