15.12 Desulfonation. Mechanism of protonation

100-175 oC with aqueous acid

This desulfonation is the exact reverse of the sulfonation process by which

the sulfonic acid was originally made.

Removal of the relatively volatile hydrocarbon by steam distillation shift the

equilibrium toward hydrocarbon

Sulfonation is unusual among electrophilic aromatic substitution reactions:

- its reversibility

- Ordinary hydrogen (protium) is displaced from an aromatic ring about twice as

fast as deuterium

+

+

electrophile

The mechanism of desulfonation must be the exact reverse of the mechanism of sulfonation.

the reaction is protonation or, more specifically, Protodesulfonation.

15.13 Mechanism of electrophilic aromatic substitution: a summary

Two essential steps are involved:

(1) attack by an electrophilic reagent upon the ring to form a carbocation

1) + Y+Z-RDS

H

Y

+ Z-

2)H

Y+ Z- Y + HZ

(2) abstraction of a proton ion from this carbocation by some base.

Slow

Fast

Y = as carring positive charge, or it can be neutral (SO3)

15.14 Mechanism of electrophilic aromatic substitution: the two

steps

- How do we know that electrophilic aromatic substitution involves two steps,

instead of just one

- How do we know that, of these two steps, the first is much slower than the

second?

Slow

Fast

Lars Melander studies (university of Gothenberg)

D(T) H

Same rate

bromination there is no significant

isotope effect.

A difference in rate (or position of equilibrium) due to a difference in the isotope

present in the reaction system is called an isotope effect.

primary isotope effects: Isotope effects due to the breaking of a bond to the isotopic atom.

C-D bond is broken more slowly than a C-H bond. and a C-T bond more slowly yet

the rates of replacement of the various hydrogen isotopes , are the same.

the reactions whose rates we are comparing do not Involve the breaking of a carbon- hydrogen bond.

there is no isotope effect here

The rate of the overall substitution is determined by the slow attachment of the electrophilic reagent to the aromatic ring to form the carbocation.

Once formed, the carbocation rapidly loses hydrogen ion to form the products.

Step (1) is thus the rate-determining step

Since it does not involve the breaking of a carbon- hydrogen bond, its rate- and hence the rate of the overall reaction- is independent of the particular hydrogen isotope that is

present.

If substitution involved a single step, as in (1a):

This step would necessarily be the rate-determining step (since it involves breaking of the carbon hydrogen-bond),

an isotope effect would be observed. Or,

if step (2) of the two-step sequence were slow enough relative to step(1) to affect the overall rate, again we would expect an isotope effect.

Thus the absence of isotope effects establishes not only the two-step nature of electrophilic aromatic substitution.

but also the relative speeds of the steps.

Figure 15.2 Nitration. Formation of

carbocation is the rate-controlling step; it

occurs equally rapidly whether protium (H) or

deuterium (D) is at the point of attack. All the

carbocations go on to product. There is no

isotope effect, and nitration is irreversible.

Figure 15.2 Nitration. Formation of

carbocation is the rate-controlling step; it

occurs equally rapidly whether protium (H) or

deuterium (D) is at the point of attack. All the

carbocations go on to product. There is no

isotope effect, and nitration is irreversible.

nitration and reactions like it are not reversible.

Figure 11.3 Sulfonation. Some carbocations go

on to product, some revert to starting material.

There is an isotope effect, and sulfonation is

reversible.

Figure 11.3 Sulfonation. Some carbocations go

on to product, some revert to starting material.

There is an isotope effect, and sulfonation is

reversible.

Unlike most other electrophilic substitution reactions, sulfonation is reversible

15.15 Reactivity and orientation

reactivity and orientation are both matters of relative rates of reaction

Slow: rate-determining +

Any differences in rate of substitution must therefore be due to differences in the rate of this step.

we expect the more stable carbocation to formed more rapidly

+

+

+ +

the intermediate carbcation is a hybrid of structures I, II, and III, Positive charge distributed to

ortho and para

+

+

+ +

Y should affect the stability of the carbocation by dispersing or intensifying the positive charge, depending upon its electron-releasing or electron-withdrawing nature.

15.16 Theory of reactivity

+ +

the structures of the carbocations formed from:

+

methyl group (II) tends to neutralize the positive charge of the ring

and so become more positive itself; this dispersal of the charge stabilizes the carbocation

the inductive effect stabilizes the developing positive charge in the transition state and thus leads to a faster reaction

The -N02 group, on the Other hand, has an electron-withdrawing inductive effect (III);

this tends to intensify the positive charge, destabilizes the carbocation, and thus causes a slower reaction.

Reactivity in electrophilic aromatic substitution:

A group that releases electrons activates the ring

A group that withdraws electrons deactivates the ring

G releases elections:

stabilizes carbocation,

activates

G withdraws elections:

destabilizes carbocation,

deactivates

Electron release of these group is due not to their

inductive effect but to resonance

25 times as reactive as

C6H6

1/3 times as reactive as

C6H6

15.1

7 T

heo

ry o

f ori

en

tati

on

+ + +

+ + +

An activating group activates all

positions of the benzene ring

It directs o- and p- simply because

it activates the o- and p- positions

much more than it does the m-

+ + +

+ +

+

+ + +

15.18 Electron release via resonance

+ +

+

+

+ + +

+ + + +

15.19 Effect of halogen on electrophilic aromatic substitution

-Cl withdraws electrons:

Destabilizes carbocation,

deactivates ring

15.20 Relation to other carbocation reactions

the more stable the carbocation,

the faster it is formed;

the faster the carbocation is formed,

the faster the reaction goes

reactivity and orientation in electrophilic aromatic substitution ( or electrophilic addition to alkenes)

Why do substituent groups on a benzene ring affect the reactivity and orientation in the way they do?

electronic effects, “pushing” or “pulling” electrons by the substituent.

Electrons can be donated (“pushed”) or withdrawn (“pulled”) by atoms or groups of atoms via:

Induction – due to differences in electronegativities

Resonance – delocalization via resonance

N

H

H

unshared pair of electrons on the nitrogenresonance donating groups(weaker inductive withdrawal)

N

R

R

N

R

H

NR

R

Rstrong inductive withdrawal(no unshared pair of electrons on thenitrogen & no resonance possible

H3C C

O

N

H

resonance donation(weaker inductive withdrawal)

H

Oresonance donation(weaker inductive withdrawal)

R

Oresonance donation(weaker inductive withdrawal)

resonance donation

H3Cinductive donationsp3 sp2 ring carbon

inductive withdrawal X—

H

C

O

HO

C

O

RO

C

O

R

C

O

resoance withdrawal andinductive withdrawal

N

O

O

resonance andinductive withdrawal

resonance andinductive withdrawal

CN

-NH2

-OH

-OCH3

-CH3

Alkyl

-H

-F

-Cl

-Br

-I

-CH

O

-COCH3

O

-COH

O

-CCH3

O

-SO3H

-C N

-NO2

Benzene

-NHCCH3

O

Reactivity

-NR3+

o– and p- directing activators

o– and p- directing deactivators

m- directing deactivators

15.21 Electrophilic substitution in naphthalene

Nitration and halogenation occur almost exclusively in the a- position