Separação de Misturas Orgânicas

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Suggested solutions for Chapter 8 Problem 1 If you wanted to separate a mixture of naphthalene, pyridine, and p-toluic acid, how would you go about it? CO₂H N naphthalene pyridine para-toluic acid Purpose of the problem Revision of simple acidity and basicity in a practical situation. Suggested solution This is a contrived problem - Pyridine is a weak base 5.5) and can be dissolved in aqueous acid. p-Toluic acid is a weak acid you cannot usually separate about 4.5) and can be dissolved in aqueous base. Naphthalene is neither an acid nor a base and three compounds in this way. remains insoluble in water at any pH. So, dissolve the mixture in an organic solvent immiscible with water (say ether, Et₂O, or CH₂Cl₂) and extract with aqueous acid (pH 6) which will extract the p-toluic acid as its anion. You now have three solutions. Evaporate the organic solution of naphthalene and recrystallize it to get pure naphthalene. Acidify the basic solution of p-toluic acid to precipitate the free acid and recrystallize it. Finally, add base to the pyridine solution, extract the pyridine with an organic solvent, (say ether, or CH₂Cl₂), evaporate the solvent, and distil the pyridine. It is just as good to extract with base first and acid second. Problem 2 In the separation of benzoic acid from toluene we suggested using NaOH solution. How concentrated a solution would bei necessary to ensure that the pH was above the of benzoic acid 4.2)? How would you estimate how much solution to use? Purpose of the problem You can check your understanding of the relationship between pH and concentration. Suggested solution Even a very weak solution of NaOH will have a pH > 4.2. By the calculation on p. 184 of the chapter, for a pH of 5 we should need [H₃O⁺] = 10⁻⁵ mol dm⁻¹. We know that the ionic product of water is [H₃O⁺] [OH⁻] = and so for pH 5 we need 10⁻⁵ mol dm⁻¹ of NaOH. This is very dilute! The trouble is that you need one hydroxide ion for every molecule of benzoic acid so if you had, say, 1.22 g PhCO₂H (= 0.01 equiv.) you would need 1000 litres (dm⁻¹) of your NaOH solution. It makes more sense to have a much more concentrated solution, say, 0.1M. This would give an unnecessarily high pH of 13 but you would need only 100 ml (cm³) to extract your benzoic acid.