The document discusses the structure and properties of benzene. It explains Kekulé's suggestion that benzene has alternating double and single bonds in a planar cyclic structure. However, benzene's properties are better explained by the resonance hybrid model, where the pi electrons are delocalized around the ring. Aromatic compounds have delocalized pi electrons in a cyclic planar structure according to Hückel's rule of 4n+2 pi electrons. Examples of aromatic and non-aromatic compounds are given. The document also discusses the nomenclature, reactions, and properties of aromatic compounds including electrophilic aromatic substitution.

1. BY
VANA JAGAN MOHAN RAO M.S.Pharm, MED.CHEM
NIPER-KOLKATA
Asst.Professor, MIPER-KURNOOL
Email: jaganvana6@gmail.com

2. Benzene : Resonance Description
Structure:
Kekulé suggested that benzene was...
i)PLANAR
ii)CYCLIC
iii)Had Alternating Double And Single Bonds
Thus These Double Bonds Are Described As
Conjugated Bonds.
Primary analysis revealed benzene had...
 molecular mass of 78
 molecular formula of C6H6
INTRODUCTION

3.  However, all bond lengths in benzene to be equal and
intermediate between single bond and double bond lengths (1.39
Å) and the ring is more stable than expected.
 To explain the above, it was suggested that the structure oscillated
between the two Kekulé forms but was represented by neither of
them. It was a RESONANCE HYBRID ( average of two structures
that differ only in the placement of the valence electrons).
Contributing Structures
Resonance hybrid
The energy calculated for a
resonance hybrid is lower
than the energies of the two
alternative structure.

5. one way to overlap
adjacent p orbitals
delocalised pi
orbital system
another
possibility
6 single bonds

7. CHARACTERISTICS OF AROMATIC COMPOUNDS
*  electron cloud delocalized all over the ring
* The resonance picture this helps to explain lack of
reactivity of benzene (substitution not addition )
Aromatic compounds are compounds that resemble
benzene in chemical behavior thus they tend to react by
substitution rather than by addition and fulfill the
aromaticity requirements.

8. To be classified as aromatic, a compound must have :
1) Cyclic structure.
2) Planar structure.
3) Each atom of the ring must have a p orbital to form a
delocalized π system i.e. no atoms in the ring can be sp3
hybridized instead all atoms must be sp2 hybridized (N.B.
carbocation and carbanions are sp2 hybridized or an
unshared pair electrons).
4) Fulfill Huckel rule i.e. the system must have 4n + 2 pi
electrons : thus by calculating n value it will be an integral
number i.e. n=0, 1, 2, 3,
CHARACTERISTICS OF AROMATIC COMPOUNDS

9. EXAMPLES OF AROMATIC COMPOUNDS
N
H
N
H
N
O
4n+2=6 4n+2=6 4n+2=6 4n+2=6 4n+2=6 4n+2=6
n=1 n=1 n=1 n=1 n=1 n=1
4n+2=2 4n+2=10 4n+2=10
n=0 n=2 n=2

10. 10
EXAMPLES OF NON AROMATIC COMPOUNDS


carbon indicted by is sp3
n=1/2 n=1/2 n=1/2
4n+2=8
n=1.5

11. NOMENCLATURE OF AROMATIC COMPOUNDS
1. Monosubstituted Benzenes
a. IUPAC Names
They are named as derivatives of benzene. One side group is
named as a prefix in front of the word benzene.
 No number is needed for mono-substituted benzene.
tert-Butyl-benzene Ethyl-benzene Nitro-benzene Chloro-benzene
C(CH3)3 CH2CH3 NO2 Cl

12.  Benzene ring has priority over :side chains with alkyl, alkoxy
groups, halogens, double and triple bonds
 In some cases the side chains on aromatic ring contain functional
groups of higher priorities (NH2, OH, CHO,C=O, COOH, COOR) thus
in this case the aromatic ring will be considered as a substituent
and the side chain will be used to give the root name. Two
aromatic radials are known
CH2
Benzyl group
(C6H5-)
phenyl group
Vinyl-benzene Allyl-benzene Ethynyl-benzene Butyl-benzene
C CH
OCH3
Methoxy-benzene

13. b. Common Names Of Monosubstituted Benzenes
Toluene Styrene Phenol Benzaldehyde Benzoic acid Aniline
CH3 CH=CH2
OH NH2H O HO O OCH3
Anisol
Any aryl group (Ar) is the aromatic group that remains after
removal of hydrogen atom from an aromatic ring. When the
benzene ring is named as substituent, it is called a phenyl
group (often abbreviated Ph).
A benzyl group (phenyl methyl group) is the seven carbon
unit consisting of benzene ring and methylene (-CH2-).

14. A hydrocarbon composed of one saturated chain and one
benzene ring is usually named as a derivative of the larger
structural unit. However, if the chain is unsaturated
compound may be named as a derivative of that chain,
regardless of ring size. The following are examples:

15.  All disubstituted benzenes (two groups are attached to benzene),
can give rise to three possible positional isomers.
 When the substituents are different, they are of equal priorities
they will should be listed in alphabetical order.
2. Nomenclature of Disubstituted and polysubstituted Benzenes
X
Y
X X
Y
Y
Common:
IUPAC:
orth- meta para
1,2- 1,3- 1,4-
1-Chloro-2-ethylbenzene 1-Bromo-3-nitrobenzene 1-Fluoro-4-iodobenzene
C2H5
Cl
NO2
Br
o-Chloroethylbenzene m-Bromonitrobenzene p-Fluoroiodobenzene
IUPAC:
Common:
FI

16.  If one of the substituents is part of a parent compound, then the
disubstituted or polysubstituted benzene is named as a derivative of
that parent compound i.e. priorities determine the root name and
substituents.
CH3 CH3 CH3
CH3
CH3
CH3
Common:
IUPAC:
o-Xylene m-Xylen p-Xylene
1,2-Dimethyl-benzene 1,3-Dimethyl-benzene 1,4-Dimethyl-benzene
OH
Cl
COOH NO2
Br
CH3
Common:
IUPAC:
o- Chlorophenol m-Bromobenzoic acid p-Nitrotoluene o-Methoxybezaldehyde
2-Chlorophenol 3-Bromobenzoic acid 4-Nitrotoluene 2-Methoxybezaldehyde 2,4,6- Trinitrotoluene
CHO
OCH3
CH3
NO2
NO2O2N

17. Electrophilic Aromatic Substitution Reactions:
REACTIONS OF BENZENE

23. SIDE-CHAIN REACTIONS OF BENZENE DERIVATIVES
a. Halogenation of an Alkyl Side Chain
CH3
Br2
UV light
Toluene
CH2Br
+ BrH
Benzyl Bromide
1)Halogenation
CH2CH3
Cl2/ UV
CHClCH3
CH2CH2Cl
Major Minor

24. b. Oxidation of an Alkyl Side Chain
CH3
KMnO 4, OH
-
, H2O, heat
Toluene
COOH
Benzoic acid
CH2CH3
CH3
NO2
KMnO 4, OH
-
, H2O, heat
COOH
COOH
NO2
+ + OH2CO2
H3O
+
H3O
+
2- Ethyl-1-methyl-4-nitrobenzene 4- Nitrophthalic acid

25. ORIENTATION EFFECTS OF SUBSTITUENTS IN ELECTROPHILIC
AROMATIC SUBSTITUTION REACTIONS OF MONOSUBSTITUTED
BENZENES
 Alkyl groups and groups with lone pairs (electron donating groups) direct new groups to
ortho-, para-positions and speed-up the reaction (i.e. o & p directors and activating groups).
 Halogens direct new groups to ortho-, para- positions but they slow down the
reaction (i.e. halogens are o & p directors and deactivating groups).
 Electron withdrawing groups such as nitro, nitrile, and carbonyl direct new
groups to the meta-position and slow the reaction down (i.e. i.e. m directors and
deactivating groups).
 Thus the order of reactivity of benzene and monosubstituted benzene derivatives in
E.Ar.sub. is as in the following chart
Substituted benzene with o,p directors > Benzene > Halobenzene derivatives >
Substituted benzene with m- directors

26. Meta directorsOrtho , para directors
-NO2
-SO3H
-COOH, -COOR
-CHO, -COR
-CN
-OH, -OR
-NH2, -NHR, -NR2
-C6H5
-CH3, -R (alkyl)
-F, -Cl, -Br, -I
OH
HNO3 / H2SO4
OH OH
+
NO2
NO2o-Nitrophenol
53 % p-Nitrophenol
47 %NO2
SO3 / H2SO4
NO2
m-Nitrobezenesulfonic acid
SO3H

27. O
Cl
O
AlCl3
OO O
O
+

28. DDT (DICHLORO DIPHENYL TRICHLORO
ETHANE
IUPAC NAME: 1,1’-(2,2,2-Trichloroethane-1,1-diyl) bis(4-chlorobenzene)
FORMULA: C14 H9 Cl5
PROPERTIES:
Colourless
Tasteless
Odourless
Crystalline
Chemically inert
Non-biodegradable
Insoluble in water
USES: Insecticide and pesticide

29. SACCHARIN
IUPAC NAME: 1,1-dioxo-1,2-benzothiazol-3-one
OTHER NAMES: Benzoic sulfimide
FORMULA: C7 H5 NO3 S
PROPERTIES:
White crystalline solid
Density: 0.828g/ml
Sparingly soluble in water
USES:
Artificial sweetener (used to sweeten drinks, candies, cookies, medicines and
tooth pastes)

30. BHC (BENZENE HEXACHLORIDE)
IUPAC NAME: (1R,2R,3S,4R,5R,6S)- Hexachlorocyclohexane
SYNONYMS: Lindane, Gamma-hexachlorocyclohexane
FORMULA: C6 H6 Cl6
USES:
Stomach and contact Insecticide
Used as a soil insecticide against termite, white grubs etc.

31. CHLORAMINE
IUPAC NAME: (1R,2R,3S,4R,5R,6S)- Hexachlorocyclohexane
SYNONYMS: Lindane, Gamma-hexachlorocyclohexane
FORMULA: C6 H6 Cl6
USES:
Drinking water infection
Swimming pool disinfection
Used to improve odour and flavour of the water