KINETIC PARAMETRS OF IONIZING POWER OF THE SOLVENTS. NATURE OF SOLVATION EFFECTS ON THE HETEROLYSIS OF 2-ARYL-2-CHLOROADAMANTANES

4 Danish Scientific Journal No102, 2025 CHEMICAL SCIENCES KINETIC PARAMETRS OF IONIZING POWER OF THE SOLVENTS. NATURE OF SOLVATION EFFECTS ON THE HETEROLYSIS OF 2-ARYL-2-CHLOROADAMANTANES Koshchii I. PhD in Chemistry, Associate Professor Department of Organic Chemistry and Technology of Organic Substances National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute” Vasilkevich O. PhD in Chemistry, Associate Professor Department of Organic Chemistry and Technology of Organic Substances National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute” Klimko Yu. PhD in Chemistry, Associate Professor Department of Organic Chemistry and Technology of Organic Substances National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute” https://doi.org/10.5281/zenodo.17740974 Abstract The rate of heterolysis of tertiary substrates, especially adamantyl derivatives, is highly dependent on the effect of nucleophilic solvation. This effect increases with increasing spatial complications. The use of 2-aryl-2halogenadamantanes as reference points is impractical because the conjugation of the positive charge with the phenyl strongly depends on the nature of the solvent. The least sensitive to the effects of specific solvation is t-BuCl. Unlike tertiary, the heterolysis rate of secondary compounds does not depend on the nucleophilicity of the solvent. Keywords: heterolysis, solvation effects, tertiary substrates, kinetic parameters, solvent ionizing ability, solvent nucleophilicity, solvolysis, correlation analysis. Introduction Tertiary substrates are of little use as benchmarks for determining the kinetic parameters of the ionizing ability of solvents (Y), since the rate of their heterolysis decreases with increasing nucleophilicity of the solvent. This effect is enhanced with increasing steric hindrance. The rate of heterolysis of 2-aryl-2-chloroadamantanes does not depend on the nucleophilicity of the solvent, however, in them the degree of conjugation (conjugation) of the nascent positive charge with phenyl strongly depends on the nature of the solvent, which makes them unsuitable as benchmarks. The rate of heterolysis of secondary compounds does not depend on the nucleophilicity of the solvent and correlates well with the solvatochromic parameters ET and Z, which characterize the ionizing ability of solvents better than Y. Materials and methods. Generalization of the kinetic data of tertiary substrates obtained by different methods (conductometric, chromatographic, verdazyl) using correlation analysis. Results and discussion. The rate of monomolecular heterolysis reactions (SN1, E1, F1, solvolysis, ʋ = k[RX]) strongly depends on the polarity, or rather, the ionizing ability of the solvent (parameters Y, Z, ET) [ 1,2]. The kinetic parameters of the ionizing ability of the solvent are often used for comparative analysis of solvation effects, in particular, to identify the influence of the nucleophilicity of the solvent on the rate of heterolysis. In this case, substrates are chosen as benchmarks in which nucleophilic attack from the back of the covalent substrate is very difficult (tert-alkyl compounds) or impossible (adamantanes) [1,3]. The term “heterolysis” for SN1 and E1 reactions means that these reactions begin with the heterolytic cleavage of a covalent bond under the influence of a solvent. This gave rise to the idea of nucleophilic assistance of the solvent [1, 4-7] as a result of the formation of a linear quadrupole between the dipoles of the substrate and the solvent b+ solv b- → Rb+ Xb-, which should help form a contact ion pair. The problem of the influence of the nucleophilicity of the solvent on the rate of monomolecular heterolysis has been the subject of constant research and discussion for many years [1-30]. Some researchers believe that the nucleophilicity of the solvent increases the reaction rate [1,4-9], others that it decreases [2,17, 18, 20-23], and others that it has no effect [25-27]. Therefore, there is still no unanimous opinion regarding the existence of nucleophilic solvent assistance in the heterolysis of not only tertiary, but also secondary and primary substrates, where there are no steric hindrances for nucleophilic solvation from the rear [2, 16]. The main evidence for nucleophilic solvent assistance is the increase in the ratio of the solvolysis rate constants k t-BuX : k 1-AdX with increasing solvent nucleophilicity [5, 28] and the influence of steric hindrances for nucleophilic solvation from the rear on this ratio [29, 30]. For example, when going from EtOH to CF3CH2OH, this ratio increases a thousandfold. Since it is assumed that the rate of solvolysis of adamantyl substrates does not depend on the nucleophilicity of the solvent due to the impossibility of nucleophilic solvation from the rear of the covalent substrate, conclusions Danish Scientific Journal No102, 2025 5 are drawn about the presence of nucleophilic solvent assistance in the solvolysis of t-BuX [28]. The conclusion about the presence of nucleophilic solvent assistance in the solvolysis of tert-alkyl compounds is only an assumption based on unreliable arguments – the ratios of rate constants and the obvious absence of the effect of solvent nucleophilicity in the solvolysis of 1AdX [16]. In [31] it was shown that the rate of solvolysis of t-BuCl and p-methoxyneophytosylate (I) decreases with increasing solvent nucleophilicity. Later, these results were confirmed in [22, 23, 32, 33], which used different sets of solvents, different parameters of solvent nucleophilicity, and also different correlation equations. The fact that the nucleophilicity of the solvent does not increase, but decreases the rate of heterolysis of tertiary substrates, and the effect increases with increasing steric hindrance for nucleophilic solvation from the rear, was shown in [16-18, 20]. Heterolysis of secondary substrates, where nucleophilic attack of the covalent substrate from the rear is not hindered, does not depend on the nucleophilicity of the solvent [2, 18, 20, 21]. The rate of heterolysis of secondary substrates is described by the polarity parameters f(ɛ)=(ɛ - 1)/(ɛ + 1) and electrophilicity E or by the solvatochromic parameters of the solvent ionizing ability Z(ET), which also correlate satisfactorily with the polarity and electrophilicity parameters. For 3-bromocyclohexene (II) and Ph2CHBr [25, 34] we have: lg kII = -19.5 + 0.0476 Z; R 0.984, S 0.323, N 30 (1) lg kII = -9.64 + 2.72f (ɛ) - 0.0700 E; R 0.972, S 0.352, N 32 (2) lg k Ph2CHBr = -10.2 + 2.88f (ɛ) + 0.0978 E; R 0.979, S 0.331, N 27 (3) lg k Ph2CHBr = -21.9 + 0.086 ET; R 0.978, S 0.334, N 27 (4) The rate of heterolysis of tertiary substrates [e.g., 2-bromo-2-methyl-adamantane (III) and cumyl chloride (IV)] decreases with increasing nucleophilicity of the solvent [32, 35]: lg kIII = -6.32 + 3.94f (ɛ) + 1.12 E - 2.02 B; R 0.979, N 15. Without B R 0.889 (5) lg kIV = -0.362 + 0.182 Z -0.378 B; R 0.976, S 0.142 N 10. Without B R 0.938 (6) The effect of nucleophilic solvent promotion is clearly evident in SN2 ion pair reactions, where the limiting step is a nucleophilic attack on the contact ion pair. Thus, the rate of solvolysis of PhCH2Cl and PhCOCl in H2O, MeOH, EtOH, AcOH, HCO2H does not correlate well with the parameters Z and ET (R~0.92), however, with additional consideration of the nucleophilicity parameter B, the correlations become excellent [36], the coefficient at B is positive, which indicates the presence of nucleophilic solvent assistance: lg kPhCOCl = -30.5 + 0.291Z + 0.0128B; R 0.999, S 0.086 (7) lg kPhCH2Cl = -31.6 + 0.255Z + 0.0149B; R 0.999, S 0.085 (8). 6 Danish Scientific Journal No102, 2025 Protic solvents Aprotic solvents Fig. 1. The lgk–Z dependence for the heterolysis of 2-bromo-2-methyladamantane 3 and 3bromocyclohexene 2 in protic and aprotic solvents (25°C): 1 – MeOH, 2 – AcOH, 3 – EtOH, 4 – PrOH, 5 – BuOH, 6 – 2-PrOH, 7 – 2-BuOH, 8 – cyclohexanol, 9 – tBuOH, 10 – t-PentOH, 11 – propylene carbonate, 12 – MeCN, 13 – DMSO, 14 – γ-butyrolactone, 15 – sulfolane, 16 – acetone, 17 – PhNO2, 18 – EtCOMe, 19 – PhCN, 20 – 1,2-dichloroethane, 21 – PhCOMe, 22 – CH2Cl2, 23 – cyclohexanone, 24 – CHCl3, 25 – PhCOOEt, 26 – o-dichlorobenzene, 27 – MeCOOEt, 28 – THF, 29 – PhCl, 30 – PhBr, 31 – PhOEt, 32 – PhI, 33 – MeCCl3, 34 – Ph2O, 35 – o-xylene, 36 – PhMe, 37 – p-xylene, 38 – cyclohexane Fig. 1 shows typical examples of the lg k – Z dependence for tertiary and secondary substrates. In the case of a tertiary substrate [bromide (III)], the rate of heterolysis of which decreases with increasing nucleophilicity of the solvent, the lg k – Z dependence consists of two lines: one for aprotic, the second for protic solvents [37]. In protic solvents, the reaction rate is two orders of magnitude lower. The lg k values in BuOH and t-PentOH occupy an intermediate position. This can be explained by the steric hindrances that arise during nucleophilic solvation of the nascent carbocation by the tertiary alcohol from the rear [38]. Indeed, when such solvation is absent due to steric reasons, the reaction rate increases greatly. Thus, when going from 2methyl-2-adamantyl-p-nitrobenzoate (Va) to 2-tert-butyl-2-adamantyl-p-nitrobenzoate (Vb), the rate increases by 2.3•105 times [16]. Danish Scientific Journal No102, 2025 7 OMe CH OTs Br Me Br Cl R ONO 2 2 C O CH Cl 2 R - Me (a), t-Bu (b) I II III IV V VI The dependence of lg k – Z (Fig. 1) for the secondary substrate [bromide (II)], the rate of heterolysis of which does not depend on the nucleophilicity of the solvent, forms one common line for protic and aprotic solvents [25]. In propylene carbonate (Z = 303), bromide (II) is one and a half orders of magnitude more active than bromide (III). With a decrease in the value of Z, the difference decreases, and in cyclohexane (Z = 218) these bromides react with the same rate, lg k25≈ - 9.6. In protic solvents, the opposite dependence occurs: the difference in heterolysis rates decreases with increasing Z. Thus, in cyclohexanol (Z = 314), bromide (III) is ~ 5 times more active than bromide (II), and in MeOH (Z = 350) they react with practically the same rate, lg k25≈ -3.2. In protic solvents, the convergence of the activities of such substrates occurs with increasing ionizing power of the solvent, which has a stronger effect (especially electrophilicity) on the less active substrate. The reason for this is the greater tendency of the less active substrate to electrophilically promote the solvent by forming an H-complex with the nucleofuge. In an aprotic medium, the approach occurs with a decrease in Z, which has a stronger (especially polarity) effect on the more active substrate. In this case, the reason is the greater polarization of the C-X bond, which promotes dipolar solvation [39]. Monomolecular heterolysis occurs through the sequential formation of four ion pairs: contact (A), spatially separated (B), separated by one solvent molecule (B), and solvation-separated (D) [Scheme 1] [2, 16, 18, 40-43] RX ‚⇌ R+X- ⇌ R+ | Ο | X- ⇌ R+ | Solv | X- ⇌ R+ |Solv, Solv| X- → Products of the reaction A B C D Scheme 1. The course of molecular heterolysis The free carbocation is formed in water or aqueous solutions [44]. The lack of nucleophilic solvent assistance indicates that ion pairs are formed after the limiting step, and the negative effect of nucleophilic solvation is due to the solvation of ion pair A, which is formed before the limiting step. This stabilizes the intermediate and complicates the separation of ions in the transition state. Nucleophilic solvation of the contact ion pair complicates the removal of the nucleofuge by the SN1 mechanism [45, 46]. In the first stage of the reaction, a contact ion pair is formed by transferring an electron from the MO of the covalent substrate to the orbital of the nucleofuge. The resulting ion pair A is stabilized by the formation of a coordination complex with the solvent molecule b+solvb- → R+X-. This occurs at a rate of 1011 - 1013 s-1 [44, 47, 48], which is two to three orders of magnitude higher than the diffusion rate (~5•109 l∙mol -1∙s-1) [10,49]. Thus, ion pair A is formed without nucleophilic assistance from the solvent. In the rate-limiting step, the solvate of ion pair A interacts with the solvent cavity (Ο). This process is accompanied by desolvation of the intermediate. Pair B is formed, which at a rate 100 times higher than the diffusion rate is converted into ion pair B. Pair B is also rapidly converted into pair D, and the latter reacts rapidly and quantitatively with the nucleophile [2, 16, 18, 42]. The rate of heterolysis of secondary substrates, where solvation from the rear is sterically accessible, does not depend on the nucleophilicity of the solvent [2, 16-18]. Stereochemical evidence for solvation of ion pair A is known, when solvolysis of optically active compounds leads to the formation of products with partially or completely preserved configuration [50]. Solvation of the A pair, on the one hand, increases its lifetime, and therefore increases the probability of encountering the solvent cavity (enthalpic control), and on the other hand, increases the volume of solvate A, which complicates its interaction with the solvent cavity (entropic control). As a result of the compensation effect ∆H≠ - ∆S≠, the rate of heterolysis of secondary substrates does not depend on the nucleophilicity of the solvent [2, 43]. In the case of tertiary substrates ∆S≠ increases more strongly than ∆H≠ due to spatial obstacles to nucleophilic solvation during the formation of a coordination complex, which causes a decrease in the rate with increasing nucleophilicity of the solvent [16]. The rate of heterolysis of t-BuCl depends relatively little on the nucleophilicity of the solvent. For the entire set of protic and aprotic solvents, it is roughly described by the dependence of lg k-ET (Fig. 2). There is a correlation with the parameters of nonspecific solvation and electrophilicity. For 22 protic and 22 aprotic solvents, it was obtained [2]: lg kt-BuCl = -19.7 + 13.9f(ɛ) + 11.7f(n) +0.0380 E; R 0.960, S 0.780, N 44 (9) 8 Danish Scientific Journal No102, 2025 ЕТ, kJ/mol Protic solvents Aprotic solvents Fig. 2. The dependence of lgk – ET for the heterolysis of t-BuCl in protic and aprotic solvents (25 °C): 1 – (CF3)2CHOH, 2 – H2O, 3 – CF3CH2OH, 4 – HCOOH, 5 – glycerin, 6 – HCONH2, 7 – ethylene glycol, 8 – MeOH, 9 – AcOH, 10 – HCONHMe, 11 – PhOH, 12 – MeCONHMe, 13 – EtOH, 14 – PrOH, 15 – BuOH, 16 – 2-PrOH, 17 – HexOH, 18 – OctOH, 19 – cyclohexanol, 20 – PhNH2, 21 – t-BuOH, 22 – t-AmOH, 23 – propylene carbonate, 24 – MeNO2, 25 – MeCN, 26 – ДМСО, 27 – γbutyrolactone, 28 – sulfolane, 29 – Ac2O, 30 – DMF, 31 – DMAA, 32 – N-methylpyrrolidone, 33 – acetone,34 – PhNO2, 35 – PhCN, 36 – CH2ClCH2Cl, 37 – PhCOMe, 38 – CH2Cl2, 39 – t-BuCl, 40 – cyclohexanone, 41 – CHCl3, 42 – dioxane, 43 – Et2O, 44 – heptane Sometimes, even in the set of protic and aprotic solvents, a small negative effect of nucleophilic solvation is manifested. For example, for 14 protic and 14 aprotic solvents, the following results were obtained [2, 51]: lg kt-BuCl = -11.5+ 0.289ET +3.67б2 – 0.00351B; R 0.960, S 0.064, N 28. (10).Without B R 0.949. This is due to the fact that in protic solvents the nucleophilicity of the solvent reduces the rate of heterolysis of t-BuCl [2] due to lg kt-BuCl = -13.6+ 16.6f(ɛ) - 0.0370Е – 1.24B; R 0.964, S 0.63, N 20. (11) intensive solvation of the contact ion pair, which makes it difficult to separate ions in the transition state. % vol organic solvent Fig. 3. Dependence of the value of the kinetic parameter of the ionizing ability of the solvent Y on the nature and composition of the mixed solvent: 1 – MeOH + H2O; 2 – EtOH + H2O; 3 – acetone + H2O; 4 – dioxane + H2O Danish Scientific Journal No102, 2025 9 The lg k-Y curves (Fig. 3) depend quite strongly on the nature and composition of the mixed solvent. The diversity of the curves is called dispersion and is explained by the different influence of the solvent on the rate of internal and external return of ion pairs or by the nucleophilic assistance of the solvent. However, the analysis of the lg k-Y dependences for tertiary substrates showed a clear negative effect of nucleophilic solvation. For example, in the solvolysis of neophyllium chloride (VI) where nucleophilic solvation from the rear is impossible, the solvolysis rate at Y=0 varies in a series of binary solvents as follows [52]: Solvent: HCO2H-AcOH AcOH-H2O MeOH – H2O EtOH-H2O Dioxane-H2O lg k, s-1: 7.8 8.2 8.4 8.5 9.2 It is clear that with increasing nucleophilicity of the solvent, the reaction rate decreases - the effect of nucleophilic solvation is manifested. The higher the nucleophilicity of the solvent, the lower the solvolysis rate and the greater the dispersion effect [53]. In [1], the effect of mixed and individual solvents on the rate of solvolysis of t-BuBr and 1-Ad-Cl was analyzed and it was concluded that these compounds react by the same mechanism with nucleophilic solvent assistance for t-BuBr and its absence for 1-Ad-Cl. The authors proposed using adamantyl substrates to determine Y. Later studies showed that during heterolysis of these compounds a stronger negative effect of nucleophilic solvation is observed. Indeed, a satisfactory correlation for nodal adamantane derivatives is observed only when B with a minus sign is taken into account [54-57]. lg k 1-AdІ = -15.4 + 20.9 f(ɛ) + 0.00162 δ – 2.98B; R 0.972, S 0.143, N12. Without B R 0.473 (12) lg k 1-AdBr = -28.1 + 0.421 ET - 0.0092 B; R 0.981, S 0.124, N10. Without B R 0.957 (13) lg k 1-AdOPic = -9.89 + 16.6 f(ɛ) + 5.1 f(n) – 0.0167 B; R 0.976, S 0.179, N8. Without B R 0.200 (14) lg k 1-AdOTs = -5.97 + 0.264 ET – 0.807 B; R 0.960, S 0.435, N18. Without B R 0.930 (15) ЕТ,kJ/mol Protic solvents Aprotic solvents Fig. 4. Dependence of lgk – ET for heterolysis of 1-AdI in protic and aprotic solvents (25 °C): 1 – (CF3)2CHOH, 2 – H2O, 3 – CF3CH2OH, 4 – HCOOH, 5 – MeOH, 6 – AcOH, 7 – EtOH, 8 – PhCH2OH, 9 – 2-PrOH, 10 – HexOH, 11 – cyclohexanol, 12 – t-BuOH, 13 – propylene carbonate, 14 – MeCN, 15 – γbutyrolactone, 16 – acetone, 17 – PhNO2, 18 – PhCN, 19 – cyclohexanone The dependence of lg kET for 1AdІ (Fig. 4), as for bromide (III) (Fig. 1), has two lines - one for protic, the second for aprotic solvents, which confirms the presence of a negative effect of nucleophilic solvation [58]. The use of adamantyl substrates for the determination of Y did not eliminate the dispersion, but only complicated the problem, since the magnitude of the negative effect of nucleophilic solvation for adamantyl substrates is much larger than for tert-butyl ones. To confirm the nucleophilic promotion of the solvent, the two-parameter Grunwald-Weinstein equation is often used [4]: lg k = mY +LN, (16) where L is the sensitivity of the solvolysis rate to the nucleophilicity of the solvent N. For the SN2 reaction L=1, for the SN1 and E1 reaction L=0 - nucleophilicity does not affect, and at L= 0.1 – 0.3 nucleophilic solvent assistance is noticeable. Often in the solvolysis of neopentyl, adamantyl and tert-alkyl substrates, in which nucleophilic solvation from the back is impossible or difficult, L has a negative value, while in the solvolysis of secondary and primary substrates, where nucleophilic solvation from the back is available, L ≥ 0; [2]. For example, for α-methyl-α-isopropyl-m-chlorobenzyl chloride (VII) 10 Danish Scientific Journal No102, 2025 OMe Cl Cl Cl Cl R (Me CCH ) C Me Cl 23 2 a) R - Ph b) R - Me c) R - Cl VII VIII IX X L=-0.28, and for 4-chloro-2,2,4,6,6-pentamethylheptane (VIII) L= -0.32, for p-methoxybenzyl chloride (IX) L= +0.1, for Ph2 CHCl L=+0.11 [2]. Therefore, equation (16) is of little use for elucidating the role of nucleophilic solvation in monomolecular heterolysis reactions. Liu et al. [59, 60] proposed to use 2-aryladamantanes derivatives as benchmarks for determining the influence of solvent nucleophilicity on the rate of heterolysis. Published data on the kinetics of heterolysis of 2-aryl-2-chloroadamantanes obtained conductometrically [61], by the verdazyl method [62,63] and chromatographically [23]. A correlation analysis of solvation effects in the heterolysis of chlorides (Xa, Xb, Xc) was performed using the three-parameter equation (17) and the Camlet-Taft equation (18) [2]. "lg k = а0 + a1 ET" (Z)" + a2 B + a3 " "n2-1" /"n2+2" " (17)" lg k = "a0 + a1 π* + a2 α + a3 β " (18) where n is the refractive index, "π*" is the dipolarity (polarity + polarizability), "α" is the electrophilicity, "β" is the nucleophilicity of the solvent. For chloride (X"a"), using equation (17), a good three-parameter correlation was obtained: lg kХа= -26.8 + 0.146 ET+1.47 f(n) – 0.00277 B; R 0.976, S 0.60, N12 (19). The parameters B and n are insignificant. Their removal gives a satisfactory one-parameter dependence. lg kХа= -26.9 + 0.112 ET; R 0.976, S 0.66, N12 (20). A similar dependence was obtained when using the parameter Z: lg kХа= -281 + 14.5 Z; R 0.984, S 0.55, N12 (21) For 7 protic solvents we have a satisfactory oneparameter dependence. lg kХа= -28.5 + 0.112 ET; R 0.977, S 0.72, N7 (22). For 5 aprotic solvents we obtained: lg kХа= -18.6 + 0.066 ET; R 0.86, S 0.49, N5 (23) Taking into account the nucleophilicity and polarizability parameters, the correlation becomes excellent: lg kХа= -12.5 + 0.0986 ET - 4.48 f(n) – 0.854 B; R 0.995, S 0.45, N5 (24) Using equation (18) gives similar relationships: lg kХа= -10.3 + 3.82" π*" + 5.77 α; R 0.961, S 0.88, N12 (25) lg kХа= -10.3 + 4.22" π*" + 5.77 α; R 0.920, S 1.3, N7 (26) lg kХа= -12.1 + 5.72 "π*" + 10.7 α; R 0.963, S 0.30, N5 (27) For 2-(p-chlorophenyl)-2-chloroadamantane (Xv) in 6 solvents (MeOH, EtOH, 2-PrOH, CF3CH2OH, AcOH, HCONH2) a good correlation was obtained lg kXv= -32.0 + 0.14 ET; R 0.981, S 0.51, N6 (28) With additional application of nucleophilicity and polarity parameters R 0.988. Thus, the correlation analysis shows that the rate of heterolysis of 2-aryl-2-chloroadamantanes is well described by the solvatochromic parameters ET and Z, as well as the parameters "π*" and α and does not depend on the nucleophilicity of the solvent. In [62], in the heterolysis of 2-phenyl-2-chloroadamantane (Xa), it was shown that the effect of the phenyl group depends very much on the nature of the solvent, changing, for example, by 3 orders of magnitude when going from 1-BuOH to sulfolane. Therefore, the use of 2-aryl-2-chloroadamantanes as a benchmark for determining the effect of the nucleophilicity of the solvent on the rate of heterolysis can lead to gross errors. In [23], a correlation analysis of solvation effects in the heterolysis of 2-methyl-2-chloroadamantane (Xb) at a temperature of 60 °C was performed using the Camlet-Taft equation and it was shown that the nucleophilicity of the solvent reduces the reaction rate. Among the tertiary substrates we have considered, which are commonly used as benchmarks in the study of solvation effects in heterolysis reactions, only t-BuCl provides some reliable information. Adamantyl substrates give a distorted picture of solvation effects, since the rate of their heterolysis strongly depends on the nucleophilicity and electrophilicity of the solvent, as well as on the internal return reaction of the ion pair. The use of 2-aryl-2-halodamantanes as benchmarks is impractical because the conjugation of the nascent positive charge with the phenyl strongly depends on the nature of the solvent. Conclusions. Tertiary substrates are unsuitable as benchmarks for determining the ionizing ability of solvents due to the negative effect of nucleophilic solvation. Secondary substrates are less sensitive to the effects of specific solvation. 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