Organic Chemistry

656 CHAPTER 16 Reactions of Substituted Benzenes

a.

RCCR

b.

Figure 16.4N good overlap poor overlap
Orbital pictures of the bond formed
(a) by overlapping porbitals in a
normal triple bond and (b) by
overlapping orbitals in the
distorted “triple bond” in benzyne.

sp^2

The incoming nucleophile can attack either of the carbons of the “triple bond”of

benzyne. Protonation of the resulting anion forms the substitution product. The overall

reaction is an elimination–addition reaction: Benzyne is formed in an elimination

reaction and immediately undergoes an addition reaction.

Substitution at the carbon that was attached to the leaving group is called direct

substitution. Substitution at the adjacent carbon is called cine substitution(cine comes

from kinesis, which is Greek for “movement”). In the following reaction,o-toluidine is

the direct-substitution product; m-toluidine is the cine-substitution product.

Benzyne is an extremely reactive species. In Section 6.3, we saw that in a molecule

with a triple bond, the two sphybridized carbons and the atoms attached to these car-

bons are linear because the bond angles are 180°. Four linear atoms

cannot be incorporated into a six-membered ring, so the system in

benzyne is distorted: The original bond is unchanged, but the orbitals that form

the new bond are not parallel to one another (Figure 16.4). Therefore, they cannot

overlap as well as the porbitals that form a normal bond, resulting in a much weak-

er and much more reactive bond.

p

p sp^2

C¬C‚C¬C

(C¬C‚C¬C)

+ NH 2

NH 2

− HNH 2

NH 2

− NH 2

−

+ NH 2

−

NH 2

HNH 2

NH 2

−

benzyne

**

*

*

*

3-D Molecule:

Benzyne

NH 2

CH 3

Br NH 2

o-bromotoluene

+ NaNH 2

m-toluidine

cine-substitution product

o-toluidine

direct-substitution product

NH 3 (liq) +

CH 3 CH 3

Although benzyne is too unstable to be isolated, evidence that it is formed can be

obtained by a trapping experiment. When furan is added to a reaction that forms a ben-

zyne intermediate, furan traps the benzyne intermediate by reacting with it in a

Diels–Alder reaction (Section 8.8). The product of the Diels–Alder reaction can be

isolated.

Br

O

O

the product of the

Diels–Alder reaction

−NH 2

benzyne

a dienophile

furan

a diene

NH 3 (liq)