In the multistage synthesis of 4-bromobenzenamine, why must the solution be rendered alkaline prior to the steam distillation after the hydrolysis of 4-bromo-N-ethanoylbenzenamine?

  • $\begingroup$ Which compound are you distilling, the 4-bromoaniline or the 4-bromo-N-ethanoylphenylamine? $\endgroup$ – LDC3 May 26 '15 at 2:37
  • $\begingroup$ the 4-ba might be a zwitterion and in the acidic medium it may be attracted to the cation forming a salt of whatever acid was used.This would raise the boiling point of the 4-ba enough to not come over to the receiving flask. When made basic with a reagent like naoh the sodium ion will react with the cation from the 4-ba salt and the 4-ba will be left as the free base which is more volatile and can be distilled . $\endgroup$ – Technetium May 26 '15 at 6:43

In principle, it is difficult to distill salts (ionic compounds). Therefore you first make sure that you have a free base in the solution. This is guaranteed in alkaline solution. For the steam distillation the compounds should be immiscible, which is another reason for having a free base. Let’s compare boiling points and solubility for aniline and its salt:

  • Aniline b.p. 184 °C, solubility in water: 36 g/L
  • Anilinium chloride b.p. 245 °C, solubility in water: 1070 g/L

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