Unit 8 the Preparative Techniques

UNIT 8 THE PREPARATIVE TECHNIQUES 8.1 Introduction . Objeaives 8.2 Heating and Agitation Heating .%gitatian 8.3 Apparatus with Interchangeable Ground Glass Joints (Quickfit) 8.4 Refluxing 8.5 Controlled Addition of a Reagent Reflw Heating with Controlled Addition Reflw Heating with controlled Addition and Stirring 8.6 Summary 8.7 Terminal Questions 8.8 Answers 8.1 INTRODUCTION The quality and diversity of our life today, as opposed to man3 experiences of two thousand years ago, stems largely from our ability to modify the environment in which we exist. The importance of chemistry in this advance is crucial, as this science offers the possibility of bringing about change at a molecular level. Either in the laboratory or on a much larger industrial scale, chemical changes can be engineered to produce, under controlled conditions, tailor-made substances that underpin social, medical and economic progress. The materials which are required for the synthesis of drugs, fertilizers or polymers necessary to support our late-twentieth century life style, are rarely in the finished form. The processes of extraction, separation, preparation and finally purification that is necessary are often costly and complex. In this unit and other units of this block, we will examine how some of these techniques can be carried out in the laboratory. Objectives After studying this unit, you should be able to @ describe methods of heating and agitation used during the preparative processes, @ discuss the advantages of using an apparatus with interchangeable pund glass joints, @ explain the reflux process, and @ describe the apparatus necessary for controlled addition of a reagent. 8.2 HEATING AND AGITATION In chemical shorthand, a chemist could reduce a given process to a convenient equation such as: L.bwatory Techniques II This tells us that two substances, A and B, after being mixed for a period of time under suitable conditions, are chemically changed to two new substances, . C and D. Thus the formation of the strongly smelling organic chemical, ethyl ethanoate (ethyl acetate) could be written: CH3COOH + CH3CH20H +CH3COOCH2CH3 + Hz0 (ethanoic acid) (ethanol) (ethyl ethanoate) (water) The above equation tells us that if we mix equimolar amounts of ethanoic acid and ethanol, then ethyl ethanoate and water are formed. This would of course be an unnecessary and expensive way to prepare water. However, ethyl ethanoate is a fairly useful chemical as it is used as a solvent for lacquers. What the equation does not tell us is anything about the conditions of this reaction. Is it necessary, for instance, to apply any heat? Should we stir or agitate the reaction vessel? Does the reaction take place more quickly if a suitable catalyst is used? In addition the reaction could be reversible. In other words, will the products when formed tend to react together to re-form the original reactants, so that a mixture of ethanoic acid, ethanol, ethyl ethanoate and water will be present? This reaction is indeed reversible and is therefore more correctly written as follows: CH3COOH + CH3CH20H + CH3COOCH2CH3 + H20 Here onwards, we will be concerned more with the techniques that may be used to achieve this reaction. 8.2.1 Heating You can refer back to Sec. 1.3 of Unit 1, Block 1 where the apparatus used for heating has been discussed. You will find that a variety of techniques are available to you for heating in the lab. The technique which you select will depend on the following factors: (1) The apparatus to be heated; (2) The substance to be heated; (3) The temperature range required; and (4) The period of heating. The following are the more usual techniques used in, for example, a typical educational laboratory. (1) Direct Burner Typically this will be a hand-held Bunsen burner. It is used in a simple distillation where a flask is heated directly. Heating with direct burner requires a careful control as very often the flammable liquids are involved. Also many organic compounds are unstable at high temperatures which can result from localized overheating. '(2) Burner With Tripod and Gauze It involves a bench top burner placed beneath a metal tripod which supports a wire gauze. The gauze spreads the heat of the burner flame heating an object above the gauze more evenly than the direct flame. This technique is used for most experiments, distillations, and-particularlyfor reflux work (see Sec.8.4). (3) Air Bath The Preparative Techniques This method is useful for destructive distillation and where direct heating of solid matter leads to extensive charring. It is also suitable for any application which requires quick cooling, good temperature control, cheap and simple apparatus, and inspection of the contents. (4) Sand Bath Heating using a sand bath is more uniform than with using a gauze, but temperature control is difficult since sand is a bad conductor and takes a long time to heat up and cool down. This is not a routine lab technique, but it may be used where high temperatures are required. (5) Water Bath 'This technique is primarily used for liquids with a low boiling point. Electrically heated baths are used for flammable liquids. The temperature range is ambient to 85" C. (6) Oil Bath Oil baths operate on exactly the same principle as the water bath and may be heated by a burner or electrically. Usually oil baths are set up in a fume cupboard to remove any toxic fumes generated by the high temperatures used. The paraffin is odourless A thermometer must always be used to avoid excessive heating. The and colourless when clean. apparatus being heated is always immersed in the oil which is generally the paraffin. This technique is used where a temperature range higher than that available with a water bath is required, and where substances need to be held at an elevated temperature for a substantial time. The temperature range is 80- 250°C (max.) depending on the type of oil used. (7) Hot Plate A hot plate for laboratory use consists of a heavy iron plate which contains an electric heating element . It can be free standing, for mounting on a retort stand, or mounted on its own base incorporating temperature controls and a thermostat. Hot plates are used where a naked flame is unsuitabIe. (8) Heating Mantle Heating mantles are designed to fit snugly flasks which are of a nominal capacity, typically in the range 10 cm3 to 2 dm3. A temperature control and a thermostat are common features of many heating mantles. Heating mantles are useful for flammable liquids with a low boiling point and where even heating is required for long periods. The temperature range is ambient to 450°C. Combined heating mantles/mechanical stirrers (see sub.-Sec.8.2.2) are also commercially available. ACTIVITY 1 Compile a database of types of heating apparatus in use in your laboratory. Include as much relevant information as you can: such as heating range, what is normally heated on it, capacities, etc. Laboratory Techniques I1 Now try the following SAQ. SAQ 1 List the heating techniques you would use to boil (i) a flammable mixture with a boiling point of 73" C. (ii) a flammable mixture with a boiling point of 120' C. ........................................................................................ 8.2.2 Agitation Normally homogeneous solutions do not require agitation, but in some cases e.g. when a substance has to be added in portions or drops and immediately brought into contact with the entire solution, you will have to use an agitation technique. This is particularly important when a precipitate is formed rnc! absorption may occur, or locally generated heat may cause decomposition of a sensitive preparation. For heterogeneous mixtures, agitation is a must. You can shorten reaction times, accurately control temperature and increase yields in this way. There are two basic methods of mechanical agitation-shaking and stirring. Shaking is quick, cheap and easy, and it may be a more appropriate technique for small quantities of homogeileous solutions. Mechanical stirring is undoubtedly more efficient but you need to set up special apparatus for this. There are several kinds of apparatus used for stirring. The main kinds of stirring apparatus are as follows. (1) Geireral Purpose Mechanical Stirrer lnduction n controller Fig.8.1: General Purpose Mechanical Stirrer. : P This consists of a small electric motor, usually an induction motor which The Preparative Techniques avoids the ignition hazard of sparking brushes, with a drive connected to a chuck (see Fig.8.1). The chuck can accommodate a variety of stimg rods made of glass rod, teflon or stainless steel (see Fig.8.2). Teflon stirrers have the advantage that they do not.break themselves or the reaction flask. Fig.8.2: The Stirring Rods. The stirrer is mains powered and can be mounted on a retort stand over the stirring vessel which may be closed or open. Stirring speeds can be controlled by a rheostat control on the stirrer. The more usual lab stirrers, with a nominal power output of about 6 W, are used with low viscosity liquids, e. aqueous solutions and light oils and can handle volumes up to about 60 dm5 . Viscous liquids and suspensions require more powerful stirrers, which may have a twin drive to accommodate two stirring rods (see Fig.8.3). -Heavy Duty Stirrer. Fig.8.3 :Heavy Duty Stirrer. Laboratory Techniques I1 (2) Magnetic Stirrer This instrument dispenses with the need for stirring rods as depicted in Fig.8.3. It operates by revolving a bar magnet horizontally (see Fig.8.4) in a container on which stands the reaction vessel. One or more magnetic 'fdllowers' (needles) made of a magnetic slug encased in glass, PTFE or ~lypropylene, are placed in the stirring vessel.

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