Vapor phase techniques

Introduction of a 3-bromosubstituent onto thiophene is accompHshed by initial tribromination, followed by reduction of the a-bromines by treatment with zinc/acetic acid, thereby utilizing only one of three bromines introduced. The so-called halogen dance sequence of reactions, whereby bromothiophenes are treated with base, causing proton abstraction and rearrangement of bromine to the produce the most-stable anion, has also been used to introduce a bromine atom at position 3. The formation of 3-bromotbiopbene [872-31-1] from this sequence of reactions (17) is an efficient use of bromine. Vapor-phase techniques have also been proposed to achieve this halogen migration (18), but with less specificity. Table 3 summarizes properties of some brominated thiophenes. 

Vapor-Phase Techniques. Vapor-phase powder synthesis teclmiques, including vapor condensation, vapor decomposition, and vapor—vapor, vapor—Hquid, and vapor—soHd reactions, employ reactive vapors or gases to produce high purity, ultrafine, reactive ceramic powders. Many nonoxide powders, eg, nitrides and carbides, for advanced ceramics are prepared by vapor-phase synthesis. 

Vapor-Phase Processing. Optical fiber preforms are prepared by vapor-phase techniques because of the superior clarity of the products. 

This technology also allows the control of refractive index profiles by doping. AH vapor-phase techniques use a vapor stream of volatile haUdes such as SiCl, GeCl, BCl, or POCl, and gases such as CI2 or O2. The reactants are oxidized and deposited onto a substrate to produce a soHd glass preform which is then drawn into a fiber. The variations of the technique differ in the way the reactants are oxidized (16). 

Polycrystalline ZnSe nanoparticles were synthesized from Me2Zn NEts and H2Se gas diluted in H2 [120] using a flexible vapor-phase technique [121]. In a counter-flow jet reactor (CJR) the vapors of Me2Zn NEts and H2Se react to form nanoparticles of ZnSe. This method provides a direct vapor-phase route for nanoparticles preparation. 

Armstrong, A. A., and H. A. Rutherford A vapor phase technique for radiation-induced grafting of vinyl monomers to fibers. Text Res. J. 33, 264... 

The improved aging characteristics noted with the iodide present indicate that it may be functioning as an antioxidant for the paper as well as a means of inhibiting the production of hydrogen peroxide. Therefore, the introduction of antioxidants into the paper by vapor phase techniques will be investigated as a possible means of further improvement of the expected life of the treated paper. 

Industrial vapor phase techniques enjoy a virtual monopoly today. However, interest is now being revived in liquid phase processes, after a long period of neglect... 

The flow sheet piindple is similar to those of vapor phase techniques, with the additional possibUity of recj diog the trimetbylbenzenes. The aromatics yield of the operation is greater than 95 molar per cent... 

It should be noted that in addition to solution-phase methods, quantum dots are frequently synthesized using molecular-beam epitaxy or other vapor-phase technique. For example, see Wang, X. Y. Ma, W. Q. Zhang, J. Y Salamo, G. J. Xiao, M. Shih, C. K. Nano Lett. 2005, 5, 1873, and references therein. 

CFC-113a can be fluorinated to CFC-114a and CFC-115 using vapor-phase techniques [reaction (13)]. Vapor-phase reduction of CFC-114a [reaction (14)] gives HCFC-124 and HFC-134a, both of which are under development as CFC alternatives. A similar reduction of CFC-115 gives its possible replacement HFC-125 [reaction (15)]. 

The catalytic properties of Au depend markedly on the preparation method, because it can bring about a large difference in the size of Au particles and the interaction with supports. Until now six methods, including solid-, liquid-, anH vapor-phase techniques, have proven effective for preparing active gold catalysts. 

At the industrial level, the processes employing these catalyst systems were fust abandoned quickly in favor of vapor phase techniques. In 1968, Mitsubishi Gas Chemical commercialized a technique employing the BF3 —HF mixture, both as an extractant and an isomerization catalyst This is achieved by a simple elevation in temperature to about 100°C (see Section 4.3.4). 

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