Determining Which Mechanism Predominates

The four factors that determine which mechanism predominates. 

In order to draw the products of a specific substitution reaction, we must first identify the reaction mechanism as either Si,j2 or S l. This information is important in the following two ways  

The trend in Si i2 reactions is due to issues of steric hindrance in the transition state, while the trend in Sj,jl reactions is due to carbocation stabihty. The bottom Hne is that methyl and primary substrates favor Sj,j2, while tertiary substrates favor Sjvjl. Secondary substrates can proceed via either mechanism, so a secondary substrate does not indicate which mechanism wiU predominate. In such a case, you must move on to the next factor, the nucleophile (covered in the next section). 

Allylic halides and benzylic halides can react either via 5 2 or via Sjsjl processes  

These substrates can react via an 5 2 mechanism because they are relatively unhindered, and they can react via an S l mechanism because loss of a leaving group generates a resonance-stabilized carbocation  

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B. Better tools available, but no consensus on mechanism or active site—1980 to 2006. Rhodes et al.291 published a comprehensive review on the heterogeneously catalyzed water-gas shift mechanism in 1995. Included in that discussion was the copper/zinc oxide/alumina system. The conclusion was that this system appears to be constructed of small metallic islands of copper resting on a zinc oxide alumina phase. Zinc oxide may exert some impact on catalytic activity, but it was suggested in the review that the contribution is small. It was indicated that strong evidence exists to support either a formate or a redox mechanism, and the authors even suggest the possibility that both mechanisms might occur, though insufficient data exist to determine which mechanism predominates. 

Several proposals have been made to fit the borderline reactions into a well-defined mechanistic scheme. Most of these adopt one of two viewpoints either (1) borderline substrates undergo concurrent SnI and Sn2 processes, with the particular system determining which mechanism, if either, predominates or (2) all Sn reactions are related by essentially the same mechanism, which differs from case to case in the detailed disposition of electrons in the transition state. In this view pure SnI and Sn2 processes are merely the extreme limiting forms of a single mechanism, and the borderline mechanism is a merged process having some features of both. 

The optical-optical double resonance technique was applied to determine which mechanism is predominant in the pumping of TPPZn (27, 28). Because of the large difference in the lifetimes of and T ... 

Substitution and elimination reactions are almost always in competition with each other. In order to predict the products of a reaction, it is necessary to determine which mechanisms are likely to occur. In some cases, only one mechanism will predominate ... 

When the reagent functions exclusively as a nucleophile (and not as a base), only substitution reactions will occur (not elimination), as illustrated in Figure 8.26. The substrate will determine which mechanism operates. Si,j2 will predominate for primary substrates, and S l will predominate for tertiary substrates. For secondary substrates, both Sivf2 and S l pathways are viable, although Si,j2 is generally favored. The rate of an Sn2 process can be further enhanced by using a polar aprotic solvent, as described in Section 7.8. 

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. 

A similar concerted mechanism accounts for the water-catalyzed reaction, which becomes predominant at pH values lower than 4. The transition in rate-determining step has not been observed for the other two enamines. This point will meet attention in Section III. 

Now that we have seen all four factors individually, we need to see how to put them all together. When analyzing a reaction, we need to look at all four factors and make a determination of which mechanism, SnI or Sn2, is predominating. It may not be just one mechanism in every case. Sometimes both mechanisms occur and it is difficult to predict which one predominates. Nevertheless, it is a lot more common to see situations that are obviously leaning toward one mechanism over the other. For example, it is clear that a reaction will be Sn2 if we have a primary substrate with a strong nucleophile in a polar aprotic solvent. On the flipside, a reaction will clearly be SnI if we have a tertiary substrate with a weak nucleophile and an excellent leaving group. 

The diagram then clearly shows the situation for a particular temperature and so can be used to determine the most important conductivity mechanism for a particular operating condition at the temperature of the diagram. Similar diagrams must be constructed for other temperatures, which, when stacked up, will give a three-dimensional representation of the way in which the predominant conduction mechanism changes with temperature. Another set of diagrams will also need to be constructed for other dopant concentrations. 

There are times when you can have substitution and elimination reactions occurring at the same time. In fact, this is often the case. In those situations, you need to determine which of the four mechanisms (Sn2, S l, E2, or El) is predominant. We will see this in Chapter 12 (Section 12.3). 

The rates for going over and through the barrier do not depend in the same way on its height and shape. Thus, as barrier heights are determined from inversion rates (see below), it is necessary to know which mechanism is operative or predominant in a particular case. 

As a whole, these investigations provided valuable criteria to differentiate between the contributions ofthe various labeling mechanisms, and to determine their relative weight in the formation of the tritiated products. The currently accepted view (cf. Evans, 1966) is that the Wilzbach labeling method is essentially based on radiation-promoted processes, which largely predominate over the reactions of the HeT+ ions, as demonstrated by the fact that several T atoms are incorporated following each /8-decay. Under typical preparative conditions, the... 

What are the principal fundamental mechanisms by which thermal degradation, sintering and redispersion of supported metals occur What factors determine which of these mechanisms predominates or controls the sintering process What do we leam from model catalyst studies regarding these mechanisms ... 

The mechanisms for this reaction are discussed in the chapter on kinetics (Chapter 9). It is a combination of first- and second-order reactions, which is not solvable anal5dically because of the nonlinear terms following the rate constants koH and kcoj.r- The rate constants were determined in the laboratory by choosing the experimental conditions in which one of the two mechanisms predominated. pH values of natural waters, however, often fall in the range 8-10, in which the reaction with both water and OH can be important. To determine the life time of CO2 as a function of pH, one must derive the solution to the reaction rate equation. This is facilitated by employing the DIG and carbonate alkalinity, Ac, (Eqs. (4.15) and (4.26)) to eliminate the concentration of bicarbonate [HCOJ], in the CO2 reaction rate equation. This substitution results in an expression... 

According to the analysis in the previous sections, the primary particle size in flame reactors is determined by the relative rates of particle collision and coalescence. For highly refractory materials, the characterislic coalescence time (12.6) depends on the solid-state diffusion coefficient, which is a very sensitive function of the temperature. The mechanisms of solid-.staie diffusion depend in a complex way on the structure of the solid. For example, a perfect cubic crystal of the substance AB consists of alternating ions A and B. Normally there are many defects in the lattice structure even in a chemically pure single crystal defect types are shown schematically in Fig. 12.8. The mechanism of diffusion in cry.stalline solids depends on the nature of the lattice defects. Three mechanisms predominate in ionic... 

In the E2 mechanism the nature of the leaving group often determines which product will predominate — Saytzev or Hofmann. In the El mechanism the leaving group is expelled in the first step, before the double bond is formed, and not surprisingly it has little influence over the products of the reaction. 

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