figure 1. The Lancaster kit
Chemical Education Journal (CEJ), Vol. 7, No. 2 /Registration No.7-17/Received September 30, 2003.
Department of Biological Sciences, Faraday Building, Lancaster University, Lancaster, LA1 4YB, U.K.
E-mail: s.breuer@lancaster.ac.uk
Introduction
Organic chemistry
Scaling down
The Lancaster contribution
Preparative experiments
Engaging the mind
Getting help
Microscale or not microscale?
References
Introduction
Laboratory work has always occupied
a central position in the teaching of chemistry. With the growing
numbers of students participating in higher education its continuation
has come under increasing scrutiny from administrators and controllers
of budgets. At the same time the growing attention paid to the
aims, processes and outcomes of teaching in general caused academics
themselves to examine the practice of laboratory work to answer
the usual set of five questions:
Why are we doing it?
How are we doing it?
Why are we doing it that way?
How do we know it is the best way?
How do we know it works?
In order to protect in our courses an
activity that is expensive, time-consuming and potentially hazardous,
we need to be sure that it has real value and benefits that are
not obtainable any other way. [1] There is much information in the literature to
guide our thinking. Johnstone and Al-Shuaili [2] recently reviewed
the literature dealing with the purposes, strategies and assessment
of laboratory work. The aims of practical work have been under
examination for a long time. Kerr produced a major report on this
in 1963; [3] although this dealt with the matter in the context
of secondary education, the issues at the tertiary level are much
the same, as shown by the conclusions of authors examining university
courses. [4], [5] Thus, the principal aims of laboratory courses
were identified [6] as needing to encourage students to gain
* manipulative skills
* observational skills
* the ability to interpret experimental data
* the ability to plan experiments
Laboratory instruction styles have also
come under scrutiny [7] according to whether the outcome, approach and
procedure are pre-determined or not. It is recognised that in
both the UK [3] and US [7] educational systems the large majority of chemistry
laboratory work is of a kind where all three of these descriptors
are pre-determined: the instructor defines the topic, selects
the method by which it will be approached and specifies the expected
outcome. This type of exercise is classified [6] as 'expository instruction'
and it has come in for much criticism [4],
[5], [8], [9] on the grounds that
* it places little emphasis on thinking.
* its prescriptive nature emphasises the following of specific procedures.
* it gives no practice to students the planning of an experiment.
* it is unrealistic in its portrayal of the processes of science.
In its defence it can be said that it can deliver a laboratory experience to a large number of students in a limited amount of time, the instructor knows that the experiments (exercises) work and are safe, all the equipment and materials are ready and available as and when required, the teaching assistants have been prepared so that they can offer sound advice to students who need them, and there is a very good chance that the students will get the positive reinforcement from an outcome in accord with the expectations. In addition, with a good set of pre- and post-lab exercises [10] they can provide considerable intellectual as well as manual involvement. While it is undesirable that a student's entire laboratory experience should be of this kind, at the early stages of a course of study when the basic techniques and principles are first encountered, it is important not to ask too much too early from the learners. Overall it is important that students' laboratory experience should meet all the aims listed above, but it is unrealistic to expect that every single experiment or even every single laboratory course should address every aim on the list.
Organic chemistry
In organic chemistry the earliest
laboratory experience usually involves an introduction to techniques
of purification, separation and characterisation by physical methods.
This is then followed by preparative exercises in which the basic
techniques are used for the purpose of making new compounds. Such
a course will have the objective of introducing students to very
simple laboratory operations: measurement and material transfer,
distillation, recrystallisation, the characterisation of materials
by m.p., b.p. and spectroscopy, to look for the relationship between
chemical changes and their physical manifestations, to learn to
follow the progress of a reaction by chromatography, to give some
thought to experimental design and to learn to write down what
they see as completely and as accurately as possible.
Such exercises are typical examples of the expository style and yet they can be of significant educational value. At the very least they give students valuable practice in manipulative skills and the satisfaction of having prepared a material with properties in agreement with expectations. (This is sometimes dismissed as 'cookbook stuff', but cookbooks have their defenders [11] when the successful production of a desired material (chemical or dish) is important.) At best such preparative exercises can help to show students what organic chemistry is about: to gain an understanding about materials, the relationship between their chemical structures and their properties, their interconversions and about the nature of our knowledge about these things.
From any group of students only a minority will ever become practising chemists, so a laboratory course must do much more than prepare students for a life at the bench. There are some fundamental ideas about chemistry that will also be of use to those who will not be practitioners, while benefiting those who will be. A well-chosen set of preparations, particularly when accompanied by suitable pre- and post-lab exercises, [10] can also help students' thinking in trying to answer questions, such as:
Why are some materials solids and some liquids?
Why are some materials soluble in water and some in organic solvents?
How can we tell how long a reaction takes?
How can we tell whether the reaction has worked?
Why does the same reaction go faster with one compound than with
another?
How do we know whether we are dealing with a pure material or
a mixture?
How can we get information about the structure of a compound?
Scaling down
In the mid-1980s a new development
entered the practice of teaching preparative organic chemistry.
Mayo, Pike and Butcher published their pioneering book [12] in
which they set out the arguments for reducing the scale of the
preparations carried out by undergraduates to15-150 mg of starting
materials. They called this approach 'microscale' [13] and
they developed specialised apparatus, techniques and experiments
to support this approach.
Microscale preparations have been familiar to many organic chemists over the years, particularly to those engaged in natural product chemistry and in the synthesis of isotopically labelled compounds. What was groundbreaking in this approach was the proposition that inexperienced undergraduates can also carry out such preparations successfully. This generated much incredulity amongst those who watched students struggling and failing to produce the desired compounds, having started with 5 g or more of starting materials. Nevertheless, it became clear, that with the right preparation and the right equipment students can handle such amounts successfully and bring such preparative efforts to satisfactory conclusions. One of the many advantages of the microscale approach is that preparations are much faster, so if something goes wrong, there is often time to have a second attempt within the same laboratory session.
The first book was soon followed by another [14] and this also introduced specialised equipment, appropriate techniques and a wide range of experiments that can be carried out using these. Between them these two approaches began to be adopted in the US and now about two thirds of universities and colleges there run at least some of their organic chemistry practical classes in microscale mode.
The Lancaster contribution
We at Lancaster became aware of
this development in the late 1980s and recognised the force of
the arguments for microscale based of increased safety, speed,
economy and the reduction of chemical waste. One problem remained,
the cost in the UK of the recommended glassware. To address this,
I attempted to devise simpler and therefore cheaper equipment
that was still effective for the carrying out of all the basic
experimental operations:
a. Measurement and material transfer
b. Carrying out reactions
c. Purification of solids
d. Purification of liquids
e. Partition and extraction
This was achieved by a combination of commercially available equipment and the aid of the technical staff in the department, and the result is shown in Figure 1. It comprises four pieces of equipment with ground-glass joints, a reaction vessel, capacity 3 mL, a Hickman still, a Liebig condenser and a piece that can serve as a vacuum take-off or a drying tube. In addition there is a home-made magnetic follower, a test-tube with a side arm, a Hirsch funnel with a rubber washer, a dropping pipette with a short, wide tip, a plain test tube, two specimen tubes, 4 and 6 mL capacity respectively and a wooden block, to support the various pieces.
figure 1. The Lancaster kit
These are the individual kits issued to students. In addition there are available in the laboratory boxes of Pasteur pipettes, top-loading balances weighing to an accuracy of 1 mg or less, electric hotplates with sand-baths for heating, water pumps and rolls of PTFE tape. With this equipment students can carry out all the standard handling operations as well as a wide range of preparative experiments.
To carry out reactions in many cases the
simplest of equipment is satisfactory; one can do quite sophisticated
chemistry in a test tube, specimen tube or simple flask although
there are reactions where the need for a reflux condenser, drying
tube or an inert atmosphere will demand more elaborate equipment.
When standard taper joints are used they must not be greased,
else the grease may contaminate the small amounts of product;
instead we find that a short length of PTFE tape smoothed on the
joint before attaching the two pieces provides an air-tight, firm
seal that never sticks.
Stirring of reaction mixtures is best done magnetically. There are commercially available spin-vanes to fit the flasks, but in our experience they are often too small to achieve effective stirring of viscous media and are too easily lost down the sink during washing up. It is easier and much cheaper to make oneself larger ones from commercially available small stirrer bars and home made PTFE spacers.
The purification of solids is done mainly by recrystallisation, usually in a specimen tube or test tube, followed by filtration. This is best done under suction on a Hirsch funnel with a plate diameter of less than 10 mm and we find that the transfer of crystalline solids suspended in a mother-liquor is efficiently done with a Pasteur pipette with a very short, wide stem (internal diameter about 2 mm; Figure 2) which avoids it being blocked up by the solid. This technique has the advantage of versatility in terms of the scale: volumes of liquid to be used, amounts of material to be transferred; it works equally as well on 10 mg or less as on a gram or more.
The purification of liquids must be done
by distillation; with care, amounts of 100 mg or more can be distilled
efficiently. The decisive consideration is that the vapours should
have to travel as short a distance as possible from the distilling
flask to the collector so as to minimise the loss caused by the
wetting of the equipment surface. For this reason a small, short,
wide distilling flask fitted with a Hickman type of still is best
(Figure 3); in these even inexperienced workers can distil 2-300
mg amounts with good recovery. In tests amounts of about 0.4 mL
could be distilled with 80% recovery or better both at atmospheric
and at reduced pressure.
Partition and extraction can be carried out very easily in small specimen tubes or test tubes. The process of mixing the phases is done by repeatedly taking up one or both into a Pasteur pipette and squirting it back into the tube. When the phases in the tube have separated, either the top or the bottom layer can be removed with a pipette. [12]
Preparative experiments
The putting together of this equipment
was followed by the development of suitable preparative experiments
that could be carried out using it. Some fifty were devised, illustrating
the preparation and reactions of all the main functional groups
up to and including the synthesis of heterocyclic compounds, involving
techniques from simple transformations to the use of organometallics
in an inert atmosphere. These were accompanied by exercises introducing
the basic techniques. A chapter describing the main manipulative
techniques, the techniques exercises and the preparative experiments
were published as a book. [15] Although in these preparations yields are not
given, other (unpublished) work by the author showed that, in
experienced hands, yields in microscale preparations were very
similar to those carried out on larger scale. The reason for not
emphasising yields in microscale exercises for students is that
the purpose is to demonstrate that the preparation/reaction works
and that products with the expected physical and spectroscopic
properties can be obtained. If they get enough for m.p./b.p. and
an infrared spectrum, it counts as success.
Engaging the mind
For a laboratory class to make
a real contribution to the teaching of chemistry, students' minds
have to be prepared so that they have a better chance of knowing
ahead of time what they are doing and what they are supposed to
get out of it, during the experiment they should know what is
the purpose of the various actions they are carrying out, and
at the end they should be required to reflect on what they did,
why they did it, why they did it that way and what was going on
at the molecular level when they observed some visible changes.
It also helps their appreciation of the everyday context of chemistry
if their attention is drawn to links between what they do and
the world of their everyday experience. The means of achieving
this is by the use of pre- and post-labs[10] and the clear
labelling of the various parts of the operating procedure. This
approach can be illustrated by one of our experiments, done by
our first year students in the third week of their course.
The preparation is that of 2-hexanol by the hydration of 1-hexene. Before they come to class they have to answer the following questions.
Pre-lab
1. You have samples of salt, sugar and butter and as solvents, bottles of water and ethyl acetate (ethanoate), used in the first week's experiments. Which would you expect to dissolve in which solvent and why? Give your reasons.
2. In the preparation below, you are starting with 0.7 ml of 1-hexene (density: 0.673). What is the mass of this 1-hexene and how much 2-hexanol would you get if you had a 100% yield?
At the start of the class their answers are checked to see if they have done them. Producing answers is a precondition of starting the lab work, but the quality of the answers is marked at a later stage, alongside the experiment itself. In other words, they don't have to have them 'right' but they must have made a serious attempt.
The lab script includes the following.
What I hope you will get out of this session.
1. To note that chemical reactions are often accompanied by observable changes.
2. That the solubility of a compound in a particular solvent or solvent pair is determined by the structures of both the solvent(s) and the compound.
This is followed by an introductory paragraph
describing the chemistry involved, a page reference to their organic
chemistry text, and tabulated hazard data on the chemicals they
use. The preparative procedure (see Box) is broken up into sections,
identified as preparation, work-up and purification and characterisation,
to help students in identifying the exact point in the process
they have reached. It doesn't guarantee that they will know what
they are doing but increases its chances. In the script the word
'VIDEO' appears in a number of places. This refers to an additional
form of help that is available, to be discussed in a later section.
 Reaction Work-up and isolation of the product What observations tell you that chemical changes have taken place? What chemical changes do they indicate? *Note. Sometimes, when after the addition of the water the mixture has been insufficiently heated, the curious phenomenon of three immiscible layers is observed at this point. If this happens, pipette off and keep the pentane layer, boil the remaining two layers for a further five minutes and return the pentane to the cooled mixture. At this point there should only be two layers and the work-up continued. |
Once the students have completed their preparative work they have a number of questions to answer. These are marked and this makes a contribution to their assessment.
What do you think is the reaction taking place when you boil the water solution of the initial product?
Why do you think pentane (rather than hexane, heptane, octane or methanol, ethanol, etc.) is used in the isolation of the 2-hexanol?
What do you think is the purpose of washing the pentane solution of the product with potassium carbonate solution? What might happen if you don't do it?
When three layers are formed, what do you think the third layer might be? What would prevent a compound being soluble in either pentane or water?
Why do you have to be careful when boiling off the pentane?
How else could you prepare this product?
Getting help
Even with the best-prepared and
written lab scripts students need help. This is generally provided
by the instructor present and by post-graduate students acting
as teaching assistants or demonstrators. A further and very valuable
addition to the range of human help has been the development of
a series of nineteen programmes16 describing all the main laboratory
manipulative techniques. We have them in the form of laser disks
(they show the best quality pictures), but they are available
as VHS tapes or CD-ROMs as well. Three of these deal with microscale
techniques and at the beginning of the organic lab course our
students are shown the relevant parts of these programmes as an
introduction to the techniques. The objective is not to expect
the students to remember in detail what they have seen, but to
be aware of what is available and how to access it when they need
it.
The video disk player is available in our labs and the scripts (and staff) encourage the students to look at the relevant part when they encounter a technique that is new to them or whose details they don't quite remember. This is a very popular tool, because the video disk player has infinite patience, doesn't mind going through the same explanation again and again, it shows very clearly what needs to be done or is expected to be seen, and it always "gets it right", something student demonstrators, being human, sometimes don't.
Microscale or not microscale?
At the time of writing about two
thirds of US universities and colleges and about one third of
UK universities use microscale experiments to some extent. It
has been considered in other places too, but some people have
reservations about the approach, and this is often expressed in
particular comments that seem to come up with remarkable regularity.
"Our students are not skilful enough." This is almost certainly untrue. In our experience complete beginners have carried out these preparations successfully. This is true not only of university students, but also of secondary school students for whom this is their first encounter with preparative chemistry. The barrier is the lack of belief of teachers in the capabilities of their students.
"This way students don't meet standard equipment." This would be true if there were any institutions where all experimental work was carried out on microscale. I am not aware of any following such a policy. In any case, we need to distinguish between the various objectives for practical courses at different times of a student's career. In the first, introductory course what students need is the principles of the basic operations. A distillation is a purification method in which an impure liquid is evaporated in one place and condensed in a purer form in another. It doesn't absolutely require the use of a still head, a Liebig condenser and a receiver adaptor, as well as the two flasks to qualify. For the majority of students, who are not progressing towards a career as a professional chemist, it is quite unimportant that they should know what a Claisen adapter looks like. In the second year and later, students (in smaller classes) do meet the standard equipment and they do learn what they are and how to use them when working with larger amounts of materials. Moreover, the experiments and laboratory tasks they get then will also require them to be more independent in designing their experiments and evaluating their results.
"This is inappropriate training for research and employment; industry doesn't work on microscale." This is only partly true. Some industry does work on a much larger scale than any likely undergraduate experiment. On the other hand in pharmaceutical research the scale of operation is often very small indeed. As far as academic research is concerned, a look at a journal, such as Tetrahedron Letters, will quickly show that a lot of papers describe synthetic work done on less than a millimole and for many organic compounds this falls into the microscale range. In any case, even if the comment were wholly true, it would be irrelevant. Undergraduate courses are generally not designed as specific training programmes for particular types of industry. We try to teach general principles and to provide solid foundations on which a wide variety of professional careers can be built.
"Students like to see something in their hands for their efforts, not just a drop of liquid or a crystal or two." This is also true and has been said by more than one student. On the other hand, others have spoken about their sense of achievement in mastering the skills necessary to isolate the right product in reasonable quantities when working on a small scale. Any approach will please some and displease others.
"The equipment is expensive." It needn't be. Our experience is that with some thought and planning one can devise a wide range of chemical experiences using the simplest of equipment. Much interesting chemistry can be done in a test tube! The kit devised in Lancaster can take us a very long way and anyone with access to a glass blower and a workshop can produce all the necessary equipment. The equipment has the advantage that, being small, it is more robust and will survive better than larger pieces. Even if the introduction of the scheme requires a significant investment, the cost of replacing broken items is much less than with standard glassware.
"Students may come to believe that all chemistry is safe." It is up to us to make it clear to students what the hazards are without letting them come to any harm. It would be extremely difficult and dangerous to organise lab accidents that are severe enough to be 'educational' but not so bad as to cause real harm. In any case, safety is an issue of ever growing importance, and anything we can do to enhance the students' learning experience while keeping them safe should be done.
To sum up, microscale working is worth considering for any institution teaching preparative organic chemistry. This is particularly true for the larger, introductory classes where the emphasis is on basic principles and where considerations, such as reducing the demand on consumables and the need to dispose of unwanted products are significant. There is great attraction in the attitude: "if you need 100 mg, make 100 mg, don't make 5 g and throw away 4.9." It would be a mistake to suggest that all laboratory work should be done this way, but the approach has an important part to play as one of the components in a chemistry student's overall experience.
References
[1] S. W. Bennett, Educ. Chem., 2000, 37, 49.
[2] A.H. Johnstone and A. Al-Shuaili, U. Chem. Ed., 2001, 5, 42
[3] J.F. Kerr, Practical Work In School Science: An Account Of An Inquiry Into The Nature And Purpose Of Practical Work In School Science Teaching In England And Wales, Leicester University Press, Leicester.1963.
[4] For example, M.A.M. Meester and R. Maskill, First Year Practical Classes in Undergraduate Chemistry Courses in England and Wales, The Royal Society of Chemistry, London, 1993.
[5] S.W. Bennett and K. OÕNeale, U. Chem. Ed., 1998, 2, 58.
[6] J.G.Buckley and R.F. Kempa, School Science Review, 1971, 53(182), 24.
[7] D.S. Domin, J. Chem. Ed., 1999, 76, 543.
[8] J.J. Lagowski, J.Chem.Ed., 1990, 67, 541.
[9] M. Pickering, J. Chem. Ed., 1987, 64, 521.
[10] J. Carnduff and N. Reid, Enhancing Undergraduate Chemistry Laboratories, The Royal Society of Chemistry, London, 2003.
[11] A. Ault, J. Chem. Ed., 2002, 79, 1177.
[12] D.W. Mayo, R.M. Pike and S.S. Butcher, Microscale Organic Laboratory, Wiley, Chichester, 1986.
[13] As defined by Cheronis in N.D. Cheronis, Semimicro Experimental Organic Chemistry, Hadrion Press, New York, 1958.
[14] K.L. Williamson, Macroscale And Microscale Organic Experiments, D.C. Heath, Lexington, Mass. 1989.
[15] S.W. Breuer, Microscale Practical Organic Chemistry, 2nd ed. Lancaster University, 1996. The text of the book is available in electronic format free from the author.
[16] A.J. Rest ed., Basic Laboratory Chemistry, details of contents and availability can be found at: http://www.soton.ac.uk/~chemweb/cvc/
Top
CEJ Vol. 7, No. 2, Contents