Organic Chemistry

29

Pericyclic Reactions

101

Vitamin D

R

eactions of organic

compounds can be divided

into three classes—polar reactions,

radical reactions, and pericyclic reactions. The

most common and the ones most familiar to you are

polar reactions. A polar reactionis one in which a

nucleophile reacts with an electrophile. Both electrons in

the new bond come from the nucleophile.

A radical reactionis one in which a new bond is formed using one electron from each

of the reactants.

A pericyclic reactionoccurs as a result of reorganizing the electrons in the reactant(s).

In this chapter we will look at the three most common types of pericyclic reactions—

electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements.

29.1 Three Kinds of Pericyclic Reactions

An electrocyclic reactionis an intramolecular reaction in which a new (sigma)

bond is formed between the ends of a conjugated (pi) system. This reaction is easy

to recognize—the product is a cycliccompound that has one more ring and one fewer

pbond than the reactant.

p

s

a radical reaction

CH 3 CH 2 + Cl Cl CH 3 CH 2 Cl + Cl

HO− + CH 3 OH + Br−

a polar reaction

CH 3 Br

+ −