Question

draw a mechanism that explains how the nitro-substituted aromatic products observed in your reaction were formed

Answer 1

Let us consider nitration of toluene.

This is an Electrophilic Aromatic Substitution Reaction(EASR).

Strong inorganic acids HNO3 and H2SO4 forms a strong highly reactive Nitronium ion (NO2⁺)- an electrophile.

?-electron density of toluene attacks this electrophile NO2⁺ ion and forms first ?-complex which subsequently gives rise to a carbocation called ?-complex.The formation of ?-complex i.e. a carbocation is selective process which governs and governed by the orientation of substituent on benzene ring i.e. the position electrophile get bonded to benzene ring.

Electrophile get position on such a C of benzene which gives rise to a more stable carbocation. In any case the carbocation will have allylic stability but there is extra stability associated with the carbocation formed by attack of electrophile at C ortho or para relative to methyl(-CH3) substituent on ring. We can see the resonance forms corresponding to the ortho and para position where one of the structure shown +ve charge on C bearing –CH3 group. This canonical form has some extra stability than other viz. +ve Inductive effect and hyperconjugation stability. Hence there are more chances of existence of these forms of ?-complexes(than those have no +ve charge on –CH3 bearing C. This would be in case of meta attack).

These stable ?-complexes then undergo aromatization through loss of H on adjacent C and finally we get the mixture of ortho- and para-nitrotoluene compounds.

The reaction mechanism: