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प्रश्न
Describe the mechanism of Nitration of benzene.
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उत्तर
It is prepared by the action of a mixture of con. HNO3 and con. H2SO4 (nitrating mixture) on benzene maintaining the temperature below 333 K.
Nitration:
Sulphuric acid generates the electrophile – NO2+, nitronium ion-from nitric acid. This is an example of aromatic electrophilic substitution reaction.
The generation of nitronium ion
To the nitronium ion (being an electron-deficient species) the π bond of benzene, donates a pair of electrons forming a 6-bond. A species with a + ve charge is formed as an intermediate. This is called ‘arenium ion’ and is stabilised by Resonance.
In the last step, the hydrogen atom attached to the carbon carrying the nitro group is pulled out as a proton, by the Lewis base HSO4–, so that stable aromatic system is formed.
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\[\begin{array}{cc}
\ce{CH2 - CH2 ->[Zn/C2H5OH]}\\
|\phantom{.......}|\phantom{..............}\\
\ce{Br}\phantom{.....}\ce{Br}\phantom{..............}
\end{array}\]
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\[\begin{array}{cc}
\phantom{...............................}\ce{CH3}\\
\phantom{............................}|\\
\ce{CH3 - C = CH2 + HBr ->[benzoyl][peroxide]CH3 - CH - CH2Br}\\
|\phantom{....................................}\\
\ce{CH3}\phantom{..................................}
\end{array}\]
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\[\begin{array}{cc}
\phantom{..........................}\ce{CH3}\\\phantom{........................}|\\\ce{CH3 - C = CH2 + HBr ->[benzoyl][peroxide] CH3 - CH - CH2Br}\\
|\phantom{......................................}\\
\ce{CH3}\phantom{....................................}\end{array}\]
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