Structure of the Leaving Group

The magnitude of these effects might be related to the nature of substituents a to N or P and to the structure of the leaving group, as exemplified by several observations that will be described and discussed below. 

The structure of the leaving group is a decisive factor for donor properties. Natural substrates for glycosidases are polysaccharidic chains and the corresponding disaccharides are still used in transglycosylation reactions as... 

What effect does the structure of the leaving group have on the rate of reaction ... 

Although kinetic studies of Inhibition by monomethylcarbamates Indicate these compouxids to be poor substrates there Is evidence that carbamyla-tion and the actual structure of the "leaving group" Is of most importance.92 That carbao latlon does occur is supported by the destruction of... 

We saw m Section 8 2 that the rate of nucleophilic substitution depends strongly on the leaving group—alkyl iodides are the most reactive alkyl fluorides the least In the next section we 11 see that the structure of the alkyl group can have an even greater effect... 

In aqueous dioxane, the endo-anti isomer gave a product mixture consistent of alcohol N and the corresponding ester (derived from capture of the leaving group p-nitrobenzoate). The other isomers gave much more complex product mixtures which were not completely characterized. Explain the trend in rates and discuss the structural reason for the stereochemical course of the reaction in the case of the endo-anti isomer. 

We will discuss shortly the most important structure-reactivity features of the E2, El, and Elcb mechanisms. The variable transition state theoiy allows discussion of reactions proceeding through transition states of intermediate character in terms of the limiting mechanistic types. The most important structural features to be considered in such a discussion are (1) the nature of the leaving group, (2) the nature of the base, (3) electronic and steric effects of substituents in the reactant molecule, and (4) solvent effects. 

Saunders10 and by Sims and coworkers11 have shown that the magnitude of the leaving-group heavy-atom isotope effect varies linearly with the extent of C—X bond rupture in the transition state for concerted elimination reactions and for nucleophilic substitution reactions, respectively. Since the magnitude of the isotope effect is directly related to the amount of C—X bond rupture in the transition state, these isotope effects provide detailed information about the structure of the transition state. 

The differences in the rates of alkaline hydrolysis of /3-lactams have been shown to depend primarily on the basicity of the amino leaving group [76]. The rate of alkaline hydrolysis increases as the pKa of the leaving group increases. Several structural features affecting the pKa of the leaving amino group are summarized in Table 5.4, the details of which follow. 

Direct nucleophilic SN2 exchanges F/leaving group (mesylate, tosylate, nosy-late, triflate, iodide or bromide) in a complex structure using [ Fj fluoride anion are more frequent in aliphatic than in aromatic series. The choice of the leaving group is strongly dependent on the substrate to label [206] and it is often neces-... 

The different resonance structures represent a Meisenheimer complex. The loss of the leaving group reestablishes the stable aromatic system. 

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