Cold-trapping

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

After the addition of the propyne the thermometer-gas outlet combination was replaced with a "cold finger" filled with dry-ice and acetone. The top of this reflux condenser was connected via a plastic tube with a cold trap (-75°C) containing 50 ml of dry THF. The cooling bath was removed and the conversion of propyne started... 

After the air in the flask had been completely replaced with nitrogen, it was cooled in a liquid nitrogen bath and a solution of 25 g of acetylene in 160 ml of dry THF was introduced. The solution had been prepared by dissolving acetylene (freed from acetone by means of a cold trap) in THF cooled at -80 to -90°C. A solution of 0.21 mol of butyl lithium in about 150 ml of hexane was added in 5 min to the vigorously stirred solution. During this addition the temperature of the mixture was kept between -80 and -100°C by occasionally dipping the flask into the liquid nitrogen. To the white suspension were successively added at -80°C a solution of 10 g. of anhydrous lithium bromide (note 1) in 30 ml of THF and 0.20 mol of freshly distilled benzaldehyde. The reaction mixture was kept for 3 h at -69°C, after which the temperature was allowed to rise to +10°C over a period of 2 h. 

Apparatus 1-1 three-necked, round-bottomed flask with a dropping funnel, a gas--tight mechanical stirrer and a very efficient condenser. The top of the condenser was connected to a cold trap (-80°C). 

Apparatus See Fig. 7 size of the flask, 2 1 a gas-tight stirrer should be used the top of the condenser was connected uia a plastic tube to 2 tubes filled with lumps of CaCl2- These tubes in turn were connected to 2 cold traps (-80°C) each of which contained 40 ml THF or CH3OCH2CH2OCH3 (free from peroxides). 

Nxylylene system, substituents affect it only to a minor extent. AH parylenes are expected to have a similar molar enthalpy of polymerization. An experimental value for the heat of polymerization of Parylene C has appeared. Using the gas evolution from the Hquid nitrogen cold trap to measure thermal input from the polymer, and taking advantage of a peculiarity of Parylene C at — 196°C to polymerize abmptiy, perhaps owing to the arrival of a free radical, a = —152 8 kJ/mol (—36.4 2.0 kcal/mol) at — 196°C was reported (25). The correction from — 196°C to room temperature is... 

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. 

Niobium Penta.fIuoride, Niobium pentafluoride is prepared best by direct fluorination of the metal with either fluorine or anhydrous hydrofluoric acid at 250—300°C. The volatile NbF is condensed in a pyrex or quartz cold trap, from which it can be vacuum-sublimed at 120°C to yield colorless monoclinic crystals. It is very hygroscopic and reacts vigorously with water to give a clear solution of hydrofluoric acid and H2NbOF ... 

Liquid nitrogen is used in cold traps to remove and recover solvents or volatile organic compounds from gas streams to reduce atmospheric emissions. Liquid nitrogen can be used to accelerate the cooldown time for process reactors (29). 

Techniques for handling sodium in commercial-scale appHcations have improved (5,23,98,101,102). Contamination by sodium oxide is kept at a minimum by completely welded constmction and inert gas-pressured transfers. Residual oxide is removed by cold traps or micrometallic filters. Special mechanical pumps or leak-free electromagnetic pumps and meters work well with clean Hquid sodium. Corrosion of stainless or carbon steel equipment is minimi2ed by keeping the oxide content low. The 8-h TWA PEL and ceiling TLV for sodium or sodium oxide or hydroxide smoke exposure is 2 mg/m. There is no defined AID for pure sodium, as even the smallest quantity ingested could potentially cause fatal injury. 

The role, design, and maintenance of creepproof barriers in traps, especially those in oil DPs, remain to be fully explored. In general, uncracked oil from a DP is completely inhibited from creeping by a surface temperature of <223 K. On the other hand, a cold trap, to perform effectively in an ordinary vacuum system, must be <173 K because of the vapor pressure of water, and <78 K because of the vapor pressure of CO2. For ultracontroUed vacuum environments, LN temperature or lower is required. CO2 accumulation on the trap surface must be less than one monolayer. The effectiveness of a LN trap can be observed by the absence of pressure pips on an ionization gauge when LN is replenished in the reservoir. 

Diffusion pumps operate at veiy low pressures. The ultimate vacuum attainable depends somewhat upon the vapor pressure of the pump liquid at the temperature of the condensing surfaces. By providing a cold trap between the diffusion pump and the region being evacuated, pressures as low as 10 mmHg absolute are achieved in... 

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). 

Methylenecyclopropane, b.p. 11° (760 mm.), is volatile at room temperature all adapter fittings must be carefully checked. The checkers recommend the use of two cold traps in series. 

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. 

In order to manufacture such polymers, it is first necessary to produce a very pure form of formaldehyde. This is typieally produced from an alkali-precipitated low molecular weight polyformaldehyde which has been carefuly washed with distilled water and dried for several hours under vacuum at about 80°C. The dried polymer is then pyrolysed by heating at 150-160°C, and the resultant formaldehyde passed through a number of cold traps (typically four) at -15°C. Some prepolymerisation occurs in these traps and removes undesirable... 

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. 

O. Nishimura, Application of a thermal desorption cold trap injector to multidimensional GC and GC-MS , J. High Resolut. Chromatogr. 18 699-704(1995). 

When columns of the same polarity are used, the elution order of components in GC are not changed and there is no need for trapping. However, when columns of different polarities are used trapping or heart-cutting must be employed. Trapping can be used in trace analysis for enrichment of samples by repetitive preseparation before the main separation is initiated and the total amount or part of a mixture can then be effectively and quantitatively transferred to a second column. The main considerations for a trap are that it should attain either very high or very low temperatures over a short period of time and be chemically inactive. The enrichment is usually carried out with a cold trap, plus an open vent after this, where the trace components are held within the trap and the excess carrier gas is vented. Then, in the re-injection mode the vent behind the trap is closed, the trap is heated and the trapped compounds can be rapidly flushed from the trap and introduced into the second column. Peak broadening and peak distortion, which could occur in the preseparation, are suppressed or eliminated by this re-injection procedure (18). 

Freshly distilled cyclohexanone, 10.8 g (0.11 mole), is added toO. 10mole of methylene-triphenylphosphorane, and the reaction mixture is stirred at room temperature for 30 minutes followed immediately by distillation under reduced pressure to give about 8 g (85%) of methylenecyclohexane, bp 42" (105 mm), which is collected in a cold trap. 

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