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SolutionsSynthesis Strategies

1. Synthesis from benzene

Propose a synthetic sequence for the following compounds, using benzene as the only aromatic precursor:

Four compounds: cyclohexyl 4-isopropylphenyl ketone, m-bromonitrobenzene, 2-bromo-6-chlorotoluene and 6-chloro-1,1,3,3-tetramethylindan-5-amine
Retrosynthetic analysis and synthesis of the four compounds
  1. Alkylation with propene and H2SO4 (cumene) and Friedel-Crafts acylation with cyclohexanecarbonyl chloride, which enters para.
  2. Nitration and then bromination: NO2 directs meta.
  3. Methylation (toluene), sulfonation (blocks the para position), chlorination and bromination at the two positions ortho to the methyl and, finally, removal of SO3H with dilute H2SO4.
  4. Double alkylation with 2,4-dimethylpentane-2,4-diol and BF3 (tetramethylindane), nitration, reduction to the amine and chlorination ortho to NH2.

2. Order matters

Propose syntheses of m-chloronitrobenzene and p-chloronitrobenzene from benzene.

  • m-Chloronitrobenzene: nitration first, then chlorination (NO2 directs meta).
  • p-Chloronitrobenzene: chlorination first, then nitration (Cl directs ortho-para); the major para isomer is separated.

3. Diazonium salts

How would you prepare 1,3,5-tribromobenzene from aniline?

1) Br2 in water: 2,4,6-tribromoaniline (NH2 activates so strongly that no catalyst is needed). 2) NaNO2/HCl: diazonium salt. 3) H3PO2: the diazonium group is replaced by H.

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