Nucleophilic reactions constant selectivity relationship

The N+ scale, which is derived from nucleophile/electrophile combination reactions, differs from the nucleophihcity scales generated from rates of Sn2 reactions cf. Table 5-15). The N+ equation, an example of a so-called constant selectivity relationship , has been applied to many other electrophile/nucleophile reactions in solution, although often with only Hmited success. 

In the majority of reactions studied, a constant selectivity relationship does not apply. In the addition of morpholine, glycine, and MeO- to CH2=CHX systems, X=PO(OEt)3, CONH2, CN, C02Me, S02Me, COMe, COPh, and CHO plots of log k versus pKa (MeX) are linear but not parallel (28) thus, the reactivity ratios for each pair of nucleophiles are not constant. 

The rate constants for reactions of highly stable triphenylmethyl and diphenylmethyl cations with various ionic and neutral nucleophiles have been measured (Gandler, 1985 McClelland etal., 1986) in aqueous acetonitrile and discussed from the view point of a reactivity-selectivity relationship. 

The nucleophilicities are found to be dependent on electronic, steric, and symbiotic effects, and limited series obeyed a constant selectivity , a reactivity-selectivity or a dual-parameter linear free-energy relationship. The conclusion made was that because of different blends of the effects, the construction of a substrate-independent nucleophilicity scale was impossible at present, but an approximate scale was presented. In nucleophilic reactions on relatively long lived vinyl cations, the steric effects predominate, but at constant steric effects, reactivity-selectivity relationships were found for very short series of substrates. Additional data are required for constructing more reliable nucleophilicity scales toward neutral and positively charged vinylic carbons. 

Four different probes gave short reactivity orders toward vinyl cations (1) common ion rate depression in solvolysis (2) competitive capture of solvolytically generated ions (3) direct reaction of a vinyl cation with nucleophiles and (4) competition between intra- and intermolecular nucleophilic capture. A short reactivity order is obtained in each case, but because of the different solvents and conditions the orders cannot be combined to a single series. However, a selectivity rule that governs the relative reactivities toward different vinyl cations in terms of a constant selectivity or a reactivity-selectivity relationship can be determined. 

Because systematic variations in selectivity with reactivity are commonly quite mild for reactions of carbocations with n-nucleophiles, and practically absent for 71-nucleophiles or hydride donors, many nucleophiles can be characterized by constant N and s values. These are valuable in correlating and predicting reactivities toward benzhydryl cations, a wide structural variety of other electrophiles and, to a good approximation, substrates reacting by an Sn2 mechanism. There are certainly failures in extending these relationships to too wide a variation of carbocation and nucleophile structures, but there is a sufficient framework of regular behavior for the influence of additional factors such as steric effects to be rationally examined as deviations from the norm. Thus comparisons between benzhydryl and trityl cations reveal quite different steric effects for reactions with hydroxylic solvents and alkenes, or even with different halide ions... 

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