INTRODUCTION TO PRACTICAL ORGANIC CHEMISTRY – MAKING AND PURIFYING ORGANIC COMPOUNDS Stage 1 – Deciding how much reagent to use Most organic reactions do not go to completion; most reach an equilibrium position with significant quantities of reactants still remaining. In addition, separating the desired product from the rest of the reaction mixture, and then purifying it, will result in the loss of more of the product. The amount of product obtained expressed as a percentage of the theoretical maximum amount possible and is known as the percentage yield. Percentage yield = Actual mass of product × 100 Predicted mass of product
When deciding on quantities of reactants to use, you must consider: how much product you require the likely percentage yield
Stage 2 – Apparatus and getting started Reactions which need to be heated strongly over a period of time require either reflux apparatus or distillation apparatus (Fig. 1).
Figure 1: Distillation apparatus
Reflux is required if the reactants, catalyst or solvent is volatile and likely to escape. Distillation is preferred if the product is volatile, or if there is a danger of further reaction. Many reactions are very vigorous at the beginning and can react violently. Often it is necessary to keep the mixture cool as the reactants are being mixed (using a cold water bath or even ice bath). A sensible precaution is to use a dropping funnel (Fig. 2) to add one of the reagents. After the reactants have been mixed, it is then necessary to heat the mixture to whichever temperature results in the best yield.
Stage 3 – Separating the product from the reaction mixture
Figure 2: Dropping funnel Solids can be separated from the reaction mixture by filtration. This is usually best achieved using a Buchner apparatus (funnel and flask) connected to a vacuum pump (Fig. 3).
Liquids can be separated from the reaction mixture using a separating funnel (if the product is immiscible with the reaction mixture) or by distillation (if the product is miscible with the reaction mixture. Figure 4: Separating Funnel
Figure 3: Buchner apparatus connected to a vacuum pump Stage 4 – Purification of the product Solids are purified by a process called recrystallization (Fig. 5): The solid is dissolved in the minimum possible quantity of hot solvent, specifically chosen so that the solid is soluble in the solvent when hot but not when cold. A few impurities (such as grit) will not dissolve in the hot solvent. These can then be removed, if necessary, by filtering the mixture through a pre-heated funnel. The solution is then cooled to below room temperature using an ice-bath. The pure solid should crystallise out of the cold solution. The solid can be separated by filtration under reduced pressure using a Buchner apparatus. A small amount of cold solvent can be added to wash the crystals. The crystals must then be dried between filter paper, and then placed in an oven to dry completely.
Figure 5: Recrystallisation Stage 5 – Testing the Purity of the Product A simple method for testing the purity of the product is to carry out a melting point determination. Pure substances melt at a fixed temperature. Impurities cause the product to melt at a lower temperature and over a broader range of temperatures. Melting point determinations are carried out in the following way: Break off around 5 cm of a capillary tube and seal one end in a Bunsen burner. Page 2
Pack 1 cm of the solid as densely as possible into the sealed end of the capillary tube. Place the capillary tube in the melting point apparatus (Fig. 6). Ensure that the apparatus heats up very slowly as the expected melting point of the substance approaches. Record the temperature when the liquid starts to melt and when it finishes melting. Compare the melting point range to the known melting point of the pure solid. A pure sample will melt sharply at a...