By Anup Pokhrel
Compounds in which nitro group ( -NO2) is directly bonded to benzene (aromatic) ring are called aromatic nitro compounds or simply nitroarenes. These are also regarded as nitro derivatives of arenas.
Principle: In the lab, nitrobenzene is prepared by the nitration of benzene with the mixture of conc. HNO3 and conc. H2SO4 (called nitrating mixture) at 60oC.
|NOTE: The purpose of controlling temperature is to prevent further nitration of nitrobenzene is to prevent further nitration of nitrobenzene to m-nitrobenzene.
Refluxing: The process of the heating reaction mixture in a flask fitted with long glass tube so that the vapor or boiling liquid are condensed and returned to the same flask is called refluxing and condenser so fitted vertically is called reflux condenser. Refluxing is done when the reaction mixture requires heating for a long time.
Process: About 50 ml of benzene is placed in RB flask . A well cooled acid mixture of conc. HNO3 and conc.H2SO4 ( 70ml) is added into it, a little by little with shaking and cooling contents of the flask if necessary to keep the temperature below 60oC. After the complex addition of acid mixture, the flask fitted with a reflux condenser (as shown in the figure) and is refluxed (boiled) on the water below 60oC till the yellow oily liquid of nitrobenzene appears on the surface.
Isolation and separation
The contents of the RB flask are now transferred into a separating funnel and the nitrobenzene forms upper layer and is separated from the lower acid layer. It is treated with the Na2CO3 solution to remove excess acid and is then washed with water. The lower layer of nitrobenzene is again separated and is dried over anhydrous CaCl2. It is finally distilled using air condenser and collected in fraction in the temperature range of 208oC to 212oC as pure and dry nitrobenzene
i) It is a pale yellow liquid having the smell of bitter almond.
ii) Its boiling point is 210oC.
iii) It is insoluble in water but soluble in organic solvents
Nitrobenzene undergoes the reduction in different media to form various products depending on the nature of reducing agent used and the reaction condition .
a) Reduction in acidic medium
Nitrobenzene is reduced to aniline by metal like Zn, Fe, Sn and conc.HCl through the formation of intermediate nitrosobenzene and N-phenyl hydroxylamine.
b) Reduction in natural medium
Nitrobenzene is first reduced to nitrosobenzene and then to N-phenyl hydroxylamine by the neutral reducing agent like Zn and aq.NH4Cl.
c) Reduction in basic medium
Nitrobenzene undergoes the bimolecular reduction in basic medium to form successively azoxybenzene, azobenzene, and hydrazo benzene depending on the nature of the reducing agent used.
d) Electrolytic reduction: Nitrobenzene, on electrolytic reduction in weak acid solution forms aniline and in strong acid solution, it forms N- phenyl hydroxylamine which rearranges to give p-aminophenol.=
e) Catalytic reduction (H2, Ni/Pt) : Reduced to aniline on catalytic reduction
f) Reduction with lithium aluminium hydride (LiAlH4)
Reduced to azobenzene
II ) ELECTROPHILIC SUBSTITUTION REACTION
Electrophile: It is defined as an electron deficient species which accepts an electron pair from an electron rich center of a substrate.
The reaction involving substitution of hydrogen of the benzene ring by the initial attack of an electrophile is called electrophilic aromatic substitution reaction which is common in the reaction of aromatic compounds.
In nitrobenzene, benzene ring contains delocalized 6π electrons. So, it is an electron rich system and is thus a good site for the attack of electrophile. Nitrobenzene undergoes electrophilic aromatic substitution in which hydrogen is replaced by various electrophiles.
Electrophilic aromatic substitution reaction in benzene occurs in meta position which is explained by resonance. Nitrobenzene may be represented by following resonating structures.
Because of the resonance effect, the nitro group (-NO2) withdraws electrons from the benzene ring and thus decreases electron density more at ortho and para position so that incoming electrophiles attack at electron rich meta position and is thus called meta director.
Nitro group ( -NO2), being electron withdrawing group withdraws electrons from the benzene ring and thus deactivates it for further electrophilic aromatic substitution and is thus called ring deactivator.
Nitrobenzene undergoes electrophilic aromatic substitution like halogenation, nitration, sulphonation on metal positions.
I) Halogenation: The substitution of hydrogen of the benzene ring by the halogens is called halogenation.
a) Chlorination: Nitrobenzene undergoes chlorination with chlorine in the presence of Ferric chloride (FeCl3) to m – chloro- nitrobenzene.
Nitrobenzene undergoes bromination with bromination in the presence of Ferric bromide (FeBr3) to m-Bromo nitrobenzene.
II) Nitration: Nitrobenzene, on nitration with the mixture of conc.HNO3 and conc.H2SO4 form m- dinitrobenzene.
III) Sulphonation: The substitution of hydrogen of benzene ring by sulphonic group ( -SO3H) is called sulphonation. Nitrobenzene undergoes sulphonation with conc. H2SO4 on heating to from m – nitrobenzene sulphonic acid.
USES OF NITROBENZENE
i) It isused in the manufacture of aniline.
ii) It is used in making polish and explosives like TNT and TNB.