In: Chemistry
Propose a nucleophilic aromatic substitution mechanisms for the reactions of
a) 2,4-dinitrochlorobenzene + sodium methoxide
b) 2,4-dimethylchlorobenzene + sodium hydroxide, 350 degrees C
c) p-nitrobromobenzene + methylamine
d) 2,4-dinitrochlorobenzene
Aromatic compounds undergo nucleophilic substitution reaction if an aromatic ring is sufficiently deactivated i.e. if is substituted with strong electron withdrawers like –NO2, -X(Cl, Br etc).
Generally a nucleophile(Nu) attacks the C-X bond as it is relatively weaker than C-NO2 bond (C-NO2) bond is having partial double bond character and hence though a Nu attack C-NO2 bond it’s a reversible attack i.e. Nu detaches back.
Moreover In case of C-Br attack by a Nu the leaving group Br is far better leaving group than our Nu and (would be) leaving group NO2- .
Hence In nucleophilic aromatic substitution reaction of Halo-nitro benzenes nucleophilic substitution occur at C bearing a halo group.
a)Sodium ethoxide provides strong nucleophile ethoxide ion (EtO-) which attack C-Cl bond and ultimately on elimination of –Cl finally 1-Methoxy-2,4-dinitrobenzene results.
Reaction mechanism shown here,
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b)2,4-Dimethylchlorobenzene is not much deactivated aromatic hence very strong nucleophile OH and very high temperature is needed to be used. The reaction mechanism shown here,
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c) 4-Nitrobromobenzene is much deactivated and a sufficiently nucleophilic methyl amine attacks C of C-Br bond and forms the product N-Methyl-4-nitrobenzenamine.
Reaction mechanism shown here,
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