Benzene
By Anup Pokhrel
KEKULE’S STRUCTURE OF BENZENE
The molecular formula of benzene is C6H6. It has hexagonal cyclic structure used for representing benzene in resonance hybrid structure. Each and every bond in benzene ring is intermediate of double bond ( 1.34A0) and single bond (1.54A0) and its bond length is 1.39A0
Each and every carbon atom in the benzene is sp2 hybridized. So, each carbon contains three sp2 hybrid orbitals . Two sp2 hybrid orbitals of each carbon overlap withsp2hybrid orbitals of adjacent two carbon forming ‘ C – C ‘ sigma (σ) bond. The third sp2 hybrid orbital of each carbon overlap with s orbital of hydrogen to from C – H bond . Hence, there are six C – Cσ bond and six C – Hσ bond in benzene.
Each and every carbon of benzene still contains unhybridized p – orbitals. Unhybridized p orbital of each carbon laterally overlap with each other to form π – electron cloud above and below the plane of σ bond.
NOMENCLATURE
The mono substituted benzene can be named by writing the substituent as a prefix before benzene.
In disubstituted benzene, substituents are mentioned as prefix
ISOMERISM
All the hydrogens in benzene are equivalent. So, it has single monosubstituted derivatives.
It gives three distributed isomers which are positional isomers of one another
ORIENTATION IN BENZENE DERIVATIVES
In electrophilic substitution, reaction in aromatic compounds, the position of the new incoming electrophile is decided by the group or atom already present in benzene. This is called as orientation. This effect is called directive effect and the groups already present in benzene are called directors or directing group. There are two types of directing group
i) Ortho/Para (O/P) directing group
ii) Metal (M) directing group
i) Ortho/Para (O/P) directing group :They are those type of groups in which directly bonded atom contains lone pair of electrons. These type of atom or groups make benzene more reactive towards electrophilic substitution reaction. Due to the presence of these groups, the incoming electrophile takes ortho and para posiiton.
ii) M – directing group
Those groups in which directly bonded atom are already bonded with more electronegative atom by multiple bonds are called M- directing group. They withdraw electron from benzene ring or aromatic system. Due to the presence of such group, the new incoming electrophile takes meta position with respect to the group already present in the benzene.
Note: O/P groups are ring activators. They activate the benzene ring for electrophilic substitution reaction i.e. they release electron towards benzene and increase its electron density ( except halogens). Halogens, though are O/P directors, deactivate the ring due to Negative Inductive ( -I) effect
Meta directing groups are electron withdrawing species and they deactivate the benzene ring for electrophilic substitution eaction i.e. they are ring deactivators.
PREPARATION OF BENZENE
i) By decarboxylation :Sodium benzoate on heating with soda lime ( NaOH + CaO) gives decarboxylated product i.e. benzene
ii) From phenol
Vapors of phenol, when passed through zinc dust gives benzene as a reduced product
iii) From chlorobenzene
Chlorobenzene, on heating with reducing agent like Lithium Aluminium Hydride ( LiAlH4) gives reduced product as benzene
iv) From acetylene
When acetylene gas is passed through red hot copper tube, it gives benzene as a polymerized product
PHYSICAL PROPERTIES
i) Benzene is a colorless liquid
ii) It has characteristic aromatic odour.
iii) Its boiling point is 80.50C.
iv) It is soluble in organic solvents like ether, chloroform, etc. but insoluble in water
v) It behaves as a solvent for organic and inorganic compounds like fat, iodine, etc.
Chemical Properties following
Benzene undergoes following different types ofg chemical reactions
A) Electrophilic substitution reaction
B) Addition Reaction
C) Oxidation
A) Electrophilic substitution reaction
Benzene commonly undergoesESR rather than addition reaction. It is due to the unexpected extra stability of benzene and it is resonance hybrid of Kekule’s structures. Electrophile, being electron deficient , can easily attack in benzene due to its high electron density
a) Nitration :When benzene is treated with nitrating imixture ( conc. HNO3 + conc.H2SO4) gives nitrobenzene. The product of introducing – NO2 group in a compound is called nitration
b) Sulphonation:Benzene, when treated with fuming sulphuric acid at room temperature gives benzene sulphonic acid
c) Friedel Crafts reaction
There are following two types of Friedel Craft reaction
i) Friedel Craft alkylation
ii) Friedel Craft acylation
i) Friedel Craft alkylation :Benzene, when treated with alkyl halide in the presence of Lewis acid (AlCl3) and ether gives alkyl benzene
ii) Friedel Craft Acylation :Benzene, when treated with acyl halide, acid halide or acetic anhydride in the pesence of Lewis acid and ether gives acyl benzene
B) Addition reaction
a) Catalytic hydrogenation:Vapors of benzene and hydrogen, when passed over heated catalyst like nickel, platinum at a temperature of about 2000C gives cyclohexane
b) Halogenation :Benzene undergoes addition reaction in ordinary condition or in the presence of sunlight with chlorine or bromine to give benzene hexachloride or benzene hexabromide respectively.
Benzene Hexachloride is used as insecticide which is commercially calledGammexane.
c) Ozonolysis ( Adition of Ozone)
Benzene undergoes addition reaction with ozone to give benzene triozonide which on hydrolysis in the presence of zinc gives 3 molecules of glyoxal
C) Oxidation
a) Combustion:Benzene undergoes complete oxidation in the presence of oxygen to give carbon dioxide and water
b) Catalytic oxidation:When vapor of benzene is passed through Vanadium pentoxide at a temperature of about 5000C gives Maleic acid, which on dehydration gives Maleic anhydride
USES OF BENZENE
i) It is used as a solvent
ii) It is used asa a solvent for the extraction of fats and oil
iii) It is used for the preparation of various organic compounds like phenol, ethylbenzene, benzene hexachloride, drugs, dyes, explosives, etc.