The first really interesting reaction seen by Organic Chemistry students is the Diels-Alder [4+2] cycloaddition, an example of a concerted process where all the bond making and breaking take place concomitantly. The SN2 is also a concerted process, however the Diels-Alder reaction is a gem because of the high density of relative stereochemistry that can be established in a single synthetic step. The most interesting and unique of all organic reactions learned fall within the narrow category of sigmatropic rearrangements, intramolecular pericyclic processes wherein one σ-bond is exchanged for another σ-bond. Example reactions are the Cope and Claisen rearrangements.
