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...
