Mechanistic progression

Until now, discussions have focused only on how carbanions and carbocations behave under conditions favorable for nucleophilic substitutions. However, these species may undergo other types of reactions in which unsaturation is introduced into the molecule. Such reactions are called elimination reactions and should be considered whenever charged species are of importance to the mechanistic progression of a molecular transformation. In previous chapters, SN1 and SN2 reactions were discussed. In this chapter, the corresponding El and E2 elimination mechanisms are presented. 

More and more attentions have been paid to H-bonding-based electron transfer in recent years. These systems have been extended to the donor-acceptor assemblies linked by two-point H-bonds, triple H-bonds, or multiple H-bonds. In this section, we will first present the important mechanistic progress in the last two decades and then discuss recent representative examples based on H-bonding. 

We hope you have appreciated the smooth mechanistic progression so far in this chapter, from Wagner-Meerwein to pinacol and semipinacol through dienone-phenol to benzilic acid. 

The exact mechanistic sequence of the ANRORC transformation is dependent on the system studied. Throughout this chapter, proposals are presented to provide an overview of the varied mechanistic progression. 

It is worthwhile to briefly discuss the history of investigations into the mechanism of diazotization. Its progression between 1894 and 1958 demonstrates that it may take more than 60 years to correct a false mechanistic interpretation of good experimental results followed by many supporting conclusions. 

Further mechanistic evidence was provided by Benkeser and Krysiak658, who determined the effects of added salts and water on the rates of cleavage of xylyltrimethylsilanes by p-toluenesulphonic acid in acetic acid at 25 °C, the progress of the reaction being followed by dilatometry the first-order rate coefficients are given in Table 227. Clearly the addition of water retards the reaction, as... 

The Br0nsted coefficient /3 lies in the range 0 < /3 < 1, and has much the same mechanistic significance as a. It is a measure of progress along the reaction coordinate. The closer /3 is to unity, the more complete proton transfer is from substrate to base in the transition state. 

In this chapter, we have summarized (recent) progress in the mechanistic understanding of the oxidation of carbon monoxide, formic acid, methanol, and ethanol on transition metal (primarily Pt) electrodes. We have emphasized the surface science approach employing well-defined electrode surfaces, i.e., single crystals, in combination with surface-sensitive techniques (FTIR and online OEMS), kinetic modeling and first-principles DFT calculations. 

Although impressive progress has been made in unraveling the mechanism of ORR catalysis by cofacial porphyrins, much remains to be learned before we can understand how this mechanism relates to those in heme enzymes and simple metalloporphyrins and use our mechanistic knowledge to rationally design improved metalloporphyrin catalysts for the ORR. 

In 1978, Sugasawa et al., at Shionogi Pharmaceutical Co. reported ortho-selective Friedel-Craft acylation with free anilines with nitrile derivatives [4]. Sugasawa reported that the reaction requires two different Lewis acids (BC13 and A1C13) and does not proceed when N,N-dialkyl anilines are used. He proposed that boron bridging between nitriles and anilines led to exclusive ortho-acylation but a conclusive mechanism was not elucidated. The report did not offer any reason why two different Lewis acids were required and why the reaction did not progress with N,N-dialkyl anilines. Therefore, we initiated mechanistic studies. 

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