THE JOURNAL OF
P H Y S I C A L CHEMISTRY (Registered in U. 8. Patent Office) (Copyright, 1962, by the American Chemical Society)
Founded by Wilder D.Bancroft E'EBRUARY 15, 1962
VOLUME56
NUMBER 2
THE YltEPARATICN AND SOME PHYSICAL COXSTANTS OF CADhlIUJI DIMETHYL BY R. DEANANDERSON AND H. AUSTINTAYLOR New York University, New York 63,N . Y . Receined July 6 , 1060
Pure cadmium dimethyl prepared by a Grignard reaction requires careful fractionat,ion to remove ether which i R the cliief impurity. Measurement shows the triple point to be -2.4" and 5.5 mm. and the normal boiling point to be 105.7'. The and the vapor pressure, log p = d e d i t y is found to be 2.479 0.001631T; the coefficient of cubical expansion, 1.0 X -3116.7lT 8.0668log T 31.905. The heat of vaporization is given by AH, = 14,300 16T and the boiling point constant IS 4.9".
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In synthesizing cadmium dimethyl for other studies it became apparent that what is at present the accepted technique of preparation could be vastly improved both as to quality of product and safety for the operator. Gilmanl by the use of anhydrous cadmium chloride improved the procedure of Krause2 who obtained cadmium dimethyl by the Grignard reaction of methylmagnesium iodide and cadmium bromide in anhydrous ether. The product has a boiling point of 105.5" at 758 mm., a, freezing point of -4.5", density (17.9'/4") of 1.9846 and index of refraction using the sodium D lines of 1.5488. Hers3 reported the boiling point as 105.7', the density (0') as 2.500 and the refractive index no as 1.786. Recently Barnford, Levi and Kewjtt4made the alkyl by the Krause synthesis and made vapor pressure measurements by the static method reporting a linear relationship between the logarithm of the pressure in mm. and the reciprocal of the absolute temperature, namely, log p = 7.764 18501T. They also report the boiling point as 105.7" and the freezing point, -4.2'. Of the dangers inherent in the preparation and use of cadmium dimethyl, repeated mention is made in the literature of ita explosive nature when over-
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(1) H. Gilman, Rec. trou. chim., 66 518 (1936); J . A m . Chem. Soc., 68, 618 (1946). (2) E . Krause, Ber., 60. 1813 (1917).
(3) W. Hen. 2.anorg. Chem., 189, 173 (1929). (4) C. 11. Barnford. D. L. Lcviand D. M. Newitt, J . Chem. Soc., 40,
468 (1946).
heated. Gilman reports that it may detonate spontaneously if heated above 90'. Its physiological effects on the user include extreme irritation of the eyes on exposure to the vapor with distortion of vision after a few hours. Light sources appear to be surrounded by bright halos and the effect persists for a day or so. Experimental Synthesis.-From experience gaincd a i l h preliminary preparations, the synthesis was carried out in a production unit, shown in Fig. 1, mounted in a hood having shatterproof glass. Methylmagnesium iodide was first made in thc convmtional way using 512.0 g. of C.P. methyl iodide and 87.8p. of magnesium turning. A 2-1. reaction flask was fittcd with a 20-inch reflux condenser connected to the atmmphcre through a drying train and a nitrogen reservoir. A dropping funnel and heavy duty stirrer were connected to the flask. All equipment had either standard taper or ball and sorkct type joints. An inlet tube was provided so that nitrogm could be lcd into the reaction vessel beneath the ether surface during the reaction. A bath of ice-watcr was used to check the reaction when it became too vigorous. About one liter of anhydrous ether was placed in the flask with the magnesium turnings. The apparatus was flushed out with nitrogen before and after this addition. The methyl iodide was introduced through a dropping funnd, in small portions requiring one hour for the addition. The condenser was maintained at -10' by the use of a 25% prestone-water solution pumped through a freezing unit. After the methyl iodide had been added the reaction mixture was kept a t moderate reflux for a half-hour on an oil-bath with heating mantle. Stirring and heating were discontinued and the system allowed to cool in an atmosphere of nitrogen. Thc ether solution of the Grignard rcagcnt was siphoned into the production unit in a, mtrogen atmosphere through
161
R. DEANANDERSON AND H.AUSTINTAYLOR
162
Fig. 2.-Automatic
Fig. 1.-Production
of cadmium dimethyl.
an assembly replacing the stirrer, C, in Fig. 1. Anhydrous cadmium chloride, 33.10 g. in weight, previously heated at 120" overnight and cooled in a vacuum desiccator was transferred to the hopper B which contained a I / * inch wood bit ground to fit into the glass bearing. All bearings and ground joints were lubricated with high vacuum grease. A reduction motor rotating the screw at about 12 r.p.m. was used intermittently as required. All heaters and motors were operated from outside the hood which was not opened until the reaction, once begun, was com lete. The condensers E and F maintained at -10" in t f e manner previously indicated were lagged to revent water condensation. With the unit assembled aa $own and the stirrer C operating, the whole in a nitrogen atmosphere, reaction was started by adding the cadmium chloride intermittently every five minutes for about five and one-half hours. The heat of reaction was sufficient to maintain a steady reflux of the ether which in turn kept the entrance to the vessel clear of cadmium chloride. The addition completed, artificial heating was supplied for ten hours. The ether solution of cadmium dimethyl after cooling was siphoned into a 2-1. distilling flask, the residue in the reaction flask being extracted twice with 150-ml. portions of ether and the whole subjected to a preliminary fractionation through a vacuum-jacketted column in a nitrogen atmosphere. The first fraction coming over at 34.7" was ether. As soon as the temperature at the top of the column started to go higher distillation waa stopped, the apparatus allowed tq cool and a second receiver in the form of a trap set in a bath of Dry Ice in acetone was substituted. Distillation was then continued for several hours under reduced pressure sufficient to maintain the temperature at the top of the column at 70" or less. About 350 ml. of clear liquid was thus collected and transferred to the kettle, R, of the fractionation unit shown in Fig. 2 which carried a thermometer well S, and nitrogen inlet tube, M, attached by a rubber sleeve, N. The capillary, M, was drawn down to a fine bore such that under reduced pressure only two or three minute bubbles entered the li uid per second to ensure smooth boiling. The column, comprised three concentric cylinders of Pyrex glass. The inner one waa packed with a/s inch Pyrex helices and the middle one carried a heating coil mounted on vertical strips of asbestos cord. Thermometers were inserted between these two tubes to record the temperatures at the top and bottom. The outer tube served as insulation. The takepff head was set into the top of the column. Vapor from the column rose through the vacuum-jacketted esction, G , ita temperature being noted by a thermometer in the well F, and condensed on the surfaces of D and E. Set for total reflux all the condensate falls through funnel, C, returning to the column through the coil, H, where mild preheating occurs. When set for intermittent takeoff, electromagnet 13 is activated at regular intervals to tilt funnel C, and divert the reflux into the receiver connected through
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Vol. 56
take-off head and fractionating column,
A. An automatic time switch with two-minute cycle or any fraction thereof actuated the magnet. Fractionation at a 3% take-off ratio was begun at 34.7" and 760 mm. and at the end the pressure had been reduced to 230 mm. to maintain boiling in the kettle at 70". The contents of the kettle was then transferred by simple distillation into a receiver cooled in liquid nitrogen. In this way about 135 ml. of cadmium dimethyl was collected. A hydrometer check on the density gave a reading in agreement with the reported value. Freezing was observed at -4.7' over a range of about half a degree. Vapor pressure readings were made and finally the li uid boiled at 107.4" and 760 mm.. It was ap arent that %e boiling point was some two degrees higher &an the repdrted value though the freezing point was in agreement with previous findings. The whole sample was therefore fractionated again through the same apparatus, Fig. 2. Artificial heating of the column was necessary to maintain the column at about a ten-degree r d i e n t Cooling solution at -10" waa used in the reflux ead. After a two-hour reflux the automatic take-off head was set to collect one drop out of 750. The first distillate came over a t 50" and 230 mm. and when 3 ml. had been collected distillation was proceeding smoothly at 67.2" and 218 mm. An additional milliliter was collected and then the column shut down and the receiver changed. On resuming fractionation, tot,al reflux was maintained for one hour, and the head temperature being 67.2" at 218 mm. take-off at four drops per two-minute cycle was resumed. When no further drifting in temperature and pressure occurred this rate was increased to sixteen drops per cycle or about 2% of the total reflux; Over a 15-hour period 80 ml. of pure alkyl was collected. The actual distillation time was seven hours, the rest of the time being necessary for shutdowns and regainin equilibrium conditions. At one such shutdown the caphary was changed while the kettle was at 70" and although the rubber sleeve waa pinched off as. the capillary was withdrawn, some air apparently entered the kettle and the vapor exploded. Freezing Point Determination.-The freezing point ?as determined in a flask of about 60-ml. capacity into whch 40 ml. of cadmium dimethyl had been distilled. A low range thermometer marked in 0.1" and calibrated, waa inserted through a rubber sleeve into the liquid. A side arm on the flask waa used for evacuation. An acetone-bath cooled by addition of acetone from an acetone-Dry Ice mixture was used as coolant in a two quart dewar. Investigation showed that a ten-degree temperature gradient was necessary to give a well defined cooling curve. The apparatus was rotated manually while in the bath and ternperatures were taken at half-minute intervals. Coohng curves with small undercooling and marked plateaus extending through several minutes showed the corrected freezing point to be -2.4". A boilin point determination of the same 40-ml. portion gave a vafue of 105.7" at 760 mm. Compared with the previous impure sample this showed a fall of 1.7" in boiling point and a rise of 2.3" in the freezing point. A greater change in the freezing point is generally to be expected. Density Determination.-A Pyrex pyknometer consisting of a tube of volume about 9 ml. carried side arms bent up-
Feb., 1952
PREPARATION AND PHYSICAL CONSTANTS OF CADMIUM DIMETHYL
163
wards at about 75". They were sections from micro-pipets 1 1J of 0.2-ml. capacity graduated in 0.01 ml. the graduations being about l-cm. apart. The open ends carried ground glass covers. A small foot waa sealed to each side of the main body so that the pyknometer would stand on a balance pan for weighing. A loading arm ground to re lace one of the cover caps was bent downwards such that wien ita open end was placed beneath a liquid, and vacuum applied to the other arm of the pyknometer, the liquid filled the vessel without leaving any gas trapped. The pyknometer was calibrated by weighing water SUEcient at 20" to fill it to the graduation markings. For cadmium dimethyl the vessel was first flushed with nitrogen, filled as described and capped and weighed. The pyknometer was then placed in a series of different temperature thermostats at each of which volume readings were made. The correction for volume due to the coefficient of expansion of Pyrex was negligible over the fourteen degree range covered. The density of cadmium dimethyl in g. per ml., corrected for buoyant effect of air was found to be Fig. 3.-Ramsay-Young vapor pressur? apparatus. dvac.= 2.479 - 0.001631T over the temperature range 10-24', using the method of function of the temperature integration of the Clapleast squares on fifteen volume readings. The volume eyron-Clausius equation gives a relation of the measurements showed that the coefficient of cubical ex- form: log p = A / T B log T C. Using this pansion of the liquid was 1.0 X 10-3. Vapor Pressure Determination.-The apparatus used for equation and the method of least squares on the the vapor pressure measurements is shown in Fig. 3. Below thirty-five measurements made, the following val44" the static method of vapor pressure measurement was ues, good to six significant figures were found: used; above 44' the Ramsay-Young method. The apparatus consists of a system of two flasks and three condensers A = -3116.74, B = -8.06680, C = 31.9054. The which can be rotated to either of two positions in plane at log terms in the equat,ion make all these figures in right angles to the standard taper joint, I. This permits A , Band C important for the evaluation of the presa back and forth distillation of the liquid between the two sure at a given temperature. The effect of sucflasks A and F. The manifold leads to an absolute mercury cessively rounding off these values is shown at a manometer, M, a nitrogen supply, J, a McLeod gage, K, a trap, L, cooled by liquid nitrogen and thence to a diffusion few selected points in Table I. It can be seen that five significant figures reproduce the observed valpump system. Before loading, the entire apparatus was alternately ues almost as well as six, but that further approxievacuated ( 5 X 10-6 mm.) and flushed out with nitrogen mations yield progressively poorer agreement heto remove oxygen and water vapor. Cadmium dimethyl was distilled through H into flask, A, in a nitrogen atmo- tween observed and calculated pressures. sphere at 150 mm. Flask, A, was then rotated to the position shown in the diagram and its heating bath put in place, TABLE I F, the receiver, was cooled in li uid nitrogen. The con- A = -3116.74 -3116.7 -3117 -3120 densers, D, were water-cooled w d e E was cooled as before 8.07' to - 10". Pressure in the system was regulated by control B = - 8.06680 - 8.0668 - 8.067 of J and temperature readings of the boiling liquid were C = 31.905 31.9054 31.91 31.9 made on a calibrated thermometer graduated in 0.1' into Pobad. Poslod. 8 PoJod. 8 Pcalcd. 6 Podod. 8 serted in paraffin oil in the well, C. Distillation w1t9 so 2 . 4 5 . 5 5 . 9 0 . 4 6.9 0 . 4 5 . 9 0 . 4 5 . 0 0 .5 smooth that a closed system was used a t each pressure and 86.0 86.2 0 . 2 86.1 0 . 1 87.1 1 . 1 79.4 6.6 45.7 when equilibrium was established simultaneous pressure 71.5 246.8 247.8 1.8 247.7 1.7 251.2 5 . 2 251.2 5 . 2 and temperature readings were made. 86.4 418.0 417.5 0 . 5 416.9 1 . 1 416.9 1.1 398.1 19.9 For temperatures below 44', the heating mantle, B, was 98.6 617.0 613.9 3 . 1 613.8 3 . 2 616.6 0 . 4 631.0 14.0 not used. Instead the product was frozen overnight and evacuated to zero pressure on the manometer. For each 105.7 760.0 756.8 3 . 2 756.8 3 . 2 758.6 1 . 4 794.3 34.3 pressure reading a constant temperature bath was placed For the minimum number of figures in the conaround the cadmium dimethyl and the equilibrium pressure read. For temperatures below room temperature and down stants A , B and C to maintain agreement with the to the freezing point a dewar flask containing chilled acetone observed values the following equation is best was used for thermostating. 3117.7/T - 8.0668 log T 31.905 log p Thirty-five pressure measurements were made
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covering the temperature range -2.4' to 106.7'. A plot of log p against 1/T does not give a straight line and deviation from linearity is more marked in the low temperature range indicating an increase in heat of vaporization with decreasing temperature. Assuming the heat of vaporization to be a linear
From this equation the heat of vaporization of cadmium dimethyl is found to be AH, = 14,300 - 16 T with values at the freezing point -2.4' and boiling point 105.7", 9970 and 8240 cal./mole or 70.0 and 57.8 cal./g., respectively. A molecular boiling point constant of 4.9"then results.
