After mechanisms had been developed which brought order and reason to many organic reactions, a few were still elusive.  They did not appear to proceed with a polar or radical pathway.  A famous example of one such reaction is the Diels-Alder reaction:

This involves the addition of an alkene to a conjugated diene.

Reactions such as the Diels-Alder have one very important feature about them; they are concerted, i.e. all bond breaking and making processes occur at the same time.  There is a cyclic transition state, in this case that it involves 6 pi electrons.  Pericyclic is the term used to describe such reactions that are concerted and proceed via a cyclic transition state.

It is quite often the case that these reactions can be induced thermally, but not photochemically, or vice versa.  An example of a photochemically induced reaction is:

Some molecules can be induced to react thermally and photochemically, but end up with different products depending on which method is used. e.g.:

It is also found that these types of reactions are often highly stereospecific.

We will now look at some of the reasons for these "odd" behaviours.