SolutionsPreparation
1. From benzene
Propose a synthesis of aniline from benzene.
Nitration (HNO3/H2SO4) to give nitrobenzene, then reduction of the nitro group (Fe/HCl, Sn/HCl or H2/Ni). After reduction with a metal in acid the anilinium salt is obtained; free aniline is released with NaOH.
2. Order matters
How would you prepare 3-bromoaniline from benzene? And 4-bromoaniline?
- 3-Bromoaniline: nitration, bromination (Br2/FeBr3; the nitro group is deactivating and meta-directing) and reduction of the nitro group.
- 4-Bromoaniline: bromination then nitration (bromine directs ortho and para; the para isomer is separated) and reduction. Alternatively: aniline → acetanilide, para bromination and deprotection.
The amino group is activating and ortho-para-directing; the nitro group is deactivating and meta-directing. Reducing the nitro group at the right moment changes the orientation of the following substitutions.
3. Why not Friedel-Crafts?
Why does aniline not undergo Friedel-Crafts alkylation or acylation?
Because the basic nitrogen coordinates to the Lewis acid (AlCl3). The complex has a positive charge on the nitrogen, which strongly deactivates the ring. To carry out the reaction, the amino group is first protected as acetanilide.