Organic Chemistry

304 CHAPTER 8 Reactions of Dienes • Ultraviolet and Visible Spectroscopy

C

a. b.

C

H

C

H

H H

CH 3

H

H 3 C

H

C C C

CH 3

H

H 3 C

H

C C C

Figure 8.1

(a) Double bonds are formed by porbital–porbital overlap. The two porbitals on the

central carbon are perpendicular, causing allene to be a nonplanar molecule.

(b) 2,3-Pentadiene has a nonsuperimposable mirror image. It is, therefore, a chiral

molecule, even though it does not have an asymmetric carbon.

8.4 How Dienes React

Dienes, like alkenes and alkynes, are nucleophiles because of the electron density of

their bonds. Therefore, they react with electrophilic reagents. Like alkenes and

alkynes, dienes undergo electrophilic addition reactions.

Until now, we have been concerned with the reactions of compounds that have only

one functional group. Compounds with two or more functional groups exhibit reac-

tions characteristic of the individual functional groups if the groups are sufficiently

separated from each other. If they are close enough to allow electron delocalization,

however, one functional group can affect the reactivity of the other. Therefore, we will

see that the reactions of isolated dienes are the same as the reactions of alkenes, but the

reactions of conjugated dienes are a little different because of electron delocalization.

8.5 Electrophilic Addition Reactions

of Isolated Dienes

The reactions of isolated dienes are just like those of alkenes. If an excess of the elec-

trophilic reagent is present, two independent addition reactions will occur, each fol-

lowing the rule that applies to all electrophilic addition reactions: The electrophile

adds to the carbon that is bonded to the greater number of hydrogens.

The reaction proceeds exactly as we would predict from our knowledge of the mecha-

nism for the reaction of alkenes with electrophilic reagents. The electrophile

adds to the electron-rich double bond in a manner that produces the more stable car-

bocation (Section 4.4). The bromide ion then adds to the carbocation. Because there is

an excess of the electrophilic reagent, both double bonds undergo addition.

(H+)

CH 2 CHCH 2 CH 2 CH CH 2 + HBr CH 3 CHCH 2 CH 2 CHCH 3

1,5-hexadiene excess Br Br

sp^2

p

CH 2 CHCH 2 CH 2 CH CH 2 + H CH 3 CHCH 2 CH 2 CH CH 2 CH 3 CHCH 2 CH 2 CH CH 2

+

Br

−

H

+

CH 3 CHCH 2 CH 2 CHCH 3 CH 3 CHCH 2 CH 2 CHCH 3

Br

Br Br

mechanism for the reaction of 1,5-hexadiene with excess HBr

+

−

Br Br + Br

Br