Number of constitutional isomers for a given carbon skeleton

aldehyde

ketone

Ketones and aldehydes are compounds that contain a carbonyl group (C=O).

Aldehydes have the carbonyl group at the end of a carbon chain.

Ketones have the carbonyl group in the middle of the chain.

There are two fewer hydrogens at the carbonyl carbon than there would have been in the corresponding alkane (CH₃CH₂CH₃).

C – C – C – C

The number of different aldehydes for a given carbon skeleton depends on the number of different terminal carbons.

There is only one aldehyde for a four carbon chain because the terminal carbons are equivalent (see structure).

A linear C₄ carbon skeleton has two different bonding environments for carbon (red and blue).

	C \|
C —	C — C

The number of ketone isomers depends on how many non-equivalent carbons are in the middle of the chain.

There is also only one ketone for a linear four-carbon chain because the two central carbons are equivalent (see structure).

When assessing the number of different bonding environments for a carbon skeleton, it is important to realise that

any linear chain is symmetrical about its centre,
thus bonding enviroments for carbon are the same to the left and the right of the centre.

Thus there is only one ketone isomer and one aldehyde isomer possible for a four-carbon chain.

branches of the same composition that are bonded to the same carbon are equivalent.

There is only one C₄ branched carbonyl compound (aldehyde) because all terminal carbons are equivalent.
There are no C₄ branched ketones since the carbon at the centre of the chain is already bonded to three carbons.