chem EAS

1. Electrophilic Aromatic Substitution
Definition:
Electrophilic aromatic substitution is an organic reaction in which an atom that is
attached to an aromatic system (usually hydrogen) is replaced by
an electrophile. The arene system contains an electron rich C=C system which
react with electrophiles via a substitution pathway (to preserve aromaticity) via
what is known as electrophilic aromatic substitution (EArS):
Reaction Mechanism:
In the first step of the reaction mechanism for this reaction, the electron-rich
aromatic ring, which in the simplest case is benzene, attacks the electrophile A.
This step leads to the formation of a positively charged cyclohexadienyl cation,
also known as an arenium ion. This carbocation is unstable, owing both to the
positive charge on the molecule and to the temporary loss of aromaticity.
However, the cyclohexadienyl cation is partially stabilized by resonance, which
allows the positive charge to be distributed over three carbon atoms.
In the second stage of the reaction, a Lewis base B donates electrons to the
hydrogen atom at the point of electrophilic attack, and the electrons shared by the
hydrogen return to the pi system, restoring aromaticity.
An electrophilic substitution reaction on benzene does not always result in
monosubstitution. While electrophilic substituents usually withdraw electrons
from the aromatic ring and thus deactivate it against further reaction, a

2. sufficiently strong electrophile can perform a second or even a third substitution.
This is especially the case with the use of catalysts.
Examples Table of EAS:
Reaction Reagents Electrophile Product Comments
Nitration HNO3 / H2SO4 NO2+ E+ formed by loss of
water from nitric acid
Sulfonation
H2SO4 or SO3 /
H2SO4
SO3 Reversible
Halogenation
Cl2 / Fe or
FeCl3
Cl+ E+ formed by Lewis
acid removing Cl-
Br2 / Fe or
FeBr3
Br+ E+ formed by Lewis
acid removing Br-
Alkylation R-Cl / AlCl3 R+ E+ formed by Lewis
acid removing Cl-
R-OH / H+ R+ E+ formed by loss of
water from alcohol
C=C / H+ R+ E+ formed by
protonation of alkene
Acylation RCOCl / AlCl3 RCO+ E+ formed by Lewis
acid removing Cl-
RCO2COR /
AlCl3
RCO+ E+ formed by Lewis
acid removing RCO2-

3. Examples of electrophilic aromatic substitution:
Nitration of Benzene:
The sourceof the nitronium ion is through the protonation of nitric acid by
sulfuric acid, which causes the loss of a water molecule and formation of a
nitronium ion.
Sulfuric Acid Activation of Nitric Acid
The first step in the nitration of benzene is to activate HNO3with sulfuric acid to
producea stronger electrophile, the nitronium ion.
Because the nitronium ion is a good electrophile, it is attacked by benzene to
produceNitrobenzene.
Mechanism
(Resonance forms of the intermediate can be seen in the
generalized electrophilic aromatic substitution)

4. Sulfonation of Benzene:
Sulfonation is a reversible reaction that produces benzenesulfonic acid by
adding sulfur trioxide and fuming sulfuric acid. The reaction is reversed by
adding hot aqueous acid to benzenesulfonic acid to producebenzene.
Mechanism
To producebenzenesulfonic acid from benzene, fuming sulfuric acid and sulfur
trioxide are added. Fuming sulfuric acid, also refered to asoleum, is a
concentrated solution of dissolved sulfur trioxide in sulfuric acid. The sulfur in
sulfur trioxide is electrophilic because the oxygens pull electrons away from it
because oxygen is very electronegative. The benzene attacks the sulfur (and
subsequent protontransfers occur)to producebenzenesulfonic acid.
Halogenation of Benzene:

5. MECHANISM FOR HALOGENATION OF BENZENE
Step 1:
The bromine reacts with the Lewis acid to form
a complex that makes the bromine more
electrophilic.
Step 2:
The π electrons of the aromatic C=C act
as a nucleophile, attacking the
electrophilic Br, and displacing iron
tetrabromide. This step destroys the
aromaticity giving the cyclohexadienyl
cation intermediate.
Step 3:
Removal of the protonfrom the sp3 C
bearing the bromo - group reforms
the C=C the aromatic system,
generating HBr and regenerating the
active catalyst.
Orientation:
The rate data shows that toluene is more reactive or activated with respect to
benzeneand the productdistribution shows that the methyl group directs the
new substituent to the ortho- and para- positions
In contrast, trifluoromethylbenzene is less reactive or deactivatedwith respect
to benzene and directs the new substituent to the meta position.
Here is a table that shows the effect of substituents on a benzene ring have on
both the rate and orientation of electrophilic aromatic substitution reactions.

6. There are two main electronic effects that substituents can exert:
RESONANCE :
Resonance effects are those that occurthrough the system and can be
represented by resonance structures. These can be either electron donating
(e.g. -OMe) where electrons are pushed toward the arene or electron
withdrawing (e.g. -C=O) where electrons are drawn away from the arene.
INDUCTIVE :

7. Inductive effects are those that occurthrough the system due to
electronegativity type effects. These too can be either electron donating (e.g. -
Me) where electrons are pushed toward the arene or electron withdrawing
(e.g. -CF3, +NR3) where electrons are drawn away from the arene.
A simplified approachto understanding substituent effects is given here, based
on the "isolated molecule approach". The text uses the more rigorous approach
of drawing the resonance structures for the intermediate formed by attack at
each of the o-, m- and p- positions.
Ortho/para directors
Groups with unshared pairs of electrons, suchas the amino group of aniline, are
strongly activating and ortho/para-directing. Such activating groups donate
those unshared electrons to the pi system.
When the electrophile attacks the ortho and para positions of aniline,
the nitrogen atom can donate electron density to the pi system (forming
an iminium ion), giving four resonance structures (as opposedto three in the
basic reaction). This substantially enhances the stability of the cationic
intermediate.

8. Compare this with the case when the electrophile attacks
the meta position. In that case, the nitrogen atom cannot donate electron
density to the pi system, giving only three resonance contributors. For
this reason, the meta-substituted productis produced in much smaller
proportion to the ortho and para products.
Other substituents, such as the alkyl and aryl substituents, may also
donate electron density to the pi system; however, since they lack an
available unshared pair of electrons, their ability to do this is rather
limited. Thus, they only weakly activate the ring and do not strongly
disfavor the meta position.
Halogens are ortho/para directors, since they possessan unshared
pair of electrons just as nitrogen does. However, the stability this
provides is offset by the fact that halogens are substantially
more electronegative than carbon, and thus draw electron density
away from the pi system. This destabilizes the cationic intermediate,
and EAS occurs less readily. Halogens are therefore deactivating
groups.
Directed ortho metalation is a special type of EAS with special ortho
directors.
Meta directors:
Non-halogen groups with atoms that are more electronegative than
carbon, such as a carboxylic acid group (CO2H) draw substantial
electron density from the pi system. These groups are
strongly deactivating groups. Additionally, since the substituted
carbonis already electron-poor, the resonance contributor with a
positive charge on this carbon (produced by ortho/para attack) is less
stable than the others. Therefore, these electron-withdrawing groups
are meta directing.

9. Reaction mechanism:
In the first step of the reaction mechanism for this reaction, the
electron-rich aromatic ring, which in the simplest case is benzene,
attacks the electrophile A. This step leads to the formation of a
positively charged cyclohexadienyl cation, also known as an arenium
ion. This carbocation is unstable, owing both to the positive charge on
the molecule and to the temporary loss of aromaticity. However, the
cyclohexadienyl cation is partially stabilized by resonance, which
allows the positive charge to be distributed over three carbon atoms.
In the second stageofthe reaction, a Lewis baseB donates electrons
to the hydrogen atom at the point of electrophilic attack, and the
electrons shared by the hydrogen return to the pi system, restoring
aromaticity.
An electrophilic substitution reaction on benzene does not always
result in monosubstitution. While electrophilic substituents usually
withdraw electrons from the aromatic ring and thus deactivate it
against further reaction, a sufficiently strong electrophile can
perform a second oreven a third substitution. This is especially the
case with the use of catalysts.
Five membered heterocyclic compunds:
Furan, thiophene, pyrrole and their derivatives are all highly
activated compared to benzene. These compounds all contain an
atom with an unshared pair of electrons (oxygen,sulphur,
or nitrogen) as a member of the aromatic ring, which substantially
increases the stability of the cationic intermediate. Examples of
electrophilic substitutions to pyrrole are the Pictet–Spengler
reaction and the Bischler–Napieralski reaction.

10. Reference:
https://en.wikipedia.org/wiki/
www.chem.ucla.edu/harding/notes
www.chem.ucalgary.ca/courses
www.mhhe.com/physsci/chemistry
www.chemguide.co.uk/mechanisms
www.leah4sci.com/
www.saplinglearning.com/