E24 PURIFICATION OF ORGANIC COMPOUNDS Distillation, recrystallisation, melting and boiling point determination

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1 E24 PURIFICATION OF ORGANIC COMPOUNDS Distillation, recrystallisation, melting and boiling point determination THE TASK To learn the main techniques of purifying organic compounds. THE SKILLS By the end of the experiment you should be able to: assemble distillation apparatus and use it to purify liquids, use recrystallisation to purify solids, use melting and boiling points to test purity. OTHER OUTCOMES You will develop an appreciation of the techniques used in organic and pharmaceutical science to obtain and test for purity INTRODUCTION Each year, many thousands of new or unknown organic compounds are prepared or extracted from natural sources such as plants, fungi and animals. In order to identify an unknown organic compound or the components of a mixture of organic compounds, it is necessary firstly to separate and purify the compounds. At any given temperature, a certain number of molecules of a substance in the liquid state have sufficient energy to escape from the surface to create a vapour pressure. This tendency becomes greater as the kinetic energy of the molecules is increased by raising the temperature. When a liquid is heated to a temperature at which the vapour pressure equals that of the surrounding atmosphere, the liquid boils, and this temperature is known as the boiling point. The boiling point of a pure substance at a specified pressure is thus a characteristic property of that substance, and can be used as a criterion of purity and a means of identification. The boiling point of a mixture of liquids is more complex and is dependent on the actual components and their relative proportions. In most cases the composition (mole fraction) of the vapour will be different from that of the mixture. Therefore, if the vapour is condensed and collected, the condensate will be enriched in one of the components. As the composition of the mixture changes, so too does its boiling point and the composition of the vapour. Distillation is the process of vaporising a sample, condensing the vapour, and collecting the condensate in another vessel. Each volatile component of the mixture can thus be distilled from the reaction mixture in an increased state of purity. If the properties of the components of the mixture do not permit a clean separation, the products of the various fractions can be redistilled to improve their purity. In this way, mixtures of liquids with different vapour pressures at a given temperature can be separated or recovered from less volatile contaminants. Distillation can be used to purify sea-water to drinking-water standard. The term evaporation is used when the residue, rather than the distillate, is of importance, and the distilling vapours are often permitted to escape uncondensed into a fume hood. Table salt is isolated from sea-water by evaporation. E24-1

2 E24-2 Separation of a mixture of solids or purification of solids is most readily achieved by recrystallisation. This process involves identifying a suitable solvent in which the material to be recrystallised or purified has high solubility at elevated temperature (typically near the boiling point of the solvent) and low solubility at low temperature. The solid (or mixture) is dissolved in a minimal volume of hot solvent, filtered to remove any insoluble components in the mixture and the solution is slowly cooled to allow the product to crystallise. The formation and growth of crystals is a complex process that involves the removal of molecules from solution (molecule by molecule) and stacking them into a regular condensed state where the molecules are packed in a crystal lattice. Like molecules tend to pack more efficiently, so the growth of crystals tends to bring together molecules of the same type. The effect of dissolving a solid then permitting it to crystallise is that the crystals that form contain less impurities (defects) than the material that was originally dissolved. Successive recrystallisations can produce extremely pure crystalline samples. Recrystallisation is the purification method of choice where the solid is highly crystalline and forms crystals readily. It may also be carried out in industrial chemistry on a massive scale, e.g. the purification of white sugar from raw sugar. Impure compound Dissolve in hot solvent Filter Filtrate Insoluble impurities (discard) Cool, solid crystallises Filter Filtrate - soluble impurities (discard) Pure Compound

3 E24-3 SAFETY A knowledge of the nature of a compound allows it to be handled effectively and safely. Consequently it is important we know of hazards which may be associated with the chemicals handled in order to take any necessary precautions. In each experiment you are required to fill in the Nature of the material in the safety table and understand the implications this has before handling the material. Relevant information is provided in Appendix A2. Materials urea ethanol impure liquid Nature highly flammable Indicate, by signing, that you have understood the information in the safety table. I understand the safety information Demonstrator s Initials LAB-WORK Date 20 Experiment 1. Determination of boiling point of ethanol Set up a distillation apparatus as shown below.

4 E24-4 Place about 15 ml of ethanol in the quick-fit flask and two or three boiling chips (antibumping granules). Heat the ethanol to boiling over the steam bath. Observe the thermometer until a steady temperature is reached and distillation is proceeding smoothly at about 1 drop per second. You may have to moderate the steam downwards from its initial setting. Once you are satisfied you have measured the boiling point, switch off the steam and allow the flask to cool. Never distil all the liquid out of the flask, as the temperature can then rise rapidly, causing decomposition and charring. Observed boiling point of ethanol C Discard the remaining ethanol and the distillate you have collected, into the ethanol residues bottle provided. Do not return this liquid to the storage bottle from which it came. Experiment 2. Purification of a liquid Purify the unknown compound ( 20 ml) by distillation and determine its boiling point. Remember all the precautions, including not contaminating one compound with another. You must, therefore, clean and dry the apparatus between samples, since any impurities, including water, will affect your results. Fresh boiling chips must be added. Observed boiling point of the volatile sample C Once cool, return any undistilled liquid and distillate to the appropriate residues bottles. Experiment 3. Purification of a solid The basic principle employed in recrystallisation is the dissolution of the crude material in a suitable hot solvent, filtration while hot to remove any insoluble impurities, then cooling of the solution so that the required compound preferentially crystallises, leaving the soluble impurities in the solution (known as the mother liquor ). Set up a 100 ml conical flask, short-stem funnel and flute a filter paper. Pour a small volume of ethanol into the flask and then place the flask and funnel on the steam bath to warm. The "urea" sample you are given is contaminated with a soluble impurity and an insoluble impurity. Weigh out 10 g of the impure urea into a clean 100 ml conical flask and add two boiling chips. In another 100 ml conical flask add ethanol ( 60 ml, estimate only) and two boiling chips and heat to boiling on the steam bath. Once the ethanol is hot, add 30 ml to the urea sample, swirl the flask to mix the contents and warm. CARE: The flask is hot! Continue to add hot ethanol in 5 ml portions, with warming between each addition, until all the urea has dissolved. Once no more solid dissolves, add 20% more ethanol (i.e. 10 ml), boil again, and carefully filter the boiling solution through the hot fluted filter paper/funnel into the clean 100 ml conical flask, while the flask remains on the steam bath.

5 E24-5 Leave the flask, now containing the clean, clear filtered solution (filtrate), on the heat-proof mat to cool slowly. Meanwhile, prepare a vacuum filtration apparatus using a Buchner funnel. Your hot filtrate should soon begin to deposit crystals. When your solution has cooled to about 40 C (i.e. warm if held in your hand) place the flask in a beaker of ice-water and allow it to cool for 5-10 minutes to complete crystallisation. Colour and shape of crystals Dampen the filter paper circle with a little ethanol, apply the vacuum and check that a good seal is obtained. Then collect your crystals by swirling the flask to suspend them and pouring the suspension into the filter funnel, attempting to leave as little product behind in the flask as you can. Your cold filtrate is a saturated solution of urea in ethanol - it may be used to assist in the transfer of the crystals as the urea product will not dissolve in it. Do not, however, use fresh solvent for this procedure - urea has an appreciable solubility in pure ethanol and much of your product will dissolve and the yield of product will decrease accordingly.

6 E24-6 Leave the crystals to dry by vacuum for 5-10 minutes. Using the base of a small beaker, squash the crystals between two filter papers to remove any further solvent, and then transfer the crystals to a labelled, pre-weighed, clean 100 ml beaker. Use the top loading balance, not an analytical balance, for this weighing. Determine the mass of dry urea recovered. Mass of 100 ml beaker and recrystallised sample g Mass of 100 ml beaker g Mass of sample g The melting point of a sample is used as a means to determine its purity. You will encounter this technique in higher years of chemistry study. POST-WORK Calculate the percentage loss that occurred during the recrystallisation. Where is this lost material? Demonstrator s Initials

AQA Chemistry A-level

AQA Chemistry A-level Required Practical 10 Preparation of a pure organic solid, test of its purity, and preparation of a pure organic liquid Reflux Reflux: continuous boiling and condensing of a reaction