Direct replacement reactions

C04-0038.Metallic titanium is produced from mtile ore (Ti02) by a direct replacement reaction with... 

The adducts B10H12L2 (structure 40) can be prepared by direct reaction of B10H14 with L or by ligand replacement reactions ... 

The classical theory of methylation with diazomethane was developed by Arndt from a different basis. It depends on the postulate (which can be traced back to von Pechmann " 0 of direct methylation mobile hydrogen in an acid compound is directly replaced by the methyl group, i.e., the methyl group appears in the place which the hydrogen previously occupied. For the reaction of tautomeric substances with diazomethane, the following equation is applicable ... 

The production of aromatic bromo-compounds is rarely an end in itself. More commonly the bromo-compounds are precursors for some further synthetic steps. Often, the bromine is replaced either in a direct substitution reaction or via... 

Carbon monoxide insertion is not restricted to transition metal-carbon bonds, although M—C is by far the most common substrate involved. Reactions have also been reported which lead to insertion of CO into M—O (114) and M—N (199) bonds. 1,1-Additions of M—H (27, 114) and M—M (104) linkages to CO have been postulated, too. However, direct replacement of CO, without rupture of the W—H bond, is indicated for the reaction between CpW(CO)3H (or -D) and PPhj (5) ... 

The argument against Scheme 1 is a negative one. Its basis derives from extensive studies carried out on the ligand replacement reactions of oxorhenium complexes of the family MeReO(dithiolate)L (20,34-37). Those studies (Sections V.B and V.C) show that all such processes studied to date proceed by direct displacement reactions without a recognizable intermediate from unassisted Re-L dissociation. (Indeed, in an early work, a dissociative step was written, but that formulation has since been revised see Section V.D.)... 

Although anthraquinone is the starting point for the preparation of many derivatives, involving substitution and replacement reactions, certain compounds are obtained directly by varying the components in the above synthesis. Thus, for example, replacement of benzene with methylbenzene (toluene) leads to the formation of 2-methylanthraquinone. A particularly important variation on the phthalic anhydride route is the synthesis of 1,4-dihydroxyanthraquinone (6.6 quinizarin) using 4-chlorophenol with sulphuric acid and boric acid as catalyst (Scheme 6.3). The absence of aluminium chloride permits hydrolysis of the chloro substituent to take place. 

The Fischer- Helferich method, as a direct anomeric-oxygen replacement reaction (Scheme 1, path A), has been very successfully applied for syntheses of simple alkyl glycosides. However, because of its reversibility, it has not gained general importance in the synthesis of complex oligosaccharides and glycoconjugates (1). 

How is the course of halogen substitution in the benzene nucleus to be explained It is not at all probable that direct replacement of hydrogen occurs, such as we must assume in the formation of benzyl chloride and in the reaction between methane and chlorine, since the hydrogen attached to the doubly bound carbon atom of olefines exhibits no special reactivity. However, various facts which will be considered later (p. 164) indicate that benzene reacts with halogen in fundamentally the same way as does ethylene. The behaviour of ethylene towards bromine is the subject of the next preparation. 

Zinc metal reacts spontaneously with an aqueous solution of copper sulfate when they re placed in direct contact. Zinc, being a more reactive metal than copper (it s higher on the activity series of metals presented in Chapter 8), displaces the copper ions in solution. The displaced copper deposits itself as pure copper metal on the surface of the dissolving zinc strip. At first, the reaction may appecir to be a simple single replacement reaction, but it s also a redox reaction. 

No reaction is known which offers the slightest evidence that bromine or iodine is capable of directly replacing chlorine in its oxy-aeids and from a theoretical point of view, such a reaction, implying as it does the direot substitution of an element joined to a number of oxygen atoms, is exceedingly improbable.—A. Michael and W. T. Conn. 

Nucleophilic reagents can also react with 2- and 4-aminopyridines at the carbon atom which carries the amino group in a replacement reaction (e.g. 738 — 739) similar to, but far less facile than, that undergone by chloro and alkoxy compounds, etc. In this way aminopyrimidines can be converted into pyrimidinones by direct acidic or alkaline hydrolysis under rather vigorous conditions. 

The lithium-copper oxide cell is voltage compatible (OCV = 1.5 V), i.e. it may be used as a direct replacement for conventional Leclanche or alkaline zinc cells. CuO has a particularly high volumetric capacity (4.2 Ah/cm3) so that cells are characterized by high specific energy -300 Wh/kg (700 Wh/dm3). The discharge curve shows a single step which may be attributed to the simple displacement reaction ... 

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