Cold finger traps

With all these cold finger traps the rate of loss of refrigerant is much greater than from the Dewars whose inner siufaces are silvered to cut down radiation losses. 

Cold finger refrigerant traps are often used in vacuum lines as a substitute for the more efficient total immersion traps which, however, tend to cut down the pumping speed of the system. In constructing a cold finger trap a Dewar seal is first made, then, before... 

Figure 52, 111, shows how a quite satisfactory cold finger trap can be made with the Dewar seal replaced by a straightforward internal seal. As before, the side arm must be added immediately the internal seal is completed and the whole annealed together. 

The competition between elimination and substitution channels when an alkyl halide is allowed to react with a nucleophile in the gas phase is a difficult problem to tackle, since in most gas-phase experiments only the ionic products of reaction are monitored (a few exceptions are reported below). Thus, for example, when w-propyl bromide is allowed to react with methoxide ion in the gas phase, the bromide ion produced can arise either by elimination (a) or by substitution (b) and the two pathways cannot be distinguished from the ions alone (Scheme 34). In this specific case it was possible to establish that the reaction follows exclusively the elimination channel through collection and analysis of the neutral products246. The experiments were performed on a FA apparatus configured with a novel cold finger trap coupled to a GC/MS system. Material collected by the trap was separated by capillary gas chromatography and the individual components identified by their retention times and El mass spectra246. 

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... 

The Dewar seal is important and useful, although a ring seal can sometimes be substituted in simple apparatus—as, for example, in the cold finger liquid air trap shown in Figure 52, III. In the making... 

Figure 52. Construction of cold finger refrigerant traps...

As the gases emerge from the heated area of the reactor, 2-phenyl-benzo[ ]azete can be trapped out on a cold finger maintained at -78 C. However, if the temperature is allowed to rise to about -40 C, the compound forms a dimer. 

Apparatus for CHjOeCH, C HjCsCH and H2C=CHCsCH a 1-1 three-necked, round-bottomed flask, equipped with a gas inlet tube, a mechanical stirrer and a "cold finger" reflux condenser (-75 C, fig. 17), cooled with dry ice and acetone the top of the condenser is connected to a cold trap ... 

The sodamide for dehydrobromination of dibromohendecanoic acid is prepared according to the method of Khan et al.,2 with some modifications (Note 2). A 3-1. three-necked flask is equipped with a stirrer and a Dry Ice-acetone cold finger reflux condenser attached through a drying tube to a trap and a bubbler filled with a saturated ammonia solution. After the con-... 

Difluoromcthylcnecydopropane (20, R = F 2.30 g, 25.6 mmol) was sealed under vacuum in a thick-walled 165-mL glass tube. The tube was heated to 312 C for 3.5 h, then cooled to liquid N2 temperature when opened and the contents (1.75 g, 76%) were transferred to a cold finger under vacuum. A quantity of polymeric material remained in the tube. The material that collected on the cold finger was thawed and trap-to-trap distilled to yield 0.38 g of a ca. 70 30 mixture of 20 (R = F) and 2,2-difiuoromcthylenecyclo-propane (by I9F NMR) and a white solid residue. The solid residue was sublimed under reduced pressure to give crystalline 21 (R = F) yield 0.68 g (30%) mp 64-65 C. 

Reaction between CF2and radicals has not been demonstrated in the gas phase but recently Mastrangelo65 has shown that -CF3 and CF2 trapped upon a liquid nitrogen cold finger, may interact on warming up to yield perfluoropropane and perfluorobutanc. Perfluoropropane has also been detected as a by-product in the photolysis of hexafluoroacetone. It is possible that all these reactions take place upon the walls of reaction vessels. 1,4-Dichlorooctafluorobutane was also detected in the high conversion photolyses but it was present only in trace quantities. From estimates of the concentration of these products, an approximate value for the disproportionation/combination ratio for -CFaCl radicals may be calculated. A value of 0.04 is obtained which is remarkably constant and independent of concentration and light intensity. 

All connections between the pieces of equipment are made with a in. copper tubing. Whenever the fluorocarbon obtained in the reaction is volatile at room temperature, a cold finger-type trap is placed between 8 and 9 to catch the product. The temperature of the trap must be high enough for passage of... 

KOH pellets (112 g, 2 mol) were heated to 110 C, under a slow stream of 02-free N2, in a 750-mL Pyrex flanged flask equipped with a Nichrome high-torque stirrer and a double-surface water-cooled reflux condenser topped with a cold finger (— 23 "C) condenser. 3-Bromo-l,l,3,3-tetrafluoroprop-l-ene (10 131.0 g. 0.68 mol) was added dropwise to the KOH, and fresh KOH pellets were added (total, 224 g) as discoloration occurred. Distillation of the product, collected in traps at — 72 and — 196 C, gave 11 yield 25.1 g (33%) bp — 37.6 C. 

Aromatic aldehydes (10 mmol) and trimethylorthoformate (20 mmol) was added to a mixture of sulfonamide (10 mmol), finely powdered calcium carbonate (9 g) and K-10 clay (2 g). The solid homogenized mixture was placed in a modified reaction tube which was connected to a removable cold finger and sample collector to trap the ensuing methanol and methyl formate. The reaction tube is inserted into Maxidigest MX 350 (Prolabo) microwave reactor equipped with a rotational mixing system. After irradiation for a specified period, the contents were cooled to room temperature and mixed thoroughly with ethyl acetate (2 x 20 mL). The solid inorganic material was filtered off and solvent was evaporated to afford tlie residue which was crystallized from the mixture of hexane and ethyl acetate. 

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