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Derivatives of Benzene

One of the main preoccupation of 18th and 19th century technology in United Kingdom was the utilization of one of the country’s main natural resource, coal. Not until the end of end of the 18th century that work was begun to explore possible uses for both the gaseous products and the tar. 

The observation that coal gas was flammable. Very few people in 1850 could have expected that coal tar would provide the raw material for a striking new industrial development and no one anticipated that within this evil smelling sticky, around 250 different compounds would be identified having very specific useful applications.

The birth of organic chemistry and related industry is completely based on careful distillation of coal tar. Further fractional distillation of these mixtures led to the isolation of individual compounds like benzene. Large scale production of benzene and its derivatives led to further studies of these compounds which finally gave more insight into the details of benzene compounds.

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Benzene Derivatives Definition

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When a chemist wants to prepare a complicated aromatic compound, whether in lab or on an industrial scale a benzene ring is not built up from non-cyclic reactants but a simpler benzene derivative is modified to obtain the required structure. Benzene being the parent hydrocarbon of a very large class of organic compounds we get to see these as aromatic compounds. The origin of this classification is simple and as the name stems from the fact that many of these compounds are isolated from natural sources and have one thing in common, the pleasant odour. Hence the term aromatic was adopted for benzene and all its derivatives.

But we need to understand that not all compounds of aromatic derivatives have pleasant smell. Carbolic acid or phenol contains a benzene ring and an OH group. Phenol contains a benzene ring and an OH group. Phenol is corrosive and is also considered as an antiseptic. To understand the structure and properties of benzene came up from the knowledge that its molecular formula is $C_{6}H_{6}$ and this requires four double bonds or four rings or two triple bonds but more likely to have multiple bonds and combinations of rings. However benzene does not easily undergo addition reactions but readily undergo substitution reactions in which an electrophile X+ which leads to a C-X bond being formed and a C-H bond being broken with H+ as the leaving group.

Furthermore only one substitution product $C_{6}H_{5}X$ can be formed requiring all six hydrogen atoms are in chemically identical environment. Since the double bonds oscillate in between the two resonance forms can contribute equally to the actual structure so that each carbon – carbon is intermediate in nature between a single bond and double bond showing a delocalised form around ring. The idea of resonance stabilisation is useful to understand the structure and properties as well as the derivatives of benzene. Benzene derivatives have a strong tendency to maintain intact the six electron pi system. 

In benzene each carbon atom is bonded to three other atoms and can be described as sp2 hybridised form and molecule is set up for a planar σ bond skeleton. The p orbitals can interact equally strongly with either neighbour and the end result is delocalised p electron system above and below the plane of the benzene ring. Most common characteristic reaction of benzene and aromatic systems is classed as substitution reaction. 
Substitution Reaction
Here, the X+ is an electrophile and the positive charge indicates that X+ has a low electron density. The chemical behaviour looks for electron rich molecules and this clearly shows an electrophilic reagent. The general characteristics of aromatic compounds is not only a substitution reaction but as the attacking reagent is electrophile, it is considered as an electrophilic aromatic substitution reaction. 
Electrophilic Aromatic Substitution Reaction
Generation of electrophile is either carried out spontaneously or is induced to do so by another reagent or by a catalysts.

$XY \rightarrow X^{+} + Y^{-}$

The general mechanism of electrophilic aromatic substitution reactions consists of two steps, which is a slow and rate limiting step. 

Rate Limiting Step
Nitration

The electrophilic aromatic substitution reaction is carried out with a mixture of concentrated nitric and sulfuric acid and the so called nitrating mixture. The product of the reaction with benzene is nitrobenzene. 
Nitration
The electrophile is $NO_{2}$ which replaces an H atom. The nitronium ion $NO_{2}$ is present in the nitrating mixture. The sulphuric acid is a stronger acid than nitric acid and hence can protonate $HNO_{3}$ and lose water to form $NO_{2}$. The presence of nitronium ion is further helped by the formation of the intermediate carbocation.

Halogenation

In halogenation reaction the halogen $Cl_{2}$ or $Br_{2}$ reacts with benzene in presence of a catalysts. The attacking reagent is $Cl^{+}$ which is considered to be a readily available electrophilic species. The chloride $(Cl^{+})$ ion formed through the action of a catalyst composed of a metal chloride behaves as an acceptor for a non-bonded electron pair on a $Cl^{-}$ ion. The chlorination of benzene shows that one molecule each of benzene, the halogen and Lewis acid are involved in slow rate determining step. 
Chlorination
Chlorination Mechanism
Fluorine, bromine and iodine can go similar reactions with benzene as chlorine. Bromination takes place in the similar manner as chlorination with proper catalyst but the reaction with fluorine does not produce fluoro benzene and instead the carbon – carbon bond ruptures and other products are formed. Iodine does not react with benzene as it is much less reactive than other halogens.

Sulfonation

Benzene reacts with concentrated sulfuric acid at around 150 C to produce benzene-sulfonic acid. This reaction mechanism involves the initial formation of sulphur trioxide from sulfuric acid and although sulfur trioxide is a neutral molecule rather than a cation and is considered a relatively powerful electrophile. 
Sulfonation
The generation of sulfur trioxide from sulfuric acid and the sequence still forms either bisulphate or water. 

Sulfonation Mechanism
Mechanism of Sulfonation
Formation of Benzene Sulfonic Acid
When the sulfur trioxide electrophile is generated with concentrated sulfuric acid, the position of equilibrium favours the formation of benzene sulfonic acid.

Friedel crafts reaction

The Friedel crafts reactions represent one of the main methods of adding carbon chains to aromatic rings. The electrophiles X+ are necessary for the formation of C-X bonds in electrophilic aromatic substitution reactions results in two types of reactions, acylation and alkylation.

Benzene and 2 chloro propane reacted together in presence of $AlCl_{3}$ to form 2 phenyl propane to show the alkylation reaction. This is directly analogous to other electrophilic aromatic substitution reaction is specific for rate limiting step.

Friedel craft alkylation use chloro alkane / $AlCl_{3}$ combination to generate the carbocation but in industry using zeolite as catalyst which avoids generation of hazardous acid waste.

The influence these substituents have on the benzene ring also effects the rate of electrophilic aromatic substitution. Alkyl groups activate the ring to further substitution which means that alkyl benzenes undergo substitution faster than benzene itself.  

Friedel crafts acylation

These are again the typical electrophilic aromatic substitution reactions in which the electrophile is an acylonium ion or equivalent. The generation of acylonium ions is straightforward and involves a similar equilibrium to acylation reactions.

The advantage of using acylation in synthesis are:
  • Acylonium ions do not rearnage
  • Acyl group deactivates the ring to further substitution
The acyl substituted aromatic compound is a very versatile synthetic intermediate. 

Benzene Derivatives Nomenclature

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Replacing one or more hydrogen atoms from benzene ring with specific and ideal other groups helps in producing the benzene derivatives. Compounds having the alkyl groups or atoms of halogen which are attached to the benzene ring are very common. Those molecules with alkyl groups or even with halogen atoms attached to the benzene ring are common everywhere. Naming these benzene derivatives with just one substituent is always preferred first followed by naming of those with two substituents and then finally naming of molecules with three or even more of such substituents.

Benzene derivatives with one substituents

The IUPAC system of naming mono substituted benzene derivatives uses the name of substituents as a prefix to the name benzene.

Fluoro benzene
Fluoro Benzene
Chloro benzene
Chloro Benzene
Isopropyl benzene

Isopropyl Benzene
Ethyl benzene
Ethyl Benzene
Few mono substituted benzenes have names where the substituted part along with the benzene ring are considered together to give a new parent name. 

Toluene 
Toluene
Styrene

Styrene
Both toluene and styrene are important industrial chemicals and these mono substituent benzene structures are drawn with these substituents at 12 O clock position but because all the carbon atoms in benzene are equivalent so any positions other than 12 O clock are also acceptable. For mono substituent benzene rings that have group attached but these are not easy to name as specific substituent, then the benzene ring is preferred as the group which is attached to these substituents. So in such cases the benzene ring attached is ideally termed as phenyl group and the molecule is then nomenclated as per the rules we apply for naming of alkanes, alkenes or even alkynes. 
 
3 phenyl but-1-ene

Benzene derivatives with two substituents

When two substituents which are either of the similar groups or different groups, get attached to a benzene ring, then three specific isomeric forms can be drawn. 
Benzene Derivatives with Two Substituents
In case we are looking at halogen substituents then it might have nomenclature coming up as 1, 2 di chloro benzene, 1, 3 di chloro benzene and 1, 4 di chloro benzene. 

We can also put these names as ortho di chloro benzene, meta di chloro benzene and para di chloro benzene.

The prefix system uses the prefixes ortho, meta and para or o-, m-, and p- forms.

Ortho means 1, 2 di substitution where the substituents are considered on the preferred side of carbon atoms.

Meta means 1, 3 di substitution, where the substituents are placed on alternate carbon from each other.

Para means 1, 4 di substitution where the substituents are placed two carbons away from each other.

When one of the two susbtituents in disubstituted benzene imparts a special name to the compound, the compound is named as a derivative of the parent molecule. 

When we get to see neither of the substituent group imparting special name, then the substituents are mentioned in their alphabetical order and finally ending with benzene. The atoms of carbon of the ring of benzene which bears these substiuents with such alphabetical order gets the prioroity and that becomes carbon 1.

The IUPAC rules for naming phenols are simply extensions of the rules used to name benzene derivatives with hydrocarbon or halogen substituents.
Methyl and hydroxy derivatives of phenol have IUPAC accepted common names. 

Methyl phenols are called cresols and the name applies to all three isomeric methyl phenols.

Ortho cresol
Ortho Cresol
Meta cresol
Meta Cresol
Para cresol
Para Cresol
For hydroxy phenols, each of the three isomers have different common name.

Catechol
Catechol 
Resorcinol
Resorcinol 
Hydroquinone
Hydroquinone
Benzene derivatives with ether system, the ethers are named as substituted hydrocarbons. Smaller hydrocarbon attachment and oxygen atom are called an alkoxy group and is considered as a substituent on the larger hydrocarbon group. The alkoxy group is –OR group, an alkyl group attached to an oxygen atom. 

Once the longest carbon chain is identified, it is used as base name. Change the –yl suffix to –oxy. Place the alkoxy name with locator number in front of base chain name.

Methoxy benzene

Methoxy benzene
Benzene derivatives with thiols

Thiols are named in exactly the same manner as alcohols in IUPAC system. The only exception is the –ol becomes thiol.

The suffix of thiol shows the substitution of a sulfur atom for an oxygen atom in a compound.

Thio phenol or benzene thiol. 

Benzene Thiol

Benzene Derivatives List

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1, 3 dimethyl Benzene or m – xylene 
1, 1 di ethyl methyl benzene or benzene 1, 3- bis 
2, 4 di methyl styrene or benzene 1, 3 di methyl 2-ethyl
1, 4 di methyl Benzene or p – xylene 
2, 5 dimethyl styrene or benzene 1, 4 dimethyl 2-ethynyl
1 butenyl Benzene
Butyl benzene 
1, 3 di ethyl benzene 
1, 2 di methyl benzene or O-xylene 
C3 alkyl benzene 

Common Benzene Derivatives

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Benzene derivatives may be considered as molecules in which one or more hydrogen atoms have been replaced with heteroatoms, substituents or functional groups. When a hydrogen atom on the benzene ring is replaced with an alkyl group such as methyl, ethyl etc then the resulting molecule is termed as arene.

Arene with alkyl name of benzene becomes an alkyl benzene or simply methyl benzene but the common name is toluene.
There are many such common benzene derivatives like toluene (methyl benzene) or Ethyl benzene.

When the parent compound benzene ring is attached to an alkane carbon chain of five or more carbon atoms, the benzene ring is treated as a substitute. It is named as 2- phenyl benzene.

Other common derivatives are as follows:

Styrene
Styrene
Vanilla
Vanilla
Tri nitro toluene
Tri Nitro Toluene
Cumene
Cumene
Nitro benzene
Nitro benzene
Aniline
Aniline
Benzoin
Benzoin
Benzaldehyde
Benzaldehyde
Phenol
Phenol 
Caprolactum
Caprolactum
Sodium benzoate
Sodium benzoate
Anisole
Anisole
Acetophenone
Acetophenone
 
Benzoic acid
Benzoic Acid
Benzene sulphonic acid
Benzene Sulphonic Acid
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