Jerry 1. Mills and Larry C. Flukinger Texas Tech Universitv Lubbock, 79409
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The Tendometric Titration
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of Trimethylamine with Boron Trifluoride
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A n advanced inorganic laboratory experiment
The utilization of high-vacuum lines is becoming progressively more common in all areas of experimental chemistry. It is therefore important t h a t chemistry majors should be familiarized with the use of high-vacuum lines as a oart of their undermaduate curriculum. Most well equipped chemistry dep&tments now have a multipurpose \.acuum line assoriated with their senior inorganic laboratory, and generally. it is used for performing one experiment in the course of a one-semester laboratory. We hare found that the exoeriments desrribed in the literature for use of a high-vacuum line in a senior inorganic course consist largely of simple preparative* or Lewis acid-base2 experiments. We have devised a n experiment for our senior laboratory which, in addition to demonstrating the use of a vacuum line for gaseous or air-sensitive compound synthesis, proves the existence of a compound a t low temperature which is unstable a t ambient temperature. Discussion Brown3 found that a t -78°C the adduct formed by the reaction of triethylamine with boron trifluoride is of the stoichiometry (CzH&N:2BF3, but that a t 0°C the com~ . proposed the low temperature plex is ( C Z H S ) ~ N : B FHe structure to be
1 .o mmoles BF3/(CH313N
We have found that trimethylamine behaves similarly. At -78"C, BF3 forms a 2 : l adduct with trimethylamine, and a t O"C it forms a 1:1adduct. The student can monitor the stoichiometry of the reaction hy performing a tensiometric titration,4 whereby the pressure of a system a t constant temperature (either 0°C or -78°C) is recorded as the mole ratio of BF3 t o (CHdsN is varied. A plot of pressure versus mole ratio of BF3 t o (CH3)3N will have a break in the curve representing the proper stoichiometry. Plots showing typical student data for the tensiometric titration are shown in Figure 1. At 0°C the initial pressure reading indicates the pressure of free (CH3)3N plus solvent (dichloromethane). As increments of BF3 are added, the pressure shows a regular decrease due to the formation of the nonvolatile adduct (CH3)sN:BFa. AS the mole ratio of BF3 to (CH3)sN reaches one, the pressure is the lowest because all of the free (CH&N is complexed. As more BF3 is added, the pressure rises drastically, and there are no further inflection points in t h e plot. When the experiment is carried out a t -78"C, the pres1 Spielman, John R., J.
CHEM. EDUC., 47,225 (1970). 2Angelici, Robert J., "Synthesis and Technique in Inorganic Chemistry," W. B. Saunders Co., Philadelphia, 1969, p. 170. 3Brown, H. C., Stehle, P. F., and Tierney, P. A,, J. Amer. Chem. Soc., 79,2020 (1957). * Shriver, D. F., "The Manipulation of Air-Sensitive Com'pounds," McGraw-Hill Book Co., New York, 1969. 636 /Journal of Chemical Education
2.0
Figure 1. The tensiometric titration of (CHsIsN with BF3 at O°C and -78'C.
sure remains nearly constant until the mole ratio reaches one. This is because the solvent, the free (CH3)3N, and the complex each have no appreciable vapor pressure a t this temperature. The pressure exhibits a slow, regular increase a t -78°C as the mole ratio of BF3 to (CH3)aN changes from 1to 2. This pressure is that of free BF3 arising from the equilibrium
The pressure then rises sharply as the mole ratio of BF3 exceeds 2. Experimental Angelici's textZ has an excellent description of a multi-purpose vacuum line and instructions for its use in preparing the Lewis acid-base complex (CH&N:BFs. We chose to use the same Lewis acid and Lewis base as in Angelici's experiment so that the general instructions in his text are still specific for this eaperiment. A description of several other tensimeters and tensiometric techniques are described in Shriver's excellent text,' in addition to further information on vacuum lines and their use. The tensimeter which we used is shown in Figure 2. The total volume of the tensimeter is approximately 50 ml. The closed-end manometer is constructed of capillary tubing for strength, ta minimize mercury weight, and to minimize the hazard involved with
Teflon ~topcoekB
Fischer-Porter Solv- Seal
Figure 2. A high-vacuum line tensimeter.
surging mercury caused by sudden pressure changes. The highvacuum stopcock is constructed of Teflon, and the coupling utilized Viton O-rings to minimize contact with grease. Approximately 5.0 ml of dichloromethane are placed into the tensimeter along with a small magnetic stirring bar. The tensimeter is then attached to the vacuum line at the inlet manifold (in Angeliei's experiment, at JI). The dichloromethane is then degassed by repeated freezing with liquid nitrogen, pumping, and thawing. The (CH&N, which has previously been introduced into a vacuum line trap cooled to -196°C (liquid nitrogen), is allowed to warm and expand into a calibrated section of the vacuum line until the pressure corresponds to 1 mmole, according to the ideal gas The 1 mmole of ( C H h N is then transferred
into the tensimeter by condensation at -196'C, and is allowed to equilibrate to reaction temperature (either 0" or -78°C). (The reaction temperature can he conveniently controlled by placing a polystyrene cup around the lower part of the tensimeter. The cup may he filled with ice water far the 0°C reaction or Drv Ice-petroieum ether slush for the -78°C reaction. This allowsa ma'etic stirrer to be used, which is not the case if a Dewar flask is used.) The pressure on the manometer is read and recorded. From a trap on the line into which BFs has previously been introduced, a known quantity (0.2-0.4 mmole, as calculated from the ideal gas law) is then condensed with liquid nitrogen into the 7-mm side arm, the stopcock is opened, and the BFI is allowed to warm and diffuse into the solution of (CH&N. The solution is vigorously stirred, and the pressure is recorded when the system has reached equilibrium (about one-half to one hour). This is repeated until about 2.5 mmoles of BFI has been added. A plot of the pressure versus the male ratio of BFs to (CH&N is made for the O T run, and another plot is made for the -78T run. Some 1. tvnical student data is illustrated in Fimre " The experrmcnt at each temperature requrrrs approximately fne to srx huun to perform becauze of the tlme unolved m equh brntmn For thtq reaaon thc mstrtlctor may wsh for the student to do the experiment at only one temperature. Then two students who performed the experiment at different temperatures may share data.
Conclusion This experiment allows a student t o hecome well familiarized with a vacuum line. Specifically, it illustrates t h e handling of air-sensitive gases, demonstrates preparative techniques, allows determination of reaction stoichiometry, a n d offers proof of a compound which is unstable at ambient temperature.
Acknowledgment T h e authors would like t o acknowledge t h e generous support of t h e Robert A. Welch Foundation a n d t h e Research Corporation.
Volume 50, Number 9, September 1973 / 637
