Vacuum stopcocks

The distinguishiag features are (i) the special stopcock is constructed to turn easily under a high vacuum (ii) a moulded arrow on the stopcock indicates the open position, euid a quarter turn from this position closes the desiccator and (iii) a special liquid container of the non-spill type. 

The addition of liquids or solutions to the reaction vessel can usually be carried out by use of a simple dropping funnel surmounted, if necessary, by a drying tube. For reactions carried out under pressure, vacuum, or an inert atmosphere, a pressureequalizing dropping funnel is required (Fig. A3.3). Care must be exercised in either case since a single setting of the stopcock will result in a decreased rate of addition as the liquid head diminishes. 

Fig. A.5. An apparatus for the lsO-labclling of the CO2 produced in biolumines-cence reactions. The stopcocks A-E and the ground joint of reaction vessel are lubricated with high-vacuum grease, and the connections between the glass tubes are made with short pieces of thick-walled silicon rubber tubing. The stopcocks C and D are high-vacuum type, such as Ace Glass, Cat. No. 8197-04 and 8195-236, or Corning, Cat. No. 7473-3.

A suspension of 3.324 g (lO.Ommol) of (/ )-2-hydroxy-1,2,2-triphenylelhyl acetate in 40 mL of THF is stirred under nitrogen at — 78°C in a 250-mL. two-necked flask equipped with a three-way stopcock connected to a vacuum pump and a nitrogen line, septum, and magnetic stirring bar. After dropwise addition of the LDA solution, the mixture is stirred in an ice bath for 1 h to complete the double deprotonation. 

Today, the glassware required consists of either a round-bottom flask or a Schlenk tube serving as the reaction chamber. This chamber is equipped with a magnetic stirbar and a Teflon high-vacuum valve (or glass stopcock) which provides for easy vacuum control after attachment to a vacuum line (Fig. 8.7). 

The submitters used a two-necked, round-bottomed flask equipped with a magnetic stirbar, three-way stopcock connected to vacuum and a N2 source, and a solid addition tube. The addition tube was charged with methyl 4-iodobenzoate, tris(dibenzylideneacetone)dipalladium(0), and tri-o-tolylphosphine. After the zinc reagent was formed and the ice bath was removed, the solids contained in the addition tube were added in one portion by inverting the tube. 

The application of vacuum and its release must at all times be gradual, so that none of the crystalline Compound 118 or its dihydrotriazole is swept out by a surging air stream. A two-way stopcock may be conveniently used for this purpose. 

Fig. 10. Apparatus for the sublimation of decaborane. (A) cold finger (B) ground-glass joint for connection to high-vacuum system (C) high-vacuum stopcock ...

In a typical experiment, a 12.18-mmol sample of hydrogen chloride is frozen into a 500-ml. reaction bulb on the vacuum system. (Liquid nitrogen is the refrigerant.) An 8.22-mmol sample of (dimethylamino)difluorophosphine is then frozen into the bulb, the stopcock to the bulb is closed, and the system is allowed to warm slowly to 25°. Reaction is indi-... 

---