Nucleophilic reactivity Swain-Scott equation

Table 1.6 shows nucleophiles ranked by one measure of nucleophilicity. These nucleophilicities are based on the relative reactivities of the nucleophile and water with methyl bromide at 25°C. The nucleophilicity n is calculated according to the Swain-Scott equation ... 

The Swain-Scott equation (Equation 42) " compares the reactivity of nucleophiles in an Sn2 mechanism with the reaction of nucleophiles with methyl bromide (Equation 42) as a reference reaction. Water is taken as the standard nucleophile (n = 0) and the equation can be written where the nucleophilicity (n) of a given nucleophile is defined by setting the reaction constant (5) for the reference reaction at unity (Equation 43). 

The final chapter of this section is by Rappoport and is concerned with nucleophilic reactions at vinylic carbon. Two reaction types are considered, those of neutral vinyl derivatives and those of vinyl cations. Correlation of rates for these reactions with both Ritchie and Swain-Scott equations was attempted without success. Rappoport concludes that these reactions are subject to a complex blend of polar, steric, and symbiotic effects and that a quantitative nucleophilicity scale toward vinylic carbon cannot be constructed . This conclusion is reminiscent of the earlier observation of Pearson (see the introduction to the section on the Brpnsted equation) and the later observation of Ritchie (Chapter 11) regarding the difficulty of correlating nucleophilic reactivity with a single equation. Rappoport finds another familiar situation when he explores the relationship between reactivity and selectivity for the vinyl substrates sometimes the RSP is obeyed and sometimes it is not. 

If we can assume that nucleophile reactivity is governed by the Swain-Scott equation and that general acid catalysis is governed by Brensted s law then the... 

The Bronsted a (or jS) value may also be related to the reactivity of the nucleophile by employing the Bronsted and Swain-Scott equations for j catalysts and k nucleophiles (equations 47 and 48)... 

Use of this equation permitted correlation of a fair amount of data (5, 21, 22) and led Edwards and Pearson (22) to discover the much-discussed a effect (enhanced reactivity for nucleophiles having an unshared pair of electrons on the atom adjacent to the nucleophilic atom) as yet a third factor controlling nucleophilicity. Subsequently, Pearson et al. (5) compared Swain-Scott n values for methyl iodide and trans-Pt(py)2Cl2 reacting with a diverse set of nucleophiles and found no relationship between the n values. Attempts to use the Edwards and related equations to correlate the results also failed, and the conclusion was made that present understanding was inadequate to permit quantitative prediction of rates for a wide variety of substrates. This pessimistic conclusion still holds, but work continues in the search for the key physiochemical properties controlling nucleophilic reactivity (23). 

Bentley has proposed alternative equations to those of Mayr for nucleophile-electrophile combinations." He suggests that nucieophilicity depends on the reactivity of the electrophile and accounts for this using a Swain-Scott type of response... 

In contrast to organic chemistry, where extensive studies have attempted to correlate nucleophilicity with various parameters (e.g., the Swain-Scott, Edwards, and Ritchie equations), there is currently little quantitative information concerning nucleophilic reactivities toward coordinated ir-hydrocarbons. Recent kinetic studies of reactions shown in equations (10) and (11) have established... 

Swain and Scott found satisfactory correlations with Equation (27) which provided 5 values for a number of reactants. However, as indicated in Scheme 33, for the limited number of substrates conveniently studied,158,186 variations in 5 did not show a clearly discernible pattern (and no obvious correlation with reactivity). Moreover, Pearson and Songstad demonstrated that the correlations break down if extended to extremes of soft and hard electrophilic centers such as platinum, in the substitution of trara,s-[Pt(pyridine)2Cl2], or hydrogen in proton transfer reactions.255 Despite this, Swain and Scott s equation has stood the test of time and it is noteworthy that a serious breakdown in the correlations occurs only when the reacting atoms of both nucleophile and electrophile are varied. In this chapter we will restrict ourselves to carbon as an electrophilic center, and particularly, although not exclusively, to carbocations. 

Far from confirming a dependence of nucleophilic selectivity on the reactivity of the carbocations, Ritchie observed that selectivities were unchanged over a 106-fold change in reactivity.15 He enshrined this result in an equation (29) analogous to that of Swain and Scott, but with the nucleophilic parameter n modified to N+ to indicate its reference (initially) to reactions of cations, and with the selectivity parameter s taken as 1.0, that is, with no dependence of the selectivity of the cation on its reactivity (as measured by the rate constant for the reference nucleophile, kn2o for water). 

A breakthrough came when Swain and Scott (5) proposed a two-parameter equation that calibrated the reactivity of nucleophiles by reference to their rate of reaction toward methyl iodide, taken to be a standard electrophile. This equation was based, in effect, on the well-known order of reactivity in SN2 reactions I- > Br-, Cl- > F- and on the work of Foss (6),... 

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