Rate constants and products for solvolyses of chlorodiphenylmethane (Ph 2 CHCl) and p-methoxybenzyl chloride in 2,2,2-trifluoroethanol (TFE)/water and TFE/ethanol are reported, along with additional kinetic data for solvolyses of tert-butyl and other alkyl halides (RX) in 97% w/w TFE/ water and in 97% w/w hexafluoropropan-2-ol/water (HFIP). Results are discussed in terms of the solvent ionizing power (Y) and the solvent nucleophilicity (N), and contributions from other solvation effects are considered. Comparisons with other S N 1 solvolyses show that solvolyses of Ph 2 CHCl in TFE mixtures are unexpectedly fast, but product ratios are unexceptional. An additional solvation effect influences solvolyses leading to delocalized cations, and a delocalized cationic transition state for concerted elimination may explain the recent results of Takeuchi et al., (J. Org. Chem. 1997, 62, 4904) without the need to postulate additional specific solvation effects for adamantyl systems, such as Bronsted-base solvation of Rand -hydrogen atoms; concerted elimination may occur because simple tertiary alkyl cations are too unstable to form in predominantly aqueous media. Iodide/bromide and bromide/chloride rate ratios are very similar for 1-adamantyl halides and corresponding solvolyses of tert-butyl halides; these ratios decrease in the order aq EtOH > TFE > HFIP, as expected for an electrophilic solvation effect (this effect can readily be incorporated into Y values). From kinetic data for a series of tertiary alkyl chlorides in 97% TFE/water, it is shown that the susceptibility of rates of solvolyses of RCl to N decreases with an increase in steric hindrance or with an increase in charge stabilization. Also, the small kinetic solvent isotope effects for typical solvolyses (e.g., methyl tosylate) indicate that nucleophilic attack lags behind heterolysis of the C-X bond. 4654

[reaction: see text] Rate constants and product selectivities (S = ([ester product]/[acid product]) x ([water]/[alcohol solvent]) are reported for solvolyses of chloroacetyl chloride (3) at -10 degrees C and phenylacetyl chloride (4) at 0 degrees C in ethanol/ and methanol/water mixtures. Additional kinetic data are reported for solvolyses in acetone/water, 2,2,2-trifluoroethanol(TFE)/water, and TFE/ethanol mixtures. Selectivities and solvent effects for 3, including the kinetic solvent isotope effect (KSIE) of 2.18 for methanol, are similar to those for solvolyses of p-nitrobenzoyl chloride (1, Z = NO(2)); rate constants in acetone/water are consistent with a third-order mechanism, and rates and products in ethanol/ and methanol/water mixtures can be explained quantitatively by competing third-order mechanisms in which one molecule of solvent (alcohol or water) acts as a nucleophile and another acts as a general base (an addition/elimination reaction channel). Selectivities increase for 3 as water is added to alcohol. Solvent effects on rate constants for solvolyses of 3 are very similar to those of methyl chloroformate, but acetyl chloride shows a lower KSIE, and a higher sensitivity to solvent-ionizing power, explained by a change to an S(N)2/S(N)1 (ionization) reaction channel. Solvolyses of 4 undergo a change from the addition/elimination channel in ethanol to the ionization channel in aqueous ethanol (<80% v/v alcohol). The reasons for change in reaction channels are discussed in terms of the gas-phase stabilities of acylium ions, calculated using Gaussian 03 (HF/6-31G(d), B3LYP/6-31G(d), and B3LYP/6-311G(d,p) MO theory).

Additional specific rates of solvolysis have been determined for acetyl chloride and diphenylacetyl chloride. These are combined with literature values to carry out correlation analyses, using the extended Grunwald–Winstein equation with incorporation of literature values for solvent nucleophilicity (NT) and solvent ionizing power (YCl). Parallel analysis are carried out using literature values for the specific rates of solvolysis of trimethylacetyl chloride, chloroacetyl chloride, phenylacetyl chloride, and α-methoxy-α-trifluoromethylphenylacetyl chloride (MTPAC). Chloroacetyl chloride and MTPAC react by an addition-elimination pathway, with the addition step rate-determining, over the full range of solvents. Acetyl chloride reacts over the full range of solvents by an ionization pathway, with considerable nucleophilic solvation. The other three substrates can solvolyze with the domination of either mechanism, depending on the properties of the solvent. Reports concerning the use of product selectivity values, kinetic solvent isotope effects, and computational studies as additional probes of the mechanism of solvolysis are discussed.Key words: Grunwald-Winstein equation, acyl chlorides, mechanism of solvolysis, solvent nucleophilicity.

No abstract

Abstract:Determinations of the specific rates of solvolysis of 1-adamantyl bromide and 1-adamantyl iodide in 1,1,1,3,3,3-hexafluoro-2-propanol-water mixtures, in conjunction with earlier reported values in 80% ethanol, have led to additional YBr and YI solvent ionizing power values. These new values will be especially important in avoiding multicollinearity when the extended Grunwald-Winstein equation (extended by addition of a term involving solvent nucleophilicity) is used to correlate solvent-induced changes in the specific rates of solvolyses involving a bromide or iodide ion leaving group.

The solvolyses of benzoyl and p-nitrobenzoyl p-toluenesulfonates (tosylates) are considerably slower than those of the previously studied mixed anhydride of acetic and p-toluenesulfonic acids (acetyl tosylate), which, even with application of rapid-response conductivity, could only be studied at considerably reduced temperatures. For the presently studied compounds, the specific rates over a wide variety of solvents could be conveniently studied at -10 °C. For solvolyses of benzoyl tosylate, application of the extended (two-term) Grunwald-Winstein equation gives sensitivities to changes in solvent nucleophilicity and solvent ionising power consistent with an ionisation (S N l) pathway. Indeed, a good correlation is obtained against only solvent ionising power. For the solvolyses of the p-nitro-derivative, very different sensitivities are obtained, with an appreciable dependence on solvent nucleophilicity, and a dominant biomolecular pathway for the substitution is proposed for all of these solvolyses, except for those in solvents rich in fluoroalcohol. Studies of solvent deuterium isotope effects in methanolysis, of leaving-group effects relative to a halide and of temperature variation effects are consistent with the proposed mechanistic pathways.

The specific rates of solvolysis of acetyl p-toluenesulfonate have been measured by a rapid-response conductivity technique at temperatures in the temperature range of À10 to À55 C. For 13 solvents at À39.6 C, an extended Grunwald-Winstein equation correlation led to sensitivities to changes in solvent nucleophilicity of 0.56 and to changes in solvent ionizing power of 0.61. In 89.1% acetone at À20 C, the comparison with acetyl bromide solvolysis led to a k OTs / k Br ratio of 1.4. In methanol and methanol-d at À39.6 C, the solvent deuterium isotope effect k MeOH /k MeOD is 0.99. These results are consistent with an S N l reaction with appreciable nucleophilic solvation or an S N 2 reaction with a loose transition state. These two approaches lead to similar structures for the transition state. A solution of the acetyl p-toluenesulfonate was conveniently prepared by the rapid reaction of acetyl chloride with silver p-toluenesulfonate in acetonitrile.

The specific rates of solvolysis of methanesulfonic anhydride have been measured conductometrically at −10 °C in 41 solvents. Use of the extended Grunwald–Winstein equation, with the NT scale of solvent nucleophilicity and the YOTs scale of solvent ionizing power, leads to sensitivity to changes in solvent nucleophilicity (ℓ value) of 0.95 and a sensitivity to changes in solvent ionizing power (m value) of 0.61, with a multiple correlation coefficient (R) of 0.973. Product selectivity values (S) in binary hydroxylic solvents favor alcohol attack in EtOH–H2O (a value of 1.2 in 90% EtOH rising to 4.0 in 40% EtOH) and in MeOH–H2O (a value of 3.7 in 90% MeOH rising to 6.0 in 50% MeOH). In 2,2,2,‐trifluoroethanol–H2O, the S values are much lower at about 0.1. Entropy of activation values are appreciably negative. Literature values for the specific rates of solvolysis of methanesulfonyl chloride have been extended to fluoroalcohol‐containing solvents (titrimetric method) and, at 45.0 °C, for an overall 43 solvents values are obtained (using NT and YC1 scales) of 1.20 for ℓ and of 0.52 for m (R = 0.969). It is proposed that both substrates solvolyze by an SN2 pathway. Copyright © 2007 John Wiley & Sons, Ltd.