A series of 21 benzhydrylium ions (diarylmethylium ions) are proposed as reference electrofuges for the development of a general nucleofugality scale, where nucleofugality refers to a combination of leaving group and solvent. A total of 167 solvolysis rate constants of benzhydrylium tosylates, bromides, chlorides, trifluoroacetates, 3,5‐dinitrobenzoates, and 4‐nitrobenzoates, two‐thirds of which have been determined during this work, were subjected to a least‐squares fit according to the correlation equation log k25 °C = sf(Nf + Ef), where sf and Nf are nucleofuge‐specific parameters and Ef is an electrofuge‐specific parameter. Although nucleofuges and electrofuges characterized in this way cover more than 12 orders of magnitude, a single set of the parameters, namely sf, Nf, and Ef, is sufficient to calculate the solvolysis rate constants at 25 °C with an accuracy of ±16 %. Because sf≈1 for all nucleofuges, that is, leaving group/solvent combinations, studied so far, qualitative discussions of nucleofugality can be based on Nf.

The correlation equation log k(25 degrees C) = sf(Nf + Ef), where sf and Nf are nucleofuge-specific parameters referring to leaving group/solvent combinations and Ef are electrofuge-specific parameters referring to the incipient carbocation R+, are used to predict ionization rate constants of alkyl derivatives R--X. We show how to employ the Ef parameters of reference electrofuges and the sf and Nf parameters of reference nucleofuges reported in the preceding article for determining further sf, Nf, and Ef parameters. Since sf is usually close to 1.0, one comes to the semiquantitative rule that at 25 degrees C, compounds R--X for which Nf + Ef>-2 will solvolyze with half-lives of less than a minute, while the solvolysis half-lives will exceed 1 month if Nf + Ef<-6.5.

The LFER equation log k = s(f)(E(f) + N(f)) was used to derive the nucleofuge specific parameters (N(f) and s(f)) for dimethyl sulfide in the series of aqueous alcohols, using the S(N)1 solvolysis rate constants obtained for X-substituted benzhydryl dimethyl sulfonates 1-5. The slope parameters (s(f)) are practically independent of the solvent used, while N(f) parameters slightly decrease as the polarity of the solvent increases.

The solvolysis rates of X‐substituted benzhydryltetrahydrothiophenium ions (1) in pure and aqueous alcohols were determined at 25 °C and compared with the rates of the corresponding benzhydryldimethylsulfonium ions (2). The linear free energy relationship equation log k = sf(Ef + Nf) has been used to relate quantitatively the leaving group abilities of tetrahydrothiophene (THT) and dimethyl sulfide (Me2S). It has been demonstrated that although generating a stronger base by heterolysis, substrates 1 solvolyze over lower barriers than 2. Steric and electronic influences that determine the relative reactivities of sulfonium salts have been examined computationally at B3LYP level of theory by calculating the energy of exchange of electrofuges with different substituents between THT and dimethyl sulfide. Because of more efficiently delocalized positive charge in THT moiety, tetrahydrothiophenium ions are more stable than the corresponding dimethylsulfonium ions, regardless of an electrofuge. The Hammond–Leffler coefficient is negative (α < 0) for the rate determining heterolysis of sulfonium salts 1 and 2. Copyright © 2011 John Wiley & Sons, Ltd.

In this short authors' review, a method for determining experimental and calculated nucleofugalities of leaving groups in a given solvents according to LFER equation log k = sf (Nf + Ef) is presented. Also, a comprehensive overview of the experimental and calculated nucleofuges specific parameters (Nf and sf) for various negatively charged and neutral leaving groups is shown. Some applications of the above method have been demonstrated: use of the electrofugalitiy and nucleofugality scales to estimate the reactivities of a variety of substrates in various solvents, as well as assessment whether a given substrate is stable in a given solvent for a sufficent amount of time, which may indicate if the substrate can be handled in the solvent of choice during synthetic and other procedures. The method can also be used to establish whether the relative reactivity of leaving groups depends on the electrofuge moiety of the substrate.