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

Answer:

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Since the net formula of a #C_4# alkane is #C_4H_10#, apparently two #H#'s have been replaced by a double-bonded #O#.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):
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#CH_3-CH_2-CH_2-CHO#
butanal or (butyric aldehyde)
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#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.

Aug 17, 2015

Answer:

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:
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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.