Post on 29-Apr-2018
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the sulfonic acid was originally made.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