The Mechanisms of Elimination

From the thorough studies of the mechanism of elimination of hydrogen halides from vicinal fluorohalo compounds, it follows that the result of elimination depends on the stabihty of the carbanionic species in which the negative charge is on the carbon P to fluorine and a to chlorine or bromine [43, 44, 45, 46, 47, 48, 49, 50, 51. 52, 53, 54]... 

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. 

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. 

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. 

In discussing the mechanism of eliminations over solids, the nomenclature which has been developed for homogeneous reactions will be used. Therefore the basic mechanisms of olefin formation have first to be outlined and their meaning in heterogeneous catalysis defined. 

The opposite of an addition to a double bond is a 1,2-elimination reaction. In solution, where the reaction is promoted by solvent or by base, the most common eliminations (and those to which we shall limit our discussion) are those that involve loss of HX, although loss of X2 from 1,2-dihalides and similar reactions are also well known. The mechanisms of eliminations of HX are of three main types (1) The Ex (elimination, first-order), shown in Equation 7.22, which is the reverse of the AdE2 reaction and which has the same first, and rate-determining,... 

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. 

Because alkyl tosylates have good leaving groups, they undergo both nucleophilic substitution and P elimination, exactly as alkyl halides do. Generally, alkyl tosylates are treated with strong nucleophiles and bases, so that the mechanism of substitution is Sn2 and the mechanism of elimination is E2. 

When the ammonium salt is heated in Step [3], OH removes a proton from the P carbon atom, forming the new k bond of the alkene. The mechanism of elimination is E2, so ... 

In dehydrohalogenation of alkyl halides (Sec. 5.13), we have already encountered a reaction in which hydrogen ion and halide ion are eliminated from a molecule by the action of base there —H and —X were lost from adjacent carbons, and so the process is called p-eHmination. In the generation of the methylene shown here, both —H and -X are eliminated from the same carbon, afid the process is called a-eliminaiion. (Later on, in Sec. 24.12, we shall see some of the evidence for the mechanism of -elimination shown above.)... 

In many instances (notably the last of those mentioned above), elimination is the predominating reaction. It should always be kept in mind, however, that elimination and substitution processes compete with each other and usually occur simultaneously. The one which predominates depends upon environmental as well as structural factors. It is the purpose of this chapter to discuss the mechanisms of elimination, and to... 

Factors which may control the reactivities of crystalline hydrates and hence determine the mechanisms of elimination of water fi om the solids are summarized below. Many have been mentioned in describing specific reactions above. 

XR provides a general indication of the mechanism of elimination for the compound of interest, and is a reliable monitoring parameter for drug interaction assessment... 

In general, it is difficult to accurately quantify drug reabsorption of a compound. A useful calculation to assess net excretion mechanisms is renal excretion ratio (XR). XR is calculated as the ratio of renal clearance and filtration clearance. Therefore, excretion ratio provides a general indication of the mechanism of elimination for the compound of interest. An XR greater than one is indicative of a net secretory process. Net reabsorption can be inferred when an XR value is less than one, and when renal clearance is dependent on urine flow rate or urine pH. 

The question is now what effects does a strong base have on an elimination or substitution reaction To answer this we consider what a base does. By far the most important use of a strong base is to abstract protons. Thinking now of the mechanisms of elimination and substitution, we see in an E2 reaction the abstraction of a proton 6 to the halide. Thus we can conclude that the addition of NaOH (a strong base) to the reaction of t-butyl chloride with water would increase the ratio of elimination to substitution. 

Note that a syn orientation refers to the mechanism of elimination and not the stereochemistry of the subsequently formed alkene. A syn elimination can result in a trans product, as was shown earlier. The Cope elimination differs from the Hofmann elimination of quaternary ammonium salts which is a trans elimination. 

Little is known about the mechanisms of elimination from other classes of compounds. Deuterium labeling is probably the best technique for studying these reactions, but this involves considerable synthetic work, and such has not yet been reported. 

The double dehydrohalogenation reaction is usually run in the presence of a strong base, such as KOH or NaNH2, and proceeds in two stages. In the first, an intermediate bromoalkene is formed, which can be isolated under more mildly basic conditions. In fact, this reaction is a valuable route to vinyl halides. The mechanism of elimination involves the abstraction of the proton on the carbon atom p to the halogen. The E2 mechanism, which operates under these strongjy basic conditions, is fastest when it involves removal of a proton, antiperipla-... 

At about the same time as his studies of the mechanisms of elimination reactions. Sir Christopher Ingold and his collaborator Edward D. Hughes applied the tools of kinetics and stereochemistry to nucleophilic substitution. 

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