Cold trapping preparative

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. 

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 ... 

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. 

For high vacuum work, where it is desirable to shorten the pumping path to a minimum, a useful built-in cold trap is shown in Figure 52, V. The outer jacket is prepared from a round-bottomed flask to which the side arms are attached, as shown in Figure 52, IV. The inner tube is shaped with a flare at its upper end and its heated... 

To add to the silver salt (Eq. 19) and peroxymercuration (Eq. 34-36) methods of preparing dialkyl peroxides, a third mild alkylation procedure has been developed that involves the use of alkyl trifluoromethane sulphonates (triflates)55). The peroxide transfer reaction between bistributyltin peroxide and the bistriflate 58 of cis-1,3-cyclopentanediol provided one of the first syntheses of 2,3-dioxabicyclo[2.2.1]heptane 9 (Eq. 44, H for D Tf = 02SCF3)56). Because of the sensitivity of 9, it was necessary to carry out the reaction in vacuo with rapid transfer of the volatile products to a cold trap to avoid decomposition a yield of 22% was achieved. 

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). 

Instead of AES the molecular emission of SnO can be stimulated in an oxycavity placed in a H2/N2 flame, and measured at 408 nm. The recommended sample preparation consists of hydride generation and concentration by cold-trap collection LOD 80 pg Sn(II)/L in a 1 mL sample36. 

Hertler16 was the first to report the preparation ofpoly(tetrafluoro-p-xylylene) by a multistep synthesis as shown in Scheme 2. Pyrolysis (330°C, 0.025 Torr) of dibromotetrafluoro-p-xylene (B CgFL,) over zinc led to deposition of the polymer film in a cold trap. 

In addition to the analytical columns (columns used mainly for analytical work), so-called preparative columns may also be encountered. Preparative columns are used when the purpose of the experiment is to prepare a pure sample of a particular substance (from a mixture containing the substance) by GC for use in other laboratory work. The procedure for this involves the individual condensation of the mixture components of interest in a cold trap as they pass from the detector and as their peak is being traced on the recorder. While analytical columns can be suitable for this, the amount of pure substance generated is typically very small, since what is being collected is only a fraction of the extremely small volume injected. Thus, columns with very large diameters (on the order of inches) and capable of very large injection volumes (on the order of milliliters) are manufactured for the preparative work. Also, the detector used must not destroy the sample, like the flame ionization detector (Section 12.6) does, for example. Thus, the thermal conductivity detector (Section 12.6) is used most often with preparative gas chromatography. 

Vinylacetylene can also be prepared in high yields by slow addition (over 45 min) of rranj-dichlorobutene (0.45 mol), to a vigorously stirred mixture of 3 g MeN+O Cl", 250 g of KOH and 250 ml of water, heated at 100 C (bath temperature). N2 is slowly (200 ml/min) introduced both during and for 30 min after the addition of dichlorobutene. The contents of the cold traps are then "redistilled" as described above. 

Similarly, the ( + )-trans-2,3-dimethyl derivative gives (-)-(M)-2,3-pentadiene yield 53 mg (20%), after isolation of the volatile material in a cold trap and preparative GC 29% op [aj 5 -25.3 (CC14). 

In the preceding section, we emphasized that the surface interaction of metallic particles with liquid molecules is a very important parameter in the dispersion behavior of the sol. Since the surface nature is very sensitive to the surface modification, we can easily regulate it with the use of surfactant. As seen in Figure 9.4.23, almost all metallic particles cannot be dispersed in hexane as a suspension liquid. In this section, we show what kind of surfactant is effective in dispersion. The sample was prepared by the gas flow-cold trap method. We tested three surfactants, dimethyldi-... 

Compound 1 (500 mg, 1.39 mmol from photolysis of stilbene and 2-methylmer-capto-benzothiazole) was sealed under vacuum in a thick-walled Pyrex tube (the volume was about 100 mL) and heated to 200 °C for 20 h in the dark. Methan-ethiol (from the [2,l,3]-elimination) and dimethyldisulfide were condensed to a cold trap (-196 °C) at a vacuum line (65 mg, 97%). The residue was separated by preparative TLC on 200 g SiC>2 with benzene to give 340 mg (78%) of 2 (mp 108 °C, methanol) and 96 mg (22%) of 3. 

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