Swain-Scott relationship

A completely empirical LFER can also be constructed with recourse only to kinetic data. This has been the case in the setting up of a scale of nucleophilic power for ligands substituting in square-planar complexes based on the Swain-Scott approach. The second-order rate constants Ay for reactions in MeOH of nucleophiles Y with tra 5-Pt(py)2Cl2, chosen as the standard substrate 

On the basis of this equation, an index of nucleophilicity pt can be assigned to each nucleophile Y (see Table 4.13). It is found, moreover, that a plot against pt of logfcy, for reaction of Y with another Pt(II) neutral substrate, is also often linear. Thus, Eq. (2.168) applies, and 5 is termed the nucleophilic discrimination factor (Sec. 4.7.1). Some of the departures from linearity of plots of Ary vs p, which have been observed, disappear if the Pt reference substrate chosen is of the same charge as the Pt reactants. The value of p, for a bulky nucleophile has also to be modified to allow for steric hindrance features. 

---

Explain in words what the Swain-Scott relationship describes and discuss in which cases it may be applied. 

Because you are more interested in groundwater contamination, you wonder how fast TMP would be transformed by chemical reactions at 10°C and pH 8.0 in a leachate from a waste disposal site containing 0.25 M Cl, 0.05 M Br", and 10-4 M CbT. Calculate the approximate half-life of TMP under these conditions by trusting your colleague s measurements and by assuming that all relevant reactions exhibit about the same activation energy of 95 kJ-mol"1. Also assume an s-value of 0.9 in the Swain-Scott relationship (Eq. 13-3). 

In principle, these measurements represent an application of the Swain-Scott relationship to two nucleophiles only. This is apparent from Equation (28), in which nAz corresponds to n for the azide ion and the electrophilic parameter s is seen to measure the selectivity of the carbocation between azide... 

The rate coefficient x is dependent on the nucleophilic power of the halide ion according to the Swain—Scott relationship [155]... 

Let us first look at some reactions that occur predominantly by an SN2 mechanism. We cons der the reactions of primary alkyl halides (i.e., R-CH2 X, where X = F, Cl, Br, I) with a series of inorganic nucleophiles. For a given primary alkyl halide (e.g., CH3-Br, R-CH2-C1), the relative reactivity toward various nucleophiles may be related by a LFER, the Swain-Scott relationship (e.g., Hine, 1962) ... 

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. 

Mayr initially defined a set of electrophilic parameters for the benzhydryl cations using a reference nucleophile, which was chosen as 2-methyl-1-pentene.268,269 Values of E were then defined as log k/k0, where k0 refers to a reference electrophile (E= 0), which was taken as the 4,4 -dimethoxybenzhydryl cation. Plots of log k against E for other alkenes are thus analogous to the plots of logk against p fR in Fig. 7 except that the correlation is referenced to kinetic rather than equilibrium measurements. However, they differ from plots based on the Swain-Scott or Ritchie relationships in which log k is normally plotted against a nucleophilic parameter, that is, n or N+, rather than E. 

Similar SN 2 mechanisms occur in the uncatalyzed ring-opening reactions of ethylene oxide and other epoxides with nucleophiles such as halide ions or amines [151, 154]. The dependence of the rate coefficient kx on the attacking nucleophile is the same as in SN 2 reactions of alkyl halides. It follows the Swain—Scott [155] and Edwards [156] relationships. 

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... 

In this case we employ Hammett terminology but any free energy relationship such as a Bronsted or a Swain -Scott equation could be used. 

Tables of MY and Nx values are given that permit calculation of a large number of reaction rates. The relationship of this expression to the formally similar, but empirical, Swain-Scott and Brpnsted equations is of much interest.

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). 

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. 

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. 

The relationship of to the nucleophilic constant n of the Swain-Scott equation appears to be linear. 

All else being equal, the better the nucleophile, the faster the Sn2 reaction. In Section 8.4.5 we examined a linear free energy relationship for niicleophilicity, called the Swain-Scott equation. The reference reaction for the Swain-Scott measure of nucleophilicities is an Sn2 reaction, that of methyl iodide and methanol. Therefore, the chemical intuition that most chemists rely upon for predicting relative nucleophilicity is actually based upon how the structure of the nucleophile influences Sn2 reactions (Table 8.5). 

A plot of log k lk ) against the nucleophilic constants of Swain and Scott gives a straight line. In calculating the values of kjk (Table 8) from k/k by means of eqn. (26), kjkv, = 229 was chosen in accord with the nucleophilic constant of hydroxide ion. The linear relationship for the various halide ions shows that the relative amounts by which these ions decrease the rate of the basic hydrolysis of chloroform are proportional to their relative nucleophili-cities. 

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

For nucleophilic displacement reactions, some relationship might be expected between the ability of an anion A to act as nucleophile and the dissociation constant of the acid HA the best nucleophiles should be the anions of the weaker acids. For an uncharged nucleophile B there should be a relationship with the dissociation constant of the conjugate acid BH. Nucleophilicity is often treated in terms of Bronsted-type relationships, but deviations and sometimes almost complete breakdown are found when wide ranges of substrates and nucleophiles are considered. Nucleophilicity is a more complex function of structure than acid-base behavior. This situation has led to the development for such reactions of a special system of correlation analysis, which involves scales of nucleophilicity constants. The first of these, the n scale, was proposed by Swain and Scott in 1953 and was based on methyl bromide as a standard substrate for an LFER. Other scales have subsequently been developed and applied in both simple and multiple regression. 

---