This document provides an overview of electrophilic substitution reactions on aromatic compounds. It discusses the energy curve for electrophilic substitution reactions and defines pi and sigma complexes. It then summarizes several specific electrophilic substitution reactions including nitration, sulfonation, halogenation, Friedel-Crafts alkylation, and Friedel-Crafts acylation. The document also covers activating and deactivating substituents, orientation effects, and substitution patterns in fused aromatic ring systems.

1. GOVERNMENT COLLEGE,
AJMER
Electrophilic Substitution Reaction On
Aromatic Compounds
Presented by:
Kanika Khandelwal
MSc. Chem Sem -1

2. Contents
 Electrophilic Substitution reaction
 Energy curve for E.S.R
 Nitration
 Sulphonation
 Halogenation
 Friedel craft Alkylation
 Friedel craft Acylation Reaction
 Activating and Deactivating Groups
 Introduction of third group in
Substituted Ring
 Orientation in other Ring System

3. Electrophilic Substitution Reaction
An Electrophilic Substitution reaction is a chemical reaction in
which a Hydrogen atom attached to an aromatic ring is replaced
by an electrophile (E+).
Mechanism :-

4. EnergycurvefortypicalE.S.R:-
 This curve is plotted b/w
energy and progress of the
reaction.
This curve involve 2 hills since
the E.S.R involves 2 steps for
completion.
 Rate of substitution at any
position depends on height of
energy barrier i.e. ( G).
 1st hill representsT.S 1st (pi
complex) and 2nd hill represents
T.S 2nd (sigma complex ), the gap
b/w the 2 hills represents the
arenium ion .
E.S.R –Electrophilic substitution reaction
T.S - Transition state
Delta G – Gibbs free energy

5. PIE COMPLEX SIGMA COMPLEX
Pie complex Sigma complex
This forms first in a reaction.
This does not involve actual
chemical bonding but held
near pie electron cloud of
aromatic ring.
The pie complex converts into
the sigma complex.
This involves the actual chemical
bonding between electrophile
and the carbon atom.

6. NitrationReaction
Inthistypeofreaction,theHatomofthearomaticringisreplacedbyNO2 group.
 Nitrating reagent used :-
mix acid (HNO3 +H2SO4).
 Active nitrating reagent :-
NO2
+ ion.
 Other reagent that could be
used :-
i)Conc./fuming sulphuric acid.
ii) HNO3 acid in organic solvent
(acetic acid , nitromethane).
iii)Nitronium salts (NO2
+ ,BF4
- ,
N2O5 ).
 Reaction involved in
reagent :-
H2SO4 +HO-NO2 HSO4
- +H2O+-NO2
H2O+-NO2 H2O +NO2
+
H2O + H2SO4 H3O+ + HSO4
-
 overall reaction :-
2H2SO4 +HNO3 NO2
+ + H3O+ +2HSO4
-
The above reaction is an acid base
reaction where nitric acid acts
as a base.

7. Reaction of Nitration :-
Mechanism:-
Step 1:-formation of arenium ion carbocation .
In this step , aromatic ring reacts with the NO2
+
& forms an arenium ion carbocation which is
stabilised by resonance .

8. Step2:- Formation of the substituted product by removal of a
proton from the arenium carbocation formed in the step 1.
This step is fast step ,and aromaticity of the compound
regained in this step.

9. Sulphonation Reaction
In this type of reaction , the H atom of the aromatic ring is
Substituted by SO3 group.
Reagent used in this reaction :- fuming sulphuric acid (oleum)
concentrated H2SO4 .
 2H2SO4 SO3 + H3O+ + HSO4–
Active electrophile :-SO3
 Reaction of sulphonation :-

10. Mechanism:-
Step 1:- In this step , the electrophile (SO3 ) attacks the benzene
/aromatic ring to give the resonance stabilised arenium ion
intermediate .This step is slow since aromaticity of the ring is
disturbed in this reaction and thus is the rate determining step.
Step 2:- Removal of a proton from arenium ion by a base
to give substitution product i.e . Benzenesulphonic acid.

11. Reverse Sulphonation Reaction:-
This reaction is a reversible reaction especially at high
temperature.
For eg:-
Mechanism for reverse Sulphonation Reaction:-

12. Halogenation Reaction
In this reaction , H atom of the aromatic ring is substituted by
halogen atom .
Active electrophile is either a halogen L.A complex or a
positive halogen.
 Aromatic rings can be chlorinated / brominated with Cl /Br in
presence of lewis acid (L.A) such as FeCl3 ,FeBr3 , AlCl3 etc.
Cl-Cl +FeCl3 Cl-Cl+-Fe-Cl3 Cl+ +FeCl4
-
(L.A complex)
Reaction of Halogenation:-

13. Mechanism:-
Step 1:- Lewis acid complex or the positive halogen
group itself attacks the aromatic ring to form
resonance stabilised arenium ion intermediate .
Step 2:-Proton is removed from the arenium ion
intermediate by a base to give the halogenated product.

14. Friedel Crafts Alkylation Reaction
 In this reaction alkyl group substitutes the H atom of aromatic
ring .
Reaction is brought about by R-X , alkenes , alcohols in
presence of lewis acid catalyst .
Example :- AlCl3 , AlBr3 , HF , H2SO4 , ZnCl2 etc.
 Order of reactivity [R-X] :- R-F > R-Cl > R-Br > R-I
 Active electrophile :- alkyl halide – lewis acid complex (1:1) or
alkyl carbocation .
Reaction of F.C Alkylation

15. Mechanism of reaction
Step 1 :- Formation of Carbocation :-
a) From Alkyl halides :-
R-Cl +AlCl3 R+ +AlCl4
-
b) From Alcohol and Lewis acid :-
R-OH +AlCl3 ROAlCl2 R+ + OAl-Cl2
c) From alcohols and proton acids :-
R-OH + H+ R+OH2 R+ + H2O
d) from alkene :- In this ,proton is always required .
C C + H+ C+ C H
Step 2:- Attack of carbocation on aromatic ring :-

16. Step 3 :- Removal of the proton to form the.
substituted product.
Limitations of F.C Alkylation :-
 Polysubstitution :- Since in these reaction, the entering group is an
activating group , di and polyalkylation occurs frequently.
 Aryl halides cannot be used instead of alkyl halides.
 Aromatic compound containing meta directing group do not
undergoes this reaction.
The activating groups such as OH ,NH2, OR etc do not facilitate
this reaction.
 Rearrangement may occur in alkylating group , this is possible
even with Lewis acid complex therefore usually its not possibe to
add primary alkyl group (except Me &Et).

17. Friedel Crafts Acylation Reaction
 This reaction involves introduction of acyl group into aromatic ring
This is most important method in preparation of aryl ketones .
 Reagent used :- Acyl halides , Carboxylic acids , anhydride ,
Ketenes in Lewis acid catalyst.
More than 1 mol. Of catalyst is required since the first mole
co. ordinates with the Oxygen of reagent.
 Order of reactivity : RCOI >RCOBr >RCOCl > RCOF
 Active electrophile :-Carbocation , acylium ion ,
acyl halide –lewis acid complex (1:1).

18. Mechanism of Reaction
Step 1 :- Generation of an electrophile
Step 2 :- Attack of electrophile to the aromatic ring , this is a
slow step and hence R.D.S.
Step 3 :- removal of proton and formation of substituted product.

19. Activatingoro/pdirectinggroup
 These groups are more reactive then benzene towards E.S.R .
They direct the incoming electrophile towards o & p position.
Eg :- O- ,NR2 , NHR , NH2 , OH , OR ,NHCOR , OCOR , SR ,Alkyl
& aryl groups.
Explanation for orientation and reactivity :-
If atom present on
key atom has lp
electrons .
Electron density
increase at o/p position
due to +R /+M.
Electrophile will
bind at o / p
position.
Overall electron
density of benzene
increases.
Therefore activates the
aromatic ring.

20.  If R(alkyl ) group is present , electron density increases by +I
or hyperconjugation effect .
 If aryl group is present on the aromatic ring , then also it shows
ortho para directing effect.

21. META DIRECTING OR DEACTIVATING GROUP
 These groups are less reactive then benzene .
They direct the incoming electrophile towards meta position .
 Eg :- NO2 , CF3 ,CN ,SO3H , CHO , COR , COOH , COOR ,
CONH2 , CCl3 etc.
Key atom attached
to more E.N atom
by = /triple bond.
Attracts electron from o
& p position due to –R/
-M effect.
Meta position will
have relatively
higher electron
density
Attacking
carbocation
attaches at ,meta
position
Hence overall electron
density of the ring
decreases .
Ring deactivates.

22. Meta directing groups with positively charged key atoms
(eg :- N+R3 ,N+H3 etc) attracts electron from o/p position
through strong (-I) effect.

23. Curious case of Halogens
Halogens are ortho para directing yet deactivating .
Do youknow
???
Here two effects (+R) &(-I) both come into
play and since these both groups are
opposing each other .
-I group withdraw electron
from the ring more strongly then
they donate electron by
resonance hence deactivated
the ring.
Deactivation is partial at o/p
position due to electron
donation by resonance (+R)
but meta position cannot be
benefited therefore they are
ortho para directing.

24. Introduction of third group in Benzene
Case 2:- If both groups present are meta directing then third group will
bind acc. to less powerful group.
Order of directing power :- Me3N >NO2 >CN . SO3H >CHO >COMe
>COOH
Case 3 :-If one group is o/p and other is meta directing ,orientation
reinforce each other , 3rd group will bind at 1st position.

25. Case 4 :- If both groups present are o/p and meta directing but o/p
group have more directive influence then para orientation of o/p
group would be preferred
Case 5 :-If both groups occupy meta position with respect to each
other , 3rd group would least likely to enter between these both
groups due to steric hinderance .

26. Orientationin other ring system
 In fused ring system ,the position are not equivalent and there are
preferred orientation even in unsusbstituted hydrocarbons .
Eg:- In Naphtalene , the alpha position is more preferred then
beta , since at alpha position intermediate would be stabilised by 2
resonating structures whereas in beta attack stabilised by only 1
resonating structures.

27. Sulphonation of naphtalene
At 80 degree
 Reaction doesn’t reach
equilibrium
 product :- alpha naphtalene
sulphonic acid , less stable because
of steric hinderance by SO3H and
H group.
At 160 degree
 Reaction obtained
equilibrium here.
 Product :-beta naphtalene
sulphonic acid ,
thermodynamically more
stable.
 Naphtalene , anthracene , phenanthrene etc are more reactive then
benzene in E.S. R due to extensive delocalisation of positive charge .
 Orientation predicted on basis of stability of carbocation formed by
attack of electrophile .
 Eg :- Furan ,Thiophene ,Pyrrole substitutes at 3rd position which
are more reactive then benzene .
Pyridine substitution occurs at 3rd position .

28. References
 Advanced organic chemistry -
Jagdamba Singh & L.D.S
Yadav
 March’s Advanced Organic Chemistry
Organic chemistry by G. Marc Loudon

29. Thankyou