11

Conditions

for less reactive arenes use oleum (30% SO3
available.

dissolved in H2SO4)

commercially

Electrophile: SO3

•strong Lewis Acid; electron deficient.
for example forms strong Lewis acid Lewis base adducts with PPh3.
Generated by:

Mechanism:

Sulphonation is reversible

the reactions shows a primary deuterium isotope effect, i.e. rate of C6D6approx. 2
times slower than for C6H6, i.e. C-D bond is broken in r.d.s.

Some intermediates go on to to become product, some revert to starting material.
k-1= k2(approx.) andEact2(D) > Eact2(H)!

For both intermediates the barrier behind is of the same height, but for the forward
reaction the barrier for C-D bond breaking is higher than for C-H bond breaking. More
deuterated σ-complexes revert to starting material than for normal hydrogen σ-
complexes, i.e. the reaction is less fast.

Sulphonation of naphthalene

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conc. H2SO4

SO3H

80oC

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+

H3O+

2 H2SO4

SO3

+

HSO4-

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SO3

SO3
H

SO3

SO3H

+H

SO3H

+SO3

Wheland intermediate
σ-complex
arenium ion

Free
Energy

Reaction coordinate

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SO3H

O3S

H(D)

C-D bond broken

C-H bond broken

+

SO3

k-1

k1

k2

Eact

Eact

k-1

k2