Organic Chemistry

nitric acid

N

O

HO O−

nitronium ion

OON

+

+

610 CHAPTER 15 Aromaticity • Reactions of Benzene

To generate the necessary electrophile, sulfuric acid protonates nitric acid. Loss of

water from protonated nitric acid forms a nitronium ion, the electrophile required for

nitration. Remember that any base present in the reaction mixture (

solvent) can remove the proton in the second step of the aromatic substitution reaction.

15.12 Sulfonation of Benzene

Fuming sulfuric acid (a solution of in sulfuric acid) or concentrated sulfuric acid

is used to sulfonate aromatic rings.

As the following mechanism shows, a substantial amount of electrophilic sulfur triox-

ide is generated when concentrated sulfuric acid is heated, as a result of the

electrophile losing a proton. Take a minute to note the similarities in the mech-

anisms for forming the electrophile for sulfonation and the electrophile

for nitration.

A sulfonic acid is a strong acid because of the three electron-withdrawing oxygen

atoms and the stability of its conjugate base—the electrons left behind when a proton

is lost are shared by three oxygen atoms (Section 1.19).

SO 3 H

+ +SO 3 H + HB+

mechanism for sulfonation

HO S

sulfuric acid

OH ++

O

O

HO S OH

O

O

HO S+ ++H 2 O SO 3 H 3 O+

O

O

−OSOH

O

O

HO

H

SOH

O

O

+

+ H

SO 3 H

B

+NO

2

+SO

3 H

+SO

3 H

(SO 3 )

SO 3 H

++H 2 SO 4 H 2 O

sulfonation

benzenesulfonic acid

∆

SO 3

NO 2

+ H

NO 2

mechanism for nitration

+NO

+ 2

nitric acid nitronium ion

:B

+ HB+

HO NO 2 + HOSO 3 HHONO 2

H

+

+NO

2 + H 2 O

+ HSO 4 −

(≠B) H 2 O, HSO 4 - ,