Complete thermodynamic analysis of the hydration of thirteen pyridines and pyridinium ions. The special case of 2,6-di-tert-butylpyridine

Arnett, Edward M.; Chawla, B.

doi:10.1021/ja00518a001

Cited by 53 publications

(24 citation statements)

References 5 publications

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“…Recent determinations of gas-phase proton affinities of DTBP and other alkyl-substituted pyridines showed that the basicity of DTBP in the gas phase was normal (2,3), which confirmed that its weak basicity in water was due to solvent effects on DTBP and (or) DTBPH+. A complete analysis of the thermodynamic cycles linking the protonation processes of DTBP and other pyridines in the gas phase and in aqueous solution led Arnett and Chawla (2) to conclude that there was indeed some hindrance to the hydration of DTBPH+ as reflected in its abnormally low enthalpy of hydration.…”

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confidence: 88%

“…Brown and Kanner (1) and others (2) proposed that the abnormally low basicity of DTBP was caused by steric hindrance to hydration of DTBPH+ .…”

mentioning

confidence: 99%

“…A complete analysis of the thermodynamic cycles linking the protonation processes of DTBP and other pyridines in the gas phase and in aqueous solution led Arnett and Chawla (2) to conclude that there was indeed some hindrance to the hydration of DTBPH+ as reflected in its abnormally low enthalpy of hydration. However, more recently Hopkins et al (3), after investigating the protonation of additional tertbutylpyridines and repeating the thermodynamic determinations of Amett and Chawla (2) of DTBP, concluded from their new datathat the hydration enthalpy of DTBPH+ was normal but that the corresponding entropy was abnormal; they suggested that the rotation of the water molecule attached to DTBPH+ and of -CMe3 was restricted.…”

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confidence: 99%

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2,6-Di-tert-butylpyridine: an unusually weak base in dimethylsulfoxide

Benoit¹,

Fréchette²,

Lefebvre³

1988

Can. J. Chem.

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. 66, 1159 (1988). The ionization constants of the conjugate acids BH+ of pyridine, 2-picoline, 2,6-lutidine, and 2,6-di-tert-butylpyridine (DTBP) have been determined in Me2S0. The partition coefficients of the bases B between Me2S0 and water, and the enthalpies of solution and protonation of B in Me2S0 have also been obtained. In contrast to its high basicity in the gas phase, DTBP is an abnormally weak base in Me2S0 (pK = 0.81). The factors responsible for this very low basicity are analyzed by considering correlations between the gas-phase, Me2S0, and aqueous basicities of B and by comparing the transfer parameters for B and BH+. The solvation of DTBP and DTBPH' in Me2S0 and in water differs. While the solvation of DTBP in Me2S0 is normal, the enthalpy of solution of DTBPH+ in Me2S0 is abnormally low and close to that of a cation whose solvation is nonspecific. This suggests a much reduced H-bonding between sterically hindered DTBPH+ and the large Me2S0 solvent molecule.ROBERT L. BENOIT, MONIQUE FRBCHETTE et DIANE LEFEBVRE. Can. J . Chem. 66, 1 159 (1988). On a dCterminC les constantes d'ionisation dans le Me2S0 des acides conjuguCs BH+ de la pyridine, de la picoline-2, de la lutidine-2,6 et de la di-tertio-butyl-2,6 pyridine (DTBP). On a aussi obtenu les coefficients de partage des bases B entre le Me2S0 Since it was first synthesized by Brown and Kanner (l), 2,6-di-tert-butylpyridine (DTBP) has attracted the interest of many researchers because of its unusually low basicity: with its two alkyl substituents, DTBP is nevertheless a weaker base than unsubstituted pyridine in aqueous solution. Brown and Kanner (1) and others (2) proposed that the abnormally low basicity of DTBP was caused by steric hindrance to hydration of DTBPH+ .Recent determinations of gas-phase proton affinities of DTBP and other alkyl-substituted pyridines showed that the basicity of DTBP in the gas phase was normal (2, 3), which confirmed that its weak basicity in water was due to solvent effects on DTBP and (or) DTBPH+. A complete analysis of the thermodynamic cycles linking the protonation processes of DTBP and other pyridines in the gas phase and in aqueous solution led Arnett and Chawla (2) to conclude that there was indeed some hindrance to the hydration of DTBPH+ as reflected in its abnormally low enthalpy of hydration. However, more recently Hopkins et al. (3), after investigating the protonation of additional tertbutylpyridines and repeating the thermodynamic determinations of Amett and Chawla (2) of DTBP, concluded from their new datathat the hydration enthalpy of DTBPH+ was normal but that the corresponding entropy was abnormal; they suggested that the rotation of the water molecule attached to DTBPH+ and of -CMe3 was restricted. These results and conclusions were in agreement with the gas phase studies of Moet-Ner and Sieck (4) on the attachment of one water molecule to a series of pyridinium cations including DTBPH+ .Since, as we have recently shown (3, contrasting solution and protonation thermodynamic data for bases in wat...

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mentioning

confidence: 88%

“…Brown and Kanner (1) and others (2) proposed that the abnormally low basicity of DTBP was caused by steric hindrance to hydration of DTBPH+ .…”

mentioning

confidence: 99%

mentioning

confidence: 99%

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2,6-Di-tert-butylpyridine: an unusually weak base in dimethylsulfoxide

Benoit¹,

Fréchette²,

Lefebvre³

1988

Can. J. Chem.

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“…The attenuated substituent eect by the solvent is partly attributed to the solvation at the ring nitrogen of the neutral and protonated pyridines as well as at the substituents, although it has been suggested that the attenuation lies more in the hydration of the protonated pyridines [65]. Electron-donating substituents have been shown to have larger attenuation eects than electron-withdrawing ones [51].…”

Section: Solvent Attenuation Eectmentioning

confidence: 99%

Computation of the influence of chemical substitution on the p K a of pyridine using semiempirical and ab initio methods

Chen

MacKerell

2000

Theoretical Chemistry Accounts: Theory, Computation, and Modeli

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The physical properties of chemicals are strongly in¯uenced by their protonation state, aecting, for example, solubility or hydrogen-bonding characteristics. The ability to accurately calculate protonation states in the form of pK a s is, therefore, desirable. Calculations of pK a changes in a series of substituted pyridines are presented. Computations were performed using both ab initio and semiempirical approaches, including free energies of solvation via reaction-®eld models. The selected methods are readily accessible with respect to both software and computational feasibility. Comparison of calculated and experimental pK a s shows the experimental trends to be reasonably reproduced by the computations with root-mean-square dierences ranging from 1.22 to 4.14 pK a units. Of the theoretical methods applied the best agreement occurred using the secondorder Mùller±Plesset/6-31G(d)/isodensity surface polarized continuum solvation model, while the more computationally accessible Austin model 1/Solvent model 2 (SM2) approach yielded results similar to the ab initio methods. Analysis of component contributions to the calculated pK a s indicates the largest source of error to be associated with the free energies of solvation of the protonated species followed by the gas-phase protonation energies; while the latter may be improved via the use of higher levels of theory, enhancements in the former require improvements in the solvation models. The inclusion of alternate minimum in the computation of pK a s is also indicated to contribute to dierences between experimental and calculated pK a values.

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“…(34) {values from this reference for the temperatures 400Q(T/K)Q444 are not shown; deviations for these values average approximately 7 per cent}; r, Gardner and Day; (32) ×, Weclawski and Bylicki; (31) +, Arnett and Chawla; (30) R, Hisamura et al (29) {values for the temperatures 296Q(T/K)Q314 exceed 6 per cent, and are not shown}; Q, Brown and Barbaras; (28) w, Hieber and Woerner (26) {values for the temperatures 290Q(T/K)Q316 are low by up to 14 per cent, and are not shown}; r, Meulen and Mann (27) {values for the temperatures 253Q(T/K)Q273 are low by up to 14 per cent, and are not shown}; t, Riley and Bailey (24) {values for the temperatures 279Q(T/K)Q287 are low by up to 11 per cent, and are not shown}; --, selected by Das et al;…”

Section: Saturation Densities and The Critical Densitymentioning

confidence: 99%