Separation of two components of a mixture

Written by J.A Dobado | Last Updated on April 22, 2024

Objective

To learn how different organic compounds can be isolated according to their acid-base properties, which change their solubility in organic and aqueous solvents, using the liquid-liquid extraction technique.

Background

Liquid-liquid extraction is one of the most common basic operations in the organic chemistry laboratory, since many reactions involve the use of this technique for product isolation. Carboxylic acids can react with bases such as sodium hydroxide, producing a proton and forming the corresponding water-soluble anions (carboxylates). This experiment involves the separation of the components of a mixture consisting of naphthalene and benzoic acid, which will be dissolved in an organic solvent such as CH₂Cl₂, depending on the acidic or basic character of these compounds.

Procedure

Separation of two components (mixture of an acid and neutral compound): Take 30 ml of a problem solution containing an organic solvent (CH₂Cl₂) and two different dissolved compounds with neutral and acid character, such as: naphthalene, and benzoic acid, respectively.

Place the 30 ml of the dissolution problem in a funnel of decanting of 250 ml. Add 40 ml of a 5% NaOH solution and shake vigorously (or with 20 ml twice). Let stand and then separate the two layers (the aqueous layer is the upper one and the organic layer, CH₂Cl₂, the lower one).

The organic layer 1-O (lower) is dried with anhydrous Na₂SO₄ for a few minutes. Remove the desiccant by gravity filtration. Transfer the filtrate to a dry round-bottomed flask, and evaporate the solvent to dryness at the rotary evaporator. The neutral component of the mixture is obtained as a solid of characteristic odor, naphthalene.

The aqueous layer 1-A is made acidic (use indicator paper) by adding small portions of HCl (conc.). Cool the mixture and vacuum filter (Kitasato and Büchner), the crystalline product obtained is the acid component of the problem mixture, benzoic acid.

General scheme of separation

Physico-chemical properties

This table collects data for the molecular weight (M_w), melting point (M.p.) boiling point (B.p.) and density of the reactives and compounds used in this laboratory experiment.

GHS pictograms

Hazard pictograms form part of the international Globally Harmonized System of Classification and Labelling of Chemicals (GHS) and are collected in the followinf Table for the chemical compounds used in this experiment.

Name	GHS
Benzoic acid
Naphthalene
CH2Cl2
NaOH
HCl
Na2SO4	Non-hazardous

International Chemical Identifier

The IUPAC InChI key identifiers for the main compounds used in this experiment are provided to facilitate the nomenclature and formulation of chemical compounds and the search for information on the Internet for these compounds.

References

• Isac-García, J.; Dobado, J. A.; Calvo-Flores, F. G.; and Martínez-García, H. (2015). Experimental Organic Chemistry Laboratory Manual. Elsevier Science & Technology. ISBN: 978-0-12-803893-2

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