Frank W. Walker and E. C. A S ~ ~ Y '
Georgia Institute of Technology Atlonta, Georgia 30332
Determination of the Molecular Weight of Air-Sensitive compounds
T h e determination of the molecular weight of air- and moisture-sensitive compounds by ebullioscopic techniques presents difficultiesthat are not encountered when working with less sensitive materials. For example sample addition is ordinarily accomplished by simply disassembling the boiling point apparatus, adding a carefully weighed portion of the material, reassembling the equipment and finally, determining the boiling point of the solution. However, the need for dry and oxygen-free conditions eliminates the possibility of using such convenient techniques because rather severe limitations are imposed on the design and operation of the ebullioscopic apparatus and on the techniques employed in handling the sample. The problems caused by these limitations have been solved and, to illustrate fully the method that has been developed, a description of the apparatus will be accompanied by a detailed account of it.s operation.
Figure 1.
Apporotvr far molecular weight determinations of air-sensitive
compound^.
Apparatus
Although the boiling point apparatus shown in Figure 1 has the same basic design as that of an ordinary CotSloan Fellow 1965567. To whom dl inquiries should he sent. Initial stages in the design of this apparatus were made at the Ethyl Corporation, Baton Ronge, Louisiana. as described by ASHBYE. C., AND SMITH, M. B . . J . Am. C h a . Soc., 86, 4363 (1964).
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trell boiling point apparatus, there are some significant differen~es.~One point worthy of note is t,hat the apparatus is of one piece construction such that there are only three places in the apparatus where outside connections must be made. These include two ground glass joints, one for a Beckman thermometer, one for connection to a nitrogen system, and an opening for a septum cap. The septum cap allows the periodic addi-
tion of samples (in solution via syringe) to the apparatus without cxposing the sensitive material to the atmosphere. Note that the septum cap opening is positioned between the condensate shield and the thermometer; this arrangement allows thorough mixing of the samples with the boiling solution. The condensate shield ext,ends to just below the bottom of the mercury bulb of the thermometer where it protects the bulb from contact with cold condensate. If the study is conducted with a low boiling solvent such as diethyl ether, the condenser should be connected to a recirculating motor and cooled with ice water to eliminate solvent loss and excessive solvent holdup in the condenser. The bulb of the thermometer is continually bathed with boiling solvent by the pump, and superheat,ing of the solution is minimized by the glass chips fused to the bottom of the solvent chamber. The remainder of the equipment comprises the nitrogen system which is designed to allow operation of the apparatus a t any pressure that is desired. This system is constructed with copper tubing with all connections being made with brass compression fittings. Where a connection must be made between the copper line and either hose fittings or glass tubing, a short piece of securely clamped pressure tubing is used. The surge tank has a capacity of about 20 1 and can withstand a vacuum of at least 1 mm. An oxygen storage tank of the type used by the United States Air Force meets these requirements. The surge tank acts as a damper for small pressure changes in the system and therefore aids in maintaining a constant pressure. The manometer is a precision Wallace and Tiernan gauge that can be read with high accuracy; it has an adjustment that compensates the pressure scale for temperature changes in the laboratory. The valves are brass-seated and must be of high quality in order for a constant pressure, either higher or lower than atmospheric pressure, to be maintained in the system. The oil bubbler acts as a terminal gas gauge in that it monitors the rate of nitrogen flow past the system. The function of the various components will become clearer once the operation of the system is understood. Operation
After placing the apparatus in a nitrogen system as shown in Figure 1, the properly adjusted Beckman thermometer is seated with a minimum of high vacuum stopcock grease and the septum cap is inserted and secured with a small piece of wire. On the initial run all connections must be checked for leaks by closing valves 2 and 4 and opening valve 3. Valve 1 is opened slowly until the manometer reads a pressure somewhat higher than atmospheric; then valve 1 is closed. The manometer is read and then rechecked in a few minutes to see if there is a pressure change. If some leakage is indicated the location can be determined with a soap solution. When the pressure remains constant for 5 min the system can be considered ready for operation. After determining that the system is tight, the next step is to dry the system andfillit withnitrogen; thisis accomplished by closing valve 3, starting the vacuum pump, and slowly opening valve 4. After the system is evacuated the glass portion of the apparatus is carefully
flamed with a bunsen burner. Extreme caution must be exercised here since prolonged heating at one point on the surface of the glass may crack the apparatus. For this reason, a sweeping motion with the flame is desirable. The vacuum is maintained until the glass has cooled; then the system is filled with nitrogen by closing valve 4, opening valves 1 and 2 (a fast flow of gas through the bubbler is desirable) and then opening valve 3 at a rate that does not allow the oil in the bubbler to enter the line. This procedure is repeated at least two more times. Since the next step involves the use of syringes for the first time, a brief description of this operation will be given. The syringes must be dried either by flaming under a nitrogen flow or by flaming and immediately placing the hot syringes in a dry box entry port which is then evacuated. Although a dry box simplifiesthe sampling operation, samples can be withdrawn from a flask on the bench top if the flask is fitted with a septum cap and a source of nitrogen. Regardless of the sampling technique, it is beneficial to provide some method of capping the syringe needle after the sample is withdrawn into the syringe. One method that works well is to insert the needle into a small section cut from a septum cap. When the sample is injected into the apparatus, the needle is pushed through the rubber section, through the septum cap an the ebullioscopic apparatus and on into the system for injection. This method eliminates loss of sample during the transfer. The final purging step is initiated by adding 20-25 ml of the solvent to the apparatus. When making additions to the apparatus, valves 3 and 2 should be opened with a small nitrogen flow through valve 1 since it is difficult to add a sample if the system is closed. After adding the solvent the evacuation procedure is repeated, followed by flaming and refilling with nitrogen. This final purging step sweeps the moisture out of the condenser and internal parts that are not efficiently heated from the outside. Upon completion of this step, the system is ready for introduction of the samples. The first material introduced is the solvent that will be employed in the study. The quantity of solvent that will be used depends on the efficiency of the solvent pump because it is absolutely necessary that the bulb of the thermometer be continually bathed with boiling solvent. The apparatus sketched here requires approximately 80 ml of ether for efficient operation of the pump. The system is heated with a heating mantle (a 50-ml mantle fits this apparatus), and the powerstat setting should be as low as possible to avoid superheating. Precautions against drafts on the apparatus must be taken, and a simple expedient is to wrap the solvent chamber up to the thermometer with insulating material. The initial reading that is made is the boiling point of the pure solvent at the desired pressure. Thus, if operating a t a pressure higher than atmospheric pressure, valves 1 and 4 are closed and valves 2 and 3 are opened. After the pressure has dropped below the desired pressure, valvc 2 is closed and valve 1 is opened slowly until the correct pressure is reached. At this point valve 1 is closed and the system is brought to thermal equilibrium (allow 5 min of boiling at the deVolume 45, Number 10, October 1968
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sired pressure). Recheck the pressure before reading the thermometer, being sure to gently tap the manometer and thermometer before making the reading. Thc observed boiling point is considered final only when it can be reproduced at least twice by dropping the pressure and then readjusting it to the desired level. After establishing the hoiling point of the solvent, the solution is added, following the same sequence of steps used with the solvent addition. Several additions of the sample arc desircd in order to obtain a value for the molecular weight at several concentrations, hut the amount of material that can he added is limited because the solution must never come in contact with the thermometer bulb. When contact occurs, the temperature registered is not the desired liquid-vapor equilibrium temperature but is that of the solution which may be lightly superheated. If operation at a pressure lower than atmospheric is desired, valve 4 is opened to the vacuum line and valve 3 is closed. After reaching the desired pressure, valves 1 and 2 are opened to obtain a rapid flow of nitrogen past the system. Finally, valve 3 is adjusted so as to maintain a constant pressure in the system by slowly leaking nitrogen through the system to the partially closed valve 4 which remains under vacuum. The other operations are similar to those outlined for operation at a pressure greater than atmospheric pressure. Experimental Results
Results from a study to determine the molecular weight of an ether solution of dimethylmagnesium, which is a highly air- and moisture-sensitive compound, are found in the table; these data are typical of the reData far Determination of Molecular Weiaht of Dimethvlmaanesium
Fraction W L(g)
Wdg)
ATB ('C)
m
i
Figure 2.
Molecular asrociotion of dimethylmagnesium in diethyl ether.
tion must he addcd to the initial weight of the pure solvent. This is true whenever the compound is added as a solution as it was in this example. Thus, the weight of the solvent as shown in the table increases as the weight of solute increases. The results are shown graphically in Figure 2. The calculations were made usine eon. (1) which was derived in the usual manner by assuming an ideal solution hut not necessarily a dilute solution. ~
.A
\
~
,
The additional terms include M z , the formula weight of the solute (54.39 g for dimethylmagnesium); MI. t,he molecular weight of the solvent (74.12 g for diethyl ether); and K g , the molal boiling point elevation constant (2.01 for diethyl ether a t an internal nitrogen pressure of 740.0 mm). Conclusion
sults obtained with this apparatus. The terms are defined as follows: W zis the weight of solute: 1Vl is the weight of solvent; ATg is the boiling point elevation; and m is the molality of the solution. The final term in the table represents the molecular weight of dimethylmagnesium in diethyl ether expressed as an L‘ z-value," which is simply the experimentally determined molecular weight divided by the formula weight. This manner of expressing the molecular weight is instructive in this case since the compound associates in solution; the result is an increase in the observed molecular weight as the concentration of solute increases. A significant point to note with these calculations is that the weight of solvent introduced with each frac.
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Journal o f Chemical Education
This apparatus has been used to elucidate the nature and extent of the association of Grignard reagents, dialkymagnesium compounds, and magnesium halides in diethyl ether and tetrahydrof~ran.~By following the steps outlined above for using the system, clear and colorless solutions of these very sensitive materials have been recovered unchanged after boiling for 8 hr in the apparatus. Since an experienced operator can obtain highly reproducible data from the system and since components of the system are readily available, this technique is within the capability of any laboratory interested in the accurate determination of the molecular weight of a sensitive material. B A (1967).
~E. C., ~ AND ~ WALKER, ~ , F., J . Organomelal. C h m . , 7, 17
