The use of other reagents

Pyridine hydrobromide perbromide (Pyr HBr Br2) has been employed as a convenient, easily weighed source of bromine for small scale-reactions. The chlorine analog has also been used in this connection. 

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The mechanism and stereochemistry of halogenation, physical methods / 273 Dihalogenation / 276 The use of A-halo compounds / 280 The use of other reagents / 282 The oxidation of halohydrins / 283 Vinylogous a-halo ketones / 284... 

The use of other reagents for the colorimetric detection and estimation of amino sugars has been recorded but they have not been widely employed. For example, sodium naphthoquinone 4-sulfate56 gives a stable color with glucosamine the intensity of which is dependent on the acidity or basicity of the environment. Vasseur57 has used orcinol (1,3,5-di-... 

In a similar reaction, the bisphosphoranylideneaminothiophene (360) with an aroyl chloride gives first the product (361) and then the tricyclic product (362). The use of other reagents in the second step enables various R groups to be introduced <93T58i>. 

By the use of other reagents certain oxidized ores may be floated. For the oxides and carbonates of basic metals, fatty acids such as oleic, and soaps, are frequently useful. These no doubt form an oriented thin film of the soap of the heavy metal present in the mineral, by double decomposition with the surface layer the long hydrocarbon chain is probably oriented outwards. By special activation it is said that even quartz may be floated by the judicious use of soaps, if its surface is contaminated naturally or artificially with small amounts of heavy metals. There would appear to be a vast range of possibilities in the separation of complex minerals, by proper selection of reagents capable of modifying their surfaces. A detailed study of the flotation of various silicate minerals, with oleic acid, has been given by Patek.4... 

In principle, all of the reagents known from organic synthesis as acylating reagents can be used however, acid anhydrides are used in most instances. The use of other reagents is motivated by efforts to eliminate acidic media, which can decompose the derivatives. Acyl imidazoles react with basic groups according to Scheme 4.9 on p. 61. 

In the Stobbe condensation side reactions encountered by the use of other reagents are considerable—e.g., in the condensation of benzophenone and diethyl succinate with sodium ethoxide and ether, a significant amount of benzhydrol is always obtained (16). The condensation of benzophenone with diethyl succinate, using sodium hydride dispersions, was completed rapidly and gave an excellent yield (95.7%) of )3-carbethoxy-7, 7 -biphenylvinyl acetic acid. 

Aqueous hydrofluoric acid dissolved in acetonitrile is a good catalyst for intramolecular Diels-Alder reactions [9] This reagent promotes highly stereoselective cyclizations of different triene esters (equation 8) The use of other acids, such as hydrochloric, acetic, and trifluoroacetic acid, results in complete polymerization of the starting trienes [9] (equation 8)... 

Other considerations aside, the use of dilute reagents minimizes effects of nonideality. This allows the use of concentrations in place of activities. Of course, the time scale, the sensitivity of the analytical method at different concentrations, and the use of other reaction components introduce additional considerations. Tied closely to this decision is the choice of solvent. Reaction rates may (or may not) be affected by such variables as polarity, dielectric constant, hydrogen-bonding ability, donor capacity, and viscosity. A change in solvent may change not only the rate but also the mechanism and possibly even the products. One cannot even assume that the net reaction is the... 

The method for creating acceptor sink condition discussed so far is based on the use of a surfactant solution. In such solutions, anionic micelles act to accelerate the transport of lipophilic molecules. We also explored the use of other sink-forming reagents, including serum proteins and uncharged cyclodextrins. Table 7.20 compares the sink effect of 100 mM (5-cyclodextrin added to the pH 7.4 buffer in the acceptor wells to that of the anionic surfactant. Cyclodextrin creates a weaker sink for the cationic bases, compared to the anionic surfactant. The electrostatic binding force between charged lipophilic bases and the anionic surfactant micelles... 

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