Distinguishing between solvation effects and mechanistic changes. Effects due to differences in solvation of aromatic rings and alkyl groups

Abstract

Solvolytic rate constants at 25 °C are reported for p-methoxybenzyl chloride (1) in aqueous binary mixtures with acetone, acetonitrile, dioxane, dimethyl sulfoxide, ethanol, methanol, and 2,2,2-trifluoroethanol and for benzoyl chloride and p-methoxybenzoyl chloride (2) in aqueous acetonitrile and aqueous dioxane. Product selectivities are reported for solvolyses of 1 in aqueous ethanol and methanol. Logarithms of rate constants for solvolyses of 2 correlate linearly with solvolyses of 1 (unit slope and only a small "dispersion"-the tendency for the various binary mixtures to show separate correlations), showing that the solvation requirements of SN1 reactions of aromatic carboxylic acid chlorides are very similar to those of benzylic chlorides. Similar good correlations are reported vs solvolyses of -phenylethyl chloride and vs Z values. For SN1 reactions of chlorides in binary mixtures having similar electrophilicities, the phenomenon of dispersion in Grunwald-Winstein plots (vs Y) and the poor correlation vs YCI (reported previously) appears to be due to the differences in solvation between aromatic rings and alkyl groups; this difference is largest for binary aqueous mixtures with methanol > ethanol > aprotic solvents. The complex rate-rate profile for similar solvolytic reactions of benzoyl chloride is dissected quantitatively into contributions from two competing reaction channels, one of which shows only a small "dispersion" in a correlation with solvolyses of 2; these results are consistent with a mainly dissociative reaction channel for solvolyses of benzoyl chloride in highly aqueous media.

Concurrent pathways to explain solvent and substituent effects for solvolyses of benzoyl chlorides in ethanol-trifluoroethanol mixtures

“…Solvent effects in binary trifluoroethanol (TFE) mixtures will be correlated with those for 1, Z = MeO to allow for the effects of solvent ionising power and of aromatic ring solvation. 7 It has recently been found that substituent effects for dissociative solvolyses of benzoyl chlorides correlate unexpectedly well with the original Hammett σ function. 8 The two types of correlation (for substituents and solvents) will provide the first independent estimates of the locations where each of the two pathways contributes 50% to the observed rate constant.…”

“…4 Recent DFT calculations indicate that the rate enhancing effects of both electron-donating and electronwithdrawing substituents on solvolyses of acid chlorides (1) in water can be explained by a concerted S N 2 mechanism along with large changes in transition state structure. 5 The purpose of our study is to provide further experimental evidence for the alternative mechanistic explanation 6,7 of simultaneous (concurrent) reaction pathways. Solvent effects in binary trifluoroethanol (TFE) mixtures will be correlated with those for 1, Z = MeO to allow for the effects of solvent ionising power and of aromatic ring solvation.…”

On the Importance of the Aromatic Ring Parameter in Studies of the Solvolyses of Cinnamyl and Cinnamoyl Halides

“…Koo et al have analyzed the solvolytic rate constants and product selectivities ( S ) of cinnamyl chloride [23] and cinnamyl bromide [24] in a number of binary mixtures of water with ethanol, methanol, acetone, and 2,2,2-trifluoroethanol (TFE). The close similarity of solvent kinetic isotope effects, rate-rate profiles of solvent effects on reactivity, and similar selectivity data to p -methoxybenzyl chloride [11, 25] made the authors conclude that the solvolyses of cinnamyl halides can be explained by product formation incorporating a general base-catalyzed nucleophilic attack on a contact ion-pair [23, 24]. Cinnamyl bromide was subject to electrochemical reduction using cyclic voltammetry and controlled-potential electrolysis [26] where it was shown that the substrate could be reduced to a resonance-stabilized cinnamyl radical, which could further be reduced to a carbanion depending upon the selected potential.…”

“…Bentley et al [11] favored the use of p -methoxybenzyl chloride as a similarity model to correlate the solvolyses of α -aryl halides. Liu et al [12, 13] also using a similarity model approach developed Y BnX scales for each leaving group X and Y x BnX scales [14] for x aromatic rings entering into conjugation with the reaction center.…”

Further Kinetic Studies of Solvolytic Reactions of Isobutyl Chloroformate in Solvents of High Ionizing Power Under Conductometric Conditions

“…We have carried out further work because the results reported were obtained using only 18 solvents for the correlation analysis; in particular, the experimental results were obtained without considering highly aqueous solvent systems including aqueous 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), which may influence those of 4 by inducing a mechanistic change [6][7][8][9] due to the variation of solvent composition; also, there is a potential problem of possible multicolinearity for solvolytic analyses by two-terms multiple regressions.…”