SolutionsAromatic Nucleophilic Substitution
1. Which halide reacts first?
Rank by reactivity towards NaOH: chlorobenzene, 1-chloro-4-nitrobenzene, 1-chloro-3-nitrobenzene and 1-chloro-2,4-dinitrobenzene.
1-Chloro-2,4-dinitrobenzene > 1-chloro-4-nitrobenzene > 1-chloro-3-nitrobenzene > chlorobenzene. Nitro groups at ortho or para stabilize the negative charge of the Meisenheimer intermediate by resonance; at meta they act only through the inductive effect.
2. The Meisenheimer intermediate
Draw the resonance forms of the intermediate of the reaction of 1-chloro-4-nitrobenzene with methoxide ion.
Methoxide adds to the carbon bearing Cl, which becomes sp3. The negative charge is delocalized over the carbons ortho and para to it and, in a fourth form, onto the oxygens of the nitro group. Loss of Cl− gives 4-nitroanisole.
3. Benzyne
2-Bromo-1,3-dimethylbenzene does not react with NaNH2 in NH3. Why?
To form benzyne the base must remove an H from the carbon next to the one bearing the halogen. In this compound both neighboring positions carry methyl groups: there is no ortho H and benzyne cannot form.