Skip to content
qorganica.es qorganica.comOrganic Chemistry made transparent Course map Español

SolutionsReactivity

1. Diazonium salts

Starting from aniline, how would you prepare each compound?

  1. Phenol
  2. Bromobenzene
  3. Benzonitrile
  4. Fluorobenzene
  5. Benzene

First the aniline is diazotized with NaNO2/HCl at 0-5 °C to give benzenediazonium chloride, and then:

  1. H2O, heat.
  2. CuBr (Sandmeyer reaction).
  3. CuCN.
  4. HBF4 and heat (Schiemann reaction).
  5. H3PO2 (replacement of the diazonium group by H).

2. Use the amino group and remove it

Propose a synthesis of 1,3,5-tribromobenzene from aniline.

Br2 in water gives 2,4,6-tribromoaniline (the amino group activates the ring so much that three bromines enter without a catalyst). Then it is diazotized (NaNO2/HCl) and treated with H3PO2, which replaces the diazonium group by a hydrogen.

Here the amino group serves to direct the bromines to positions that are meta to each other, and it is removed at the end.

3. Why aromatic diazonium salts?

Why are arenediazonium salts useful in synthesis at 0 °C, while alkanediazonium salts decompose at once?

In the arenediazonium ion the –N≡N+ group is conjugated with the ring, and losing it would give an aryl cation, which is very unstable (the empty orbital is sp2 and cannot be delocalized into the ring). In the alkanediazonium ion, loss of N2 gives an alkyl carbocation and is very fast.

← Back to the exercises