Introduction to benzene, orbital picture, resonance in benzene, Huckel‟s rule Reactions of benzene - nitration, sulphonation, halogenation- reactivity, Friedel- Craft‟s alkylation- reactivity, limitations, Friedel-Craft‟s acylation. Substituents, effect of substituents on reactivity and orientation of mono substituted benzene compounds towards electrophilic substitution reaction.

1. Benzene and its derivatives

2.  Introduction to benzene, orbital picture, resonance in
benzene, Huckel‟s rule
 Reactions of benzene - nitration, sulphonation,
halogenation- reactivity, Friedel- Craft‟s alkylation-
reactivity, limitations, Friedel-Craft‟s acylation.
 Substituents, effect of substituents on reactivity and
orientation of mono substituted benzene compounds
towards electrophilic substitution reaction.

3. Introduction to benzene
 Benzene and all those compounds which resemble benzene
in their chemical behavior are termed as aromatic.
 Benzene is an aromatic compound having molecular
formula C6H6.
 It contains six carbon atoms, 6 hydrogen atoms and three
conjugated double bonds.
 Aromatic compound having one or more benzene rings in
their molecules are called benzenoid compounds or
benzenoids.
 E.g benzene, toluene , xylene , cholrobenzene , phenol etc.
basic structure of benzene is

4. Nomenclature of Derivatives of
Benzene
 In IUPAC Nomenclature there are various rules for naming
benzene derivatives.
 These are follows,

7. Resonance structure of benzene
 To explain all the limitations of kekule’s structure, it has been
proposed is a resonance hybrid of two kekule’s structure (I & II).
 These two structures are canonical forms of benzene.
 In actual, the benzene is a resonance hybrid of these two
structures (A) its means that any two adjacent carbon atoms of
the benzene molecule are neither joined by a pure single bond
nor by a pure double bond.
 As a result C-C bond lengths are equal i.e 1.39 A0 and lie in
between C=C bond length of 1.34 A0 and C-C bond length of
1.54 A0
 Resonance hybrid is always more stable than its cannonical
structures.

8. Molecular Orbital (MO) structure of
Benzene
 Molecular orbital theory states that all the carbon atoms of benzene are
assumed to be SP2 hybridized.
 Each C atom form two C-C sigma bonds with adjacent C atoms and C-H sigma
bond with hydrogen atom.
 So overall there are six C-C sigma bonds and six C-H sigma bonds, which lie in
one plane and the angle between any two adjacent sigma bond is 120.

9. Aromacity & Huckel rule
 Benzene and its derivative having large resonance energies are
called as aromatic compound and their extra stability is referred to
their special property called as aromatic character or aromaticity.
 Huckel rule :
 In 1931, a German physicist, Huckel gave certain rule for defining
the aromaticity of organic compounds.
 In order to be aromatic, a compound must fulfill the following
criteria:
 1) the molecule or ion must be flat or planer
 2) it should have cyclic delocalised electron clouds above and below
the plane of the molecule.
 3) the total number of π electrons in the molecule should be ( 4n+2)
where n=0,1,2… etc
 4) the π electron clouds should encompass all the carbon atoms of the
cyclic system.
 This rule is also known as ( 4n+2) π rule.
 A molecule which does not possess or satisfy one or more of the
above conditions are said to be non-aromatic.

10.  Huckel rule examples :

11. Reactions of Benzene
 Resonance provides extra stability to benzene and other aromatic
compounds.
 So all these compounds shows substitution reaction rather than
addition reactions.
 These substitution reaction are carried out by using reagents
which are electrophillic in nature as benzene has pi- electrons.
 Benzene shows aromatic electrophilic substitution reaction :
 Aromatic electrophilic substitution reaction takes place in two
different ways:
 1) Concerted reactions:
 2) two step process:
Types of reaction benzene :
 A) Sulphonation
 B) Nitration
 C) Halogenation
 D) Friedal Craft alkylation

12.  1) Concerted reactions:
 this reaction takes place in single step.
 It involves simultaneous formation of new covalent bond between C atom of
the ring and the electrophile and breakage of C-H bond.
 2) two step process:
 In the first step, electrophile adds to the benzene ring and forms a carbocation
intermediate and
 In the Second step, the intermediate looses a proton to give the substitution
product.

13. A) Sulphonation
 It is carried out by the reaction of benzene with concentrated
sulphuric acid.
 The mechanism of sulphonation involves following steps
 1) Generation of an electrophile : Two molecules of sulphuric
acid reacts to form sul- phur trioxide which acts as
electrophile
 Formation of Carbocation intermediate : SO, attacks the
benzene ring to form 2. carbocation intermediate.

14.  3. Loss of Proton :Carbocation looses a proton to form
sulphonic acid anion.
 Addition of Proton : Proton is then added to the benzene
sulphonic acid to give final 4. sulphonated product.

15. B) Nitration
 It is carried out by reacting benzene with a mixture of Conc.
HNO3, + Conc. H2SO4, which is also known as nitrating
mixture.
 Following steps are involved in nitration of benzene.
 1) Generation of an electrophile : In presence of conc.
H2SO4, nitric acid acts as a base 1. which accepts proton to
form protonated nitric acid which looses water to form
nitronium ion (electrophile).
 2. Formation of s-complex carbocation intermediate :
NO2,* (nitronium ion) attacks the benzene ring forming a s
complex carbocatoin which is further stabilized by

16. This step is slow and hence it is the rate determining step of the
reaction.
3. Loss of Proton from Carbocation intermediate: Carbocation
looses a proton to base to form the final product i.e.
nitrobenzene.

17. C) Halogenatlon
 It is generally carried out at low temperature, in the absence of
sunlight and in the presence of lewis acids like anhydrous ferric or
aluminium chloride as a catalyst.
 Halogenations occurs by following mechanism
 Step 1: Production of an Electrophile
 Step 2: Formation of a-complex or carbocation intermdiate
 The +ve end of the polarised chloride molecule attacks the T-
electron cloud of the ben- zene ring to form a o-complex or the
carbocation intermediate which is stabilized by reso- nance

18.  Step 3: Loss of a proton from the carbocation intermediate :
 The base (AlCl4) present then abstracts a proton from
intermediate to form chlorobe zene. This step is fast and
hence does not aftect the rate of the reaction.

19. D) Friedal Crafts Alkylation
 This method is useful in the preparation of alkyl substituted
benzene by the reaction between benzene and a suitable
alkyl halide by using Lewis acid catalyst like AlCl3, BF3,
FeCl3, etc.
 Step 1: Generation of an electrophile
 The alkyl halide reacts with anhydrous AICI3, to form
polarised alkyl halide which acts as the electrophile

20.  Step 2 : Formation of an intermediate
 The +ve part of the polarized alkyl halide molecule attacks
the 1-electron claud of the. benzene ring to form
carbocation intermediate (stabilized by resonance).
 Step 3 : Loss of a proton
 Carbocation looses a proton to the base and gives final
product.

21. e) Friedal-Crafts Acylation
 This method is useful in the preparation of aromatic
ketones. Benzene and other arenes reacts with acid
chlorides on acid anhydrides in presence of anhydrous
AICI, to form aromatic ketones.
 For example :-

22.  1. Generation of an electrophile :
 The acid chloride or acid anhydride reacts with anhydrous
aluminium chloride to form acylium ion (R-C=O) which acts
as an electrophile.
 2. Formation of a carbocation intermediate :
 The acylium ion then attacks the benzene ring resulting in
the formation of carbocation which is stabilized by
resonance.

23.  3. Loss of a proton :
 The base (AlCl4) abstracts a proton from carboction to give
final product.
 This step is fast and does not affect the rate of reaction

24. Substituent's, effect of substituent's on reactivity and
orientation of mono substituted benzene
 Benzene has six hydrogen which are equal.
 If we replace any one H atom by a substituent's it gives a
monosubstituted derivative of benzene.
 When monosubstituted benzene is converted into di-
substituted in derivative, then three isomer ( ortho, Meta and
para ) are possible.
 Effect of substitution on orientation :
 A second substituted can occupy any of the remaining five
position in monosubstituted benzene.
 The position 2 & 6 are equivalent & give ortho product.

25.  Position 4 is unique and gives para product.
 There are two ortho, two meta and one para substitution with
respect to substituent which is already present.
 All the group can be divided into following classes
 1) Ortho-para directing group:
 these group direct the incoming groups to ortho and para
position.
 For example- alkyl ( R) , phenyl (C6H5) halogens (Cl ,Br )
hydroxyl (OH) Amino (NH2)
 2) Meta Directing groups :
 These group direct the incoming groups to meta position.
 Eg. Trialkyl ammounium ion ( N+ R3 ) Nitro (NO2) , cyano ( CN),
aldehyde (CHO) , ketonic (COR) , Carboxylic ( -COOH),
Sulphoic acid (SO3H)

26.  In short, substituent which contain multiple bonds,
(double/triple) are usually meta directing and which do
not contain any multiple bonds but contain one or more
pairs of electrons on the atom are ortho and para
directing.

27.  Effect of Substituent's on Reactivity :
 Ortho and para directing groups (except alkyl & phenyl) contain one
or more pairs of electrons on the atom and these electrons interact
with the m-electrons of the benzene and increases the electron
density.
 Hence the benzene ring gets activated for further electrophillic
substitution. So, all the ortho & para directing groups except
halogens are activating groups.
 Meta directing groups due to the presence of multiple bonds in them
withdraw electrons from the benzene ring and decreases the
electron density and hence deactivates the benzene ring for further
electrophilic substitution. So all the meta directing groups are called
as deactivating groups.
 A substituent which activates the benzene ring for further
substitution is called an activating substituent.
 A substituent which deactivates the benzene ring for further
substitution is called deactivating substituent.
 Ortho-para directors are activators whereas meta directors are

28.  Theory of Reactivity :
 The rate of electrophilic substitution reactions depends upon the
energy of activation i.e. difference in energy of the transition state
and ground state of reactants.
 So, the rate. depends on the availability of electrons in the benzene
ring. If the ring is electron rich (- ve), the electrophillic attack is
faster.
 If the ring is electron deficient (+ ve) the attack is slower. So, an
electron donating substituent will activate the aromatic ring while an
electron withdrawing substituent will deactivate it.
 Ortho-Para directing groups (-OH, NH2, CH3) release electrons into
the ring, makes it electron rich and hence activates the ring to
electrophillic attack.
 Meta directing groups (NO2, SO3H, COOH) withdraw electrons from
the ring and makes it electron deficient and hence deactivates the

29.  Theory of Orientation :
 Orientation or directive effect can be explained by studying
all the possible resonance structures of the o complex
formed as a result of the attack of the electrophile at Ortho,
meta and para positions for different types of substitutions.
a) Ortho-para directing groups having electron releasing
inductive effect (+I effect) :-
For example, alkyl groups has +I effect i.e. electron releasing
inductive effect.
study the o complexes or the carbocation intermediate formed
by attacked of E+ at ortho, para -and meta positions of
toluene.

31.  b) Ortho-para directing groups having electron withdrawing (-I
effect) and electron releasing (+R or +M effect) resonance or
mesomeric effects.
 For example, -NH2 group. Various 6 complex intermediates
resulting from attack at ortho, para & meta positions are given
below :

32.  c) Meta directing groups :
 All meta directing groups are electron withdrawing in nature. They have both
electron withdrawing inductive and resonance effect i.e. -I & -R effect. For
example, -NO2, CN, COOH, -CHO, -SO3H etc
 Intermediate carbocations resulting from ortho, para and meta attack are given
below :-

33.  Reactivity of Halogens :
 Halogens have electrons withdrawing inductive effect (-I effect) and electron
donating resonance effect (+R effect).
 Due to high electronegativity of halogens the inductive effect predominates
over the resonance effect, So, the overall effect is electron withdrawing.
Halogens are ortho para directing.