Substitution reactions general considerations

Mechanisms of ligand substitution reactions general considerations 325... 

Now let s consider the effect of the substrate on the rate of an E2 process. Recall from the previous chapter that Sn2 reactions generally do not occur with tertiary substrates, because of steric considerations. But E2 reactions are different than Sn2 reactions, and in fact, tertiary substrates often undergo E2 reactions quite rapidly. To explain why tertiary substrates will undergo E2 but not Sn2 reactions, we must recognize that the key difference between substitution and elimination is the role played by the reagent. In a substitution reaction, the reagent functions as a nucleophile and attacks an electrophilic position. In an elimination reaction, the reagent functions as a base and removes a proton, which is easily achieved even with a tertiary substrate. In fact, tertiary substrates react even more rapidly than primary substrates. 

Reactions and reactivity of nucleophiles with thiolsulfonates 137 Nucleophilic substitutions of sulfenyl derivatives general considerations 139 Bimolecular substitution at sulfenyl sulfur stepwise or concerted 140 Reversibility in reactions of nucleophiles with cyclic thiolsulfonates 145 Other reactions of thiolsulfonates 147... 

Cquare planar complexes are generally of the low-spin d8 type. This includes the four-coordinated complexes of Ni (II), Pd(II), Pt(II), Au(III), Rh(I) and Ir(I). The best known and most extensively studied are the compounds of Pt(II). The kinetics and mechanisms of substitution reactions of these systems have been investigated in considerable detail. Studies on complexes of the other metal ions are rather limited, but the results obtained suggest that their reaction mechanism is similar to that of the Pt(II) systems. This paper briefly surveys some of the available information, and presents the current view on the mechanism of substitution reactions of square planar complexes. 

The mechanism for the displacement reactions has also received considerable attention (Eaborn, 1960). A two-step process is generally regarded as satisfying the facts. Reasoning by analogy with conventional aromatic substitution reactions, the first step is presumed to be... 

The barrier that the reaction must overcome in order to proceed is determined by the difference in the solvation of the activated complex and the reactants. The activated complex itself is generally considered to be a transitory moiety, which cannot be isolated for its solvation properties to be studied, but in rare cases it is a reactive intermediate of a finite lifetime. The solvation properties of the activated complex must generally be inferred from its postulated chemical composition and conformation, whereas those of the reactants can be studied independently of the reaction. This is the reason why very little predictive information can be obtained, even though the explanatory power of the transition state theory is very considerable. For organic nucleophilic substitution reactions,... 

In this regard, the rule under consideration (i.e. the radical substitution reaction is generally developed via formation of the most inactive radical), fulfilled for radical substitution proceeding with the transfer of a single hydrogen atom, is untrue if the transfer of hydrogen atoms from the donor happens simultaneously [6],... 

Orientation. A multiply bonded nitrogen atom deactivates carbon atoms or to it toward electrophilic attack thus initial substitution in 1,2- and 1,3-dihetero compounds is generally as shown in structures 156 and 157. Pyrazoles 156 (Z = NH), isoxazoles 156 (Z = O), isothiazoles 156 (Z = S), imidazoles 157 (Z = NH, tautomerism can make the 4- and 5-positions equivalent), and thiazoles 157 (Z = S) do indeed undergo electrophilic substitution as shown. Little is known of the electrophilic substitution reactions of oxazoles 157 (Z = O) or compounds containing three or more heteroatoms in one ring. Deactivation of the 4-position in 1,3-dihetero compounds 157 is less effective because of considerable double bond fixation (cf. Sections 2.4.3.2.1 and 3.4.3.1.7), and if the 5-position of imidazoles or thiazoles is blocked, substitution can occur at the 4-position 158. 

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