Swain—Scott equation

Swain and Scott developed a linear free energy relationship for nucleophilicity (equation 8.43) in the same form as the Hammett equation. 

The term n is the nucleophilicity parameter for a given nucleophile, while s is the measure of the sensitivity of the substrate to the nucleophilicity of the attacking reagent. The rate constants k and ko are the rate constants for reaction of the nucleophile and of water, respectively, and the n value of water 

Equation 8.38 has been called an oxibase scale, since the E term is an oxidation term, while the H term is a basicity term Davis, R. E. in Janssen, M. J., Ed. Organosulfur Chemistry Wiley-Interscience New York, 1967 pp. 311-328. 

TABLE 8.13 Selected Values of Swain Nucleophilicity Parameter, n 

TABLE 8.14 Selected Values of Swain-Scott s Value 

---

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

Spin trap, 102 Statistical kinetics, 76 Steady-state approximation, 77-82 Stiff differential equations, 114 Stoichiometric equations, 12 Stopped-flow method, 253-255 Substrate titration, 140 Success fraction approach, 79 Swain-Scott equation, 230-231... 

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 ... 

The Hammett and Taft equations are not the only linear free-energy relationships known. We shall encounter others—for example, the Bronsted relations, and the Grunwald-Winstein and Swain-Scott equations later in this book. 

To get a better understanding of what the Swain-Scott equation means, we have rewritten it in Equation 4.22 in the form that makes the linear free-energy relationship more apparent. 

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). 

The relative reactivities of Table 9 differ from those towards saturated carbon atom, as measured by the w-values of the Swain-Scott equation,... 

McCauley and King [204] have carried out a careful quantitative study of kinetic salt effects of halides and product ratios in the acid catalyzed hydrolysis of diazoacetone in water which confirms the validity of rate eqn. (47). The fraction of halogenoacetone formed completely corresponds to the relative magnitude of the second term (dependent on [X-]) in the rate equation. The results obtained for fcHCI, hbt and ftm obey the Swain—Scott equation (54) with s = 0.35 [203]. The experimental fcH... 

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 ... 

See also electrophilicity Ritchie equation SWAIN-SCOTT EQUATION. 

Swain-Scott equation The linear free-energy... 

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). ... 

---