INORGANIC SYNTHESIS
In this experiment your objective will be to synthesize an inorganic complex compound in pure form and high yield and to discover the chemistry involved in the syntheses. You will also determine its paramagnetic susceptibility in order to find the number of unpaired electrons present in the compound to help determine its structure. You will receive specific procedures for the syntheses of your compound prior to the experiment. These procedures were developed by research groups and were obtained from the chemical literature.
The information in this write-up includes:
1.general instructions on the synthesis of your compounds
2.the location of special equipment and materials
3.general instructions for recrystallization
- general instructions for the determination of the paramagnetic susceptibility
- a pre-lab safety worksheet
6.report sheets for the synthesis and the magnetic measurements
GENERAL INSTRUCTIONS
BECAUSE OF THE VARIETIES OF ACTIVITIES OCCURRING IN THE LAB, SAFETY GOGGLES MUST BE WORN AT ALL TIMES.
You have two weeks in which to synthesize your compounds and measure the magnetic susceptibility. Your product will be turned in along with the report sheets at the end of the three weeks. The locations of special equipment and materials are listed on the following pages. You will need to check out certain pieces of equipment from the stockroom. These have been organized according to synthesis number and can be checked out as a "kit". All borrowed materials must be returned at the end of each lab period so plan your work accordingly.
At the outset of a synthesis you will weigh the compound on which your yield will be based to the nearest 0.01 g on an analytical balance. This will be the compound which contains the metal ion of the complex you are synthesizing. Your final product should also be weighed on an analytical balance, again to the nearest 0.01 g. All other weighing operations can be carried out satisfactorily on the balances located on the side counters, unless the quantities are less than about 1 g.
In general it is much easier to dissolve solids in a small volume of solvent if they are first finely pulverized in a mortar and pestle. This is usually specified in the instructions when starting materials are provided in the form of large crystals, but it should be borne in mind for other steps in the procedure as well.
Most, but not all, of the compounds which will be synthesized are obtained as nice crystals. They should be turned-in in this crystalline form and NOT ground up in a mortar and pestle. Those compounds that are naturally obtained in an amorphous form are expected to be ground up uniformly before submitting the product. In order to determine the paramagnetic suspectibility the material on which the measurement is made must be ground up, but only a very small amount of material is needed for this.
For reasons given in the RECRYSTALLIZATION instructions it is not feasible in most cases for you to actually determine the purity of your product; however, it should be rather obvious if the product is damp, or if it smells of the solvent used for recrystallization. This state of affairs would represent gross contamination.
Each product should be turned-in in a l0-dram vial with a plastic cap. These are available in the lab. Each vial should be CLEARLY labeled, in ink, with (a) your name (b) lab section (c) synthesis number (d) name of compound (e) mass of product. Label tape that is meant for this purpose is available in the lab.
LOCATION OF SPECIAL EQUIPMENT AND MATERIALS
1.Needed solvents will be available in one gallon stainless steel safety cans on the side shelf. Take only the amount that you need.
2.Needed acids and bases will be available, along with a few reagents, on special trays on the side shelf.
3.Evaporating dishes, Büchner funnels (and paper), mortars and pestles are available in the stockroom.
4.Rectangular plastic boxes that can be used to hold crushed ice for cooling baths are located on the side shelf.
5. Hot plates and steam baths are distributed throughout the lab.
6. Desiccators with CaC12 or special desiccants will be made available as needed on request to lab instructor or assistant.
7.Drying ovens set for 55°C and 110°C are located along the wall and are labeled as to their temperatures.
8.A refrigerator is available for long term crystallizations (overnight or from one week to the next). Containers must be covered with Parafilm or aluminum foil and labeled with your name.
PLEASE RETURN ALL BORROWED EQUIPMENT (EVAPORATING DISHES, FUNNELS, MORTARS AND PESTLES, ETC.) TO THE STOCKROOM IN A CLEAN, DRY CONDITION AT THE END OF EACH PERIOD.
RECRYSTALLIZATION
An impure solid may often be purified by recrystallization. This usually involves dissolving the impure solid in a hot solvent (generally at its boiling point) and then letting the desired material crystallize out as the solution cools. Low concentration impurities remain in solution. The amount of hot solvent used should be just enough to dissolve the material so that as much of the desired product as possible will crystallize when the solution is cooled. It is important to choose a solvent in which the material being purified has a high temperature coefficient of solubility, as well as low solubility when cold, so as to minimize loss of material. Usually the solvent is also chosen so that the impurities will remain in solution when cold. In some circumstances it may be desirable to choose a solvent in which the impurities are insoluble when the solution is hot, in which case they are filtered out of the hot solution before letting the desired material recrystallize.
The following solvents are commonly used for recrystallizations: acetone, cyclohexane, ethanol, ethyl acetate, hexane (sometimes referred to as "petroleum ether"), methanol, toluene, and water. Binary mixtures of these are sometimes used. Since most of these solvents are highly inflammable all heating should be done on an electric hot plate, usually set at "low".
A general procedure which may often be used for recrystallization is the following:
1.Preparing the solution. Put the impure solid in a beaker of appropriate size, add solvent in small amounts, and maintain gentle agitation and swirling on the hot plate as the solution is brought to the boiling point. If the solid does not completely dissolve, additional solvent is added and the solution is reheated to the boiling point. Cease addition of solvent when the material has just dissolved. Be sure to wait sufficiently long between additions of solvent so as to achieve saturation, otherwise too much solvent will be used and there will be excessive loss of material.
2.Cooling the solution. Cool the solution slowly, with occasional swirling, to room temperature (or to 0° in an ice bath if appropriate) in order to encourage the formation of large, easily filtered crystals of the product. Really large crystals can be expected only with very slow cooling and without any agitation at all. Unless it is necessary to do so to avoid excessive loss of the desired material, it is simpler to cool only to room temperature and the chances of reincorporating impurities is lessened. Some substances crystallize extremely slowly, and several hours or days at low temperature (refrigeration is practical) may be required to maximize your yield.
3.Filtering the solution. The recrystallized material is usually removed from the supernatant solution by suction filtration, using an appropriate filter mounted on a side-neck filter flask. Be sure to clamp the filter flask to a ring stand. The apparatus has a high center of mass and therefore an unsupported rig has an unfortunate tendency to tip over. The most common filters used for this purpose are a sintered-glass funnel or a porcelain Büchner funnel that requires a piece of filter paper. It is important that a size is chosen which covers all the holes yet lies absolutely flat (i.e., does not curl slightly up the sides of the funnel). Select a funnel size appropriate to the quantity of crystals to be filtered. When using a Büchner funnel, the positioned filter paper should be moistened with the solvent which is being employed and suction applied before starting the transfer of your crystal. If you are filtering an ice-cold solution, you should also pre-cool your filter funnel; a refrigerator is simplest, but an ice bath may be used with care taken to avoid getting water in the solution unless your solvent is water. When all of your crystals have been transferred to the filter funnel (with the help of a spatula) and sucked reasonably dry, press them down on the filter with your spatula in order to assist in the sucking off of additional solution. This step, along with the first two, may be repeated as often as it is judged necessary to remove the impurities. Each repetition will naturally incur some additional loss of desired material.
4.Washing the crystals. The small amount of adhering solution which contains soluble impurities may be removed by stopping the suction, adding a few ml of fresh, cold (ice-cold if appropriate) solvent, stirring up the crystals with your spatula, and then re-applying the suction. When you use a Büchner funnel take care that you do not poke a hole in the filter paper; a flattened glass rod spatula will minimize this possibility. This washing process may be repeated several times, as appropriate. Adding portions of wash solution while the suction is on, or without stirring up the crystals, simply leads to channeling and ineffective washing.
5.Drying the crystals. Dry the crystals as much as possible on the filter by the application of suction; then transfer them to an appropriate size of watch glass or beaker. Oftentimes complete drying can be achieved by just letting the material stand, in a relatively thin layer, in a well ventilated place for several hours. Of course if the crystals are thermally stable, do not oxidize, and do not melt, they can be dried in an oven. When a high-boiling solvent has been used, time can be saved by drying the crystals in a vacuum desiccator connected, through a protective freeze-out (liquid N2 or dry ice) trap, to a vacuum pump. Crystals that have been recrystallized from solvents other than water cannot be dried in an ordinary desiccator because desiccants commonly remove only water. Some desiccants (P4O10 and concentrated H2SO4 for example) will remove volatile basic solvents, and others (CaO and NaOH) will remove volatile acidic solvents.
6.Checking for purity. A common way to check the purity of a compound is to determine its melting point and then compare it with the value reported in chemical handbooks or research papers. If a published value is not available, it is common practice to recrystallize again the compound and to redetermine its melting point. If it is the same as before, it is likely to be pure. A compound which has a melting point that is lower than the published value (or which is not really a "point" but a "range) can be judged to be impure or to decompose on heating; recrystallization should be performed. If a compound decomposes instead of melting, the melting point can't be used for characterization. If the compound is organic in nature, it may be analyzed for % carbon and % hydrogen and these values then compared to those expected for the compound you were crystallizing. In the case of inorganic complex compounds, a common method of analysis is to determine the percentage of the central metal ion present and then to compare this value with the expected value.
Most of the compounds synthesized by the class in this experiment decompose before they melt, hence melting point cannot be used as an indicator of purity. Also, many of the compounds cannot be simply or satisfactorily recrystallized, and in these cases the directions do not include this step. And since analyzing for the central metal ion is in most cases too time-consuming, you will basically have to settle for nice looking dry crystals of uniform color and crystal form.
DETERMINATION OF PARAMAGNETIC SUSCEPTIBILITY
AND UNPAIRED ELECTRONS
An atom, molecule, or ion that contains one or more unpaired electrons will possess a permanent magnetic moment, µ. Under some circumstances another source of magnetic moment (usually very small) results from the orbital angular momentum of electrons in incompletely filled subshells. In this experiment we shall assume that the observed paramagnetic susceptibility results only from unpaired electron spins.
If a given molecular entity possesses n unpaired electrons, its magnetic moment, µ, will be given by
µ = [n (n+2)]1/2 B.M. (l)
with units expressed in "Bohr Magnetons" (B.M.). One Bohr Magneton is actually a collection of fundamental constants:
one B.M. = eh/4me = 9.27 x 10-21 erg gauss-l= 9.27 x 10-24J Tesla-l.
where e is the charge on the electron, h is Planck's constant, and me is the mass of the electron. (Knowledge of the Bohr Magneton is not needed in order to carry out the experiment.)
Normally, the molecular magnetic moments of paramagnetic substances are randomly oriented with respect to each other and there is no net observable magnetic moment. In the presence of an applied magnetic field, these magnetic moments tend to align themselves with respect to the applied field and there is an observable interaction, even though the randomizing effect of thermal kinetic motion prevents a large interaction at normal temperatures. It is this interaction, measured by means of a balance, that is studied in this experiment.
The molar magnetic susceptibility (m) has been derived from theoretical considerations and shown to be composed of two parts, as follows
m = N2 µ2/3RT + Na (2)
Here N is Avogadro's number, R is the gas constant, and T is the absolute temperature. The second term Na represents the molar diamagnetic susceptibility of the molecule and the first term represents the effective molar paramagnetic susceptibility at a given temperature. If m is measured experimentally, then the magnetic moment can be determined by rearranging equation (2) and solving, as follows.
µ2 = (3RT/N2) (m - Na)
µ = (3RT/N2)l/2 (m - Na )1/2
µ = 2.824 [T (m - Na)]1/2 B.M. (3)
In our experiment, Na is very small and can be ignored. For more accurate work, Na can be found in published tables. From the experimental value of µ it is possible to determine the number of unpaired electrons that have caused the observed paramagnetic susceptibility by using equation (1).
The experimentally observed molar magnetic susceptibility (m) needed for use in equation (3) can be obtained from first principles and an exact knowledge of applied field strengths, etc., but it is more common to calibrate the experimental apparatus using a stable, welldefined compound for which the molar magnetic susceptibility is already known. This is the method used in this experiment. The molar magnetic susceptibility is calculated from the observed gram magnetic susceptibility by simply multiplying by the molar mass, M.
m = g.M (4)
There are several methods for measuring g including the Gouy method and the Faraday method where the apparent change in mass of a sample is measured when an external magnetic field is applied. A new type of measurement of g has been developed by D. F. Evans in which the force that the sample exerts on a pair of suspended permanent magnets is measured. This is the method employed in our experiment. The MSB-1 balance measures the change in current required to keep a set of suspended permanent magnets in balance while their magnetic field interacts with that of the sample.
A reading must be taken with the sample tube filled (R) and with the sample tube empty (Ro). The gram magnetic susceptibility is then found from
g = (L/m) [C(R - Ro)] (5)
where L is the sample length in centimeters, m is the sample mass in grams, and C is the calibration constant for the balance. (We have ignored the small contribution from the magnetic susceptibility of the air in the empty sample tube.) The length L and the mass m are measurements to be made independently of the MSB1 balance measurement as explained below. The calibration constant C is determined by measuring a substance with a known g, namely Hg[Co(SCN)4]. Its gram magnetic susceptibility is given by
g = (4.985 x 10-3) / (T + 10) (6)
where T is in Kelvin. Given the values of L, m, and Ro for the calibration sample provided, measurement of R and T for this sample will allow the determination of C using equations (5) and (6). Once C is known, a measurement of R, Ro, L, m, and T for your inorganic synthesis product will permit you to determine g. The molar magnetic susceptibility, the magnetic moment, and the number of unpaired electrons for your inorganic compound can then be calculated via equation (4), (3), and (1), respectively.
EXPERIMENTAL MEASUREMENT OF MAGNETIC SUSCEPTIBILITY
1.The MSB-1 balance must be turned on and allowed to warm up for 30 minutes. (The instructor should have done this before the beginning of the laboratory period.)
2.Obtain a sample tube from the stockroom. Handle it carefully because itis easy to break and costs quite a bit. Make sure the tube is clean and dry. Using the analytical balance next to the MSB-1 balance determine the mass of the empty sample tube.
3.Make sure the MSB-1 balance is set on the 1 setting. Carefully adjusting the zero knob, zero the MSB-1 balance. Place the empty sample tube in the hole at the top of the MSB-1 balance and read Ro on the digital meter. Remove your sample tube.
4.Using a mortar and pestle, carefully crush your sample to very fine particles. Crush enough sample to fill the sample tube to a height of between 2.5 and 3.5 cm. Using the tools provided, fill your sample tube to 3.0 + 0.5 cm. Tap the bottom of your sample tube on the mat provided several times to assure good packing of your sample. Using the analytical balance, measure the mass of your sample plus sample tube.
5.Zero the MSB-1 balance. Place your sample tube now loaded with sample in the MSB-1 balance and read R from the digital meter. Take your sample out of the MSB-1 balance and tap the bottom of it on the mat several times. Place your sample back in the MSB-1 balance and again read R. Continue this process until subsequent readings agree to within + 3 Runits
6.Remove your sample tube and rezero the MSB-1 balance. Measure the length of the sample in the tube with the accurate ruler provided and determine an estimate of the uncertainty in your measurement.
7.Measure R for the standard sample provided in a manner similar to that described in 5. Remove the standard and rezero the MSB-1 balance. Record L, m, and Ro for the standard.