The cold traps

The purpose of the cold traps (see Fig. 2.1.) is two-fold (1) to trap volatile materials from the line on their way to the pumps and thus to protect the pumping system and (2) to trap the vapour and any pumping fluid before it can enter the line by back-diffusion. 

The most common coolant used for the traps is liquid nitrogen (b.p. —195.8 °C). In comparison to liquid air, which was previously used as 

For some purposes solid carbon dioxide ( dry-ice ), sublimation temperature —78.5 °C or mixtures of dry-ice and acetone (temperature —78 to —95 °C) are used as coolants. These are obviously not as efficient as liquid nitrogen and they should not be used with chemicals which have an appreciable vapour pressure at the appropriate temperatures. 

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The cold trap usually contains some unconverted alkyne, which can be passed through the reaction mixture again. 

In a world increasingly conscious of the dangers of contact with chemicals, a process that is conducted within the walls of a vacuum chamber, such as the VDP process for parylene coatings, offers great advantages. Provided the vacuum pump exhaust is appropriately vented and suitable caution is observed in cleaning out the cold trap (trace products of the pyrolysis, which may possibly be dangerous, would collect here), the VDP parylene process has an inherently low potential for operator contact with hazardous chemicals. 

NaBH4 has also been crystd from isopropylamine by dissolving it in the solvent at reflux, cooling, filtering and allowing the solution to stand in a filter flask connected to a Dry-ice/acetone trap. After most of the solvent was passed over into the cold trap, crystals were removed with forceps, washed with dry diethyl ether and dried under vacuum. [Kim and Itoh J Phys Chem 91 126 1987.] Somewhat less pure crystals were obtained more rapidly by using Soxhlet extraction with only a small amount of solvent and extracting for about 8h. The... 

Note 9) in 500 ml. of dry tetrahydrofuran i.q added to the gtirred basic mixture heated to 65° over a period of approximately 8 hours a light nitrogen stream is used to carry the methylenecyclopropane into the cold trap. After the addition is complete, the reaction mixture is stirred and heated to 65° for 3 more hours (Note 10). The trap flask contains 58 g. (43%) of methylenecyclopropane (Note 11). 

The yield is determined by weighing the cold trap before and after distillation of methylenecyclopropane. Any small amounts of tetra-hydrofuran carried into the methylenecyclopropane trap are eliminated in a subsequent distillation. By proton magnetic resonance analysis the checkers found that no tetrahydrofuran reached the cold traps the spectrum (dichloromethane) shows a triplet at S 1.00 and a quintuplet at S 5.35 in the ratio 4 2. 

The reaction mixture and contents of the cold trap are then transferred (Note 7) to a 500-ml. distilling flask attached through a short fractionating column to a water-cooled condenser which is connected in series to a receiver, a trap cooled in a dry ice-acetone bath, and a hydrogen chloride absorption trap which may later be attached to a water pump. The mixture is then distilled until the pot temperature reaches 100° and practically all of the acetyl chloride has been driven over. 

The diazirines are of special interest because of their isomerism with the aliphatic diazo compounds. The diazirines show considerable differences in their properties from the aliphatic diazo compounds, except in their explosive nature. The compounds 3-methyl-3-ethyl-diazirine and 3,3-diethyldiazirine prepared by Paulsen detonated on shock and on heating. Small quantities of 3,3-pentamethylenediazirine (68) can be distilled at normal pressures (bp 109°C). On overheating, explosion followed. 3-n-Propyldiazirine exploded on attempts to distil it a little above room temperature. 3-Methyldiazirine is stable as a gas, but on attempting to condense ca. 100 mg for vapor pressure measurements, it detonated with complete destruction of the apparatus." Diazirine (67) decomposed at once when a sample which had been condensed in dry ice was taken out of the cold trap. Work with the lower molecular weight diazirines in condensed phases should therefore be avoided. 

GC-TEA Analysis. A Bendix model 2200 GC and Thermo Electron model 502 TEA were used. The GC injector temperature was 210 C. The TEA pyrolysis furnace was operated at 450 C and the cold trap was held at -150 C in isopentane slush. Oxygen flow to the ozonator was 20 cc/min and indicated pressure was 1.5 torr at a helium flow rate of 20 cc/min. TEA output was processed by a digital integrator (Spectra Physics System I). 

The formation of the cyclodisilazane ring system can be explained by dimerization of the unstable iminosilane in the cold trap.7... 

No condensate was found in the cold trap (ice or dry ice/acetone) after any of the experiments, nor was any material recovered from the gas train by flushing with toluene and ethanol. However, the quantity of condensate expected on the basis of ashing of the heated samples is gravimetrically significant in all cases (15 mg expected at 155°C, 56 mg expected at 330°C). 

An explosion was experienced dining work up of an epoxide opening reaction involving acidified sodium azide in a dichloromethane/dimethyl sulfoxide solvent. The author ascribes this to diazidomethane formation from dichloromethane [1]. A second report of an analoguous accident, also attributed to diazidomethane, almost certainly involved hydrogen azide for the cold traps of a vacuum pump on a rotary evaporator were involved this implies an explosive more volatile than dichloromethane. It is recommended that halogenated solvents be not used for azide reactions [2]. 

At the end of the experiment the hydrocarbons were removed from the cold trap and analyzed. The cyclohexane content of the sample was determined by gas chromatography on a Perkin-Elmer Fll gas chromatograph. The remainder of the sample was separated into a benzene and a cyclohexane fraction on a preparative gas chromatograph (Varian Auto-prep). 

Fig. 17. Thermograms recorded during the adsorption of doses of oxygen at the surface of nickel-oxide samples containing preadsorbed oxygen, the cold trap being cooled (A) or not cooled (B) (71). 

Equation (44) has indeed been confirmed by electrical conductivity measurements and by the detection of carbon dioxide condensed in the cold trap (53). 

The pilot-scale SBCR unit with cross-flow filtration module is schematically represented in Figure 15.5. The SBCR has a 5.08 cm diameter and 2 m height with an effective reactor volume of 3.7 L. The synthesis gas passes continuously through the reactor and is distributed by a sparger near the bottom of the reactor vessel. The product gas and slurry exit at the top of the reactor and pass through an overhead gas/liquid separator, where the slurry is disengaged from the gas phase. Vapor products and unreacted syngas exit the gas/liquid separator and enter a warm trap (373 K) followed by a cold trap (273 K). A dry flow meter downstream of the cold trap measures the exit gas flow rate. 

Following the first run, in which components are transferred from the precolumn to the on-line cold trap, the system will reset to a second method and, on becoming ready, the cold trap is desorbed and the analytical run automatically started. 

Because the balance housing is separated from the reaction chamber (cf. Fig. 25), one diffusion pump evacuates only the balance housing whereas the other evacuates the reaction chamber. The reaction chamber is connected to the diffusion pump by a cold trap. The decomposition products can be taken from the gas outlet for analysis or condensed on the cold trap by means of a coolant. 

Receiver D is a 250-mL, round-bottomed flask with two, 15-cm long necks with a diameter of 3.5 to 5 cm (entrance neck) and 2.5 cm (exit neck) (see Figure 1). If a receiver D with a narrower entrance neck than indicated is used the neck can become plugged by condensate. A wide-bore connection is used between receiver D and the cold trap E to prevent a pressure drop during pyrolysis, which might be caused by a restriction in the HCI-gas flow. 

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