Freeze-pump-thaw

MISCELLANEOUS TECHNIQUES Freeze-pump-thaw and purging... 

Isobutene [115-11-7] M 56.1, b -6.6 /760mm. Dried by passage through anhydrous CaS04 at 0°. Purified by freeze-pump-thaw cycles and trap-to-trap distn. 

Methyl nitrate [598-58-3] M 77,0, b 65 /760mm, d 1.2322, d 1,2167, d 5 1,2032. Distd at -80°. The middle fraction was subjected to several freeze-pump-thaw cycles. VAPOUR EXPLODES ON HEATING. 

Can be purified by zone melting or by distn under vacuum at 0 , subjecting the middle fraction to several freeze-pump-thaw cycles. An impure sample containing higher nitroalkanes and traces of cyanoalkanes was purified (on the basis of its NMR spectrum) by crystn from diethyl ether at -60° (cooling in Dry-ice)(Parrett and Sun J Chem Educ 54 448 7977]. 

Alternatively the gas is passed over CuO pellets at 300° to remove hydrogen and hydrocarbons, over Ca chips at 600° to remove oxygen and, finally, over titanium chips at 700° to remove nitrogen. Also purified by freeze-pump-thaw cycles and by passage over sputtered sodium [Arnold and Smith J Chem Soc, Faraday Trans 2 77 861 1981]. 

Methanol (certified A.C.S., purchased from Fisher Scientific) was distilled from Mg(OMe)2 and was degassed by three freeze-pump-thaw cycles prior to use. 

The photodegradation of PVCa solutions has been studied to determine a mechanism for the photodegradation process and identify the excited states involved. The polymer was secondary grade (Aldrich Chemical Company) that was further purified by reprecipitation three times from methylene chloride solution by the dropwise addition into methly alcohol. Solutions in methylene chloride were found to yellow rapidly in air upon exposure to 360 nm light, whereas solutions degassed by the freeze-pump-thaw technique showed little yellowing. 

Preparation of a Typical Au-Acetone Colloid. The metal atom reactor has been described previously. (39,59, 60) As a typical example, a W-A1 0 crucible was charged with 0.5Qg Au metal (one piece). Acetone (300 mL, dried over K2C0 ) was placed in a ligand inlet tube and freeze-pump-thaw degassed with several cycles. The reactor was pumped down to 1 x 10 Torr while the crucible was warmed to red heat. A liquid N2 filled Dewar was placed around the vessel and Au (0.2g) and acetone (80g) were codeposited over a 1.0 hr period. The matrix was a dark purple color at the end of the deposition. The matrix was allowed to warm slowly under vacuum by removal of the liquid N2 from the Dewar and placing the cold Dewar around the reactor. 

Poly(styrene) and PMMA were synthesized from their respective monomers using azobisisobutyronitrile-initiated radical polymerization in benzene. Four freeze-pump-thaw cycles were used to degas the monomer solutions and polymerization was carried out for 48 hours at 60°C. The polymers were purified by multiple reprecipitations from dichloromethane into methanol. Films of these polymers were prepared and found to be free of any fluorescent impurity. 

Methoxy-2 -methylbiphenyl. o-Tolylboronic acid, 10.0 g (73.6 mmol) (Note 1), 16.8 g (71.8 mmol) of 4-iodoanisole (Note 2), and 200 mL of acetone (Note 3) are combined in a 1-L, three-necked flask equipped with an efficient stirbar, two stoppers, and a reflux condenser attached to a gas-flow adapter with a stopcock. Potassium carbonate, 25.0 g (0.180 mol), is dissolved in 200 mL of water (Note 4) in a separate 250-mL Schlenk flask. In a third flask (25-mL Schlenk flask) 3.30 mg (0.02 mmol, 0.2%) of palladium acetate (Note 5) is dissolved in 10 mL of acetone. All three flasks are then thoroughly degassed by four freeze-pump-thaw cycles. Under an argon back flow, one of the stoppers on the three-necked flask is replaced with a rubber septum, and the carbonate and catalyst solutions are added via cannula to form a biphasic mixture. The top layer turns brown upon addition of the catalyst. The septum is... 

Typically three to five evacuation/argon flush cycles are employed. This deoxygenation protocol is simpler and faster than a freeze-pump-thaw procedure. 

Under an inert atmosphere, a solution of the substrate in benzene was added to a slurry of freshly purified CuCl (5 mol%), Me2PPh (6 equiv./Cu), and t-butanol (10 equiv./Cu) in benzene (final concentration 0.4-0.8 M in substrate). After degassing with one freeze-pump-thaw cycle, the suspension was placed under a slight positive pressure of hydrogen and allowed to stir until completion, as monitored by TLC. The product was isolated and purified as described above. 

OEP deuterated at the meso positions was obtained by keeping OEP in concentrated D2SO for 24 h (31). All samples were thoroughly degassed by freeze-pump-thaw cycles on a high vacuum line. 

Triisopropylthiobenzophenone 20a and its p-methoxy derivative 20b were chosen for study (Scheme 11). Like thiobenzophenone, thiones 20 show well-separated absorption maxima in the visible (600 nm, nTi" ) and UV (320 nm, Tm" ) regions of the spectrum. In contrast, photolysis of freeze-pump-thaw-deoxygenated solutions of thiones 20a and 20b in benzene through Pyrex (>290 nm) afforded excellent chemical yields (ca. 70%) of the benzocyclo-butenethiol derivatives 21a and 21b. The same photoreaction could be brought about in the crystalline state at a much-reduced rate. 

A further advantage of this catalytic system is that the organic cycloadducts can be easily separated from the aqueous phase, allowing the catalyst solution to be isolated and directly reused. The catalyst retains its activity for the cycloaddition through several separate uses. As illustrated for the conversion of 108 to 109, the yield of the cycloaddition remained approximately constant for five reuses of the catalyst and decreased only with the sixth use (Tab. 13.9). With alkynoate substrate 94, the experiment was repeated with rigorous deoxygenation (by freeze-pump-thaw cycles) of the catalyst solution between uses, and it was found to function cleanly after eight uses (Tab. 13.8, entry 4). 

Toluene is first distilled from sodium benzophenone ketyl under argon. Prior to the reaction, the toluene is degassed by performing two freeze-pump-thaw cycles, then the flask is backfilled with argon. 

Materials. An H-ZSM-5 type zeolite (overall Si/Al = 33.5) was provided by DEGUSSA, Wolfgang, F.R.G. The well-shaped crystallites had an average size of 8.8 p x 5.2 p x 3.2 y. Pyridine, benzene and ethylbenzene were from MERCK, Darmstadt, F.R.G., spectroscopic grade, purified by distillation as well as repeated freeze-pump-thaw cycles and finally stored over highly activated 3A molecular sieve pellets. 

All test solutions, except otherwise mentioned, were degassed by hree freeze-pump-thaw cycles with a vacuum line operated under 10 torr pressure. Problems of background fluorescence originating from sulfonates themselves could be adequately minimized by proper choice of probe/sulfonate ratios. The excitation wavelength was set at 340 nm. Emission spectra were obtained in constant energy mode. The spectral resolution was 3 nm. 

I.6.2. A variety of models If the most rigorous technique is not required, for example, if a monomer is to be polymerised at, say, 80 °C by a radical initiator, then solvent and monomer are run into the mixing chamber, the catalyst is added and left to dissolve, the assembly is then attached to a vacuum Une to allow the reaction mixture to be degassed by the conventional freeze-pump-thaw process and to facilitate the filling of the body and the capillary. When this has been done, the dilatometer is thermostatted and the height of the meniscus in the capillary is monitored by means of a catheto-meter. The simple dilatometer adequate for this can be modified for more... 

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