Nucleophilic substitution rate coefficients

Unless a more specific reference is given, rate coefficients in Section VI.B have been taken from the volumes of Reference 107c Volume 3, Part 1 (first order nucleophilic substitution) Volume 4, Part 2 (aromatic nucleophilic substitution). 

The substituent effect of a methyl group on the rate coefficient of the basic hydrolysis of ethylene oxide is small. The kOH values for ethylene oxide, propylene oxide, and isobutylene oxide are almost the same (Table 9). It has been demonstrated by Long and Pritchard [150] with the aid of experiments in oxygen-18 labeled water that hydroxide ion attacks propylene oxide and isobutylene oxide predominantly at the primary carbon. Consequently, the base catalyzed hydrolysis of epoxides is a simple bimolecular nucleophilic substitution. 

For neutral nucleophiles, we have utilized a series of ring-substituted N,N-dimethylanilines. The second-order rate coefficients should now be independent of nucleophile concentration, and this was confirmed by showing that log (k/k0 obtained from the product ratios, was independent of the amine concentration for 0.008 to 0.08 M N,N-dimethyl-p-toluidine. The log (k/k0) values could also be conveniently determined for m-CH3-, H-, p-Br-, and m-Cl-substituted derivatives (equation 13). For the m-N02 derivative, even at 0.32 M, the dominant reaction is solvolysis and only an approximate value for log (k/k0) could be obtained. A Hammett plot against the tabulated a values (43) (omitting the approximate m-N02 data) led to a linear plot and a slope (p value) of —2.77 0.15 (r = —0.996). This value is similar to values for reaction with other ethyl derivatives, derived from kinetically determined k values —3.60 for reaction with ethyl iodide in nitrobenzene at... 

Fig. 8. The relationship between the rate coefficients for the bimolecular elimination k and substitution (kg) of cyclohexyl p-toluenesulphonate with various nucleophiles in a number of solvents. Solvent O, acetone at 75°C , /-butyl alcohol at 75°C A. ethanol at 35°C 0< methanol at 75°C. (Reproduced with permission from Biale e/a/. .)...

The systematic measurements of reaction rates of nucleophilic substitutions have shown that, depending on the structure of reactants, the reaction kinetics can follow either the first or the second order rate law. The chemical reaction rate can be expressed by kinetics equations in which the main parameter is the reaction coefficient (rate constant) k ... 

On the other hand, use of activity coefficients of transfer, instead of free energies of transfer, is an equivalent approach for examination of medium effects on reaction rates and equilibria. Parker has emphasized this alternative approach and the reader is referred to his writings, especially for medium effects in bimolecular nucleophilic substitution (Parker, 1969). 

Kinetic, steric, and thermodynamic results have been reviewed to argue that the rate-determining step in some aliphatic nucleophilic substitutions is the transfer of an electron. The same group carried out a systematic ranking of different nucleophiles with respect to their ability to stabilize the transition states of substitution reactions, acetonitrile and dimethyl sulfoxide being the solvents involved. The nucleophiles included enolates, phenolates, thiophenolates, hydroxide, and cyanide. The method is based on a comparison of the rate coefficient, ksm, for the substitution reaction between a given nucleophile and benzyl chloride with the rate coefficient, A et for the corresponding electron transfer from an aromatic radical anion to benzyl chloride. The ratio ksuB/ ET expresses the rate enhancement due to electronic interaction in the transition state of the substitution reaction. 

Reaction yields for substituted terephthalic acids are consistent with the Hammett coefficients, such that the more nucleophilic add salt is fevored in the reaction with the electron-poor lead chloride moiety. This is consistent with a nucleophihc attack (probably associative) on the lead atom as being the critical (rate-determining) step. 

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