Types of substitution mechanism

In inorganic substitutions, the limiting mechanisms are dissociative (D), in which the intermediate has a lower coordination number than the starting complex (equation 25.3), and associative (A), in which the intermediate has a higher coordination number (equation 25.4).  

Dissociative and associative reaction mechanisms involve two-step pathways and an intermediate. 

In an I mechanism, there is no intermediate but various transition states are possible. Two types of interchange mechanisms can be identified  

In an 4 mechanism, the reaction rate shows a dependence on the entering group. In an 4 mechanism, the rate shows only a very small dependence on the entering group. It is usually difficult to distinguish between A and 4, D and 4, and 4 and 4 processes. 

An interchange (I) mechanism is a concerted process in which there is no intermediate species with a coordination number different from that of the starting complex. 

An intermediate occurs at a local energy minimum it can be detected and, sometimes, isolated. A transition state occurs at an energy maximum, and cannot be isolated. 

In most metal complex substitution pathways, bond formation between the metal and entering group is thought to be concurrent with bond cleavage between the metal and leaving group (equation 26.7). This is the interchange (I) mechanism. 

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Consider these results with respect to the mechanisms outlined in Fig. 5.6 (p. 274). Delineate the types of substituted 1-arylethyl halides which react with azide ion according to each of these mechanisms on the basis of the data given above. 

The scope of heteroaryne or elimination-addition type of substitution in aromatic azines seems likely to be limited by its requirement for a relatively unactivated leaving group, for an adjacent ionizable substituent or hydrogen atom, and for a very strong base. However, reaction via the heteroaryne mechanism may occur more frequently than is presently appreciated. For example, it has been recently shown that in the reaction of 4-chloropyridine with lithium piperidide, at least a small amount of aryne substitution accompanies direct displacement. The ratio of 4- to 3-substitution was 996 4 and, therefore, there was 0.8% or more pyridyne participation. Heteroarynes are undoubtedly subject to orientation and steric effects which frequently lead to the overwhelming predominance of... 

In the last chapter we saw the importance of nnderstanding mechanisms. We said that mechanisms are the keys to understanding everything else. In this chapter, we wiU see a very special case of this. Students often have difficulty with substitution reactions—specifically, being able to predict whether a reaction is an Sn2 or an SnI. These are different types of substitution reactions and their mechanisms are very different from each other. By focusing on the differences in their mechaiusms, we can understand why we get Sn2 in some cases and SnI in other cases. 

For toluene fluorination, the impact of micro-reactor processing on the ratio of ortho-, meta- and para-isomers for monofluorinated toluene could be deduced and explained by a change in the type of reaction mechanism. The ortho-, meta- and para-isomer ratio was 5 1 3 for fluorination in a falling film micro reactor and a micro bubble column at a temperature of-16 °C [164,167]. This ratio is in accordance with an electrophilic substitution pathway. In contrast, radical mechanisms are strongly favored for conventional laboratory-scale processing, resulting in much more meta-substitution accompanied by imcontroUed multi-fluorination, addition and polymerization reactions. 

But Ingold s triumph came in finally seeing the advantages of Robinson s explanation system, revising it, and substituting a new and clearer language and classification of types of reaction mechanisms. Lapworth, Robinson, and their collaborators referred to Ingold s "conversion" experience, a conversion in which Paul eventually helped create the myth of his role not as saint but as savior. 

The aquated Co(III) ion is a powerful oxidant. The value of E = 1.88 V (p = 0) is independent of Co(III) concentration over a wide range suggesting little dimer formation. It is stable for some hours in solution especially in the presence of Co(II) ions. This permits examination of its reactions. The CoOH " species is believed to be much more reactive than COjq Ref. 208. Both outer sphere and substitution-controlled inner sphere mechanisms are displayed. As water in the Co(H20) ion is replaced by NHj the lability of the coordinated water is reduced. The cobalt(III) complexes which have been so well characterized by Werner are thus the most widely chosen substrates for investigating substitution behavior. This includes proton exchange in coordinated ammines, and all types of substitution reactions (Chap. 4) as well as stereochemical change (Table 7.8). The CoNjX" entity has featured widely in substitution investigations. There are extensive data for anation reactions of... 

The above reactions are relatively straightforward in terms of mechanism. There are, however, a number of important transformations based on 1,3,5-triazine as starting material, and which result in formation of either pyridine or pyrimidine derivatives. 1,3,5-Triazine reacts very readily with nucleophiles, probably as outlined in equation (197) if X in (31) is a suitable electrophile, cyclization can take place. Thus, when X = CN, 4-amino-5-cyanopyrimidine is obtained, and other illustrative examples are shown in equation (198). This procedure is particularly useful for the preparation of 2-unsubstituted pyrimidines, a class of compound which is not readily accessible by other types of ring formation. The reverse type of transformation, i.e. of pyrimidines to 1,3,5-triazines, is also an important synthetic method, and one which has been studied in detail. Two types of substituted... 

Dessy et al.28-2 do not distinguish between aromatic substitution and aliphatic substitution and their conclusions as to the mechanism of reaction (21) apparently include both of these types of substitution. 

Several reactive chloro compounds have been used to attempt to effect the controlled monochlorination of aromatic amines. One such reagent is N-chlorosuccinimide, with which the chlorination of aniline, for example, can be largely restricted to monosubstitution, although a mixture of isomers (orthopara, 1.9 1) is obtained.24 One approach to the achievement of specific ortho chlorination is illustrated by the synthesis of o-chlorobenzanilide (Expt 6.61), readily hydrolysable to o-chloroaniline. The anilide is formed, by a type of Swi mechanism indicated below, when AT-phenylhydroxylamine is benzoylated and the product is treated with thionyl chloride.25 The reaction has been successfully applied to several substituted JV-phenylhydroxylamines, prepared by the controlled reduction of the corresponding substituted nitro compounds (cf. Expt 6.87). 

We shall discuss several types of no-mechanism reactions first and some-mechanism reactions second. From an orbital viewpoint, all heterolytic rearrangements would be subsumed under sigmatropic reactions heterolytic substitutions and additions (eliminations) turn out to be versions of cycloadditions (cycloeliminations). Nevertheless, we have retained categories on the basis of mechanism, because these are familiar, and because the trend is from the relatively simple to the complex mechanism. 

On of the most important types of substitution reaction is a nucleophilic substitution, an Sn2 reaction in which a halide is added to a molecule. These reactions can lead to a variety of new functional groups. The mechanism for these reactions involves the attack of a nucleophile on a central carbon atom. Simultaneously, (3-elimination of a leaving group occurs. 

This type of substitution is called an SN1 reaction, for Substitution, Nucleophilic, unimolecular. The term unimolecular means there is only one molecule involved in the transition state of the rate-limiting step. The mechanism of the SnI reaction of tert-butyl bromide with methanol is shown here. Ionization of the alkyl halide (first step) is the rate-limiting step. 

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