Elimination reactions, general characteristics

As a general rule, nucleophilic addition reactions are characteristic only of aldehydes and ketones, not of carboxylic acid derivatives. The reason for the difference is structural. As discussed previously in A Preview of Carbonyl Compounds and shown in Figure 19.14, the tetrahedral intermediate produced by addition of a nucleophile to a carboxylic acid derivative can eliminate a leaving group, leading to a net nucleophilic acyl substitution reaction. The tetrahedral intermediate... 

However, the E2C mechanism has been criticized, and it has been contended that all the experimental results can be explained by the normal E2 mechanism. McLennan suggested that the transition state is that shown as 18. An ion-pair mechanism has also been proposed. Although the actual mechanisms involved may be a matter of controversy, there is no doubt that a class of elimination reactions exists that is characterized by second-order attack by weak bases. " These reactions also have the following general characteristics (1) they are favored by good leaving groups (2) they are favored by polar aprotic solvents (3) the reactivity order is tertiary > secondary > primary, the opposite of the normal E2 order (p. 1319) (4) the elimination is always anti (syn elimination is not found), but in cyclohexyl systems, a diequatorial anti elimination is about as favorable as a diaxial anti elimination (unlike the normal E2 reaction, p. 1302) (5) they follow Zaitsev s rule (see below), where this does not conflict with the requirement for anti elimination. 

As we have seen (Section 4, p. 191) the range of effective molarities associated with ring-closure reactions is very much greater than that characteristic of intramolecular general acid-base catalysis the main classification is therefore in terms of mechanism. By far the largest section (I, Tables A-D) gives EM s for intramolecular nucleophilic reactions. These can be concerted displacements (mostly at tetrahedral carbon), stepwise displacements (mostly addition-elimination reactions at trigonal carbon), or additions, and they have been classified in terms of the nucleophilic and electrophilic centres. 

Just like the ability to bind HCl, this exchange reaction is a general characteristic of all efficient PVC heat stabilizers and stabilizer systems. An essential condition of this exchange reaction, is of course, that the transferred groups - in this case a mercaptocarbonic acid ester group - have a lower tendency to be eliminated than the chlorine atom. 

In Chapters 11 and 12 we return to the presentation of a new functional group the carbon-carbon double bond. This functional group differs from those seen so far in that it hicks strongly polarized covalent bonds. Instead, its reactivity arises from specitil characteristics of electrons in so-called it bonds. The properties of the.se electrons and their consequences are discussed in the next chapter. Chapter 11 is restricted to a general description of alkenes as a compound class and a presentation of methods of preparation of double bonds. Most of the reactions are ones you have already seen, because the major methods of alkene syntheses are the same elimination reactions of alcohols and haloalkanes that were presented in Chapters 7 and 9. Only some finer details have been added. 

This is the first of three chapters dealing with an in-depth study of the organic reactions of compounds containing C-Z a bonds, where Z is an element more electronegative than carbon. In Chapter 7 we learn about alkyl halides and one of their characteristic reactions, nucleophilic substitution. In Chapter 8, we look at elimination, a second general reaction of alkyl halides. We conclude this discussion in Chapter 9 by examining other molecules that also undergo nucleophilic substitution and elimination reactions. 

The following reactions describe characteristic chemical properties of tetrazoles. Tetra-zoles, in general, should be handled with care, since even crystalline derivatives are liable to decompose explosively with elimination of nitrogen. 

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