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Synthesis and Characterization of (CH3)2NPF4 "Phosphorane" Me3SiNMe2 + PF5 → Me3SiF + Me2NPF4 References Hewson, M. J. C.; Schmutzler, R. Inorganic Synthesis 1978, 18, 179. This experiment introduces you to sophisticated high vacuum techniques that are used to handle gases, to multinuclear NMR spectroscopy, and to the chemistry of M-X compounds. Simple bond strengths drive the formation of Si-F bonds (hint for one of the questions) in much the same way that formation of a salt (e.g., LiCl) drives many reactions. The system you will use is sketched below (Figure 1) and the actual system is shown in the photo in Figure 2. Note that all amounts of gases (think in moles) are not easily determined from mass, so the Ideal Gas Equation becomes important. One of the most difficult techniques is transferring a gas to the vacuum system. See the Experiment Setups on the website for a detailed description. Figure 1. Schematic of the high vacuum system used in this experiment. Note that the dark dots represent stopcocks and all lines are hollow tubes. Mechanical vacuum pump Muck trap Figure 2. Actual system used in this experiment. Note that the mechanical vacuum pump is not shown, but is located on the floor to the left of the hood. You have been introduced to H-1 and C-13 NMR spectroscopy in your organic classes. In principal, P-31 and F-19 are similar and both nuclei have spins of ½ giving rise to similar splitting patterns. Here is one useful link to some information on P-31 and F-19 NMR spectroscopy. You can almost certainly find others that will help you answer some the following questions. The reference listed above is the original report in a series of books in the library called Inorganic Synthesis. All procedures are actually checked in independent laboratories. In many cases you will need the original reference to answer the question below. Finding it is part of your chemistry education. Questions 1. In order to measure amounts (moles) of gases, the volume and pressure of the container must be known. This is done by a calibration process. a. The first step is to get the calibration compound (tetramethylsilane), TMS, into the system. It is a volatile liquid, but air is dissolved in the liquid and will contaminate the TMS and make getting a true pressure reading for a known amount of TMS impossible. Read about "freeze-pump-thaw"; describe it briefly, and then discuss how to get the TMS into the vacuum system. b. Now that the TMS is in the system, use the sketch of the vacuum system above, and the data in the table below, explain in detail (use bullet or numbered steps and use the labels on the sketch in Figure 1) how the volume of a vacuum system is determined. c. Complete the table shown above by filling in your calculated volumes. Show all work for one example calculation, e.g., the volume of just the manifold. manometer reading mm Hg (Torr) (trial 1) manometer reading mm Hg (Torr) (trial 2) Calculated volume (trial 1) Calculated volume (trial 2) Average Calculated volume initial height 636 636 manifold 242 194 manifold + trap A 334 297 manifold + traps A &B 394 364 manifold + traps A, B, C 435 410 manifold + traps A, B, C, and bulb 558 548 grams of (CH3)4Si expanded into system at ca. 23 °C 0.64 g 0.72 g d. If a much larger amount (e.g., triple or quadruple) of TMS [(CH3)4Si] was used in the calibration, what effect would this have on the calculated volume of of the system above? Explain your answer. 2. a. Correctly draw the 2-dimensional shape of Me2NPF4 as you were taught in a highquality General Chemistry class. Use a chemical drawing program to do this with correct bond angles. a. Are the F atoms equivalent or is the space they occupy different? b. Suggest two alternate syntheses of Me2NPF4. Be sure to write balanced reactions. c. Why is the method used in this course preferable? 3. The related compound, (Me3Si)2NPF4 is unstable and cannot be isolated. Suggest a reason for this. (See: Inorg. Chem. 1977, 16, 1460-1463.) 4. a. At -90 °C, the 19F NMR spectrum of Me2NPF4 is considerably different than the room temperature spectrum. Explain briefly. b. Using the low temperature 19F NMR data given on page 184 in Inorganic Synthesis, 1978, 18, 179ff, (note this is a series of BOOKS in the library) draw a "stick" diagram of the spectrum showing the correct spacings and intensities of all the signals in the spectrum. Use graph paper (you can print some from the Internet) and draw this to scale (Hz) (rough sketches will not be acceptable! Even simple software like Paint will work.) [Note: The reference a series of BOOKS in the library. If you cannot find it, I will send you a scanned copy, but please make the walk to the library first. Do not check out the book, since the rest of the class will need it too.] 5. There are two very common impurities found in this experiment: Me2NP(O)F2 and the disubstituted pentacoordinated phosphorane product, (Me2N)2PF3. These may only be clearly observable in the 1H NMR spectrum. a. Sketch the expected 1H NMR, 19F and 31P NMR spectra for these compounds [using stick diagrams]. (absolute values of coupling are not important, only the splitting patterns). Assume the 31P spectrum is proton-decoupled and proton-fluorine coupling is not observed in the 19F spectrum. Look up the spectra and/or see the instructor. Explain the multiplets observed in the spectra. b. There are no significant differences between the room temperature and low temperature spectra of (Me2N)2PF3; this is different than for Me2NPF4. Offer an explanation. 6. There is a very useful chart that can help you ESTIMATE the boiling point of a liquid at a certain pressure (at or below atmospheric pressure), if you know the boiling point at another pressure. This is sometimes called a “Boiling Point Nomograph” and is supposedly based on the Clausius-Clapeyron Equation and Trouton’s Rule. See The Chemist’s Companion or find the “Boiling Point Nomograph” on the Internet. Although there are electronic calculations, learn to use a hard copy and a ruler. Practically, the nomograph is based on an empirical observation that boiling points of “normal” liquids all vary similarly with pressure. a. What is the Clausius-Clapeyron Equation? Trouton’s Rule? b. Using the nomograph, estimate the boiling point of a liquid at 1 Torr (1 mm Hg) if its normal boiling point is 220 °C. c. Describe how you would determine the molar heat of vaporization (∆Hvap) of a volatile liquid on the vacuum line using different temperature slush baths. d. A variation of the Clausius-Clapeyron Equation can help in “trap-to-trap” distillations. A compound will generally be trapped at a temperature at which its vapor pressure is 1 Torr (1 mm Hg). Suppose a pure liquid obeys the following equation: log P = 6.89 – 1274.69/T where P is the vapor pressure in Torr and T is the temperature in K. Describe (just a number is not enough) how to purify this compound.

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