Mechanisms of elimination reactions

In Chapter 9, we saw that we can prepare alkenes by dehydrohalogenation. The dehydration of alcohols also gives alkenes, but this reaction occurs with rearrangement of the carbocation, and gives mixtures of products having different carbon skeletons. The elimination of a hydrogen hahde from an alkyl halide is a complex process. We must consider both rcgiochemistry and stereoelectronic effects. These effects are related to the mechanism of the reaction, which may be either E2 or El. 

In this section, we use concepts developed in our discussion of and S l reactions, and we see how experimental variables affect the product formed in a dehydrohalogenation reaction. The structural features that control and Sj l substitution and E2 and El elimination reactions are related. 

Like the reaction mechanism, the E2 mechanism is a one-step, concerted process. In an E2 dehydrohalogenation reaction, the base—which is also a nucleophile— removes a proton from the (3-carbon atom. As the proton is removed, the leaving group departs, and a double bond forms. The rate of an E2 reaction depends on the concentrations of the substrate and the base. If the substrate concentration is doubled, the reaction rate also doubles, as in processes. Thus, the rates of E2 and Sj 2 

An El mechanism occurs in two steps, and the rate-determining step is the formation of a carbocation intermediate. An reaction also proceeds in two steps, and the rate-determining step is formation 

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For reviews, see E. Baeiocehi in Chemistry of Halides, Pseudo-HaUdes and Azides, Part 2, S. Patai and Z. Kappopjit, eds., John Wiley Sons, New York, 1983, Chapter 23 W. H. Saunders, Jr., and A. F. Coekerill, Mechanisms of Elimination Reactions, John Wiley Sons, New York, 1973 D. J. McLennan, Tetrahedron 31 2999 (1975). 

W. H. Saunders, Jr., and A. F. Cockerill, Mechanisms of Elimination Reactions, John Wiley Sons, New Vbik, 1973. 

In the course of work on the mechanism of elimination reactions, the author and his co-workers have measured reaction-rate constants for the second-order elimination of hydrogen chloride from six dichloroethyl compounds of type Ar2CHCHCl2 and three monochloroethyl compounds of type Ar2CHCH2Cl (7). Samples of each of these materials were furnished to the Bureau of Entomology and Plant Quarantine for insecticidal testing, and the author is indebted to C. C. Deonier and I. H. Gilbert for permission to use certain of their data in this paper. The rate constants and larvicidal results are given in Table I. 

Saunders, W.H. and Cockerill, A.F. (1973). Mechanisms of Elimination Reactions, Wiley, New York... 

K. A. Cooper, E. D. Hughes, and C. K. Ingold, "The Mechanism of Elimination Reactions. Part III. Unimolecular Olefin Formation from Tert.-Butyl Halides in Acid and Alkaline Aqueous Solutions," JCS 140 (1937) 1280. 

Mechanism of Elimination Reactions. PartX. Kinetics of Olefin... 

Kaldor, S.B., Eredenburg, M.E. and Saunders, W.H. (1980). Mechanisms of elimination reactions 32. Tritium isotope effects and tunnel effects in the reaction of 2,2-diphenylethyl-2-t derivatives with various bases. J. Am. Chem. Soc. 102, 6296-6299... 

Studies of structure effects on rate have helped substantially to bring researchers to the present deep understanding (72,13) of the mechanism of elimination reactions. Beside stereochemical evidence, successful linear correlations have yielded the desired information. The published series of reactants and correlations are summarized in Table II. The fit of straight lines to experimental data is usually good or very good, and only a few points deviate significantly. Details of the correlations may be found in the original literature here we will concentrate on the values of the slopes. 

The study of heterogeneous catalysis with the emphasis on the effects of reactant structure stimulates consideration of the reacting system in terms of mutual interactions. Modification of the catalyst surface by the action of reactants is a part of these interactions. This idea is not new, but hitherto little evidence supported it now it is an inherent component of the accepted mechanism of elimination reactions. In general, the working surface may be quite different from the initial surface. Even the solvent may participate in the mechanism, as the results of the Delft school (125, 161, 162) indicate, by temporally accommodating hydrogen species formed in a reaction step from the reactants or hydrogen molecules on the surface. 

Hughes, E. D., C. K. Ingold, and V. J. Shiner jr. Mechanism of elimination reactions. Part XVII. The comparative unimportance of steric strain in unimolecular olefin elimination. J. chem. Soc. [London] 7953, 3827. 

For discussion, see Saunders Cockerill Mechanisms of Elimination Reactions , Wiley New York. 1973, pp. 548 554. 

Saunders WH, Jr, Cockerill AF. 1973. Mechanisms of elimination reactions. New York Wiley. 

In this chapter we will talk about the mechanisms of elimination reactions—as in the case of substitutions, there is more than one mechanism for eliminations. We will compare eliminations with substitutions—either reaction can happen from almost identical starting materials, and you will learn how to predict which is the more likely. Much of the mechanistic discussion relates very closely to Chapter 17, and we suggest that you should make sure you understand all of the points in that chapter before tackling this one. This chapter will also tell you about uses for elimination reactions. Apart from a brief look at the Wittig reaction in Chapter 14, this is the first time you have met a way of making alkenes. 

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