Electrophilic reactions aliphatic substitution

A more detailed classification of chemical reactions will give specifications on the mechanism of a reaction electrophilic aromatic substitution, nucleophilic aliphatic substitution, etc. Details on this mechanism can be included to various degrees thus, nucleophilic aliphatic substitutions can further be classified into Sf l and reactions. However, as reaction conditions such as a change in solvent can shift a mechanism from one type to another, such details are of interest in the discussion of reaction mechanism but less so in reaction classification. 

In Part 2 of this book, we shall be directly concerned with organic reactions and their mechanisms. The reactions have been classified into 10 chapters, based primarily on reaction type substitutions, additions to multiple bonds, eliminations, rearrangements, and oxidation-reduction reactions. Five chapters are devoted to substitutions these are classified on the basis of mechanism as well as substrate. Chapters 10 and 13 include nucleophilic substitutions at aliphatic and aromatic substrates, respectively, Chapters 12 and 11 deal with electrophilic substitutions at aliphatic and aromatic substrates, respectively. All free-radical substitutions are discussed in Chapter 14. Additions to multiple bonds are classified not according to mechanism, but according to the type of multiple bond. Additions to carbon-carbon multiple bonds are dealt with in Chapter 15 additions to other multiple bonds in Chapter 16. One chapter is devoted to each of the three remaining reaction types Chapter 17, eliminations Chapter 18, rearrangements Chapter 19, oxidation-reduction reactions. This last chapter covers only those oxidation-reduction reactions that could not be conveniently treated in any of the other categories (except for oxidative eliminations). 

Much less work has been done on electrophilic aliphatic substitution mechanisms than on nucleophilic substitutions, and the exact mechanisms of many of the reactions in this chapter are in doubt. For many of them, not enough work has been done to permit us to decide which of the mechanisms described in this chapter is operating, if indeed any is. There may be other electrophilic substitution mechanisms, and some of the reactions in this chapter may not even be electrophilic substitutions at all. 

For example, U-Sn2[7][MnAe] indicates a nucleophilic aliphatic substitution following an Ugi reaction to give a 7-membered ring, where the additional nucleophile is embedded in the amine component and the leaving group (electrophile) is introduced in the carboxylic component. When more than one new ring is formed,... 

As has been exemplified in this chapter, fluorinations with fluorine-18 can be classified into two categories (1) the nucleophilic reactions, which usually involve no-carrier-added [ F]fluoride of high-specific radioactivity as its Kf FIF-K complex and include substitutions in the aliphatic and the /lomoaromatic series and (2) the electrophilic reactions, which mainly use moderately low-specific radioactivity molecular [ F]fluorine, or other reagents prepared from it, such as acetyl [ F] hypofluorite, and include addition across double bonds, reactions with carbanions and especially fluorodehydrogenation and fluorodemetallation reactions. 

Wide synthetic possibilities for modification of coordinated ligands are opened up by the classic reactions of electrophilic and nucleophilic substitution in complexes of aliphatic, aromatic, and heterocyclic compounds [314,359,418 422]. For example, the transformations (3.196) were known long ago [419] ... 

Cationic intermediates are considered the active species in many organic reactions, as well as in cationic polymerizations. For example, cationic intermediates are postulated in both electrophilic addition and elimination reactions. They are also involved in electrophilic aromatic substitutions and in some nucleophilic aliphatic substitutions. The latter reactions may involve either onium or carbenium ions. The current understanding of... 

Analogous to the Y scale and based on a nueleophilic substitution reaetion, Gielen and Nasielski [51] suggested that a solvent polarity seale could be based on electrophilic aliphatic substitution reactions such as the reaetion of bromine with tetra-methyltin shown in Eq. (7-22). 

For aliphatic electrophihc substitution, we can distinguish at least four possible major mechanisms, which we call Sgl, Se2 (front), Se2 (back), and Sei. The Sgl is unimolecular the other three are bimolecular. It is noted that the term SeAt has been proposed to represent electrophilic aromatic substitution, so that the term Se2 refers exclusively to electrophihc substitutions where a steric course is possible. To describe the steric course of an aliphatic substitution reaction, the suffixes ret and inv were proposed, referring to retention and inversion of conhguration, respectively. 

In al this we have estimated the stability of a carbonium ion on the same basis the dispersal or concentration of the charge due to electron release or electron withdrawal by the substituent groups. As wc shall see, the approach that has worked so well for elimination, for addition, and for electrophilic aromatic substitution works for still another important class of organic reactions in which a positive charge develops nucleophilic aliphatic substitution by the S l mechanism (Sec. 14.14). It works equally well for nucleophilic aromatic substitution (Sec. 25.9), in which a negative charge develops. Finally, we shall find that this approach will help us to understand acidity or basicity of such compounds as carboxylic acids, sulfonic acids, amines, and phenols. 

This duality of mechanism does not reflect exceptional behavior, but is usual for electrophilic aromatic substitution. It also fits into the usual pattern for nucleophilic aliphatic substitution (Sec. 14.16), which—from the standpoint of the alkyl halide—is the kind of reaction taking place. Furthermore, the particular halides (T and methyl) which appear to react by this second mechanism are just the ones that would have been expected to do so. 

The mechanisms that occur in aliphatic electrophilic substitution reactions are less well defined than those that occur in aliphatic nucleophilic substitution and aromatic electrophilic reactions. There is still, however, the usual division between unimolecular and bimolecular pathways the former consisting of only the SE1 mechanism, while the latter consists of the SE2 (front), SE2 (back) and the SEi mechanism. 

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