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. As the bar magnet revolves beneath the stirring vessel, the followers are induced to move with the magnet and so agitate the mixture in the stirring vessel. The followers are removed with forceps or with another encased magnet having a handle.
Magnetic follower
I a I Bar magnet
Fig.8.4: Magnetic Stirrer,
This apparatus is used in closed systems where: (a) Gas volume changes have to be observed, (b) Air has to be excluded to prevent oxidation, or (c' An anhydrous environment is required and in open systems as well.
It is also used in containers where the introduction of a stirring rod is difficult. Combined hot platelmagnetic stirrers are also commercially available,
(3) Hershberg Stirrer This is a kind of stirring rod (see Fig.8.5) and it consists of a hollow glass tube sealed with a glass ring. The ring is threaded with 1 mm diameter wire, e.g. chromel, nichrome or tantalum, as illustrated. The stirrer is easily passed through narrow necks, and the wire paddle closely follows the contour of the
Fig.8.5: Hershberg Stirrer. reaction vessel, e.g. a round-bottomed flask. It is therefore useful for stirring The Preparative Techniques pastes and solids which cling to the stirring vessel wails.
(4) Mercury Sealed Stirrer These stirrers are used for: . (a) Simultaneous stirring and refluxing of a mixture. (b) Stirring in a sealed vessel; (c) Agitation without escape of gas or vapour; and (d) Stirring in an inert atmosphere.
Fig.8.6 shows how mercury can be incorporated between an outer tube and an inner bearing tube which contains the stirring rod. In this way, a seal is formed which prevents the passage of gas fumes to or from the liquid being stirred. Bearing points are lubricated with glycerol or silicone grease.
Fig.8.6: Mercury Seated-Stirrer.
Now try the following SAQ.
SAQ 2 Write down Two methods of maintaining even and gentle boiling when using a direct burner. (i) ...... (ii) ......
You can check your answers with ours at the end of the unit.
8.3 APPARATUS WITH INTERCHANGEABLE GROUND GLASS JOINTS (QUICKFIT)
Now-a-days corks and rubber stoppers can be entirely dispensed with for joints and connections in glass apparatus assemblies. This is due to the availability of a wide range of glassware of standard shapes and sizes which rely on ground glass joints. In sub.-Sec. 1.4.1 of Unit 1 of Block 1, you have already seen a range of standard components, which can be used to built assemblies for most of the routine lab needs. Laboratory Techniques I1 One of the best-known brands of this type of apparatus is manufactured by Borosil although other manufacturers also market similar apparatus. The apparatus with ground glass joints have the following main advantages:
(1) Corrosive substances are easily manipulated without introducing impurities from the apparatus, unless the glass itself is attacked by such substances. (2) Accurate alignment of apparatus is easier. (3) Joints fit well, provided the apparatus is correctly aligned and the joints Remember that the B series of are clean prior to greasing. joints is most commonly used (4.) Rapid assembly of component parts. for general work and joints are (5) With standard part sizes, any broken parts are easily replaced. described as '814123' (6) Most common operations of organic chemistry can be carried out in - where 14 = outside diameter of apparatus built from standard components. the small end of the cone in mm (7) Wider passages are possible at joints which decreases the danger in violent and 23 = length of the ground reactions and diminishes the likelihood of choking. zone in mm. Although this kind of apparatus is quick and easy to use, you must bear the following precautions in mind:
(1) All ground glass surfaces must be clean. (2) Apparatus must be adequately supported by clamps and stands. The usual precautions listed in IJnit apply with regard to alignment, cushioning and over-tightening of clamps. (3) Joints should be lightly lubricated with silicone grease where (a) Reduced pressure is to be used, (b) High temperature distillation is to be carried out, or (c) Alkalis are in use. (4) All joints must be cleaned after use.
8.4 REFLUXING Many organic reactions occur slowly at room temperature-usually too slowly for our purposes. In order to speed up such a reaction, we apply heat to the reactants to raise their temperature, An adequate reaction rate is often achieved by elevating the temperature of the mixture to boiling point and sometimes this temperature has to be maintained for an appreciable time. Under these conditions, the volatility of the mixture can cause considerable problems due to evaporation of solvent and the flammability of escaped vapour.
To overcome these problems, it is usual to heat the reactants under reflux. Typical apparatus for this is shown in Fig.8.7 (a). A flask is filled not more than half its capacity with the reaction mixture and some anti-bumping material is added for uniform boiling. A water condenser is then fitted vertically in the neck of the reaction flask and heat is applied to the flask, using any of the usual techniques.
The temperature of the mixture is raised to boiling point ,and the mixture is allowed to simrnet. Vapour escaping from the mixture condenses in the condenser and drains back down into the flask. This apparatus is suitahle for mixtures which have a boiling point below 130°C. Above this temperature, an air condenser as shown in Fig. 8.7(b), is used. In case an air condenser is not available, a normal water condenser, without using water, can be used. Also remember to use as guard (drying) tube when it is necessary to cany out the The PreparaUve Techniques reaction in anhydrous conditions.
glasswool
Fig.8.7: Reflux Heating Apparatus: (a) Using a Water Condenser and (b) Using Air Condenser and a Guard Tube. Now try the following SAQ.
SAQ 3
Is the reflux process designed to (tick the correct answer):
(i) Recycle impure solvents; (ii) Enable heating of flammable solvents with a direct burner; or (iii) Enable heating of liquids without loss of vapour.
You can check your answers with ours at the end of the unit before continuing.
15 I hborrtory Techniques 11 8.5 CONTROLLED ADDITION OF-A REAGENT
In this section, we will show you two examples of Quickfit apparatus assemblies that allow controlled addition of a reagent. In each assembly, the common components are a Liebig condenser and a cylindrical separatory funnel.
8.5.1 Reflux Heating with Controlled Addition
Fig. 8.8 shown below gives the arrangement for heating a homogeneous reaction mixture under reflux with controlled addition of a liquid. A two way adapter has been.used with the flask to fit the condenser and the separatory funnel. But you can also use a two-necked flask instead of the too way adapter and straight away fit the condenser and the separatory funnel in the two joints of the two-necked flask. The mode of heating and use of the guard tube would depend upon the nature of the reactants. Similarly, the nature of condenser (air or water) would depend upon the refluxing temperature. Note that here the process of' boiling would ensure the mixing of the contents of the reaction mixture
Condenser Addition funnel
Magnetic stirrer bar
Fig. 8.8: Apparatus Assembly for Reflux Heating with Controlled Addition. 8.5.2 Reflux Heating with Controlled Addition and Stirring The Preparative Techniques
In case, the reaction involves a solid component or immiscible liquids, then stirring is also required along with refluxing. For this purpose, we can use the following set-up of apparatus, as shown in Fig. 8.9. Note that a three-necked flask has been used to accommodate the stirrer. Alternatively, a magnetic stirrer with hot plate can be used alongwith the two-necked flask for fitting the condenser and the separatory funnel.
Condenser
Fig.8.9: Apparatus Assembly for a Reflux Heating, Stirring end the Controlled Addition of a Reagent.
SAQ 4
Which of the following pieces of apparatus is essential in any assembly for an experiment which include the controlled addition of a reagent? Tick the correct answer.
(i) A Liebig condenser (ii) A separatory funnel (iii) A thermometer
Check your answer with ours at the end of the unit before continuing. 8.6 SUMMARY In this unit, you have learnt various methods of heating and agitation used in preparative processes. the advantages of using apparatus with interchangeable ground glass joints. about the situations when the reflux process is used. the use of apparatus necessary for controlled addition of a reagent. 8.7 TERMINAL QUESTIONS 1. What are the advantages and disadvantages of a sand bath ? I 2. (a) Write three advantages of using an apparatus with interchangeable ground glass joints. (b) What precautions would you take while using such an apparatus ?
3. Would you say that the reflux process is usually carried out in an open or a closed apparatus assembly? (Tick the right answer and give reason for your answer.) '. open Closed : Reason:
8.8 ANSWERS % Self Assessment Questions
1. (i) An electrically heated water bath, a hot plate or a heating mantle. Your choice between these alternatives will be governed by the length of time for which boiling is to be maintained (ii)An electrically heated oil bath, a hot plate or a heating mantle. Revise Sec.8.2.1 if you wrote down the wrong technique. Remember that you must not use a direct burner with flammable liquids. 2. Any of the following techniques are commonly used. (i) Addition of anti-bump material to the liquid. (ii) Use of a wire gauze on a tripod. - *_ (iii) Use of a heat resistant sheet on a tripod. You would have written down methods (i) and (ii). If you didn't ,revise sub-Sec.8.2.1.
3. The correct answer is (iii). The reflux process enables you to heat liquid; without loss of vapour by condensing the vapour and draining the condensate back to the heating vessel. Revise Sec.8.4 if your answer was wrong. 4. The correct answer is (2). A separatory funnel is essential since its The Preparative Techniques stopcok arrangement allows a high degree of control when dispensing liquid. Revise Sec.8.5 if your answer was wrong.
Terminal Questions
1. Advantages : (i) Heating is more uniform than with a wire guaze. (ii) Can be used when high temperatures are required.
Disadvantages: (i) Sand takes long time to heat up and cool down.
(a)(i) Corrosive substances can be easily manipulated. (ii) Rapid assembly of component parts. (iii) Broken components can be changed easily. (b)(i) All glass surfaces must be clean before and after use. (ii) Apparatus should be supported by clamps. (iii) Joints should be lubricated with silicone grease. i
Open. The vapour is allowed to enter a condenser with both ends open. The apparatus does not have to be closed in order to contain the vapour. Revise Sec.8.3 if your answer was wrong.