Describe exactly how you would go about separating a mixture of the three water-insoluble liquids, aniline (b.p. 184 °C), n-butylbenzene (b.p. 183 °C) and n-valeric acid (b.p. 187 °C), recovering each compound pure and in essentially quantitative yield.

This is a question from Organic Chemistry by Morrison and Boyd. My problem is I know everything that is going to apply in this problem but I'm unable to execute the solution.

Things that I know:
1. Aniline is an amine attached to benzene group, n-butylbenzene is a benzene with a butyl group attached to one of its carbon atom, Valerie acid is pentatonic acid.
2. Aniline are polar and basic, Valeric acid are acidic, benzene is an electron withdrawing group.
3. Aniline are insoluble in water due to lack of hydrogen bonding.

How can solve this problem? Is my knowledge about the problem is insufficient? I have given this problem only to illustrate my problem with reasoning in chemistry, so please illustrate your answer by giving the solution of this problem or the way you find better.

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    $\begingroup$ Knowing too many useless things can obscure your vision. True, aniline is basic; what does that really mean? $\endgroup$ – Ivan Neretin Feb 7 '19 at 5:04
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    $\begingroup$ Simple extraction by treatment with aq. acid followed by by aq. NaOH will seperate all the three compounds. $\endgroup$ – Soumik Das Feb 7 '19 at 6:32
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    $\begingroup$ @SoumikDas Of course it will, but the question was not so much about the separation per se as it was about the reasoning involved, and that's what I tried to stress in the first place. $\endgroup$ – Ivan Neretin Feb 7 '19 at 6:38
  • $\begingroup$ The question is a little strange as both aniline and valeric acid are soluble in water to the extent of a few grams/100ml which offers an immediate path to separation from the butylbenzene. $\endgroup$ – porphyrin Feb 7 '19 at 11:48

Several methods to separate substances exist in chemistry. A non-exhaustive list:

  • Distillation
  • Extraction
  • Chromatography
  • (Fractional) crystallization

which may be supplemented by chemical modification.

Let's consider some of the methods mentioned. Distillation is a classic, but the boiling points given in the problem are very close to each other and would probably prevent effective separation. No data is given on the freezing points of the substances, so direct selective crystallization may be hard. Chromatography (as in silica column chromatography) would probably work in some fashion, but is probably not the intended answer here. Other, more exotic methods such as the creation of complexes using cryptands and the like are probably possible as well.

What about extraction using water? The compounds are not soluble in water, but we can modify some of them to be so. If we add a sufficiently strong acid in a sufficient amount of water and mix vigorously, we can probably get most of the aniline into the aqueous phase. After separating in an appropriate funnel, we would make the aqueous phase slightly basic and extract the aniline using some organic solvent that does not mix with water. Likewise, a strong base (and subsequent "inverted" procedure) can be used to separate out the n-valeric acid.

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