Mechanisms of substitution reactions,

In the latter case, a subdivision of this classification is made by considering the site of the electron pair after the heterolytic scission. When the electron pair remains in the reactive centre, it is named electrophilic heterolysis and when it accompanies the leaving group, it is called nucleophilic heterolysis. 

Nucleophilic substitution reactions are one of the most important classes of reactions in organic chemistry. In particular, 8 2 reactions are among the most extensively stndied chemical processes in solution and in the gas phase, both theoretically and experimentally. The history of the study of these reactions closely parallels (and is sometimes responsible for) the development of concepts such as structure-reactivity relationships, linear free-energy relationships, steric inhibition, kinetics as a probe of mechanism, stereochemistry as a probe of mechanism and solvent effects. 

Ingold and co-workers also considered another reaction mechanism, where the rate-limiting step is the heterolytic dissociation of the RX substrate 

The reaction rate is now determined by the rate of the slow step, heterolytic dissociation of the C-X bond. 

However, estimation of the enthalpy of dissociation of tertiary butyl bromide in water 

Now let us consider the application of kinetic and other techniques to the determination of reaction mechanisms. Reactions of coordination compounds can be divided into two broad categories substitution reactions and redox reactions. In each of these, a variety of mechanistic paths may be operable. 

Substitution react ions are generally classified as having either dissociative or associative mechanisms. We will use the general octahedral substitution reaction (23) 

The extreme example of an associative mechanism, the A mechanism, equation (25), 

The symbols SnI and Sn2 were introduced by Hughes and Ingold for organic reactions. The term SnI is used for dissociative mechanisms and means substitution nucleophilic unimolecular. The term Sn2 is used for mechanisms in which bond making is important and means substitution nucleophilic bimolecular. Let us now look at several systems to see how mechanistic information on substitution reactions of coordination compounds was obtained. 

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Mechanisms of substitution reactions of metal complexes. F. Basolo and R. G. Pearson, Adv. Inorg. Chem. Radiochem., 1961, 3, 1-89 (132). 

Kinetics and mechanisms of substitution reactions of octahedral macrocyclic amine complexes. C. K. Poon, Coord. Chem. Rev., 1973,10,1-35 (130). 

The kinetics and mechanisms of substitution reactions of metal complexes are discussed with emphasis on factors affecting the reactions of chelates and multidentate ligands. Evidence for associative mechanisms is reviewed. The substitution behavior of copper(III) and nickel(III) complexes is presented. Factors affecting the formation and dissociation rates of chelates are considered along with proton-transfer and nucleophilic substitution reactions of metal peptide complexes. The rate constants for the replacement of tripeptides from copper(II) by triethylene-... 

In recent years there has been a tendency to assume that the mechanisms of substitution reactions of metal complexes are well understood. In fact, there are many fundamental questions about substitution reactions which remain to be answered and many aspects which have not been explored. The question of associative versus dissociative mechanisms is still unresolved and is important both for a fundamental understanding and for the predicted behavior of the reactions. The type of experiments planned can be affected by the expectation that reactions are predominantly dissociative or associative. The substitution behavior of newly characterized oxidation states such as copper-(III) and nickel (III) are just beginning to be available. Acid catalysis of metal complex dissociation provides important pathways for substitution reactions. Proton-transfer reactions to coordinated groups can accelerate substitutions. The main... 

We have seen above that the structure of the substrate is the most important feature that dictates the mechanism of substitution reactions. Thus, the Sn2 mechanism is favoured when the reaction takes place at a primary centre, whereas an SnI mechanism is preferred at tertiary centres, or where stable intermediate carbocations can be produced (see Section 6.2.3). 

Mechanisms of Substitution Reactions of Metal Complexes Fred Basolo and Ralph 0. Pearson... 

Mechanisms of Substitution Reactions of Cobalt (III) Cyanide Complexes... 

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 field of nickel complexes with macrocydic ligands is enormous and continuous interest in this area in recent years has resulted in innumerable publications. A number of books and review articles are also available covering the general argument of the bonding capability of the various macrocydic ligands towards transition and non-transition metals. 22 2627 Synthetic procedures for metal complexes with some tetraaza macrocycles have been reported.2628 Kinetics and mechanism of substitution reactions of six-coordinate macrocydic complexes have also been reviewed.2629... 

The significant changes imposed on the dithioaromatic ligands and complexes upon sulfur addition are illustrated in the structure of the Ni(p-/-PrPhDtaXp-(-PrPhDtaS) complex (Fig. 48) (Table XXII), determined by Fackler et al. (233, 257). The same workers explored the sulfur addition and abstraction reaction in depth (232) (see also Section IV). The rates and mechanisms of substitution reactions of square planar nickel(II) 1,1 -dithiolate complexes (502) is discussed in Section IV. 

A similar study of the rates and mechanisms of substitution reactions of the Pt(II) 1,1-dithiolate complexes, Pt(L)2 (L = /-MNT2, NED2, CDC2 and CPD2-), also has been reported (352). The mechanism proposed for the substitution reactions of the Pt(L)2- complexes with bidentate 1,1-dithio chelates is the same as that shown in Fig. 60. 

The mechanism of substitution reactions at saturated silicon centers is well studied, regarding both kinetics and stereochemistry13,14. In contrast, addition reactions to unsaturated silicon centers, such as to disilenes and silenes, are relatively unexplored. The reason is clear suitable substrates for investigations of regio- and stereochemistry and reaction kinetics are not readily available due to inherent kinetic instability of disilenes and silenes. Kinetically stabilized disilenes and silenes are now available, but these are not always convenient for studying the precise mechanism of addition reactions. For example, stable disilenes are usually prepared by the dimerization of silylenes with bulky substituents. Therefore, it is extremely difficult to prepare unsymmetrically substituted disilenes necessary for regio- and/or stereochemical studies. 

Figure 6.2 shows the standard mechanism of substitution reactions carried out on carboxylic acid derivatives in neutral or basic solutions. The tetrahedral intermediate—formed in the rate-determining step—can be converted to the substitution product via two different routes. The shorter route consists of a single step the leaving group X is eliminated with a rate constant Ad. In this way the substitution product is formed in a total of two steps. The longer route to the same substitution product is realized when the tetrahedral intermediate is proto-nated. To what extent this occurs depends, according to Equation 6.1, on the pH value and on the equilibrium constant Kcq defined in the middle of Figure 6.2 ...

Nearby groups can evidently increase the rate of substitution reactions significantly. Now, you may be thinking back to Chapter 17 and saying yes, yes, we know that —when we were discussing the mechanisms of substitution reactions we pointed out that a cation-stabilizing group at the reaction centre makes SnI reactions very fast for example—... 

The kinetics and mechanisms of substitution reactions studied in detail have been reviewed elsewhere 1-3). Here we shall summarize some recent data obtained in this field. As far as terminology is concerned, in the majority of cases that of Ingold 4) has been used, in which substitution of one ligand by another is regarded as a nucleophilic (SN) reaction. However, such a classification is rather rigid, and the term nucleophilicity is imprecise if one considers the variety of ligands from the simplest anions to olefins, acetylenes, arenes, etc. 

C tereochemistry has played a major role in the development of chemistry, and it continues to be most significant. Werner made extensive use of the information available to him on the stereochemistry of metal complexes in developing his coordination theory. He made the first meaningful attempt to understand the mechanisms of substitution reactions of these systems on the basis of the stereochemical changes accompanying such reactions. The paper 49) he wrote in 1912 is a real milestone and should be read by anyone interested in octahedral substitution reactions. It is valuable because of the large amount of experimental data it contains on reactions of cis and [Pg.408]

Explanations of the mechanisms of substitution reactions and their relation to solubility must involve consideration of two factors (1) the... 

During the last two decades, the volume of activation has become a recognized criterion to complement traditional investigations of the mechanisms of substitution reactions. At this stage, it may be useful to recall the classification... 

A perspective report emphasised the key role of the application of pressure in kinetic studies in bringing clarity to understanding the mechanism of substitution reactions of cobalamins.193 The effect of various alkyl substituents in the trans position on the kinetic, thermodynamic and ground-state properties has been studied. Cobalamins featuring in these studies were cyanocobalamin (vitamin Bi2), aquacobalamin and the complex formed when the cyano or water ligand is replaced... 

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