Comprehensive Organic Chemistry Experiments for the Laboratory Classroom

Carlos A M Afonso, Nuno R Candeias, Dulce Pereira Simão, Alexandre F Trindade, Jaime A S Coelho, Bin Tan, Robert Franzén, Carlos A M Afonso, Nuno R Candeias, Dulce Pereira Simão, Alexandre F Trindade, Jaime A S Coelho, Bin Tan, Robert Franzén

• Editorial: Royal Society of Chemistry
• Año: 2020
• ISBN:
  9781839162978
• Edición: 1
• Categoría: Physical Sciences
• Categoría: Organic Chemistry

This expansive and practical textbook contains organic chemistry experiments for teaching in the laboratory at the undergraduate level covering a range of functional group transformations and key organic reactions.The editorial team have collected contributions from around the world and standardized them for publication. Each experiment will explore a modern chemistry scenario, such as: sustainable chemistry; application in the pharmaceutical industry; catalysis and material sciences, to name a few. All the experiments will be complemented with a set of questions to challenge the students and a section for the instructors, concerning the results obtained and advice on getting the best outcome from the experiment. A section covering practical aspects with tips and advice for the instructors, together with the results obtained in the laboratory by students, has been compiled for each experiment.

Targeted at professors and lecturers in chemistry, this useful text will provide up to date experiments putting the science into context for the students.

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

*E-mail: [email protected]

Number of sessions (Duration of each session)	Hazard level	Difficulty level	Level of study
2 or 3 (3 h each)	Low	Low	Introductory
Class names Aromatic compounds, carboxylic acids, amines, hydrocarbons
Concepts involved Acid–base reactions. Fundamental experimental techniques of unitary operations: Extraction, distillation, filtration and thin-layer chromatography. Purification of compounds by recrystallization and characterization through their physical properties and IR spectra
Chemicals neededp-Toluidine, benzoic acid, naphthalene, methylene chloride, hydrochloric acid, sodium hydroxide, sodium sulfate, ethanol, petroleum ether (40–60 °C) (also known as light petroleum (40–60 °C))
Equipment and experimental techniques involved Usual laboratory glassware; extraction, filtration, distillation and recrystallization apparatus; TLC plates
Keywords Acid–base reactions, distillation, extraction, filtration, IR spectroscopy, physical properties, thin-layer chromatography

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.

Experimental Procedure

Laboratory Session 1 (3 h)

Separation (ref. 4)

1. In a separatory funnel introduce 150 mL of the mixture to be separated (this volume of solution contains 3 g of p-toluidine, 3 g of benzoic acid and 3 g of naphthalene, in methylene chloride).
2. Extract the mixture with 25 mL of an aqueous HCl (3N) solution and shake thoroughly. Allow to stand and separate the aqueous layer. Repeat this procedure twice. Finally, extract the organic layer with 25 mL of water. Collect the aqueous extracts in an Erlenmeyer flask (solution A).
3. Recover the organic phase from the previous extraction and extract it with 25 mL of an aqueous NaOH (3N) solution and shake thoroughly. Allow to stand and separate the aqueous layer. Repeat this procedure twice. Finally, extract the organic layer with 25 mL of water. Collect the aqueous extracts in an Erlenmeyer flask (solution B).
4. Keep the organic phase left after the two previous extractions in an Erlenmeyer flask (solution C) and dry it with anhydrous sodium sulfate.

Recovery

5. To the solution A add a concentrated NaOH solution until no more precipitate formation is observed. Assure that the medium is basic by testing with indicator paper. Cool the mixture in ice, filter with suction at the water vacuum pump and collect the crystals in a Büchner funnel. Wash them with a small amount of ice cold water. Let them dry at the air over filter paper.
6. To the solution B add a concentrated HCl solution until no more precipitate formation is observed. Ensure that the medium is acidic by testing with indicator paper. After this point proceed as described in step 5 for solution A.
7. Filter the solution C through a fluted filter paper and collect the filtrate in a 250 mL round-bottomed flask. Distil the methylene chloride in a rotary evapora...