Organic Chemistry

330 CHAPTER 8 Reactions of Dienes • Ultraviolet and Visible Spectroscopy

Summary of Reactions

1. If there is excess electrophilic reagent, both double bonds of an isolated dienewill undergo electrophilic addition.

If there is only one equivalent of electrophilic reagent, only the most reactive of the double bonds of an isolated dienewill undergo

electrophilic addition (Section 8.5).

2.Conjugated dienesundergo 1,2- and 1,4-addition with one equivalent of an electrophilic reagent (Section 8.6).

3.Conjugated dienesundergo 1,4-cycloaddition with a dienophile (a Diels–Alder reaction) (Section 8.8).

O

C

CH 2 CH CH CH 2 CH 2 CH C R

R

O

+ ∆

CH

Br Br

RCH CHCH CHR + HBr RCH 2 CH CHR + RCH 2 CH CHCHR

1,2-addition product 1,4-addition product

CH 2 CHCH 2 CH 2 C CH 2

CH 3

+ HBr CH 2 CHCH 2 CH 2 CCH 3

CH 3

Br

CH 2 CHCH 2 CH 2 C CH 2

CH 3

+ HBr CH 3 CHCH 2 CH 2 CCH 3

CH 3

Br Br

excess

product and a 1,4-addition product. The first step is
addition of the electrophile to one of the carbons at the
end of the conjugated system.
When a reaction produces more than one product, the
product formed most rapidly is the kinetic product; the
most stable product is the thermodynamic product. If
the reaction is carried out under mild conditions so that it is
irreversible, the major product will be the kinetic product; if
the reaction is carried out under vigorous conditions so that
it is reversible, the major product will be the thermodynamic
product. When a reaction is under kinetic control, the rela-
tive amounts of the products depend on the rates at which
they are formed; when a reaction is under thermodynamic
control, the relative amounts of the products depend on their
stabilities. A common intermediateis an intermediate that
both products have in common.
In a Diels–Alder reaction, a conjugated diene reacts with
a dienophile to form a cyclic compound; in this concerted
cycloaddition reaction, two new bonds are formed
at the expense of two bonds. The conjugated diene must
be in an s-cis conformation. The reactivity of the dienophile
is increased by electron-withdrawing groups attached to the
carbons. The HOMOof one reactant and the LUMOof
the other are used to show the transfer of electrons between

sp^2

p

[4 + 2] s

sp^2

the molecules. According to the conservation of orbital sym-

metry,pericyclic reactionsoccur as a result of the overlap

of in-phase orbitals. The Diels–Alder reaction is stereospe-

cific; it is a syn addition reaction with respect to both the

diene and the dienophile. If both the diene and the

dienophile are unsymmetrically substituted, two products

are possible because the reactants can be aligned in two dif-

ferent ways. In bridged bicyclic compounds, a substituent

can be endoor exo; endo is favored if the dienophile’s sub-

stituent has electrons.

Ultravioletand visible(UV/Vis) spectroscopyprovide

information about compounds with conjugated double

bonds. UV light is higher in energy than visible light; the

shorter the wavelength, the greater is the energy. UV and

visible light cause and electronic transi-

tions; transistions have larger molar absorptivities.

A chromophoreis that part of a molecule that absorbs UV

or visible light. The Beer–Lambert lawis the relationship

among absorbance, concentration, and length of the light

path: The more conjugated double bonds there are

in a compound, the less energy is required for the electronic

transition and the longer is the at which it occurs.

Reaction rates and values are commonly measured

using UV/Vis spectroscopy.

pKa

lmax

A=cle.

p:p*

n:p* p:p*

p