Reactions bead test

Quartz glass (silica), pyrex (borosilicates) and other household and laboratory glasses (boroaluminosilicates) owe their high chemical resistance to the (tetrahedral) network forming properties of Si, B, P and A1 (cf., the borax and phosphate bead tests in qualitative dry reactions on the salts of numerous metals). 

A useful reaction which may be carried out at this stage is the microcosmic bead test (Section II.2, test 6). This test is carried out in a loop of platinum wire exactly as for the borax bead test. The presence of a white skeleton (of silica) in the coloured glass indicates silicate. Tin(IV) oxide, Sn02, dissolves slowly in the bead may be mistaken for silica. 

Apply the microcosmic bead test If a skeleton bead is obtained, silica or a silicate is indicated. A negative result does not definitely prove that silica or a silicate is absent, as a skeleton is not always formed. The silicon tetrafluoride test should then be employed (Section IV.26, reaction 6). 

The color of the borax is due to copper metaborate, Cu(B02)2/ that has been formed. The color of many metallic borates is characteristic of the particular metal that is present. Thus, the presence of certain metals is sometimes determined by what are called borax bead tests. Sodium metaborate, NaB02, was the other product in the borax bead reaction. A different borate of sodium will be prepared below. 

Neutralization does not necessarily involve the solvent. Some discussion of the role of the solvent will be given in Chapter 8. Here we shall consider a few reactions which do not involve the solvent. Some have already been mentioned, e.g., the gaseous phase reaction between boron fluoride and amines or ammonia. Others occurring at high temperatures have been listed by Audrieth and Moeller. The formation of certain electrolytic melts, borax and metaphosphate bead tests, the manufacture of glass and cement, and the formation of slag in the blast furnace may all be classified as neutralization reactions. ... 

In contrast, there are fewer limitations from the chemical point of view. The preparation of large, well-defined, libraries that involve amino acid building blocks has been demonstrated many times. Carefully optimized reaction conditions for the preparation of other mixed libraries can also ensure that each desired compound is present in sufficient amount. However, the reaction rates of some individual selectors with the activated solid support may be lower than that of others. As a result, the more reactive selectors would occupy a majority of the sites within the beads. Since the most reactive selectors may not be the most selective, testing of a slightly larger number of specifically designed CSPs may be required to reduce the effect of falsenegative results. 

Naltrexone in combination with lactide/glycolide copolymer has been investigated (83-87). Chiang (85) reported the clinical evaluations of a bead preparation containing 70% naltrexone and 30% of a 90 10 lactide/glycolide copolymer. Each subject received a 10-mg i.v. dose of naltrexone and a 63-mg dose by subcutaneous implantation of the beads. Average plasma naltrexone levels were maintained at 0.3-0.4 ng/ml for approximately 1 month. Two out of three subjects experienced a local inflammatory reaction at the site of implantation. This unexplained problem prevented further clinical testing of... 

In related work a library of 1,458 peptide ligands and various metal salts was tested in hydrolysis reactions of (p-nitrophenyl)phosphates.35 An active substructure composed of polymer-bound histidine in combination with Eu3+ was identified by further dissecting the original hit structure. It needs to be pointed out that catalytically active polymer beads can also be tested for catalytic activity using IR-thermography. In a seminal paper this was demonstrated using 7,000 encoded polymer beads prepared by split-and-pool methods, specifically in the metal-free acylation of alcohols.36... 

Two acylation reactions as depicted in Schemes 12.7 and 12.8 had the same issue when monitored by single bead FTIR (Fig.s 12.11 and 12.12). The starting resins (13) and (15) did not have convenient signal to monitor by FTIR. A chloranil test [15] specific for the secondary amines was used to confirm the complete consumption of (13), therefore the reaction completion. An iron-chloride-pyridine test [16] was used to confirm the complete consumption of (15). In both color tests, a blue color would suggest the presence of the starting material. In both cases we observed the disappearance of the blue color that indicated the reaction completion. 

The process of orthogonal testing highlights one of the major technical challenges in solution-phase processes the separation of individual products from each other. In SPS, of course, products are, by definition, always attached to some kind of solid support, such as resin beads or pins. In solution-phase experiments, products are thoroughly mixed with each other. Even if one can identify which among all possible products are desirable, some way must he found to separate those products from those of no interest to the experimenter and from excess reagents that may he present in the reaction mixture. 

In reactions in which HC1, HBr, or HI are evolved there is a convenient way of detecting and following the course of the reaction. Gas will be evolved and a bead of silver nitrate solution in a small loop of nichrome wire placed in this gas stream will become opaque, if these gases are present. Hydrogen fluoride will not do this since silver fluoride is very soluble in water. If a simple test for hydrogen fluoride is desired, a similar bead of calcium chloride will serve very well. 

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