Chem242. Int. Inorg. Chem. UMass-Amherst
Properties of Deuterium-Labeled Hydrates1 *This lab was taken from a lab offered at Purdue University
Introduction An isotopically labeled compound contains an isotope of one or more of its component elements in higher abundance than is found in nature. Such compounds play an important role in the structural characterization of substances as well as in various mechanistic studies because the presence or location of specific types of atoms can be determined. For example, deuterium (D, the isotope of hydrogen with one proton, one neutron, and an atomic mass of 2) was used in an investigation of the mechanism of the hydrogenation of ethylene, H2C=CH2, to form ethane, CH3CH3, according to the equation:
C2H4+H2 → C2H6 This reaction was shown to be more complex than previously thought. When the reaction was run with C2H4 and D2, the product of the reaction was not C2H4D2, as expected, but was a mixture of products: all seven members of the series C2HnD6-n were observed. The mixture of products could be characterized by mass spectrometry because they range in mass from 30 to 36, a consequence of the difference in mass between a normal hydrogen atom and a deuterium atom. Isotopes are also useful in assigning infrared spectra. Changing the mass of a substituent atom by replacing it with a heavier isotope shifts the stretching frequency for normal modes involving that isotope, and thus the infrared bands. Alternatively, the location, of an isotope can sometimes be followed by infrared spectroscopy. Isotopes are also useful for investigating reaction mechanisms, as changing the mass of an atom alters the rate at which it reacts. This has to do with alterations in the stretching frequency for bonds to that atom, and with zero-point energy. Objective To monitor the extent of isotopic substitution in hydrated salts by infrared spectroscopy.
In this experiment we will use deuterium oxide, D2O (heavy water), to study the dehydration and rehydration of several crystalline hydrates. A crystalline hydrate is a stoichiometric compound, such as K2C2O4•H2O, Ba(ClO3)2•H2O, CuSO4•5H2O, or NH4Fe(SO4)2•7H2O that bonds to a metal ion. Individual water molecules can be identified in such a hydrate; the water molecule is not cleaved. One method of obtaining deuterated crystalline hydrates would be repeated recrystallization from heavy water of the protonated analogue. Even in the best case, when the compound is readily recrystallized, at least several milliliters of D2O would be required. A more rational approach proposed by Eriksson et al.2,3 is the vapor-phase exchange reaction of the water molecules with the solid dehydrated salt. If we use MXn•mH2O as the general formula for the hydrate we can write equations for the process as:
MXn•mH2O(s) → MXn(s) + mH2O(g) dehydrate
MXn(s) + mD2O(g) → MXn•mD2O(s) re-hydrate Chem242. Int. Inorg. Chem. UMass-Amherst
The greatest advantage of this approach is the very high degree of deuteration achieved using little more than stoichiometric amount of heavy water.
Experimental Work individually. You will be given a clean, dry crucible and lid. Label the crucible with your initials then weigh the crucible and its lid. Caution: Once you have weighed your crucible take care not to touch the crucible or its lid with your bare hands. You may deposit foreign objects or oil from your hands on the container. Handle it only with clean tongs. Into the crucible place a small sample (~0.150 g) of one of the following crystalline hydrates: K2C2O4•H2O, Ba(ClO3)2•H2O, CuSO4•5H2O, or NH4Fe(SO4)2•7H2O. Weigh your sample, crucible, and crucible lid together. Place your labeled crucible and its contents (without the lid) in the oven (heated to 110° C) for one hour. Record an infrared spectrum of a second sample of your hydrate in KBr. Print the spectrum on a piece of graph paper. After your solid has been in the oven for an hour, quickly transfer your sample to a desiccator so it can cool in an anhydrous atmosphere. Once it has cooled, weigh your sample and crucible (with lid). Caution: Your sample will quickly pick up water from the air. Make sure that you do not leave your sample in the air any longer than necessary.
Prepare the deuterated derivative by the reaction of your dried solid with D2O vapor. Carefully measure 1 mL of D2O into a jar with a screw cap. Place your crucible (without its lid) on the bottom of the jar in the puddle of D2O. Do not get water in the crucible. Store your sealed jar in your drawer for one week. Remember to keep the crucible lid clean during this time. During the next lab period, wipe your crucible dry, weigh your sample, crucible, and lid. Record an infrared spectrum of your deuterated crystalline hydrate in KBr. Record the spectrum on a piece of graph paper. Collect and record the following data for the other three hydrates. Salt Hydrated Dehydrated Deuterated person mass (g) mass(g) mass (g)
K2C2O4•H2O
Ba(ClO3)2•H2O
CuSO4•5H2O
NH4Fe(SO4)2•7H2O
Chem242. Int. Inorg. Chem. UMass-Amherst
DATA ANALYSIS/CALCULATIONS
• For all four samples, calculate the extent of dehydration, based upon the difference of weight between the crystalline hydrate and the dehydrated inorganic salt. Express your results as a percentage of dehydration; that is, the percent of the expected weight loss that was actually observed. • Calculate the degree of deuteration, based upon the difference of weight between the deuterated crystalline hydrate and the dehydrated salt. Express this result as a percentage of deuteration. • The two O------H stretching bands of the water molecules in a hydrate are broad and run together. Identify the average O------H stretching frequency in the infrared spectrum of your starting material. Use the information from the appendix to this experiment (“Isotope Effects”) as a guide and predict the average frequency of the O------D stretching vibration. Show your calculation. How does this compare to the observed O------D stretching frequency in your deuterated crystalline hydrate? • Using the IR spectrum of your deuterated sample, integrate the bands for the O-----H and O----D stretching vibrations. Using the following equation you can calculate the extent of isotopic exchange, χ(D). 2A(D) χ(D) = A(H ) + 2A(D) A(H) and A(D) are the areas of the ν(O-H) and ν(O-D) bands, respectively. The factor of 2 is used to account for the fact that the O-----H stretching mode absorbs about twice as strongly as the O------D stretching mode.
DISCUSSION Your discussion should address the following points in addition to analyzing and interpreting the results and relating the results to the objectives of the experiment. • You are able to quantify the extent of deuteration by two different methods, gravimetric analysis and infrared spectroscopy. Compare your results from the two methods. Discuss which you feel has less error and why. • Discuss the importance of using tetrachloroethylene or KBr for infrared spectroscopy instead of Nujol in the experiment.
REFERENCES: 1. Adapted from Ivanovska, M.; Stojanoski, K.; Zdravkovski, Z. J. Chem. Ed., 1993, 70, 603. 2. Eriksson, A.; Lindgren, J.; Stojanoski, K. J. Mol. Sruct. 1986, 143, 167-170. 3. Stojanoski, K.; Eriksson, A. Acta Chem. Scand. 1987, A41, 274-282.