Pseudobase Formation in Nonaqueous Solvents

Rate and Equilibrium Constants for Addition of Hydroxide and Alkoxide Ions to 158° 

6 Corrected for [H20] = 55 M. c Calculated from data in Gravitz and Jencks.164 

While quantitative measurements of pseudobase formation in aqueous alcoholic solutions may be used as indications of the relative susceptibilities to nucleophilic attack for closely related cations,9,53,75,218 such data are not directly comparable with equilibrium and rate data for pseudobase formation in either water or the pure alcohol (or even with data in aqueous alcoholic solutions of other compositions). Several workers218,257,261,262 have reported rate constants for hydroxide ion attack on heterocyclic cations in aqueous alcoholic solutions without any apparent attempt to consider the complications that arise in such media as a result of the competition between hydroxide and alkoxide ions as nucleophiles. The only attempt to measure the relative reactivities of hydroxide and alkoxide ions toward a heterocyclic cation appears to be the work of Gravitz and Jencks306 for the IV,0-trimethylenephthalimidium cation (158). In this study, product analysis indicated the relative proportions of hydroxide and alkoxide adduct 

307 Kavalek et al.S2 give log K = 3.27(7° + 0.07 for this correlation, with the data for X = 4-N02 not fitting the correlation line. In fact this equation does not reproduce the data of Kavalek et at.52 Equation (38) was calculated for the present review using Wells table308 of Hammett (7° values and includes the 4-N02 substituent. 

It is unfortunate that there has been so little work devoted to quantitative measurements of cation-pseudobase equilibria in methanol and ethanol since these media have several advantages over water for the determination of the relative susceptibilities of heterocyclic cations to pseudobase formation. The enhanced stability of the pseudobase relative to the cation in alcohols compared to water is discussed earlier this phenomenon will permit the quantitative measurement of pseudobase formation in methanol (and especially ethanol) for many heterocyclic cations for which the equilibrium lies too far in favor of the cation in aqueous solution to allow a direct measurement of the equilibrium constant. Furthermore, the deprotonation of hydroxide pseudobases (Section V,B) and the occurrence of subsequent irreversible reactions (Sections V,C and D), which complicate measurements for pKR+ 14 in aqueous solutions, are not problems in alcohol solutions. Data are now available for the preparation of buffer solutions in methanol over a wide range of acidities.309-312 An appropriate basicity function scale will be required for more basic solutions. The series of -(substituted phenyl)pyridinium cations (163) studied by Kavalek et al.i2 should be suitable for use as indicators in at least some of the basic region. The Hm and Jm basicity functions313 should not be assumed90 to apply to methoxide ion addition to heterocyclic cations because of the differently charged species involved in the indicators used to construct these scales. 

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