Question

Write the structural formulae of the two isomers with the molecular formula `C_4H_8O` to illustrate functional group isomerism?

Answer 1

At first, just disregard the `H`'s. You use them later on to complete the valencies of the other atoms.

Explanation:

Since the net formula of a `C_4` alkane is `C_4H_10`, apparently two `H`'s have been replaced by a double-bonded `O`.

This can be done in only two different ways: at the end or somewhere in the middle.

Your isomers are (pictures from Wikipedia):

`CH_3-CH_2-CH_2-CHO`
butanal or (butyric aldehyde)

`CH_3-CO-CH_2-CH_3`
butanone (or methyl ethyl ketone)

The functional difference between aldehydes and ketones is that only the aldehyde can easily be oxidised to form a carbonic acid, in this case butanoic acid (or butyric acid). The ketones can only be destructively oxidised, by more powerful reactants.

Answer 2

An alternative way of assessing organic formulae is to invoke degrees of unsaturation. More isomers are possible.

Explanation:

As explained in the linky, `C_4H_8O` has 1 degree of unsaturation. Each degree of unsaturation corresponds to a double bond (either `C=C`, or `C=O`), `OR` a ring (a ring junction means 2 hydrogens are no longer needed for the chain termini. The alternative answer is indeed correct to identify the ketone and aldehyde as possible structural formulae. However, `C_4H_8O` may also represent a 5-membered ring, ether tetrahydrofuran:

THF (so-called) is a solvent that is widely used in organic and organometallic chemistry. It is water soluble, and higher boiling than diethyl ether, and its structure illustrates the richness of organic chemistry: only a few atoms, but a wealth of possible structural formulae.