Swain-Scott equation, nucleophilicity

The LFER correlation according to the Swain-Scott equation for nucleophilic attack on /3-propiolactone. Data are from Ref. 13. 

The following overall nucleophilicity order for Sn2 mechanisms (in protic solvents) was given by Edwards and Pearson RS > ArS >1 >CN > OH > Nj > Br > ArO > Cl > pyridine > AcO > H2O. A quantitative relationship (the Swain-Scott equation) has been worked out similar to the linear free energy equations considered in Chapter 9 ... 

A point of key importance in study of solvolysis is the nucleophilicity of the solvent. Whereas the Y and other scales have been available for measuring ionizing power for some years, there has been no satisfactory scale for nucleophilicity. Swain, Mosely, and Bown attempted to set up an equation for correlation of solvolysis rates that included both nucleophilicity and ionizing power 112 their system did not prove particularly helpful for understanding mechanism.113 The Swain-Scott equation, discussed in Chapter 4 (p. 185), was not evaluated for solvents. 

Second-order rate constants for the reactions of phenacyl bromide with a number of anionic or neutral nucleophiles in 3 2 (v/v) acetone-water have been measured at several temperatures.141 Correlation analysis with the Bronsted equation or Swain-Scott equation is not satisfactory. Better results were obtained with the two-parameter Edwards equation. 

Solvolysis of the R,R and R,S isomers of 2-bromo-9-(l-X-ethyl)fluorenes, X = Cl, Br, I, or OBs, in 25% (v/v) acetonitrile in water has been studied with respect to rates of formation of elimination products and of substitution products (X = OH or NHCOMe).142 The parent 9-(l-X-ethyl)fluorenes and the 2,2/-dibromo-9-(l-X-ethyl)-fluorenes were also studied. Various effects of leaving group and of the presence of nucleophiles on the competition between the reactions were observed and the Bronsted equation was applied to the results for the elimination reactions. A related study of solvolysis of 9-(X-methyl)fluorenes, X = I, Br, or Bs, was also carried out, in which the Swain-Scott equation was applied to nucleophilic selectivities in the S 2 reactions.143... 

Note again that 5 here differs from s in the Swain-Scott equation which refers to the slope of a plot of log k versus the nucleophilicity parameter n (or N) rather than electrophilicity parameter (E). 

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). 

Figure 10 illustrates the application of the Swain-Scott equation to the reaction of nucleophiles with V-methoxymethyl-A,A-dimethyl-3-nitroanilinium ion (Equation 45). ... 

Equation (31) has an electrophilic ( ) and a nucleophilic (N component and the value, s, is a nucleophile specific parameter. This equation has a close family relationship with the Ritchie and Swain-Scott equations (Chapter 2) and the Edwards equation (29). The equation successfully correlates rate constants for a wide range of disparate structures and... 

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. 

The rate constants of the reaction CH3Y + X- CH X + Y in sulfolane solution are described by the Marcus equation the quadratic term contributes very little. The Marcus equation then reduces to the expression log kyx = My + Nx, where My is a property of CH3Y only and Nx is a property of CG X only. Each term includes only the identity rates and the equilibria for methylation of a reference nucleophile. The two terms are determined independently of unsymmetric rate measurements, in contrast to the Swain-Scott equation. Short tables of both terms are presented. Extension to other solvents and to other reactions including group transfers is discussed. With other alkyl groups, the simple expression may cover the continuum from elimination-addition to addition-elimination and may also cover other group transfers. 

The Swain-Scott equation (I) is probably one of the most important empirical equations in the field of nucleophilic reactions ... 

The two major differences between these two equations are as follows (1) Unlike the Swain-Scott equation, the Ritchie equation lacks a selectivity parameter. Thus, a plot of log k versus N+ always gives a unity slope. (2) Nucleophilicity ranking by the two scales is different. This difference is especially noticeable for the three nucleophiles, CN-, CH30-, and N3-. According to the N+ scale, the nucleophilicity order is N3- > CH30- > CN-, whereas the reverse is true for the Swain-Scott n scale. 

Kevill and Lin s (27) earlier study of the ethanolysis of the triethyloxonium ion, at 0.0 °C, has been extended to a consideration of the competition between the solvent and added nucleophile, either anionic or neutral, for reaction with the substrate (equation 11). This study is related to earlier studies, at 25.0 °C, of competition between water and added nucleophile for reaction with methyl bromide (36), methyl iodide (37), or the cyclic pen-tamethyleneiodonium ion (38) and of competition between methanol and added nucleophile for reaction with methyl iodide (39) or trans-Pt(py)2Cl2 (39). The nucleophilicities are usually expressed, relative to the solvent, in terms of the Swain-Scott equation (36) (equation 12). In equation 12, k and k0 are second-order rate coefficients for reaction of a substrate with the added nucleophile and with the solvent, n is a measure of the nucleophilicity of the added nucleophile, and s is a measure of the sensitivity of the substrate toward changes in nucleophilicity. The value of s is taken as unity for the standard substrate. 

Nucleophilicities relative to a standard solvent can be quantified by the Swain-Scott equation (12)66, in which k and k0 are the second-order rate constants for reactions of the nucleophile and solvent respectively, and s is a measure of the sensitivity of the substrate to nucleophilicity n. By this definition, the nucleophilicity of the solvent is zero. For all reactions examined, there will be competition between attack by solvent (present in large excess) and reaction with added anionic nucleophiles. Hence, only n values well above zero can be obtained with satisfactory reliability. In the original work66, the solvent was water and all but one of the substrates were neutral s was defined as 1.0 for methyl bromide and was calculated to be 0.66 for ethyl tosylate the lowest reliable n value reported was 1.9 for picrate anion, but a value of < 1 for p-tosylate anion was reported66 in a footnote. 

Representative s values are collected in Table 24 and n values in Table 25. The order of nucleophhcity (n) in the Swain-Scott approach bears no relationship to the pK of the conjugate acid of the nucleophile. Since in a given constant series of nucleophiles basicity and nucleophiUcity are related there have been attempts to correlate nucleophiUcity with one or more parameters. If the relative importance of these parameters is the same as in the methyl bromide reaction then the Swain-Scott equation will hold. 

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... 

Nucleophilic strength depends on two factors, the nature of the substrate (s) and the strength of the nucleophile, measured by its nucleophilicity (n). Nucleophilic strength for a given species is given by the Swain-Scott equation [ log = (s) (n) ] where n = 0 for water at 25°C, s - 2.00 for methyl... 

Table 2.12. Nucleophilic Strength with Bromoethane as Ordered by the Swain-Scott Equation...

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