1. Separation and purification of mixtures
1.1. Separation, Purification and Identification of the Components of a Mixture
Abel J. S. C. Vieira* and Elvira M. S. M. Gaspar
Faculty of Sciences and Technology, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal
Background
In this work, typical chemical reactions of organic compounds are not involved, with the exception of acid–base reactions.1 Its aim is to provide students with the knowledge of fundamental experimental techniques of unitary operations, such as extraction, distillation (under normal and reduced pressure), filtration, recrystallization and thin-layer chromatography.2,3
The mixture to be separated is a solution in methylene chloride of three different aromatic compounds: an amine (p-toluidine) a carboxylic acid (benzoic acid) and a hydrocarbon (naphthalene). It is a four-component mixture: the (liquid) solvent and the three (solid) solutes. Considering the physical state of these components, one first (easy) attempt of separation could be a simple distillation of methylene chloride, yielding the solvent and a solid residue containing the three solids. However, to isolate each one of the components of the solid residue, complex solid-phase separation techniques would be necessary, which are difficult to perform and, very often, with low efficacy. Therefore, it is much more convenient to proceed to a liquid–liquid extraction.
Although the liquid–liquid extraction is a simple and efficient separation method, in the case of the present mixture, it would be necessary to find a set of solvents (not miscible with methylene chloride) to remove selectively each one of the three solutes, which, obviously, is not easy to obtain. However, taking into account the acid–base properties of two of the solutes, water can be used as the extraction solvent (water is not miscible with methylene chloride and a convenient “green” solvent), by performing very simple acid–base reactions.
Therefore, by treating the starting organic solution (in a separatory funnel) with an acidic aqueous solution, e.g. a solution of hydrochloric acid in water, the acid reacts with the p-toluidine base, yielding p-toluidinium chloride, a water soluble salt. The two phases obtained are easily separated, the aqueous layer containing the ammonium salt of the amine and the organic layer containing the remaining components dissolved in methylene chloride. By treating now this organic phase with a basic aqueous solution, e.g. a solution of sodium hydroxide in water, the base reacts with benzoic acid, yielding sodium benzoate, another water-soluble salt. A new separation of the two phases leaves an aqueous layer containing the salt of the acid and an organic layer containing only naphthalene dissolved in methylene chloride.
At this point of the work three isolated solutions are available: one aqueous solution of p-toluidinium chloride, one aqueous solution of sodium benzoate and one organic solution of naphthalene in methylene chloride. Naphthalene can be recovered by simply distilling the solvent. The other two components (both insoluble in cold water) can be isolated by “reversing” the pH of the aqueous solutions of the corresponding salts, i.e. by adding a base to the p-toluidinium chloride solution and an acid to the sodium benzoate solution.
Each one of the three isolated solid compounds can be further purified by recrystallization in the appropriate solvent.
The diagram of the graphical abstract illustrates schematically the sequential separation process.
Additional Safety
The experiment can be performed in a regular laboratory bench, with proper safety equipment, such as protective clothing, gloves and safety goggles. Aromatic compounds are usually toxic and inhalation or contact with eyes or skin must be avoided. The same safety rules apply to methylene chloride, which is quite volatile; good laboratory ventilation is advisable.
p-Toluidine: R:23/24/25-36-40-43-50; S: 1/2-28-36/37-45-61.
Naphthalene: R: 22-40-50/53; S: 2-36/37-46-60-61.
Methylene chloride: R: 40; S: 1/2-23-24/25-36/37.
Hydrochloric acid: R: 23-35; S: 1/2-9-26-36/37/39-45.
Sodium hydroxide: R: 35; S: 1/2-26-37/39-45.
Ethanol: R: 11; S: 2-7-16.
