Solvent nucleophilicity values

Table 1. Solvent Nucleophilicity Values Based upon the Solvolysis of the Triethyloxonium Ion at 0.0 °C.

The possible mechanisms for solvolysis of phosphoric monoesters show that the pathway followed depends upon a variety of factors, such as substituents, solvent, pH value, presence of nucleophiles, etc. The possible occurrence of monomeric metaphosphate ion cannot therefore be generalized and frequently cannot be predicted. It must be established in each individual case by a sum of kinetic and thermodynamic arguments since the product pattern frequently fails to provide unequivocal evidence for its intermediacy. The question of how free the PO ion actually exists in solution generally remains unanswered. There are no hard boundaries between solvation by solvent, complex formation with very weak nucleophiles such as dioxane or possibly acetonitrile, existence in a transition state of a reaction, such as in 129, or SN2(P) or oxyphosphorane mechanisms with suitable nucleophiles. 

For lack of a better system, the ratio of rate in an ethanol-water mixture of the same Y value as acetic acid to rate in the much less nucleophilic acetic acid, ( Etoii/ acoh) yj has served as a measure of sensitivity to solvent nucleophilicity. More recently, the problem has received renewed attention, and two groups have proposed possible approaches.114 Of the two proposals, that of Bentley, Schadt, and Schleyer is easier to apply. Their scheme defines the solvent nucleophilicity, N, by Equation 5.21, where k is the solvolysis rate constant of methyl tosylate in... 

The rate of the solvolysis reaction between 3-methyl-3-chlorobut-l-ene and 13 different alcohols correlated (r = 0.977) with the solvent s ET value.84 The weak negative correlation with solvent nucleophilicity and the first-order kinetics was taken as evidence that the reaction occurs via an SN1 mechanism. 

This procedure was necessary because unique solutions were not obtained when 25 values of cx and c2 as well as 17 values of dx and d2 were derived by an iterative computer-procedure. Swain s own doubts about the results of this treatment have been emphasised by Streitwieser (1956a), who pointed out that values of do not reflect sensitivity to solvent nucleophilicity (Swain used cx/c2 to characterize reactivity). 

The value of kd was obtained from the determination of triplet lifetimes by measuring the decay of phosphorescence and found to be insensitive to changes in solvent polarity. The k2 values derived from Eqs. 10 and 11 were correlated with solvent parameters using the linear solvation energy relationship described by Abraham, Kamlet and Taft and co-workers [18] (Eq. 12), which relates rate constants (k) to four different solvation parameters (1) or the square of the Hildebrand solubility parameter (solvent cohesive energy density), (2) n or solvent dipolarity or polarizability, (3) a, or solvent hydrogen bond donor acidity (solvent electrophilic assistance), and (4) or solvent hydrogen bond acceptor basicity (solvent nucleophilic assistance). 

The c coefficients measure the substrate response to nucleophilicity, dY, and electrophilicity, d2. Again, 80% aqueous ethanol was chosen as the standard solvent, and a set of c values was chosen in accord with supposed mechanistic behavior of model substrates cY = 3c2 for methyl bromide, = c2 for tert-butyl chloride, and 3cx = c2 for (C6H5)3CF. This approach was largely unsuccessful, probably in large part because tert-butyl chloride is assumed to be equally sensitive to nucleophilicity and ionizing power (29). The equation predicts, for example, that tertiary, benzhydryl, and bridgehead substrates are more sensitive to solvent nucleophilicity than typical primary and secondary SN2 substrates (31). 

In Chapter 18, Bentley reviews development of scales of solvent nucleophilicity and applies the Bentley-Schleyer equation to determine N and Y values for sulfuric acid. Also, Bentley examines here the validity of the Taft... 

Product data cited previously (26, 35-37, 41) support independent evidence from kinetic studies (Table III) that HFIP is even less nucleophilic than TFE. Solubilities of alkali metal salts show the same trend in cation solvating power (18, 19). These diverse results warrant emphasis because Abraham et al. have implied (42) that the solvatochromic parameter P is a measure of solvent nucleophilicity. However, the P values for TFE and HFIP are both zero (43), and the relationship between p and solvent nucleophilicity is therefore questionable. 

Steric Effects on Solvent Nucleophilicity. The N0Ts values (Table III) show that TFA is, by this measure, less nucleophilic than HFIP. As the rate ratios 2-AdOTs (CH3)2CHOTs in TFA and 97% HFIP were very similar, we initially proposed (17) that solvolyses of (CH3)2CHOTs in these two weakly nucleophilic solvents were kc processes. More detailed studies (4, 50) later showed that solvolyses of secondary tosylates in 100% HFIP were closer to limiting (kc) than those in TFA. This apparent reverse of the order of nucleophilicity could be caused by steric effects (50), which could also contribute to the lower precision of the iN/mY equation (5) in correlations of rate data for secondary substrates for example, for 2-propyl tosylate, solvolyses in TFE and HFIP are predicted to be slower than observed, whereas reaction in CF3C02H is predicted to be faster than observed (3). 

For situations where solvent nucleophilicity may be a factor, Kevill (8) favors the use of the extended Grunwald-Winstein equation (equation 1). Scales of NOTs and OTs values based upon the use of methyl tosylate and 2-adamantyl tosylate as model SN2- and SNl-reacting substrates have been developed (15, 16). Also Y scales have been developed for other anionic leaving groups using 1-adamantyl or 2-adamantyl derivatives (17-19), where Sn2 reaction is impossible or severely hindered. 

Values of nAN or nBN for all solvents were obtained from equations 3 or 4, because the y value was available from equation 5, when the nucleophilicity ratio, R, for the reference solvents acetic and formic acid was chosen to be 1. The nN values for solvolysis solvents were compared with the Schadt-Bentley-Schleyer proposed set of nucleophilicity values. The lin-... 

The rates of Via and VIb in different solvents gave somewhat different correlations by the equation log (k/k0) = raY0Ts for aqueous ethanols and for other solvents, with m values of 0.82 and 0.78 for the latter, whereas Vic gave a very scattered correlation by this equation (17). Therefore, the rates were correlated by the equation log (k/k0 = mY0Ts + IN (38), where N is a measure of solvent nucleophilicity and l expresses the dependence on N. For Via, VIb, and Vic, values obtained in this way for m and l were 0.90, 0.91, and 0.93 and 0.26, 0.37, and 0.72, respectively, with correlation coefficients of 0.994, 0.994, and 0.977, repsectively. Whatever the merits of the dual correlation with Y0Ts and N, all three substrates showed enhanced rates in the ethanol solvents compared with the trends for the other solvents this result suggests that nucleophilic interactions are important. 

DFT studies of the hydrolysis of acetyl and chloroacetyl chloride and of variously substituted benzoyl chlorides supported an S 2 mechanism. An extended Grunwald-Winstein equation correlation for the specific rates of solvolysis of 3,4,5-trimethoxybenzoyl chloride gave sensitivities towards changes in solvent nucleophilic-ity (/-value) of 0.29 and solvent ionizing power (m-value) of 0.54. The low m-value allowed specific rates to be determined in highly ionizing fiuoroalcohol/H20 mixtures. A parallel correlation of the specific rates of solvolysis of 2,4,6-trichlorobenzoyl chloride revealed that solvolyses in 100% and 90% ethanol or methanol did not appreciably follow the ionization pathway indicated for solvolyses in the other solvents and it was proposed that, despite the two or//to-substituents, the addition-elimination pathway had become dominant. ... 

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