The Influence of Hydration on Base Strength. IV. Hindered Bases

KENZO HIRAOKA. Can. J. Chem. 65, 1258 (1987. An approximate linear relationship between the proton affinities of conjugate bases (B 's) and the free energies of hydration of conjugate acid ions (BH' 's) is found for B = alkyl-substituted H 2 0 and NH3. With increase in proton affinities, the free energies of hydration increase linearly with a slope of about 1 for analogous compounds. The charge delocalization in the protonated bases and the structure-making effect induced by the hydrophobic groups in the alkyl-substituted H 3 0 + and NH4' are crucial factors in the determination of the free energies of hydration.

Section: Discussionmentioning

confidence: 99%

Relationship between gas-phase proton affinities of conjugate bases B's and free energies of hydration of conjugate acid ions BH⁺'s for B = alkyl-substituted H₂O and NH₃

Hiraoka¹

1987

“…Subtraction of the values of the paranieters for the basic ionization process from those corresponding to the ionization of water lead to the thermodynamic parameters for acidic ionization represented by [7]. The values chosen for pK,, AGwO, AH;, …”

Section: ( T = K )mentioning

confidence: 99%

“…The search for structure-reactivity relationships for the ionization of amines in water has a long history (1)(2)(3)(4)(5)(6)(7)(8). The simple approach, based on a comparison of pKa values, led to the wellknown amine anomaly (9,10) where R2NH > R3N > RNH2, an order contrary to normal expectations based on inductive effects.…”

Section: Introductionmentioning

confidence: 99%

“…The simple approach, based on a comparison of pKa values, led to the wellknown amine anomaly (9,10) where R2NH > R3N > RNH2, an order contrary to normal expectations based on inductive effects. It was subsequently found that the expected order was followed in the gas phase (1 [1][2][3][4][5][6][7][8][9][10][11][12][13][14] and hence the anomaly could not be attributed to steric effects (15,16) or to other internal factors (17) but rather mas associated with solvation.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Thermodynamic parameters for the basic ionization of some cyclic amines in water

Lobo¹,

Murty²,

Robertson³

1976

SHEILA TERESA LOBO, TANIKELLA S. S. R. MURTY, and Ross ELMORE ROBERTSON. Can. J. Chem. 54, 3607 (1976).The thermodynamic parameters defining the basic dissociation in water of diethylamine, cyclohexylamine, piperidine, and quinuclidine have been determined by a dilution method based on conductance measurements. 54, 3607 (1976).Les paramktres thermodynan~iques dkfinissant la dissociation basique, dans l'eau. de la diethylamine, de la cyclohexylamine, de la pipCridine et de la quinuclidine ont Ct C determinis par une mkthode de dilution basCe sur des mesures de conductivitCs. A 25 ^C, les valeurs de ASaO et de ACPa0 pour l'ionisation acide des ions ammonium correspondants sont ASaO -8.08, -1.24, -8.51, -13.65 cal deg-I molkl et ACPa0 13.9, -2.7, 18.0, 4.2 cal deg-1 mol-1. On discute de la relation entre ces valeurs dCriv6es et les changements dans la solvation: on note les valeurs apparemment anormales du ACDaO de la cyclohexylan~ine et de la quinuclidine mais on ne peut les rationaliser.[Traduit par le journal] Introduction In this paper we report the thermodynamic quantities defining the ionization of certain cyclic amines in water and note the relationship between these quantities and structure.The search for structure-reactivity relationships for the ionization of amines in water has a long history (1-8). The simple approach, based on a comparison of pKa values, led to the well-

“…Thus to the extent that the methyl groups cause distortion from planarity in the molecule, the hyperconjugative capability of these methyls will become diminished. The second factor to be considered is steric hindrance to hydration (15). Since the protonated form of the base will be more hydrated by solvent molecules than the neutral form, the presence of the ortho methyl groups in 1 will interfere with solvation of the protonated azoxy function and hence destabilize the protonated form.…”

Section: \ mentioning

confidence: 99%

Hexamethylazoxybenzene. Structure, Basicity, and Rearrangement

Cox¹,

Buncel²

1973

2,2′,4,4′,6,6′-Hexamethylazoxybenzene (1) has been prepared and characterized. The u.v. spectrum is indicative of large distortion from coplanarity. 1 is only slightly more basic than azoxybenzene despite expectations due to electronic effect of the methyl substituents. Two factors are considered responsible for this: steric inhibition of resonance in the unprotonated species and steric hindrance to hydration in the conjugate acid.Rearrangement of 1 in moderately concentrated sulfuric acid gives rise to the alcohol 4-hydroxymethyl-2,2′,4′,6,6′-pentamethylazobenzene (6). A Bamberger type phenolic product which would result from methyl migration is not observed. Possible reaction mechanisms are given.