Nucleophilic substitution description

FIGURE 8 2 Hybrid orbital description of the bonding changes that take place at carbon during nucleophilic substitution by the Sn2 mechanism... 

The first three chapters discuss fundamental bonding theory, stereochemistry, and conformation, respectively. Chapter 4 discusses the means of study and description of reaction mechanisms. Chapter 9 focuses on aromaticity and aromatic stabilization and can be used at an earlier stage of a course if an instructor desires to do so. The other chapters discuss specific mechanistic types, including nucleophilic substitution, polar additions and eliminations, carbon acids and enolates, carbonyl chemistry, aromatic substitution, concerted reactions, free-radical reactions, and photochemistry. 

A general mechanistic description of the copper-promoted nucleophilic substitution involves an oxidative addition of the aryl halide to Cu(I) followed by collapse of the arylcopper intermediate with a ligand transfer (reductive elimination).140... 

Although replacement reactions of the pentameric system are also known, these have not been as well studied (14). The following account gives a description of some of the different types of nucleophilic substitution reactions that have been well studied. The sequential replacement of chloride substituents by fluoride has been studied, and is discussed elsewhere (14, 23). 

Generally, only a single stepwise or concerted pathway for aliphatic nucleophihc substitution is detected by experiment because of the very different activation barriers for formation of the respective reaction transition states for these reactions. The description of the borderline between stepwise and concerted nucleophilic substitution reactions presented in this chapter has been obtained through a search for those rare substrates that show comparable barriers to these two reactions and through the characterization of the barrier for nucleophile addition to the putative carbocation intermediate of the stepwise reaction in the region of this change in mechanism. 

The description of the borderline between stepwise and concerted nucleophilic substitution remains murky in cases where there is no significant stabilization of the transition state for the concerted reaction through the coupling of bond cleavage and formation. The reason is that there are no simple experimental protocols to detect the point at which the energy well for the carbocation intermediate of the stepwise reaction in the upper right hand corner of Figure 2.3 is transformed into... 

This review attempts to provide an overview of microwave-promoted metal-catalyzed transformations of aryl and vinyl halides (or pseudo halides), providing a personal selection of both pioneering and very recently published work. Covered areas include carbonylative transformations, Heck and Sono-gashira reactions, nucleophilic substitutions and cross-couplings. Because of the diversity of the microwave systems used, the reader should consult the original references for detailed descriptions of settings and instrumentation. 

There is some evidence (see Section 3.7) that the development of double bond character between Cl and 05 in actual sugars may occur with varying degrees of synchronicity. In a description of nucleophilic substitution at an oxocarbenium centre, therefore, description of the transition state really... 

The mechanistic description refers to a nucleophilic substitution via a radical intermediate that proceeds by unimolecular decomposition of a radical anion derived from the reactant. There are two families of such reactions that have been developed... 

Pyridine-1-oxide undergoes electrophilic and nucleophilic substitution reactions at the 2- and 4-positions, as predicted from its resonance description 76 ... 

The need for an apical potentiality term is well displayed in the foregoing discussion. That such a term is largely descriptive and possesses little predictive utility is also amply illustrated. But the observation that apical potentiality may differ enormously from relative apicophilicity is a significant advance in our understanding of nucleophilic substitution reactions at phosphorus. In one direction. Hall and Inch s observations serve as a catalyst for the better understanding of the associative mechanism and the dynamics of TBP intermediates. In another, they serve to delineate the applicability of Westheimer s guidelines and in particular relative apicophilicity and the preference rules. 

These problems differ from those in earlier chapters in that they directly test your knowledge of core material rather than using a descriptive passage to extend the material or introduce new ideas. The number of factors that contribute to nucleophilic substitution can be daunting. The really major ones, though, are few and readily applied to specific reactions by using the S l and Sn2 mechanisms to guide your analysis. 

This chain reaction is analogous to radical chain mechanisms for nucleophilic aliphatic nucleophilic substitution that had been suggested independently by Russell and by Komblum and their co-workers. The descriptive title SrnI (substitution radical-nucleophilic unimolecular) was suggested for this reaction by analogy to the SnI mechanism for aliphatic substitution. The lUPAC notation for the SrkjI reaction is (T -t- Dm -t- An), in which the symbol T refers to an electron transfer. When the reaction was carried out in Ihe presence of solvated electrons formed by adding potassium metal to the ammonia solution, virtually no aryne (rearranged) products were observed. Instead, reaction of 95c produced only 98 (40%) and 94 (40%) but no 99, and reaction of 96c produced 99 (54%) and 94 (30%) with only a trace of 98. ... 

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