102 CHAPTER 29 Pericyclic Reactions
Electrocyclic reactions are reversible. In the reverse direction, an electrocyclic
reaction is one in which a bond in a cyclic reactant breaks, forming a conjugated
system that has one more bond than the cyclic reactant.In acycloaddition reaction, two different bond–containing molecules react to
form a cyclic compound. Each of the reactants loses a bond, and the resulting cyclic
product has two new bonds. The Diels–Alder reaction is a familiar example of a
cycloaddition reaction (Section 8.8).In a sigmatropic rearrangement, a bond is broken in the reactant, a new
bond is formed in the product, and the bonds rearrange. The number of bonds
does not change (the reactant and the product have the same number of bonds).
The bond that is broken can be in the middle of the system or at the end of the
system. The system consists of the double-bonded carbons and the carbons
immediately adjacent to them.H 3 Cbond is broken
in the middle of
the systemsigmatropic rearrangements∆H 3 Cbond
is formedproduct and reactant have
the same number of bondsp ps ppp ps s+new bondnew bond1,3-butadienecyclohexenethe product has two fewer
bonds than the sum of the
bonds in the reactantsa cycloaddition reactionethenespp∆bond
breakscyclobutenethe product has one more
bond than the reactant1,3-butadieneps p∆new bond1,3,5-hexatriene 1,3-cyclohexadienethe product has one fewer
bond than the reactantan electrocyclic reaction3-D Molecules:
1,3,5-Hexatriene;
1,3-Butadiene;
3-Methyl-1,5-hexadiene