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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5852_Библиотеки_им_академика_М_И_Перельмана.pdf
C H A P T e r 10 u Arenes
(Combined names)
(Singular names)
Phenyl group 2-Methy-1-phenylpropene Diphenyl ether
Benzyl group Benzyl bromide 2-Benzyl-1,1-dichlorocylobutane
Nomenclature
The benzene ring nomenclature system is not as systematic as the alkenes, alkanes and alkynes. Some of the
compounds that are mono-substituted are named by benzene prexed by the group name. Some examples
are given below. mainly, benzene ring compounds are referred to by their singular names. These names are
unique and need memorization. The following are a few examples for singular-name benzene compounds:
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There are two commonly seen substituent groups which have benzene ring. These are phenyl, which is
abbreviated as Ph-, along with benzyl, which is abbreviated as Bn-.
The following are examples of some compounds that have phenyl and benzyl groups:
362
1,2-Dichlorobenzene
ortho-Dichlorobenzene
o-Dichlorobenzene
1,2-Dimethylbenzene
ortho-Xylene
o-Xylene
1,2,4,5-Tetramethylbenzene
(durene)
1,3-Dichlorobenzene
meta-Dichlorobenzene
m-Dichlorobenzene
3-Aminotoluene
meta-Toluidine
m-Toluidine
1,4-Dichlorobenzene
para-Dichlorobenzene
p-Dichlorobenzene
4-Hydroxytoluene
para-Cresol
p-Cresol
4-Bromo-1-methoxy-2-nitrobezene
PHArmACeuTiCAl OrgAniC CHemiSTry
When there is more than one substituent on a benzene ring, then the position of the substituent needs to be
found out. This is done by giving a number to each of the carbons in the ring.
In case the benzene ring has two substituted groups, ortho, meta and para prexes are used. Ortho is
1,2-substitution, meta is 1,3-substitution and para indicates 1,4-substitution.
The following examples show the names of different benzene compounds. Some toluenes with two
substituents have singular names (e.g. toluidine, cresol and xylene). The isomers of these compounds are
designated by the ortho, meta or para prex.
Some of the disubstituted benzenes also have singular names given to particular isomers (e.g. salicylic acid
and resorcinol). if a benzene ring has three or more substituent groups, the ring is numbered to assign the lowest
possible numbers to the substituents, as shown in the last row of examples. In the nal name, the substituents are
listed alphabetically . The numbering corresponds to the alphabetical order if the substitution is symmetrical.
C H A P T e r 10 u Arenes
Iso-propyl benzenen-Propyl benzene
1-Methylnaphthalene 2-Methylnaphthalene
10.3 ISOMERISM IN ARENES
monoalkyl-substituted benzene with large alkyl group can show chain isomerism. For example, propyl
benzene (C6H5 C3H7) has two isomers:
Disubstituted benzene shows position isomerism. For example, dimethyl benzene or xylene [C6H4(CH3)2]
can have three isomers.
363
These three dimethyl benzenes are also isomeric with ethyl benzene:
isomerism is not shown by mono-substituted benzene having methyl or ethyl group as the side chain,
because all the positions on benzene ring are identical.
mono-substituted polycyclic arenes exhibit positional isomerism. For example, methyl naphthalene exists
in two isomeric forms, viz. 1-methylnaphthalene and 2-methylnaphthalene.
in the case of polycyclic compounds containing more benzene rings fused together, the number of positional
isomers also increases.
364
10.4 THE AROMATIC CHARACTER (AROMATICITY)
PHArmACeuTiCAl OrgAniC CHemiSTry
Benzene and all those compounds which resemble benzene in their chemical behaviour are called aromatic
compounds. They have certain characteristic properties which are quite different from those of aliphatic and
alicyclic compounds. Some of the important properties of aromatic compounds are as follows:
1. They are usually cyclic compounds.
2. Their molecules have been shown to be planar by X-ray and electron diffraction methods. in spite of
high degree of unsaturation, these compounds are resistant to usual additional reactions of unsaturated
compounds.
3. Despite unsaturation, these compounds undergo substitution reaction. The important of these
substitution reactions are electrophilic substitution reactions like those of benzene (e.g. halogenation,
nitration, sulphonation, Friedel–Crafts alkylation and acylation).
4. They show unusual stability as shown by their low heat of hydrogenation and heat of combustion.
Their molecules have resonance stabilization. The p-electrons in the ring are delocalized.
These properties are common to all aromatic compounds whether benzenoids (derivatives of benzene) or
nonbenzenoids (having no benzene ring in them) and are collectively referred to as aromaticity or aromatic
character.
10.4.1 Huckel Rule
The distinct behaviour of all aromatic compounds, quite different from aliphatic and alicyclic compounds,
suggests that they must have some common structural features which account for their aromatic character. in
1931, Huckel formulated a simple rule predicting whether or not a given annulene (ring compounds having
an even number of CH units) or a compound would be aromatic. He derived this rule on the basis of quantum
mechanical calculations:
1. Molecule or ion must be cyclic and must be at or planar or nearly so.
2. it should have a cyclic cloud of delocalized p-electrons above and below the plane of the molecule.
3. The p-electron clouds should encompass all the atoms of the ring system.
4. The total number of p-electrons in the molecular species should be 4n + 2 where n = 0, 1, 2 , 3, …, etc.
These points are collectively known as Huckel rule or simply (4n + 2) rule. From the above points, it is
clear that delocalization is not sufcient criterion for aromaticity. The molecule must have 2 (n = 0), 6 (n = 1),
10 (n = 2), 14 (n = 3) p-electrons and molecular structure should be planar.
10.4.2 Aromatic, Antiaromatic and Nonaromatic Compounds
A compound is aromatic if its p-electrons are delocalized over the entire ring and is established by the
p-electron delocalization. One of the best methods to determine whether or not the p-electrons of a cyclic
system are delocalized is through the use of nuclear magnetic resonance (nmr) spectroscopy. it provides
direct physical evidence of whether or not the p-electrons are delocalized.
The stabilization of a compound by the delocalization of p-electrons can be calculated with the help
of heat of hydrogenation. We know that the actual heat of hydrogenation is 36 kcal/mole less than that
C H A P T e r 10 u Arenes
π-Electron energy
increase
Benzene
(6π electrons, aromatic)
π-Electron energy
decrease
calculated for the hypothetical 1,3,5-cyclohexatriene. We call this energy difference between them as
resonance energy (delocalization energy) or stabilization energy.
There is another way to establish stabilization: that is by comparing the p-electron energy of the cyclic
system with that of the corresponding open chain compound. This approach is particularly useful because it
helps us to study not only annulenes but aromatic cations and anions as well.
W e select a linear chain model molecule of sp
2
-hybridized atoms that carries the same number of p-electrons as
our cyclic compound. Then we imagine ourselves removing two hydrogens from the end of this chain and joining
the ends to form a ring. if the ring has lower p-electrons energy, than open chain, then the ring is aromatic.
if the ring and the chain have the same p-electron energy, then the ring is nonaromatic.
if the ring has greater p-electron energy than open chain, then the ring is antiaromatic. The above concept
can be summarized as a modern denition of any aromatic compound: compare the energy of the p-electrons
of the cyclic conjugated molecule or ion with that of its open-chain counterpart. if the ring closure involves
the decrease in the p-electron energy, the molecule or ion is classied as being aromatic. if it increases
the p-electron energy on ring closure, then the molecule or ion is classied as being antiaromatic, and if it
remains the same, the molecule is classied as being nonaromatic.
let us study the following examples that illustrate how this approach has been used.
1. Cyclobutadiene: For cyclobutadiene we consider the change in p-electrons energy for the following
hypothetical transformation.
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Calculations and experiments appear to conrm that the p-electron energy of cyclobutadiene is higher than
that of its open chain counterpart. Therefore, cyclobutadiene is classied as being an antiaromatic compound.
2. Benzenes: Here our comparison is based on the following hypothetical transformation:
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π-Electron energy
decrease
Cyclo pentadienyl anion
(6π electrons, aromatic)
PHArmACeuTiCAl OrgAniC CHemiSTry
Calculations and experiments conrm that benzene has a much lower p-electron energy than
1,3,5-hexatriene. Benzene is classied as being aromatic.
3. Cyclopentadienyl anion: We use a linear anion here for our hypothetical transformation.
Calculations and experiments conrm that cyclopentadienyl anion has less p-energy and hence is aromatic.
MEMORY FOCUS
The term 1. aromatic refers to the compounds which are highly unsaturated and unexpectedly (relatively)
stable towards reagents that adds to alkenes and alkynes.
Aromatic compounds are
2.
a. Cyclic
b. Fully conjugated
c. Planar so that there is a continuous overlap of all p orbitals of the ring
d. Unsaturated compounds however undergo substitution more readily than addition reactions
e. Have a closed loop of (4n + 2) p-electrons in the cyclic arrangement of p orbital, where n = 0, 1,
2, 3, . . . etc. If the compound has 2, 6, 10, or 14
similar systems, however, with 4n
anti-aromatic provided they are planar.
Huckel criteria for aromaticity:
3. Systems with 4n p-electrons (4, 8) behave like alkenes provided they
are nonplanar. Those compounds are named nonaromatic.
The term
4. arene is used to describe aromatic hydrocarbons where one hydrogen atom of benzene is
substituted by an alkyl group. For example, toluene and sec-butylbenzene.
p-electrons (4, 8) are specially unstable and are said to be
p-electrons, they are aromatic. Conversely, a
C H A P T e r 10 u Arenes
Alkyl sodium benzoate
Sodium toluate Toluene
Alkyl benzene
o-, m- and p-Methylphenol
Alkyl
Benzene
Toluene
o-, m-, p-Toluene sulphonic acid Toluene
Alkyl benzene
sulphonic acid
Alkyl benzene
Steam or dil.
Steam or dil.
10.5 PREPARATION OF ARENES
Arenes are prepared by following two methods:
1. Methods involving removal of the group already present in the nucleus
a. By heating sodium salt of aromatic acid with soda lime (naOH + CaO) (removal of –COOH)
b. By passing vapours of phenol over heated zinc dust (removal of –OH group)
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c. By hydrolyzing sulphonic acid with super-heated steam or by boiling with dilute hydrochloric acid
under pressure at 400–470 K (removal of –SO
H group)
3
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Alkyl benzene
diazonium chloride
Toluene diazonium
chloride
Toluene
Alkyl benzene
Toluene
o- and p-Xylene
Alkyl toluene
Phenyl magnesium
bromide
Alkyl toluene
Dry ether
PHArmACeuTiCAl OrgAniC CHemiSTry
d. By the action of hypo phosphorous aid, H3PO2 (reducing agent) on arene diazonium salt (removal
of –n= n+Cl– group)
2. Methods involving substitution of hydrogen of the benzene nucleus by one or more alkyl
groups
a. By the action of alkyl halide on benzene in presence of anhydrous aluminium chloride as catalyst
(Friedel–Crafts reaction)
b. By treating Grignar d reagent with alkyl halide in dry ether
C H A P T e r 10 u Arenes
770º K
Pheny bromide Methyl
iodide
o-, m- or p-Bromo toluene o-, m- or p-XyleneMethyl bromide
This reaction is useful for preparing alky benzenes with branched side chain.
c. By cyclization of long-chain alkanes obtained from petroleum: When vapours of alkanes are passed
over chromium catalyst (Cr
supported over alumina) under pressure at 775 K.
2O3
369
Similarly toluene is prepared from n-heptane and xylenes from n-octane:
Petroleum is the most important source of aromatic hydrocarbons which are formed by cyclization
of aliphatic hydrocarbons.
d. By heating halobenzene or its homologous and an alkyl halide with sodium metal in dry ether
(Wurtz–Fittig reaction)
370
•
PHArmACeuTiCAl OrgAniC CHemiSTry
e. By reduction of ketones with zinc amalgam and hydrochloride (Clemmensen reduction)
The ketone for the above reaction can be prepared from benzene by Friedel–Crafts reaction.
10.6 PHYSICAL PROPERTIES
Arenes are usually colourless, refractive liquids, insoluble in water, but miscible with organic solvents in all
proportions. Their vapours are inammable with characteristic odour and are toxic in nature. The boiling
point increases with the increase in molecular weight.
aromatic hydrocarbons Boiling point (K) Hydrocarbons Boiling point (K)
Toluene C6H5CH
3
ethyl benzene C6H5C2H
Propyl benzene C
6H5C3H7
5
373 o-Xylene C
409 m-Xylene C
432 p-Xylene C
mesitylene C6H3(CH3)
6H4
6H4
6H4
(CH3)
(CH3)
(CH3)
2
2
2
3
417
412
411
538
10.7 CHEMICAL PROPERTIES
Benzene nucleus is unusually stable but the hydrogen atom attached to it can be replaced by other atoms or
groups (substitution reaction). The side chain of arenes is aliphatic in character. Their general reactions are
as follows:
1. Combustion: Arenes are inammable liquids and burn with sooty ame, giving carbon dioxide and water
vapours.
2. Nitration: Toluene (arenes) is nitrated more easily than benzene giving o- and p-nitrotoluene as the main
product. Further nitration gives trisubstituted trinitrotoluene (TnT). This is because methyl group in toluene
is electron-releasing and activates the benzene ring.