Closed Teflon vessels

Although the ability of microwaves (MW) to heat water and other polar materials has been known for half a century or more, it was not until 1986 that two groups of researchers independently reported the application of MW heating to organic synthesis. Gedye et al. [1] found that several organic reactions in polar solvents could be performed rapidly and conveniently in closed Teflon vessels in a domestic MW oven. These reactions included the hydrolysis of amides and esters to carboxylic acids, esterification of carboxylic acids with alcohols, oxidation of alkyl benzenes to aromatic carboxylic acids and the conversion of alkyl halides to ethers. 

The pressure generated in a reaction vessel, and hence the rate enhancement, depends on a number of factors including the MW power level, the volatility of the solvent, the dielectric loss of the reaction mixture, the size of the vessel and the volume of the reaction mixture [7, 20]. Gedye et al. [20] found that, in the esterification of benzoic acid with a series of aliphatic alcohols (Scheme 4.1) in closed Teflon vessels, the most dramatic rate enhancements were observed with methanol (the most volatile solvent). 

With KF in the presence of a phase-transfer agent (18-crown-6) silyl ketene acetals react with aldimines to give -lactams within a few minutes under the action of micro-wave irradiation in closed Teflon vessels (Eq. 55). 

Moreno described the cycloaddition of 2,5-dimethylfuran (42) catalyzed by silica-supported Lewis acids under solvent-free conditions in closed Teflon vessels using a commercial microwave oven (Scheme 9.11) [28, 52]. Under these conditions coordination of the silica-supported catalyst with the oxygen bridge favors ring opening, thus leading to the aromatic compounds in one step. The use of Si (71) gave the best results for aromatic compounds. 

For the determination of arsenic by conventional inductively coupled plasma atomic emission spectrometry the samples were digested in closed Teflon vessels, similar to the technique described by Brzezinska et al. [126]. 

Decomposition in closed Teflon vessels at medium pressure (up to 8 bar) with microwave heating [43-44]. Microwave Digestion System MDS-81D (CEM Corporation, North Carofina, USA Floyd Inc., South Carolina, USA). 

Soil (total) Digestion of sample with HCI/HN03with heat in closed Teflon vessel in microwave dilution reduction with SnCI2 and hydroxylammonium chloride in H2SQ4 purging to detector CVAAS No data 90-110 Van Delft and Vos 1988... 

Indoloquinoxaline 693 was obtained in 83-90% yield by irradiating a mixture of isatin with o-phenylenediamine in acetic acid in an MW oven in either an open flask or closed Teflon vessel for 1-2 min. Carbethoxymethylation of isatin with ethyl chloroacetate in the presence of K2CO3 and Nal was carried out under MWI to give compound 694 in 70% yield within 4min. It could be converted into 695 by reaction with o-phenylenediamine in acetic acid in either an open flask (2 min) or closed Teflon vessel (Imin) (Scheme 134). Alternatively, carbethoxymethylation of 693 under MWI for 6 min afforded a 53% yield of 695. Reaction of 695 with hydrazine hydrate in ethanol under MWI gave the hydrazide 696 in 84% yield. MWI has been also used to condense 696 with aromatic aldehydes and monosaccharides to yield the hydrazone derivatives 697 in 58-99% yields (Scheme 134) (05JCR(S)299). 

A) Pressure decomposition in a closed Teflon vessel inside a steel cylinder ... 

The uranium content in plants and mushrooms grown on a uranium-contaminated site in Germany was determined by ICPMS (Baumann et al. 2014). All samples were sent to the laboratory immediately after collection and processed there. First, the samples were rinsed with deionized water and then cut with a knife and separated into liquid sap and solid residue by centrifugation. Birch leaves were hacked by a food processor and centrifuged, but the amount of sap was not always sufficient for analysis. The residues were digested in a closed Teflon vessel in a microwave oven for half an hour. The uranium content in the sap and digested samples was determined by ICPMS. Some samples were also analyzed by time-resolved laser-induced fluorescence (TREES) in a He-cryostat that is suitable for speciation studies. 

In a preliminary report, Bose et had described an efficient and rapid synthesis of number of P-lactams under microwave irradiation (MWI). Further in closed Teflon vessels using KF and phase transfer catalyst (PTC), P-lactams have been synthesized in few minutes from ketene silyl acetal and aldimines (Scheme 53). ... 

Closed vessel microwave digestion for bones, teeth, hair, and soil Specially designed closed pressurized Teflon vessels may be used for microwave digestion. Teflon is transparent to microwaves, which enhances the effect of the acids by raising the temperature and pressure within the vessel. In addition the closed vessels will retain any volatile components (e.g., Si) in solution. It must, however, be emphasized that any sealed vessels must only be heated if they have been designed for the purpose. Examples of applications include Kingston and Walter (1992), Baldwin et al. (1994), Sheppard et al. (1994), and Tamba et al. (1994). 

About 0.1-2.0g of wet sediment was placed into a Teflon vessel and 3mL of concentrated nitric acid, 0.5mL concentrated perchloric acid, and 4mL concentrated hydrofluoric acid were added. The closed vessels were kept at room temperature for lh. The samples were then placed in a pressure cooker and heated for lh on a hot plate at a temperature of 300°C. After cooling, the vessels were uncapped and the samples evaporated to 2mL on a hot plate at 250°C. After cooling, 3mL concentrated nitric acid was added. To complex the fluorides, lg boric acid was added to each sample. The solutions were transferred to lOOmT volumetric flasks and adjusted to volume with deionized water. Inorganic arsenic standards, having the same acid content as the samples, were used for calibration. 

A Teflon vessel was charged with l-phenyl-4-vinylpyrazole 1 (170 mg, 1 mmol) and dimethyl acetylenedicarboxylate 2 (426 mg, 3 mmol) and then closed and the reaction mixture irradiated in a domestic oven at 780 W for 6 min (final tempera-... 

Figure 36-1 is a schematic of a commercially available closed digestion vessel designed for use in a microwave oven. It consists of a Teflon body, a cap, and a safety relief valve designed to operate at 120 10 psi. At this pressure, the safety valve opens and then reseals. 

The size of the blank can be reduced in various ways. The equipment used for sampling and for commination of the sample should be made, if possible, from material which does not contain the element to be determined. In some cases the admixtures can be removed by the use of an appropriate solvent, which does not react with the sample but dissolves the impurities (e.g., the use of hydrochloric acid to remove iron from a sample of silicon comminuted in a steel mortar). Quartz, polyethylene or Teflon vessels are used instead of glassware. To prevent contact of the sample with laboratory air, certain chemical operations may be conducted in closed chambers (dry boxes) flushed with purified air or inert gas [95, 96]. 

Figure 6 22. High-pressure DTA apparatus of KubaMa and Schneider (75). (u) a. r = coolers. b, n = pyrophillite insulating disk c = heating block d = high-pressure vessel e. m — copper seals f, I = Zr02 blocks g = copper shield h = calorimeter block i = thermocouple k = corundum capillary o = fastening screw p = support c] = in- and outlet of refrigerant. (/ )(i) Open Pt/Ir well In) closed teflon well with brass holder lh = steel-sheathed thermocouple).

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