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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5574_Библиотеки_им_академика_М_И_Перельмана.pdf
C H A P T e r 10 u Arenes
The activating power of nHAc is more than that of CH3. in such a case, the third group enters
the p-position with respect to the more powerful group if that position is not occupied. in case the
p-position is blocked, the new entrant goes to the ortho position. The o- and p-directing groups have
been arranged in decreasing order of their directive strength:
O– > NR2 > NHR > NH2 > OH > OR > NHCOR > CH
Primary alkyl > Secondary alkyl > Tertiary alkyl > –C6H
3
5
examples: A third group enters the position shown by the arrow.
2. Both the groups already present belong to category of m-directing groups. in such a case,
tri-substitution is very difcult. The position of incoming group is decided by the group which has
less deactivating power. The strength of m-directing groups is in the order.
+
NR3 > NO2 > CF3 > CN > SO3H > CHO > COR > COOH > COOR > CONH2 > CCl3 > F > Cl > Br > I
381
The third group occupies the meta position with respect to the stronger group. in case both the
groups are of the same strength and m-position is unoccupied, tri-substitution is easier. For example:
3. The two groups already present belong to different categories (i.e. one group is activating and another
is deactivating). in such case, the o-, p-directing groups take precedence and the third group enters
almost entirely at one position and its position is governed by activating group. For example:
382
This group is
i. activating group and
ii. ortho–para directing group.
PHArmACeuTiCAl OrgAniC CHemiSTry
10.10 SOME INDIVIDUAL MEMBERS
10.10.1 Toluene, Methylbenzene, Phenylmethane (C6H5CH3)
Toluene was obtained by dry distillation of Tolu balsam from where it is derived. it is commercially known
as toluol.
Methods of preparation
it can be prepared by the general methods described earlier. However, commercially it is prepared by the
following methods:
1. From coal tar: 50% of benzol is obtained by fractional distillation of light oil present in a fraction
of coal tar distillation. light oil contains large percentage of toluene, which can be separated by
fractional distillation. The fraction collected between 331 and 383 K is toluene.
2. By catalytic dehydrogenation of n-heptane or methylcyclohexane (both are petr oleum pr oducts)
CH3(CH2)5CH3
n
-Heptane Toluene
C6H5CH3 + 4H
2
Physical properties
Toluene is colourless liquid (boiling point 384 K) with benzene-like odour. it is insoluble in water but
soluble in alcohol and ether.
Chemical properties
it shows three types of reactions:
1. reaction due to benzene ring: it undergoes electrophilic
substitution reaction
2. reaction due to side chain
3. reduction
1. Electrophilic substitution reaction: Since methyl group is ortho–para directing group, substitution
takes place at ortho or para position. in most of the cases, para is the main product due to steric
hindrance.
C H A P T e r 10 u Arenes
Conc. HNO
3
Conc. HNO3/conc. H2SO
4
Cl2/Fe, Low temp.
Dark
RX/Anhyd. AIX
3
/∆
Conc. H
2SO4
/∆
RCOCI/Anhyd. AICI
3
∆
∆
∆
383
PHArmACeuT i CAl OrgAniC CH emiSTry
384
hν
hν
hν
i.
ii.
iii.
C6H5CH
3
C6H5CH2Br
Br2/hν/∆
or NBS/CCI
4
/hν
Benzoic acid
Phthalic acid
∆
∆
C6H5–CH2–(CH2)n–CH
3
C6H5–CH2–(CH2)n–CHO
(i) CrO
2Cl2
/CS
2
(ii) H2O/H
2. Reactions due to side chain
a. Chlorination: Toluene undergoes chlorination in presence of sunlight. The chlorinated product on
hydrolysis gives benzyl alcohol, benzaldehyde and benzoic acid.
b. Bromination: Bromination is carried out either by Br2 or nBS. Since bromine is less reactive, it is
more selective reagent. Bromination takes place at benzylic carbon giving a mono bromo product.
c. Side-chain oxidation: Oxidation with hot
When oxidized with chromyl
chloride, CrO
gives benzaldehyde. Conversion
of toluene into benzaldehyde by
chromyl chloride is known as Etard
reaction.
alkaline KmnO
oxidizes side chain into
4
acidic functional group. Benzene ring is
not oxidized under these conditions.
/acetic acid or SeO2
3
& Learning Plus
Chromyl chloride oxidizes terminal methyl group of alkyl benzene into CHO group.
REMEMBER
Side-chain oxidation takes place at the benzylic carbon and
as a result of it, alkyl groups larger than methyl group are also
degraded to benzoic acid.
C H A P T e r 10 u Arenes
CHO
CH
3
(i) CrO3/Acetic anhydride
(ii) CrO
2Cl2
(iii) SeO
2
R–C6H
5
R–COOH
(i) O
3
CH3COOH
(ii) H
2O2
Minor
∆
d. Oxidation of benzene ring: On ozonolysis followed by treatment with hydrogen peroxide, the
benzene ring of an alkyl benzene is oxidized to carboxylic acid.
3. Reduction: Toluene undergoes two types of reduction.
Total reduction
385
Birch r eduction
10.2.2 Styrene (an Alkenyl Benzene), 2-Phenyl Ethane, Phenylethylene, Vinylbenzene
Methods of preparation
An aromatic hydrocarbon with a side chain containing a double bond can be prepared by the same methods
by which alkenes are prepared by 1,2-elimination. The presence of the aromatic ring in the molecule may
affect the orientation of elimination and the ease with which it takes place.
1. Dehydrogenation of side chain: Friedel–Crafts reaction
386
CH CH – COOH
−CO
2
CH CH
2
∆
CH CH
2
CH – CH
3
OH
or
CH
2CH2
OH
H
2
O
∆
ZnCl
2
,
or H/
Alc. KOH/∆
CH CH
2
CH — CH
3
+ HCl
Styrene
H
2
, Ni, 20°C, 2–3 atm
75 min
Ethylbenzene
100 min
Ethylcyclohexane
CH CH
2
CH2CH
3
CH2CH
3
H2, Ni, 125°C, 110 atm
PHArmACeuTiCAl OrgAniC CHemiSTry
2. Decarboxylation of carboxylic acid having unsaturated
side chain: Cinnamic acid, when heated in presence of
hydroquinone, gives styrene on decarboxylation:
3. Dehydration of hydroxyalkylbenzene
REMEMBER
Heating in presence of hydroquinone prevents
polymerization of unsaturated Cinnamic acid.
4. Dehydrohalogenation of haloalkylbenzene
On an industrial scale, the elimination generally involves dehydrogenation. For example, styrene, one
of the most important synthetic aromatic compounds, can be prepared by simply heating ethylbenzene to
about 600ºC in the presence of a catalyst. ethylbenzene, in turn, is prepared by a Friedel–Crafts reaction
between two simple hydrocarbons: benzene and ethylene.
Chemical properties
1. Hydrogenation: Although both the benzene ring and the C = C double bond can be hydrogenated
Thus, by selecting the proper conditions, it is quite easy to hydrogenate the side chain without touching
catalytically, the conditions required for the double bond are much less extreme.
the aromatic ring.
C H A P T e r 10 u Arenes
∆
CH CH
2
CH — CH
3
CH2CH2 X
HX
Markownikoff’s product
Anti-Markownikoff’s product
CH – CH2Cl
CH – CH
2
Cl2/FeCl
3
Cl2/FeCl
3
CH CH
2
Cl
Cl
Cl Cl
2. Oxidation: mild oxidation of the double bond yields a 1,2-diol; more vigorous oxidation cleaves
the C = C double bond and generally gives a carboxylic acid in which the COOH group is attached
directly to the ring.
3. addition of haloacid: in presence of peroxide or light, halogen acids are added and anti-markownikoff’s
product is obtained when HBr is added to styrene.
387
4. addition of halogens
388
Peroxide
Polystyrene
CH CH
2
(—CH — CH2 —)
n
o-Xylene
Boiling point
417 K
m-Xylene
Boiling point
412 K
p-Xylene
Boiling point
411 K
Ethylbenzene
Boiling point
409 K
PHArmACeuTiCAl OrgAniC CHemiSTry
5. Polymerization
10.10.3 Xylene, Dimethylbenzene (C6H4(CH3)2)
Xylene shows position isomerism and is isomeric with ethylbenzene. The four isomers represented by
molecular formula C
are as follows:
8H10
� Xylenes are prepared by the same method as other homologous of benzene are prepared.
� Xylenes are colourless liquid, insoluble in water but soluble in alcohol and ether.
� On oxidation ortho, meta, para xylene gives ortho, meta, para benzene dicarboxylic acids.
manufacture of dyes.
10.10.4 Mesitylene, 1,3,5-Trimethylbenzene (C6H2(CH3)3)
it occurs in coal tar; however, it is prepared by condensation of acetone (acetone is distilled in presence of
conc. sulphuric acid) or by polymerizing propyne in presence of concentrated sulphuric acid.
Properties:
nitric acid, three methyl groups attached to the benzene nucleus get oxidized and thus are replaced by
–COOH groups.
Xylenes are used as solvents for rubber and lacquers. Amino derivatives of xylenes are used in the
it is a colourless, pleasant smelling liquid with boiling point 337 K. On oxidation with dilute
C H A P T e r 10 u Arenes
Mesitylenic acid
REVISION QUESTIONS
1. What are aromatic hydrocarbons? What are the main differences between aromatic and aliphatic
compounds?
2. (a) Write a note on aromaticity.
(b) What is Huckel rule? Write the structure of two compounds that follow this rule.
3. What is the action of (a) ozone, (b) nitric acid and (c) sulphuric acid on toluene.
389
4. give the mechanism of Friedel–Crafts reaction. What are the methods employed for the generation
of electrophile in each of the following reactions:
(a) nitration
(b) Halogenation
(c) Sulphonation
5. Write the structures of benzene isomers with formula C8H10.
6. How is toluene prepared? Describe its important reactions.
7. (a) State the conditions under which chlorination of toluene take place in the benzene ring and in
the side chain.
(b) What happens when
(i) ethylbenzene is treated with alkaline KmnO
4
(ii) Chlorine is passed through boiling toluene
8. How will you distinguish between ethylbenzene and o-xylene?
9. give a mechanism for the reaction of toluene with chlorine in the presence of uV light.
10. How will you synthesize
390
PHArmACeuTiCAl OrgAniC CHemiSTry
(a) ethylbenzene from acetophenone
(b) Benzaldehyde from benzene
(c) p-Chlorotoluene from toluene
(d) p-nitrobenzoic acid from toluene
(e) Styrene from benzene
11. A hydrocarbon, C9H12 gave o-phthalic acid as the only organic product of oxidation. What is the
structure and name of the hydrocarbon?
Ans: The hydrocarbon is o-ethyl toluene.
12. Starting with styrene how you will prepare the following:
(a) ethyl cyclohexane (b) Benzoic acid
(c) Polystyrene (d) p-Chlorostyrene
13. How toluene is obtained in large scale? From toluene how will you prepare benzaldehyde, benzal
chloride and o-chlorotoluene?
14. What is meant by aromaticity? State Huckel’s rule and explain the stability of C7H
+
cation and C5H
7
anion.
–
5
15. Methyl group in toluene is ortho–para directing where as triuoromethyl group in triuoromethyl-
benzene is meta directing. explain why.
MULTIPLE CHOICE QUESTIONS
1. Aromatic hydrocarbons are also called
(a) Arenes (b) Huckel’s compounds
(c) Trienes (d) Alkoxy compounds
2. The carbon atoms in a benzene ring are
(a) sp hybridized (b) sp3 hybridized
(c) sp2 hybridized (d) none of these
3. The C–C bond length in benzene is
(a) greater than the C–C bond length in ethane
(b) shorter than the C–C bond length in ethylene
(c) same as that of C–C bond length in ethylene
(d) intermediate between C–C bond length in ethane and C–C bond length in ethylene