Stoichiometric technique

The simplest method to measure gas solubilities is what we wiU call the stoichiometric technique. It can be done either at constant pressure or with a constant volume of gas. For the constant pressure technique, a given mass of IL is brought into contact with the gas at a fixed pressure. The Hquid is stirred vigorously to enhance mass transfer and to allow approach to equihbrium. The total volume of gas dehv-ered to the system (minus the vapor space) is used to determine the solubihty. If the experiments are performed at pressures sufficiently high that the ideal gas law does not apply, then accurate equations of state can be employed to convert the volume of gas into moles. For the constant volume technique, a known volume of gas is brought into contact with the stirred ionic liquid sample. Once equihbrium is reached, the pressure is noted, and the solubility is determined as before. The effect of temperature (and thus enthalpies and entropies) can be determined by repetition of the experiment at multiple temperatures. 

The advantage of the stoichiometric technique is that it is extremely simple. Care has to be taken to remove all gases dissolved in the IL sample initially, but this is easily accomplished because one does not have to worry about volatilization of the IL sample when the sample chamber is evacuated. The disadvantage of this technique is that it requires relatively large amounts of ILs to obtain accurate measurements for gases that are only sparingly soluble. At ambient temperature and pressure, for instance, 10 cm of l-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) would take up only 0.2 cm of a gas with a Henry s law constant of 

5000 bar. Also, small temperature variations can cause large uncertainties. For instance, for 50 cm of gas, a temperature fluctuation of just 1 °C would also cause about a 0.2 cm volume change. For some metal apparatus of this type, gas adsorption on the metal surfaces can be an additional source of error. Thus, stoichiometric measurements are best for high-solubility gases and, in general, require excellent temperature and pressure control and measurement, as well as relatively large samples. 

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The main advantage of the gravimetric technique is that it requires a much smaller sample than the stoichiometric technique. In many cases, samples as small as 70 mg are sufficient. Accurate temperature and pressure control and measurement are still required, but gas adsorption on the metal walls of the equipment is no longer a concern because it is only the weight gain of the sample that is measured. 

Although the solubilities of gases in ILs are extremely important, at the time of this writing the number of published studies are limited. Some measurements were presented in oral and poster presentations at a five day symposium dedicated to ionic liquid research at the American Chemical Society national meeting in San Diego in April, 2001. Scovazzo et al. [14], for instance, presented preliminary results for CO2 and N2 solubility in [BMIM][PF6], and Rooney et aL [15] presented the solubihty of several gases in several different ILs as determined by the stoichiometric technique. A recent manuscript [16] presented Henry s law constants for H2 in two ILs. Given the lack of availability of other data, we concentrate below on the data collected in our laboratories. 

The value for [BMIM][PF6] corresponds to about 5700 bar, which is consistent with our measurements. However, it should be noted that Berger et al. [16] used a constant volume stoichiometric technique with a 50 cm vessel, pressurized to 50 atm. and containing 10 cm of IL. The resulting pressure drop when the gas is absorbed into the liquid would only be on the order of 0.005 atm. The authors do not report the uncertainty of their Henry s constants, nor the accuracy of their pressure gauge. Unless a highly accurate differential pressure transducer was employed, it is Hkely that these values are good order of magnitude estimates only. 

The selective nitration of phenylcarbonate into the 4,4 -dinitro isomer has been described in the literature. In this work we show that basic solids, such as CsF and CS2CO3 as well as basic organic resins, do catalyse the transcarbonation of the nitrated phenylcarbonate into para-nitrophenol and recyclable phenylcarbonate, which is a considerable advantage compared with homogeneous stoichiometric techniques. The overall sequence is an attractive alternative for the synthesis of para-nitrophenol. 

Various spectroscopic techniques can also be used to measure gas solubilities in ILs. For instance, Welton and coworkers have used proton NMR spectroscopy [16] to determine the solubility of hydrogen in a series of ILs. Since hydrogen exhibits low solubility in ILs and has such a low molecular weight, it is difficult to measure gravimetrically or by any of the stoichiometric techniques. As a result, it is particularly well suited to determination by spectroscopy. In addition, Kazarian and coworkers have measured CO2 solubility by infrared spectroscopy [17]. In general, spectroscopic techniques are quite attractive, as long as extinction coefficients do... 

Palladium-catalyzed allylic substitution may be regarded as a special case of cross-coupling with jr-allylpalladium complexes. First developed as a stoichiometric technique, this reaction was later realized in a catalytic mode, and became a valuable tool of organic synthesis, as it allows for a broad variation of both allylic substrates and nucleophiles. 

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