Organic Chemistry

Section 15.12 Sulfonation of Benzene 611

Sulfonation of benzene is a reversible reaction. If benzenesulfonic acid is heated in
dilute acid, the reaction proceeds in the reverse direction.

The principle of microscopic reversibilityapplies to all reactions. It states that the
mechanism of a reaction in the reverse direction must retrace each step of the mecha-
nism in the forward direction in microscopic detail. This means that the forward and
reverse reactions must have the same intermediates and that the rate-determining “en-
ergy hill”must be the same in both directions. For example, sulfonation is described
by the reaction coordinate diagram in Figure 15.5, going from left to right. Therefore,
desulfonation is described by the same reaction coordinate diagram going from right
to left. In sulfonation, the rate-determining step is nucleophilic attack of benzene on
the ion. In desulfonation, the rate-limiting step is loss of the ion from
the benzene ring. An example of the usefulness of desulfonation to synthetic chemists
is given in Chapter 16, Problem 19.

SO 3 H + SO^3 H

H +SO

+ 3 H

mechanism for desulfonation

+ H+

+SO

3 H

+SO

3 H

SO 3 H

+ SO 3 + H+

H 3 O+/^100 °C

benzenesulfonic acid

SOH

O

O

benzenesulfonate ion

S

O

O

pKa = −0.60

O−

+ H+

H

SO 3 H

Free energy

Progress of the reaction

SO 3 H

+ H+

+SO

+ 3 H

+

transition state for the rate-determining step

in the forward direction and for the

rate-determining step in the reverse direction

>Figure 15.5

Reaction coordinate diagram for

the sulfonation of benzene (left to

right) and the desulfonation of

benzenesulfonic acid (right to left).

PROBLEM 19

The reaction coordinate diagram in Figure 15.5 shows that the rate-determining step for sul-

fonation is the slower of the two steps, whereas the rate-determining step for desulfonation is

the faster of the two steps. Explain how the faster step can be the rate-determining step.