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

2 Answers
Jul 27, 2015

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


Since the net formula of a #C_4# alkane is #C_4H_10#, apparently two #H#'s have been replaced by a double-bonded #O#.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):
enter image source here
butanal or (butyric aldehyde)
enter image source here
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.

Aug 17, 2015

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


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:
enter image source here

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.