Chemistry 212
Aromatic Compounds - 1
Summary of Class Discussion
Spring 2012
2. Mechanisms of Reactions of Aromatic Compounds.
a. Unsubstituted Compounds (Benzene)
(1.) Data:
(a.)
+
Br
Br
CCl4
(2.) Exploration: Analysis of Reactions of Aromatic Compounds:
No Rxn
Br
(b.)
+
Br
Br
+
FeBr3
HBr
Cl
(c.)
+
Cl
Cl
+
AlCl3
O
O
S
(d.)
+
SO3
HCl
H
O
H2SO4
NO2
(e.)
+
HNO3
(f.)
CH3 C Cl
CH3
CH3
AlCl3
+
CH3
C
C
Cl AlCl3
(b.) What bonds are made and broken in each reaction? What
similarities are there among bonds made and broken in all
reactions? Propose a simple general model for the overall
reaction.
For most of the reactions, a C-H bond on the benzene and
one -bond in the reagent are broken and a -bond between
an electrophilic atom of the reagent and the carbon atom of
benzene is made while a new bond is made between an Hatom and a nucleophilic atom of the reagent.
Simple Model for Aromatic Substitution
E
H
+
+ HCl
O
O
(g.)
H2O
CH3
C CH3
CH3
+
+
H2SO4
(a.) What is the overall reaction in each example where a reaction
occurs?
The overall reaction in each example is substitution of a
halogen, a nitro group, a sulfone group, an alkyl group or an
acyl group for a hydrogen atom on an aromatic ring.
CH3 +
HCl
E
Nu
+
H
Nu
(c.) What type of reagents seem to be most common among the
examples (nucleophilic-basic, electrophilic-acidic, etc.) (FeCl3,
AlCl3 and AlBr3 are Lewis Acids.)
All of the reagents are Brønsted or Lewis Acids -Electrophiles.
(d.) Considering your previous experience with the reactivity of alkenes, use your general model to work your way, step-by-step,
through a mechanism for the overall reaction. What should be the first step? How can the final product be formed for the first
intermediate?
As with isolated C=C compounds under acidic conditions we would expect the first interaction to be between the -electrons
of the benzene ring with the electrophilic atom of the reagent forming a new -bond. In order to get to the resulting
products, the nucleophilic atom of the reagent needs to remove the H of the benzene as a proton forming the H-Nu -bond
2
Aromatic Compounds
and reforming the aromatic system of the ring. This second step is essentially the second step of the E1 elimination
mechanism. So both steps have precedents in our recent experience.
H
+
E
H
E
Nu
E+
+
Nu
H
Nu
(e.) Apply your general mechanism to each of the reactions in the data set. (NOTE: as indicated above, FeCl3, AlCl3 and AlBr3 are
Lewis Acids. As illustrated below, their positively charged metal ions associate with the lone pair e-‘s on one of the halogen or
oxygen atoms in the reagent. The nuclear charge of the metal ion lowers the energy of the e-'s of that halogen or oxygen atom. As
shown below for reactions (e.) -> (g.) the reagents combine to form an electrophilic intermediate that reacts with the aromatic ring.
Reactions b & c – Illustrated for the reaction of Br2.
H

+
Br

FeBr3
Br
Br
+
FeBr4
H
Br
+
HBr
The electrophile is a complex of the halogen with Ferric halide complex that polarizes the halogen electrons toward the iron.
Initial reaction of the electrophile with the benzene  electrons produces a delocalized cation on the ring. The nucleophile
produced is the ferric tetra-halide anion complex that releases the hydrogen halide upon protonation.
Aromatic Compounds
3
Reactions d
O
O
O
H
S
+
O
S
H
H
O
O
S
O
O
O
+
H
H
O
S
O
O
O
H
O
S
H
H
O
O
S
O
+
H2SO4
O
H
O
O
+
O
S
H
O
O
H
O
In this case SO3 acts as the electrophile and H2SO4 serves as an acid and base catalyst after the first step. (See CGWW
pp 552-553) In this case the bond broken in the electrophile is a -bond so the reagent remains a single unit and regains the
proton removed from the ring. So technically, the net reaction is addition, but the mechanism follows the same sequence as
the substitution reaction and a ring C-H bond is replaced by a C-H sulfur atom.
The following reagents react to form the actual active electrophile. These reactions are illustrated below. Complete the mechanisms using the active
electrophile.
Reaction e
H
O
+
+
N
N
+
O
O
O
HSO4
H2O
O
+
HSO4
+
H2O
+
H2SO4
+
H2O
N
O
H
HNO3
+ H2SO4
As explained in the question and illustrate in the equations, the mixture of HNO3 and H2SO4 react to form the electrophile,
NO2+, water and hydrogen sulfate anion. The after the initial electrophilic addition step, the HEE on HSO4- remove the
proton from the intermediate complex forming the nitro aromatic and reforming sulfuric acid.
4
Aromatic Compounds
Reaction f
For 2˚ or 3˚ alkyl halides, the aluminum trihalide complex dissociates to form the free carbocation, which acts as the
electrophile. (See reactions b & c above). (See also CGWW p. 553)
CH3
H
CH3
d
d
+
+
AlBr4
+
C
C
Br
AlBr
3
CH3
CH3
CH3
CH3
3Þcation has sufficient e- density that it
can form without a large energy increase.
H
CH3
CH3
CH3
C
C
HBr
+
CH3
CH3
AlBr4
CH3
H
Acyl halides (Rxn g) interact with the catalysts to form acylium ions that act as carbon electrophiles toward the aromatic ring.
Reaction h.
An Acylium Ion
O
CH3
+
CH3
C
C
O
H
Cl
O
+
CH3
C
H
+ AlCl4
AlCl3
+ AlCl4
CH3
O
C
+ H-Cl
Aromatic Compounds
(f.)
5
What problems did you encounter in applying your general mechanism to all of the reactions? Could you modify your mechanism
to work for all of the reactions? How or why not? Explain.
Explanations above.
(g.) Based upon your experience in (2) (a.) -> (f.), your understanding of alkene reactions, and the structure and energy of aromatic
compounds, propose a reason why it is reasonable that aromatic compounds might undergo substitution rather than addition as
alkenes do. You should consider (write out) the expected mechanism for addition and compare the energies of the transition
states for substitution vs. addition in each case.
The mechanism proposed in d. above still seems to be appropriate and can account for all of the products in the data set.
H
H
+
aromtic

E
E

Nu
1
2
E
H

E
Nu
partially aromtic

Nu
H
non-aromtic
+
E
+
Nu
partially aromtic
H
Nu
aromtic
The second step differs from that in electrophilic addition to alkenes because the partial aromaticity developing in the
substitution (loss of the proton makes the substitution lower in energy than the addition (Br adding to the cation)
which lacks aromaticity.
6
Aromatic Compounds
Transition States for Substitution vs Addition to Benzene
H
H
E
Br
+ Nu
H
H
+ Nu
Aromatic
Substitution
Aromatic
Addition

Br
H
Br
H
Partially Aromatic
Delocalized Partial
Cation

H
Br


Nu
H
Delocalized but
NOT Aromatic Partial
Cation
H
E
E
+
HNu
Only Product Isolated
H
Nu
Not Formed in Significant Amounts
(h,) Considering your answers and mechanisms above, try to suggest a reason why there is no reaction in example (1.) (a.), while
cyclohexene (See below) reacts under identical conditions. A reaction coordinate diagram may be helpful to your analysis.
Br
+
Br
Br
Br
+
CCl4
Br
(a.)
+
Br
Br
CCl4
No Rxn
Br
Aromatic Compounds
7
G Alkene
G Aromatic
w/ catalyst
Alkene
Aromatic
w/o catalyst
Aromatic
w/ catalyst
Alkene
Aromatic
G Aromatic
w/o catalyst
Aromatic
w/ catalyst
G
reactants
Rxn Coordinate
There should be no reaction in example (1.) (a.) because the relatively low energy of the aromatic ring e-'s together with
the relatively high energy of the e-'s on the partially negative leaving Br atom in the transition state make the G for
formation of the intermediate cation too high to produce a significant rate of reaction. The catalyst decreases the
energy of the partial negative on Br in the transition state. So with the catalyst, the relatively lower energy transition
state makes the G low enough to allow the reaction to proceed.
Because the π-electrons of the alkene have considerably higher energy than those in the aromatic π system, the G for the
addition of halogen to the C=C is low enough to occur even without the FeBr3 in the transition state.