30 • Why This Chapter?
Broad outlines of both polar and radical reactions have been in place for more than a century, but our understanding of pericyclic reactions has emerged more recently. Prior to the mid-1960s, in fact, they were even occasionally referred to as “no-mechanism reactions.” They occur largely in the laboratory rather than in biological processes, but a knowledge of them is necessary, both for completeness in studying organic chemistry and in understanding biological pathways where they do occur.
Most organic reactions take place by polar mechanisms, in which a nucleophile donates two electrons to an electrophile in forming a new bond. Other reactions take place by radical mechanisms, in which each of two reactants donates one electron in forming a new bond. Both kinds of reactions occur frequently in the laboratory and in living organisms. Less common, however, is the third major class of organic reactions—pericyclic reactions.
A pericyclic reaction is one that occurs by a concerted process through a cyclic transition state. A concerted reaction is one in which all bonding changes occur simultaneously; no intermediates are involved. Rather than try to expand this definition now, we’ll begin by briefly reviewing some of the ideas of molecular orbital theory introduced in the chapters on Structure and Bonding and Conjugated Compounds and Ultraviolet Spectroscopy and then looking individually at the three main classes of pericyclic reactions: electrocyclic reactions, cycloadditions, and sigmatropic rearrangements.