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Benzene Derivatives

 The definition of aromatic compounds it is obvious that study of their chemistry will begin with Benzene. Benzene has been in news since 1825 and its chemical and physical properties are better known than those of any single organic compound. The satisfactory structure for benzene was not available till 1931 and the all those substituents that reacted into benzene were available even much later.

The problem was not the complexity of structure of the molecule which almost made it difficult to comprehend the structure theory but mainly the accepted modification of benzene structure.

The Kekule structure was unsatisfactory and this led to the resonance which implies structures differ only by the arrangement of electrons. This also led to the vibration study of both benzene and its derivatives, their intensity and frequency of infra-red and Raman bands arising from exciting of these vibrations which finally gave away the chemical nature and place of substitution of different substituents.

The benzene nucleus dominates in the determination of the force field of the vibrating molecule which helps in getting the principles of symmetry perturbation procedures for a normal modes of benzene derivative compared to the normal vibration mode of benzene.

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Common Benzene Derivatives

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The benzene derivatives are mainly derived from the various structures of benzene which differ in the arrangement of electrons.

Considering the normal vibrations of benzene it is now well known that the individual atomic displacements apart from the degenerate pair of normal mode give us tangential, radial and out of plane vibrations which are again due to carbon – carbon stretching, in plane bending C – X (where X is the first atom of substituents), in plane bending, skeletal radial orientation, internal vibrations of polyatomic substituents etc which helps in getting the derivatives of benzene in a proper perspective. 

Benzene
 
Benzene is a very stable compound and yields simpler compounds only with great difficulty. It is unattacked by concentrated alkalis and even strong oxidising agents it has little effect.

The substitution products of benzenes are highly important and mainly the ones which are derived from concentrated nitric and sulphuric acids.
Nitration:

When benzene is gradually to concentrated nitric acid or to a mixture of equal volumes of nitric and sulphuric acid with a temperature being kept at below 40 C, a reaction takes place. This mixture is poured into excess of water a pale yellow oil separates out. This is heavier than the aqueous solution. This is washed with dilute alkali followed by water and then distilled after drying with calcium chloride. The product thus formed is mono nitro benzene.

$C_{6} H_{5} H+O NO_{2} \rightarrow C_{6}H_{5} NO_{2} + H_{2}O$
 
During the process of nitration one atom of hydrogen in the benzene molecule is replaced by the mono valent nitro $(-NO_{2})$ group.
If the mono nitro benzene be warmed on the water bath with a similar nitrating mixture, another atom of hydrogen can be readily replaced by the nitro group and di nitro benzene. 

Dinitro Benzene
 
Sulphonation:
Benzene is only very slowly attacked by concentrated sulphuric acid at ordinary temperature, but in case the two substances are heated together, the benzene goes completely into solution and the reaction is finished when no oil separates on pouring the mixture into excess of water.
If the solution is heated with barium carbonate in excess, then excess of sulphuric acid is eliminated in form of barium sulphate. The filtrate thus formed is evaporated to yield a crystalline residue of a barium salt of another acid.

In the process of Sulphonation a hydrogen atom of benzene has been replaced by monovalent sulphonic acid $(-SO_{3}H)$ group. The benzene sulphonic acid thus formed is a strong monobasic acid.
 
Halogenation: Chlorine and bromine react very slowly with benzene in absence of sunlight, giving substituted products like mono chloro benzene or mono bromo benzene and along with that a hydrogen halide is also evolved.

$C_{6}H_{6} + Cl_{2} \rightarrow C_{6}H_{5}Cl + HCl$

In the presence of suitable catalysts or halogen carriers, like iron filings or iodine, the reaction goes readily at ordinary temperatures even at dark. Higher halogenated substitution products are also obtained till C6Cl6 and C6Br6 respectively.

These halogen addition compounds of benzene correspond to the reduction or hydrogenation product of benzene which is formed when benzene is reduced with hydrogen in the presence of finely divided nickel or colloidal platinum.

The stability of benzene towards ordinary reagnets and its power of forming substitution products with chlorine and bromine indicate strong affinity to paraffin.

Naming Denzene Derivatives

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Replacing one or more hydrogen atoms on benzene ring with other substituent groups produces benzene derivatives.
  • The naming of benzene derivatives is purely based on the substitution part and hence, goes with a specific order.
  • If more than one H of benzene is replaced then isomers can arise. The systematic nomenclature for benzene derivatives involves naming substituent groups and identifying their position on the ring by numbering the six carbon atoms.
The scheme identifies isomers of di-substituted benzenes with the prefixes – Ortho, O substituent groups on adjacent carbons in benzene ring. Meta, m- substituents separated by one carbon atom. Para, p- substituent groups on carbons on opposite sides of ring.

The derivatives are as follows: Toluene, chloro benzene, benzoic acid, aniline, styrene and phenol. 
Benzene Derivatives

The positions of benzene ring. 
Positions of Benzene
 
There is different term for systematic name and common name.

Systematic name   Common name 
 1, 2 – di chloro benzene   O – chloro benzene 
 1, 3 – di methyl benzene  m – xylene 
1, 4 – di nitro benzene   p – di nitro benzene
                       
Dichloro Benzene   Di Methyl BenzenePara Dinitro Toluene
  • The benzene compounds with either an alkyl or halogen attached to the ring are taken into account.
  • The naming of such derivative with only one substituent is preferred first and then the naming of those with two attached groups and finally with three or more.
  • The IUPAC system of naming mono substituted benzene derivatives uses the name the substituent as prefix to benzene. The examples are fluoro benzene, chloro benzene, iso- propyl benzene and ethyl benzene etc. 
Mono Substituted Benzene
  • For mono substituted benzene rings which has a group attached to a substituent the naming is not easy, the benzene ring is then accepted as a group which is bonded to this substituent.
  • The attachment of benzene ring is termed as phenyl group and the resulting molecule is termed according to the rules used for naming the alkanes alkenes or alkynes.
For any Benzene derivatives consisting two substituents having the same or different substituents attached to the ring could be distinguished among three isomers, the positions of substituents relative to one another. These are done either by numbers or by using non numerical prefixes.
The prefix system:
  • Ortho: 1, 2 di substituted (substitution on adjacent carbon atoms)
  • Meta: 1. 3 di substituted (substitution are on carbon atoms separated from each other)
  • Para: 1, 4 di substituted (substitution are two carbon atoms separated from each other on opposite sides of the ring)
  • When prefix are used the three isomeric forms imparts a special name to the compound and the compound is named as a derivative of parent molecule.
  • When the substituent groups do not provide any special name, the substituent groups are arranged in the order of alphabet with suffix – benzene. While the carbon atom of the benzene ring consisting the attached group with a priority of alphabet becomes carbon 1.
  • A benzene ring bearing two methyl groups is a special base and such compounds are not named as di methyl benzenes or as methyl toluenes but are called xylenes.
  • When there are presence of more than two groups ring the substituent’s positions are indicated with specific numbers. The ring thus gets numbered in a specific way in order to obtain the lowest number possible for these carbon atoms having substituents.
  • whenever there is a choice of numbering systems, then the group is given first preference while alphabetical order is provided with a lower number.
The IUPAC rules for naming phenols are simply extensions of the rules used for naming benzene derivatives with hydrocarbons or halogen substituents. The parent name is phenol and the ring numbering always begins with the hydroxyl group and proceeds in the direction that gives the lower number to next carbon atom bearing a substituent.

List of Benzene Derivatives

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 Substituent of benzene  
 Derivative name 
 -CN  Benzonitrile
 -OH  Phenol
 $-CH_{3}$  Toluene
 -COOH  Benzoic acid
 $-C_{2}H_{2}$  Styrene
 $-OCH_{3}$  Anisole
 $-NH_{2}$  Aniline
 $-HSO_{3}$  Benzene sulphonic acid 
 $-(CH_{3})_{2}$  O – xylene
 $-(CH_{3})_{3}$  Mesitylene

Benzene Derivatives Common Names

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Benzene derivatives have been isolated and used as industrial reagents for over 100 years and many of the names are rooted in historical traditions of chemistry.

The compounds below mentioned carry the historical common names and most of the time not by the systemic IUPAC names.
  • Phenol is also known as Benzenol.
  • Toluene is also known as Methyl benzene.
  • Aniline is also known as Benzenamine.
  • Anisole is also known as Methoxy benzene.
  • Styrene is IUPAC name as vinyl benzene.
  • Acetophenone is also known as Methyl phenyl ketone.
  • Benzaldehyde has IUPAC Benzenecarbaldehyde.
  • Benzoic acid has IUPAC name as Benzene carboxylic acid.
Many compounds are named as derivatives of benzene with their substituents named as if they are attached to an alkane.

Examples are as follows:

Tert – butyl benzene, nitro benzene, ethynyl benzene or phenyl acetylene, benzene sulphonic acid etc. O- di chloro benzene can also be termed as 1, 2 – di chloro benzene, m – chloroperoxy benzoic acid can be termed as 3 – chloroperoxy benzoic acid, and p – nitro phenol can be termed as 4 – nitro phenol.

With three or more substituents on the benzene ring, the numbers attributed show the respective position. 

Di Substituted Benzene
 
The names of most methylated benzene derivatives do not include the root word benzene and the common root for such isomers are xylene. Examples are as follows:

Toluene $\rightarrow$ O – xylene, m – xylene, p – xylene, Mesitylene, and Durene. 

Mono Substituted Benzene
 
When only two groups are attached to a benzene ring, the positional relationship between them can be specified by prefixing ortho, meta, and para. The names use a root word which are different from benz- including phenol, anisole, aniline and acetophenone.

IUPAC Names for Benzene Derivatives

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The IUPAC system retains certain common names for various simpler benzene derivatives like toluene, cumene, styrene, xylene phenol aniline anisole and benzoic acid. The common names are used as parent names if the characteristics functional group are found to be present on a benzoic derivative.

In molecules other than functional group the benzene ring is named as a substituent on a parent chain. For benzene rings with two substituents the three possible constitutional isomers are 1, 2 substitution (ortho); 1, 3 – substitution (meta); 1, 4 – substitution (para).
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