Separation, Purification and Identification
120
Separation, Purification and Identification
120Paperback(BK&CD-ROM)
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Separation, Purification and Identification
By Lesley Smart
The Royal Society of Chemistry
Copyright © 2002 The Open UniversityAll rights reserved.
ISBN: 978-0-85404-685-0
CHAPTER 1
INTRODUCTION: PREPARATION OF A COMPOUND 1
Chemistry is a fundamental science that underpins much of the world around us. It is also a practical subject. Although much of what we have learnt so far may have seemed conceptual or theoretical in nature, the basis for it has all come about through centuries of experimental laboratory work performed originally by individuals in their own homes, but nowadays by chemists — technicians, undergraduates, postgraduates and advanced researchers. None of the chemistry that you have learnt so far would have been known without these skilled experimentalists.
The aim of this book is to introduce you to many of the skills and techniques that are required by the modern chemist, such as how to perform a reaction, how to purify the products and finally how to prove your results — that you have actually made what you set out to make. In the text we can only describe the various procedures, but you will be able to watch many of them on the associated CD-ROM.
The skills and techniques described here are generally applicable to the whole of chemistry, whether it be an organic or inorganic experiment. Therefore rather than subdividing the book on the basis of the different branches of chemistry, we have integrated the material as far as possible, using examples from all areas of modern chemistry.
1.1 Planning a reaction
Before chemists can perform a reaction, just as in any profession, they need to plan exactly what they are going to do. If you were to ask practising chemists, they would all agree that time spent in planning a reaction is time well spent, and invaluable to the success of the experiment.
What are the major points which you should consider when planning a reaction?
A list of most of the questions and points is given below.
• The scale of the reaction — how much product do you want to make?
• The mole ratios of the reactants; how much of each reactant to use?
• How expensive are the reagents? Are there cheaper alternatives?
• What is the most suitable solvent for the reaction?
• What temperature will be required?
• How long will the reaction take?
• Will you need to work under an inert and/or dry atmosphere?
• What equipment will be needed?
• Can the reaction be performed on the benchtop, or is a fume cupboard needed?
• What safety precautions will be necessary?
You also have to consider what you are trying to achieve during the reaction. Is the reaction probing some detailed reaction mechanism or is it preparatory — in other words, part of a long synthesis directed towards a desired product. An analytical chemist investigating a mechanism will have a very different set of priorities in planning a reaction compared to a synthetic chemist.
Chemists find that the careful keeping of a laboratory notebook is essential during their work. This involves carefully noting down everything that was done during an experiment from start to finish, recording relevant masses and other data such as temperature and timings, and noting all observations. If this is done in an orderly fashion, then it is very easy to draw conclusions from an experiment, to draw out data for a report or publication, to repeat the reaction, or simply to plan your next reaction.
An extract from a (rather idealized!) well-kept laboratory notebook should look something like Figure 1.1.
Notice the style and the various conventions that are used. The aim of the experiment and the equation for the reaction are set out clearly at the start, followed by the method and finally the results. A note is also made of any safety precautions necessary. Note that amounts of substances are placed in brackets after the compounds they refer to and are given in grams (or mls if the compound is a liquid) and also (preferably) in numbers of moles: this is conventional for formal reports and publications, so you may as well get used to it from the start.
Formal reports are always written in the past tense and the passive voice: '10 ml of water was added to the reaction' rather than 'I added 10 ml of water ...'.
A template for how you should write-up your experiment in your laboratory notebook is given in Figure 1.2 (overleaf). You may well see variations on this style elsewhere and there is nothing wrong with most of these. However, if you follow this general format, you will not go far wrong when writing-up experiments.
1.2 Assembling the apparatus: doing the reaction
Before we can consider doing a reaction, we need to learn something about the apparatus that is available to use. You may have encountered some chemical apparatus before, for example a test tube, beaker or conical flask or even a bunsen burner. These alone however are insufficient to perform most reactions. Over the years, chemists have developed specialized apparatus for performing chemical reactions. In particular, we have glassware which is capable of withstanding extreme high and low temperatures and corrosive substances, and which can be used to keep out air and moisture. This specialized glassware consists of a series of interlocking tapered ground-glass joints (Figure 1.3 overleaf), which permit various pieces of glassware and apparatus to be connected together without the need for rubber stoppers, corks or any sort of rubber tubing connectors (the joints only need to be lightly greased). Collectively, this apparatus is known as Quickfit® apparatus, due to the easy and rapid way in which the apparatus may be connected and assembled.
Illustrated in Figure 1.4 (overleaf) is a typical set of glassware and Quickfit glass apparatus which you might encounter in any modern laboratory, whether it be in a university or in industry. You should try and familiarize yourself with the names and shapes of each of these pieces of apparatus, so that when you come to follow an experimental procedure, you know exactly what apparatus you need to assemble.
Handling Quickfit apparatus is an acquired skill in its own right and it takes a while to be familiar with its use and capabilities. Quickfit apparatus comes in a variety of sizes, each perfectly adapted for large- or small-scale reactions. It is left to the experimentalist to decide which size of flask or funnel would be best for the particular reaction that is to be performed.
COMPUTER ACTMTY 1.1 Looking at Glassware
At some point in the near future you should watch the video entitled Looking at Glassware in the multimedia activity called Practical techniques on the Experimental techniques CD-ROM that accompanies this book. This activity demonstrates how to assemble various pieces of chemical apparatus and illustrates the advantages of the interlocking Quickfit style of glassware. This activity should take approximately 10minutes to complete .
Notice that when performing any experiment, normal laboratory safety procedures must always be followed, i.e. the wearing of protective clothing (usually a lab coat), safety spectacles, and gloves.
Under legislation known as COSHH (Control of Substances Hazardous to Health), a detailed risk assessment has to be made, documented and filed for every experiment performed. This may indicate that special safety precautions are deemed necessary, such as using a fume cupboard, or a face-mask. How to make these assessments is beyond the scope of this book.
Once we have assembled the apparatus , we can start the reaction. Part of a typical experimental procedure may read as follows:
'Place 2-methylpropan-2-ol (25 g; 0.34 mol) and concentrated hydrochloric acid (85 ml) in a 250 ml separating funnel and shake the mixture from time to time over 20 minutes.'
What exactly does this mean and how can we relate this to the apparatus you have just been learning about? From Figure 1.4, we can see what a separating (or separatory) funnel looks like. However, there are also certain assumptions in any given experimental procedure. For example, all apparatus should always be clamped securely (Figure l .5a) so that it does not drop or fall over, and you may have noticed this as you watched Looking at Glassware in Computer Activity 1.1. This is always assumed rather than stated, as an experienced chemist knows that a separating funnel or round-bottomed flask cannot stand on its own. If we were to write this experimental procedure out in full, it is actually telling you to:
• Put on laboratory coat, goggles and gloves.
• Clamp a 250 ml separating funnel securely and close the tap.
• Place 2-methylpropan-2-ol (25 g; 0.34 mol) and concentrated hydrochloric acid (85 ml) inside the separating funnel , pouring them in carefully from a measuring cylinder, using a funnel.
• Place a stopper in the separating funnel and shake the mixture from time to time for a period of 20 minutes. Between each shaking , invert the funnel carefully, holding the stopper tightly in place, and open the tap to release any excess pressure of gas. The reason for carrying out this last procedure, rather than the more obvious loosening of the stopper, is that if there is a pressure of gas inside the vessel, when you loosen the stopper it could blow hydrochloric acid fumes into your face. By inverting the funnel and releasing the gas through the tap, you can point it safely away from yourself. (Figure 1.5b).
* Based on your knowledge of the reactions of alcohols, write an equation for the reaction being performed in the experiment we have just described.
* The experiment described the reaction of 2-methylpropan-2-ol with concentrated hydrochloric acid, the product that we hope to obtain is 2-chloro-2-methy lpropane, via a nucleophilic substitution (SN1) reaction mechanism.
[FORMULA NOT REPRODUCIBLE IN ASCII] (1.1)
This was a fairly simple experimental procedure. Another is described below, for the preparation of carbonatotetraamminecobalt(III) nitrate from cobalt(II) nitrate, ammonia, ammonium carbonate and hydrogen peroxide, by the unbalanced equation
[FORMULA NOT REPRODUCIBLE IN ASCII]
'Dissolve (NH4)2CO3 (20 g; 0.21 mol) in distilled water (60 ml) and add concentrated aqueous ammonia (60 ml). While stirring, pour this solution into an aqueous solution of Co(NO3)2 (15 g; 0.052 mol, 30 ml of distilled water). Slowly add hydrogen peroxide (8 ml, 30% solution). Pour into an evaporating dish and concentrate to 90–100 ml over a bunsen burner (do not allow the solution to boil). During the evaporation time add (NH4)2CO3 in small portions (5 g; 0.05 mol).'
This reaction would be done in a fume cupboard because ammonia fumes are extremely pungent and lachrymatory (they make you cry). No special equipment is required and it is a case of making a sensible choice of vessels for the mixing and heating. A fuller explanation of what we would actually do in each step of the procedure is:
• Weigh the solid (NH4)2CO3 (20 g; 0.21 mol) using a top-loading balance and place in a 250 ml beaker.
• Measure 60 ml of water using a 100 ml measuring cylinder, add it to the beaker and stir with a glass rod to dissolve the solid.
• Measure 60 ml of cone. ammonia in the measuring cylinder and pour into the beaker carefully.
• Prepare the aqueous solution of Co(NO3)2 (15 g; 0.052 mol, in 30 ml of distilled water) similarly, in a small0 conical flask.
• Mount the conical flask on a magnetic stirrer, put in a magnet bar, and set the stirrer going.
• Add the ammonia solution from the beaker to the flask by pouring carefully either through a funnel or down a glass rod (Figure 1.6a).
• Measure the hydrogen peroxide (8 ml of a 30% solution) in a clean, dry 10 ml measuring cylinder and pour into the reaction mixture while maintaining stirring.
• Transfer the solution to a large evaporating dish which is supported over a bunsen burner using a tripod stand (Figure 1.6b).
• Heat very slowly and carefully to prevent spitting while gradually spooning in the previously weighed (NH4hC03 (5 g; 0.05 mol) using a spatula.
Both these experimental procedures are comparatively straightforward, since no precautions have to be taken to exclude air, moisture, heat or light. Unfortunately, this is rarely the case and more often than not, chemists have to take specific precautions to exclude at least one of these factors, most commonly air (particularly oxygen) or moisture (as water vapour in the air). Our next experimental procedure shows the precautions that must be taken when performing a moisture-sensitive reaction — in this case the preparation of the organometallic complex [{Fe(CO)2(η5-C5H5}2]
[FORMULA NOT REPRODUCIBLE IN ASCII] (1.2)
(Note that η5 (pronounced 'eta five') refers to the way in which the C5H5 ring is bonded to Fe.)
'This procedure must be carried out in a fume cupboard. Assemble the apparatus shown in Figure 1.7, and perform the reaction under an atmosphere of dry nitrogen. Add Fe(CO)5 (14.6 g, 10ml; 70.5 mmol) and dicyclopentadiene (60 g, 64 ml; 455 mmol) to the flask. Reduce the nitrogen flow and heat the reaction mixture under reflux to 135 °C for 8 to 10 hours. (It is important not to let the temperature go below 130 °C (as no reaction will occur) or above 140 °C (decomposition of the product will occur). After the reaction period, allow the mixture to cool slowly to room temperature.'
A fuller explanation of what we would actually do in each step in the procedure is:
• Assemble the apparatus as shown in Figure 1.7.
• Flush the system for 5 minutes with a rapid stream of nitrogen.
• With the nitrogen stream still flowing rapidly, remove the thermometer and add dicyclopentadiene (60 g, 64 ml, 455 mmol) to the round-bottomed 3-necked flask. To minimize your exposure to Fe(CO)5, use a syringe to measure out and introduce the Fe(CO)5 (14.6 g, 10 ml, 70.5 mmol) through the rubber septum into the flask. The constant stream of nitrogen will minimize air (which contains water vapour) entering the flask while the reactants are being added.
• Placing a reflux condenser between the flask and bubbler, turn the nitrogen flow down very low (one or two bubbles a minute). Using an oilbath, heat the reaction mixture under reflux to 135 °C for 8 to 10hours. 'Heating under reflux' means that you use a reflux condenser to prevent the volatile chemicals from escaping from the flask. The reflux condenser is cooled by circulating cold water; when hot vapours rise up through it, they meet the cold surface, condense and drip back into the reaction mixture. The reaction temperature cannot rise above the boiling temperature of the solvent (you will see this demonstrated in Computer Activity 2.3).
• Carefully adjust the thermostatic control to maintain steady boiling, checking the temperature remains between 130 °C and 140 °C.
• After the reaction period, allow the mixture to cool slowly to room temperature, increasing the nitrogen flow slightly. (The nitrogen flow will prevent air from being drawn into the reaction vessel as it cools.)
We can immediately see that such a procedure is going to take a lot more time and will also require a great deal more care and skill from the experimentalist.
Practical chemistry is a manual skill in much the same way as cookery, woodwork or embroidery. It takes time and practice to learn and develop the right skills to be able to perform a reaction or synthesis with confidence.
Simply doing a reaction is not the end of the story; it is really just the beginning of a long process as we will see in the following sections.
1.3 Summary of Section 1
1 Chemistry is a practical subject, requiring specialist apparatus to perform most chemical reactions.
2 Chemists have a unique style of describing and writing-up experiments.
3 A risk assessment must always be made before performing any experiment.
4 It is sometimes necessary to perform reactions under a dry, inert gaseous atmosphere, to exclude all traces of moisture and oxygen.
QUESTION 1.1
The following is taken from a student's badly written laboratory notebook. Can you spot the mistakes and rewrite it in a proper scientific style?
'The three chemicals were put in a flask with a white plastic bar. A change had happened after 35 minutes, so I stopped the reaction and then added solvents and separated them. I evaporated one layer to give the product. The reaction was done under dry conditions. The starting material was a white solid and the product a yellow oil, which I got lots of.'
(Continues...)
Excerpted from Separation, Purification and Identification by Lesley Smart. Copyright © 2002 The Open University. Excerpted by permission of The Royal Society of Chemistry.
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