Separating and Identifying Compounds

Organic chemists separate and identify compounds by using differences in their properties. In a liquid–liquid extraction, a mixture dissolved in an organic solvent such as diethyl ether is shaken with an aqueous solution. A compound moves into the aqueous layer if it is charged or very polar, and it stays in the ether if it is neutral and nonpolar. Acids and bases can be moved between the layers by changing the pH, because the ionized form of an acid or base is much more soluble in water than its neutral form.

A student has a solution in ether of three compounds: benzoic acid, which is a weak acid (pKa about 4.2), aniline, which is a weak base whose conjugate acid, the anilinium ion, has a pKa of about 4.6, and naphthalene, a neutral hydrocarbon. She shakes the ether solution first with aqueous NaOH (pH about 13), separates the layers, then shakes the remaining ether solution with aqueous HCl (pH about 1), and separates the layers again. She keeps the three liquids separate and labels them: the NaOH layer (Fraction 1), the HCl layer (Fraction 2) and the ether layer that is left (Fraction 3).

Part 1. To identify the compounds in each fraction, the student uses thin-layer chromatography (TLC) on a silica gel plate, which is a polar stationary phase. She spots each of the three pure compounds on the plate, develops it in a nonpolar solvent mixture until the solvent front has moved 8.0 cm from the starting line, and visualizes the spots with ultraviolet light. The retention factor, Rf, is the distance moved by the compound divided by the distance moved by the solvent front. Table 1 shows the distance each compound moved.

Table 1. Distance moved on the TLC plate (solvent front = 8.0 cm).

Part 2. The student recovers the compound from Fraction 1 by adding concentrated HCl until a white solid forms, filtering the solid and drying it. She records an infrared (IR) spectrum and a mass spectrum. Table 2 shows the main features.

Table 2. Data for the solid isolated from Fraction 1.

The student also notes that the exact positions of absorptions in an IR spectrum depend on the physical state of the sample and that a strong absorption near 1700 cm⁻¹ is a typical feature of a C=O bond, while a broad band between 2500 and 3300 cm⁻¹ is typical of an O–H bond that is strongly hydrogen-bonded. In the mass spectrum, the peak at the highest mass usually corresponds to the intact molecule that has lost one electron.