Alternative Mechanistic Hypotheses

In this section, we will examine several alternative mechanistic hypotheses that will be recognized as incorporating elements of the original Hughes-Ingold scheme. Indeed, the first two  

In their early work Hughes and Ingold considered SnI and Sn2 pathways as competitive reactions of a single substrate, and borderline behavior resulted from a blend of the two processes. For example, they found that methanolysis of 1-phenylethyl chloride at 70°C is accelerated by sodium methdxide  

The kinetic order with respect to sodium methoxide is nonintegral, so the kinetic test for molecularity fails. The reaction is neither first- nor second-order overall. The kinetic data could be fitted to the equation 

In order to fit stereochemical observations into this framework, Hughes and Ingold introduced the concept of time lag into the SnI mechanism. The hydrolysis 

Streitwieser, Jr., Solvolytic Displacement Reactions, McGraw-Hill Book Co., New York, 1962, pp. 66-69. 

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Qualitative analysis has the main objective of confirming a given mechanistic hypothesis by rejection of conflicting alternatives. This may be applied to single elementary steps, the intermediates, or how the steps are linked together. 

The acetylene-insertion reaction presumably occurs by the following mechanistic sequence (1) insertion of Pd(0) into the SCB, (2) regioselective yy -silylpalladation of the acetylenic compounds to provide seven-membered l-pallada-4-silacyclic intermediate, and (3) reductive elimination of Pd(0) to afford silacyclohexene. Alternatively, /3-hydride elimination would open the palladacycle, generating a vinylpalladium hydride species that would undergo reductive elimination to yield the ring-opened allylvinylsilane. Isotopic labeling studies provided evidence in support of this mechanistic hypothesis (Scheme 47). 

Although it has been established that the HOMO (diazoalkane)-LUMO (alkene) controlled concerted cycloaddition occurs without intervention of any intermediate for the reactions of simple diazoalkanes with alkenes, Huisgen once proposed a mechanistic alternative 4 namely an initial hypothetical nitrene-type 1,1-cycloaddition reaction of phenyldiazomethane to styrene followed by a vinylcyclopropane-cy-clopentene-type 1,3-sigmatropic rearrangement Control experiments, however, excluded this hypothesis for the bimolecular 1,3-dipolar cycloaddition reaction of diazomethane (Scheme 60).204... 

The carbene insertion route (pathway (b)) has most frequently been implied to explain the thermal cydizations of various ethynyl-substituted aromatic precursors. Although the other two alternatives cannot be excluded at present, since in many cases (see below) no careful mechanistic studies have been undertaken, the carbene hypothesis has the advantage of... 

Enol radical cation intermediates have rerantly been invoked in the ribonucleotide reductase process. According to a hypothesis by Stubbe [127], they are formed through water loss from the 3 -ribonucleotide radical. They are supposed to react subsequently with H (or alternatively via a very unlikely two-electron reduction followed by protonation) to an intermediate 3 -hydroxy radical that is finally transformed to the deoxyribonucleotide. The above mechanistic evidence on simple enol radical cation chemistry, however, argues against this mechanistic model, since deprotonation should be much faster than nucleophilic attack even under physiological conditions. 

This chapter moves toward an alternate objective and dwells primarily on the macroscopic imprint of radionuclide plumes. Specifically, we proceed on the hypothesis that quantifying such macroscopic characteristics as plume dimensionality, length, and persistence might provide useful information about the primary soil processes that control the fate and transport of radioactive contaminants and the overall risk posed by radionuclide contaminant plumes. This approach is termed a historical case analysis. It builds on the molecular approach described in earlier chapters and provides some guidance to how standard fate and transport models might be modified to reflect better mechanistic process understanding. Specifically, a historical case analysis sets some bounds on what fate and transport models should predict. 

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