Elimination reactions analyzing mechanisms

We mentioned that there are three main steps for predicting the products of substitution and elimination reactions. In the previous section, we explored the first step (determining the function of the reagent). In this section, we now explore the second step of the process in which we analyze the substrate and identify which mechanism(s) operates. 

When deciding which substitution or elimination mechanism dominates a reaction, analyze the structure of the haloalkane, the choice of the solvent, and the relative base strength of the nucleophile. 

In this chapter, we will explore elimination reactions in the same way that we explored substitution reactions. We begin with the mechanisms for El and E2 reactions, and then we move on to the factors that help us determine in each case which mechanism predominates. There is one big difference between the last chapter and this chapter. In the last chapter, most of the information was given to you, and there was very httle to look up in other sources (your textbook, your class notes, etc.). But now you know how important mechanisms are, you know that mechanisms explain everything, you know how to analyze different factors that affect reactions, and so on. So in this chapter, YOU are going to provide the key information, by filling it in the appropriate places. 

In principle, the bases Y are also nucleophiles, and, hence, they can react with the same alkyl halides and sulfonates via the SN2 mechanism. The point of reaction is the C atom that bears the leaving group. In order to carry out E2 eliminations chemoselectively, competing Sn2 reactions must be excluded. To understand the outcome of the competition (E2 elimination vs. Sn2 reaction), it is analyzed kinetically with Equations 4.1-4.3. 

In addition to isolation and characterization of the ruthenacycle complexes 18 or 32, the detailed reaction mechanism of the [2 + 2 + 2] cyclotrimerization of acetylene was analyzed by means of density functional calculations with the Becke s three-parameter hybrid density functional method (B3LYP) [25, 33]. As shown in Scheme 4.12, the acetylene cyclotrimerization is expected to proceed with formal insertion/reductive elimination mechanism. The acetylene insertion starts with the formal [2 + 2] cycloaddition of the ruthenacycle 35 and acetylene via 36 with almost no activation barrier, leading to the bicydic intermediate 37. The subsequent ring-... 

Once the necessary species have been found, the second step in reducing a mechanism is the elimination of its non-important reactions. A classical and reliable method is the comparison of the contribution of reaction steps to the production rate of necessary species. A description of this method - without the pre-selection of redundant species - is given by Wamatz [4, 93]. A more recent application of this technique for methane flames is presented in [94]. According to this method a reaction is redundant if its contribution to the production rate of each necessary species is small. This rule sounds simple and obvious but there are, however, several drawbacks and pitfalls. First, the reaction contributions have to be considered at several reaction times (or at several heights in the case of steady flames). Second, all reaction contributions to each necessary species have to be considered, and it is not easy to analyze such huge matrices. The threshold of unimportance will vary from time to time, and from species to species. Applying a uniform threshold for each time and species (e.g., minimum 5% contribution) can either result in redundant reactions being left in the scheme, or an over-simplified mechanism. 

Analyzing the frequency factor v, Bursey43 was able to show that the details of the reaction mechanisms of the electron impact induced ketene elimination from ortho substituted phenyl acetates and from acetanilides, 36, are dramatically influenced by the nature of the substituent R = F, Cl, Br, J. The results demonstrate that the decrease of v going from the voluminous iodine to the small fluorine is not caused by steric effects (which should operate in an opposite direction) but is the result of an electronic interaction of both substituents, leading to a tighter transition state in the case of the more electronegative fluorine. Additional factors which are not yet completely understood play a decisive part in the fragmentation of the anilides. 

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