Benzene and its derivatives.ppt

according to PCI Syllabus
Subject:-Pharmaceutical Organic Chemistry-II
Class/year:-S.Y.B.Pharmacy
Sem:-III

1. Benzene and its
derivatives
Mr.P.S.Kore
Assistant Professor(Research Scholar)
Pharmaceutical Chemistry Department
RCP, Kasegaon

2. Aliphatic hydrocarbons –include alkanes, alkenes,
and alkynes
Aromatic hydrocarbons–compounds that contain
one or more benzene ring –they are also called
arenes
An aromatic hydrocarbon group is called an aryl
group (Ar–)
Benzene -
the simplest aromatic hydrocarbon with the
molecular formula, C6H6.
Discovered by Faraday
undergoes substitutionreactions rather than addition
reactions

3. Nomenclature:
Dimethyl benzene have the common
nameXylene:CH
3
CH
2
CH
3
HC
CH
3
CH
3
benzenemethylbenzeneethylbenzene Cumene
Toluene
Xylene Isopropylbenzene CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
meta
-
xylene
para-Xylene
o- xylene

4. Aryl Group:
Aralkyl group:CH
3

CH
3

CH
3

meta-tolyl
para-tolyl
o- tolyl

phenyl CH
2

CH


C
-




Benzyl Benzal Benzo

5. Structure of Benzene:
1. Molecular Formula: Elemental analysis
and molecular weight determination
showed that benzene had the molecular
formula C6H6, this indicated that benzene
was highly unsaturated compared withn-
hexane (C6H14)
2. Straight-chain structure not possible:
Benzene constructed as a straight chain
or ring compound having double (C=C)
and or (C=C) bonds.

6. Benzene does not behave like alkenes or alkynes
Benzene did not decolourize bromine in carbon
tetrachloride or cold aqueous potassium permanganate
also did not add water in presence of acid.
3.Evidence of cyclic structure:
A) Substitution of benzene:benzene reacts with bromine in
the presence of FeBr3 to form monobromobenzene.
Only one monobromo and no isomeric products obtained,
becoz all hydrogens are identical. Benzene had cyclic str. Of
six carbons and to each carbon was attach. one hydrogen.Benzene
Br
2
/Ccl
4
Dilute cold Kmno
4
H
2
O/H
+
No reaction C
6
H
6
+ Br
2
FeBr
3
C
6
H
5
Br + HBr
Benzene
Bromine
Bromobenzene

7. B) Addition of Hydrogen: Benzene added three moles of
hydrogen in presence of nickel catalyst to give
cyclohexane.
It porved that presence of three carbon-carbon double
bonds.
4. Kekule’s structure of benzene:He suggested that
benzene consisted of cyclic planner structure of six carbon
with alternate double, single bond.
Each carbon attached with one hydrogen.
Objections: 1. if kekule structure is correct than dibromo
should be attached to carbon-carbon double bond.
2. in other structure the dibromo should be attached to
carbon –carbon single bond.C
6
H
6
+ 3H
2
Ni
Benzene
Hydrogen
Cyclohexane benzene
Br
Br
Br
Br
(a) (b)
2Br
2
FeBr
3

8. Overcome: kekulefurther suggested that
benzene was mixture of two forms.
Kekulefailedto explain why benzene does not
undergoes addition reaction.
5.Resonance description of benzene: The
phenomenon in which two or more str. Can be
written for a substance which involve identical
positions of atoms is called as resonance. (a) (b) (a) (b) (c)
Resonance structure
Hybrid

9. Reason behind benzene which does not
undergoes addition reactions.
Resonance hybrid is more stable than
contributing structures. C C



Bond length:1.54 A
C C


Bond length:1.34 A Bond length:1.40 A

10. Resonance Energy of Benzene:
Benzene stability is due to the formation
of delocalized sigma molecular orbital.
The energy can be measured by changes
in heat of hydrogenations that are
associated with reactions.
1. Hydrogenation of cyclohexane evolves
28.6 kcal/mol.+H
2
Cyclohexane
cyclohexene
+
28.6 kcal

11. 2. 1,3 cyclohexadiene evolves 55.4 kcal/mol,
approximately double amout of cyclohexene.
3. when 1,3,5 cyclohexatriene (kekule
structure)undergoes hydrogenation it evolves
85.8 kcal/mol.
It is hypothetical strucutre of contianing three
double bonds. We can predict the energy evolved
by 1,3,5 cyclohexatriene approximately i.e.
3*28.6 or 85.6 kcal/mol.+2H
2
Cyclohexane
+
55.4 kcal
1,3 cyclohexadiene +
3H
2
Cyclohexane
+
85.8 kcal
1,3,5 cyclohexatriene
Hypothetical

12. Cyclohexane
+ H
2
+ 2H
2
+ 3H
2
+ 2H
2
28.6 kcal
55.4 kcal
85.8 kcal
49.8 kcal
36 kcal (resonance energy) of benzene
Fig. Heats evolved on hydrogenation of one mole of cyclic compounds.
estimated
Hypothetical 1,3,5 cyclohexatriene

13. Aromaticity (HUCKEL RULE)
The aromatic compounds apparently conatains
alternate double and single bond in a cyclic
structure.
They undergoes substitution reaction rather than
addition reaction.
This characteristic behaviour is called as
Aromatic character or Aromaticity.
Criteria for Aromaticity
Rule:1-An aromatic compound is cyclic and
planar.
Rule:2-Each atom in an aromatic ring has a p
orbital. These p orbital must be parallel for
continuous overlapping around the ring.

14. Rule:3-The cyclic pi molecular orbital (electron
cloud) formed by overlap of p orbitals must
contain (4n+2) Pi electrons, where n=0,1,2,3,
etc. This is known as “Huckel rule”.
Benzene:It is a cyclic planar compound. It has p
orbital on each carbon of the ring involved in a
double bond. It has three double bonds and six
pi electrons, which is in accordance with Huckel's
rule.
Therefore 4n+2=6 or 4n=6-2
 4n =4 and n=1benzene naphthaleneCycloheptatriene cyclooctattraene

15. Preparation of Benzene:
1. From petroleum: Aromatization of C6-
C8 fraction of petroleum naptha yields a
mixture of Benzene, toluene and Xylenes.CH
3
(CH
2
)
4
CH
3
Pt/Al
2
O
3
500
0
C, 15 min
Benzene
n-Hexane
+
4H
2
CH
3
CH
3
(CH
2
)
5
CH
3
Pt/Al
2
O
3
500
0
C, 15 min
Toluene
n-Heptane
+
4H
2
CH
3
CH
3
CH
3
(CH
2
)
6
CH
3
Pt/Al
2
O
3
500
0
C, 15 min
Xylenes
n-Octane
+
CH
2
CH
3
ethylbenzene

16. Benzene is obtained on large scale from petroleum and
coal tar. The synthetic methods on laboratory scale are of
academic interest only.1. By passing acetylene through red hot tube at 500
0
C
CH CH3
benzene
Acetylene
red hot
tube
2. By heating benzoic acid or its sodium salt w ith soda lime(NaOH+CaO)
C
O
ONa+
NaOH
CaO
+
Na
2
CO
3
Sodium benzoate
3. By heating phenol w ith zinc dust
OH+
Zn +
ZnO
4. By treating chlorobenzene w ith magnesium follow ed by treatment w ith dilute HCl
Cl
Mg
MgCl
Phenhyl magnesium chloride
Chlorobenzene
H
+
/H
2
O
Benzene
Benzene
Benzene

17. Physical Properties
Benzene is colourless, liquid, B.P 80
O
C and M.P 55
O
C
Insoluble in water and immiscible with alcohol, ether
and chloroform.
Itself act as a good solvent for many organic and
inorganic compounds.e.g. resins, fat, sulfur and
iodine.
Its vapours are highly toxic and on inhalation produce
loss of consciousness.
It burns with a bluish flame.
Benzene and its derivatives show characteristic IR
spectrum.
The two band near 1600cm-1 and 1500 cm-1 have
streching of carbon-carbon bonds of aromatic ring.
The sharp bands near 3030cm-1 are caused by
aromatic C-H bonds.

18. Chemical Properties
1. Electrophilic substitution reactions.
2. Addition reactions.
3. Oxidation reactions.

19. Chemical Properties
1. Electrophilic substitution reactions.NO
2
SO
3
H
CH
3
C CH
3
O
Cl
Nitration
HNO3
Sulphonation
H
2
SO
4
Alkylation
CH
3
Cl
HalogenationCl
2
,Br
2
,I
2
,F
2
Acylation
CH
3
CCl
O
Nitrobenzene
Benzene sulfonic acid
Toluene
Acetophenone
Fig. Electrophilic substituion reaction of Benzene.
FeCl
3
H
2
SO
4
AlCl
3

20. General Mechanism:
Benzene undergoes electrophilic
substitution reactions. It is attacked by
the electrophiles, giving substitution
products.
Where E
+
is any electrophile and Nu:-
nucleophile. H
E
+
E-Nu H-Nu+
catalyst
Substitution
Product

21. General Mechanism:
1. Formation of Electrophile.
2. The electrophile attacks the aromatic ring to
form a carbonium ion.
The intermediate ion is resonance-stabilized.It is
a hybrid of all positions.i.e. ortho, meta and para
positionsE Nu+
Catalyst Nu CatalystE
+
+ H
EE
+
+
+
Carbonium ion
Benzene

22. General Mechanism:
Step 3: Loss of proton gives the substitution
product.E Nu+
Catalyst Nu CatalystE
+
+ H
E
+
Carbonium ion
E
Substitution product
+
H Nu+Catalyst
Nu-Catalyst

23. Chemical Properties
2. Addition Reactions.
A. Addition of hydrogen.
Benzene reacts with hydrogen in presence of
nickel or platinum cataylyst at 150 temp to form
cyclohexane.
B. Addition of Halogens.
Benzene reacts with chlorine(or bromine) in the
presence of ultraviolet light to form benzene
hexachloride(Hexachlorocyclohexane) +
3H
2
Ni
2+
150
0
c
benzene
cyclohexane +
3Cl
2
Cl
Cl
Cl
Cl
Cl
Cl
UV
Light
benzene
Benzene hexachloride

24. Cumene (isopropyl benzene)
It is obtained by Friedel crafts alkylation
of benzene with propene.
Reaction: cumene is a colourless liquid,
b.p 153
0
c. It is used in the mfg of phenol.+CH
3
CH
CH
2
CH
CH
3
CH
3
Propene
Benzene
Cumene
isopropylbenzene CHCH
3
CH
3
COCH
3
CH
3 OH
OH
+
CH
3
C CH
3
O
phenol
Cumene
cumene hydroperoxide
H
2
O/H
+
O
2
Acetone

25. Xylene (Dimethyl benzene)
From petroleum: Aromatization of C6-C8
fraction of petroleum naptha yields a
mixture of Benzene, toluene and Xylenes.CH
3
CH
3
CH
3
(CH
2
)
6
CH
3
Pt/Al
2
O
3
500
0
C, 15 min
Xylenes
n-Octane
+
CH
2CH
3
ethylbenzene
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
o-xylene m-xylene
p-xylene
Xylene are produced along w ith benzene, toluene and ethylbenzene.
n-octane undergoes aromatization reaction to give xylene and ethylbenzene

26. Toulene(methylbenzene)
Preparation:1. benzene by friedel crafts
reaction.
2. from bromobenzene by wurtz-fittig
reaction.CH
3
+ +
HClCH
3
I
AlCl
3
TOULENE
BENZENE Br +
2Na
BROMOBENZENE
+
Br-CH
3
dry
ether
CH
3+
2NaBr
TOULENE

27. Substitution of Monosubstituted Benzenes.
All the hydrogen atoms of the benzene
ring are equivalent.
A second substitution E, can occupy any
of the remaining H atom or position.
2-6 position called as Ortho product.
3-5 position called as Meta product.
4 is unique is called as Para product.S
Substitution
Product
-H
+S
benzene

28. Two types of Influence of substituents.
1. Directive or orientation(to arrange) effect.
2. Activity effect.
1. Directive or orientation(to arrange) effect.
Directing of second substituent by the first
substituent to the ortho,meta or para position
depending upon the nature of substituent is
called as directive or orient effect.S
benzene
S
E
E
S
EE
S
E
2-6 ortho
3-5 meta
4-para
-H
+E
1
2
3
4
5
6

29. 2. Activity effect.
The substitution already present may
activate or deactivate the benzene ring
towards further substitution is called
activity effect.
Ortho-para directing effect.
Second substitution towards ortho and
para position simultaneously is called as
ortho-para directors.
E.g. Phenol undergoes nitration reaction.OH
OH
NO
2
nitration
OH
O
2
N
OH
NO
2
o-nitrophenol
53%
P-nitrophenol
47%
m-nitrophenol
0%
Phenol

30. 2.Meta-directing effect.
The second substituent attach to the meta
position are reffered as meta-directors.
E.g Nitration of nitrobenzene gives 94%
m-dinitrobenzene and only 5% ortho and
1% para nitrobenzene.O
2
N
NO
2
NO
2
nitration
O
2
N
O
2
N
O
2
N
NO
2
o-nitrobenzene
94%
P-nitrobenzene
1%
m-nitrobenzene
5%
Nitrobenzene

31. It is observed that ortho-para
directing groups activate the
ring whereas meta directing
group deactivate the ring.
Substituent Effecton reactivity
Ortho-paradirecting
-OH,-OR,-NH2,-NR2,
-CH3, C2H5, -R
Stonglyactivating
Weekly activating
-F,-Cl,-Br,-I Deactivating
Meta-Directing
-NO2,-CHO, -COR,-
SO3H,-CN,-COOH
Stronglydeactivating