5 - The effect of heteroatoms

Summary

Heteroatoms in organic chemistry are defined as those atoms other than carbon or hydrogen. In theory, this leaves over a hundred possibilities, but in practice the number of atoms likely to be encountered in organic molecules is much more limited. We shall look at some of the most important heteroatoms in this chapter.

Heteroatoms have a profound effect on the reactivity of organic molecules. Hydrocarbons have only a limited range of reactions, and so although organic chemistry is often defined as the chemistry of carbon, it is the other elements that make it interesting. In this chapter we shall look at how heteroatoms affect the reactivity of organic molecules. While the great variety of organic molecules may at first seem bewildering, when molecules are considered in terms of their heteroatoms and the effects that these have, then the reactivity patterns observed are in fact quite logical, and consequently easy to study.

Oxygen

Oxygen has two main properties that shape its reactivity in organic chemistry: it is electronegative and has electron lone pairs. The electronegativity of oxygen means that organic molecules containing oxygen are polarized such that the oxygen atom usually bears a partial negative charge. This is particularly marked in carbonyl compounds, which have a double bond between oxygen and carbon.

This property also makes it possible for oxygen to form anions (recall from Section 2.2.2 that more electronegative atoms form anions more easily). Oxygen anions are intermediates in many of the reactions of oxygen-containing organic molecules.