General Base versus Nucleophilic Reaction

2 General Acid Catalysis Versus Specific Acid-General Base Catalysis  

The rate of a bimolecular reaction between acid (HA) and a nucleophilic substrate (Nu) could obey Equation (19). 

If the proton transfer steps are not rate limiting HA and Nu are respectively in equilibrium with their respective conjugate base and acid (Equations 20 and 21) and Equation (19) may be rewritten as Equation (22)  

In this case the evidence is confirmatory because product analysis demonstrates ether formation rather than hydrolysis. 

Equations (19) and (22) refer to mechanisms involving reaction of HA with Nu or A with HNu respectively. The respective rate constants and are related by the Equation k = and the experi- 

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A kinetic smdy of the acylation of ethylenediamine with benzoyl chloride (110) in water-dioxane mixtures at pH 5-7 showed that the reaction involves mainly benzoylation of the monoprotonated form of ethylenediamine. Stopped-flow FT-IR spectroscopy has been used to study the amine-catalysed reactions of benzoyl chloride (110) with either butanol or phenol in dichloromethane at 0 °C. A large isotope effect was observed for butanol versus butanol-O-d, which is consistent with a general-base-catalysed mechanism. An overall reaction order of three and a negligible isotope effect for phenol versus phenol- /6 were observed and are consistent with either a base- or nucleophilic-catalysed mechanism. Mechanistic studies of the aminolysis of substituted phenylacetyl chlorides (111) in acetonitrile at —15 °C have revealed that reactions with anilines point to an associative iSN2 pathway. ... 

King, J. A., Jr. Nucleophilic versus general base catalysis in carbonyl substitution reactions the influence of acylpyridinium/acylammonium salt formation on the observed reaction rate. Trends in Organic Chemistry QQ7, 6, 67-89. 

Secondary halides are borderline, and substitution or elimination may be favored, depending on the particular base/nucleophile, solvent, and temperature at which the reaction is carried out. Elimination is favored with strong bases/good nucleophiles—for example, hydroxide ion and ethoxide ion. Substitution is favored with weak bases/poor nucleophiles—for example, acetate ion. Table 7.7 summarizes these generalizations about substitution versus elimination reactions of haloalkanes. 

Based on these differences, it is possible to represent the VBCMDs for typical nucleophilic substitution reactions on Si versus C as shown in Figs. 6.16a and b. In Fig. 6.16a, the ionic curve X SiL3 + X is very stable in the pentacoordinated geometry due to the electrostatic energy of the triple ion structure, much like the case of the (FIIF) species discussed before. Consequently, the pentacoordinated (XSiL3X) species will generally be a... 

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