Organic Chemistry

314 CHAPTER 8 Reactions of Dienes • Ultraviolet and Visible Spectroscopy

C

C

H H

H

C

C

H H

H H

O

C

C

C

H H

H C

H

N

Figure 8.3N
By comparing the electrostatic
potential maps you can see that an
electron-withdrawing substituent
decreases the electron density of
the carbon–carbon double bond.

The reactivity of the dienophile is increased if one or more electron-withdrawing

groups are attached to its carbons.

An electron-withdrawing group—such as a carbonyl group or a cyano

group—withdraws electrons from the double bond. This puts a partial posi-

tive charge on one of its carbons (Figure 8.3), making the Diels–Alder reaction

easier to initiate.

The partially positively charged carbon of the dienophile can be likened to the

electrophile that is attacked by electrons from C-1 of the conjugated diene. The

other carbon of the dienophile is the nucleophile that adds to C-4 of the diene.

A Molecular Orbital Description of the Diels–Alder Reaction

The two new bonds that are formed in a Diels–Alder reaction result from a transfer of

electron density between the reactants. Molecular orbtial theory provides an insight into

this process. When electrons are transferred between molecules, we must use the

HOMO (highest occupied molecular orbital)of one reactant and the LUMO (lowest

unoccupied molecular orbital)of the other because only an empty orbital can accept

electrons. It doesn’t matter whether we use the HOMO of the dienophile and the

LUMO of the diene or the HOMO of the diene and the LUMO of the dienophile. We

just need to use the HOMO of one and the LUMO of the other.

s

sp^2

p

sp^2

+

−

CH 2 CH CCH 3 CH 2 CH CCH 3

O O

resonance contributors of the dienophile

CH 2 CH CCH 3

O

resonance hybrid

+

−

sp^2

(C‚N)

(C“O)

a 1,4-addition reaction to 1,3-butadiene

C CH

H

CH 2

CH 2

H

C

30 ºC CCH^3

O

CH

CH 2

CCH 3

O

CH 2

4

3

2

1

C

H CH 2

H

C

CH 2

4

3

2

1

electron-withdrawing group

sp^2

Kurt Alder (1902–1958)was born in
a part of Germany that is now
Poland. After World War I, he and
his family moved to Germany. They
were expelled from their home region
when it was ceded to Poland. After
receiving his Ph.D. under Diels in
1926, Alder continued working with
him, and in 1928 they discovered the
Diels–Alder reaction. Alder was a
professor of chemistry at the Univer-
sity of Kiel and at the University of
Cologne. He received the 1950 Nobel
Prize in chemistry, sharing it with his
mentor, Otto Diels.