Ionization and Proton Exchange of Amines in Acetic Acid. II. Effect of Structure on Reactivity and the Reaction Mechanism

Grunwald, Ernest; Price, E.

doi:10.1021/ja01069a002

Cited by 19 publications

(8 citation statements)

References 2 publications

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“…Therefore, the equilibrium (2) shifts to the left, and the resulting proton mobility is much lower compared with those of the other bases. [15] Thus, it was found that relatively strong bases, which have conjugated acid acidities close to that of methanol (pK a 15.5), are suitable for the dissociation of methanol and anodic methoxylation. It is notable that the current efficiency for the anodic methoxylation of 3 was greatly increased about three times compared with our previous work using a conventional supporting electrolyte, Et 4 NOTs.…”

Section: Resultsmentioning

confidence: 99%

Development of a Novel Environmentally Friendly Electrolytic System by Using Recyclable Solid‐Supported Bases for In Situ Generation of a Supporting Electrolyte from Methanol as a Solvent: Application for Anodic Methoxylation of Organic Compounds

Tajima

2005

Chemistry A European J

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We have successfully developed a novel environmentally friendly electrolytic system using recyclable solid-supported bases for in situ generation of a supporting electrolyte from methanol as a solvent. It was found that solid-supported bases are electrochemically inactive at an electrode surface. It was also found that solid-supported bases dissociate methanol into methoxide anions and protons. Therefore, in the presence of solid-supported bases, it was clarified that methanol serves as both a solvent and a supporting electrolyte generated in situ. Anodic methoxylation of various compounds with solid-supported bases was carried out to provide the corresponding methoxylated products in good to excellent yields with a few exceptions. The methoxylated products and the solid-supported bases were easily separated by only filtration, and the desired pure methoxylated products were readily isolated simply by concentration of the filtrates. The separated and recovered solid-supported bases were recyclable for several times.

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Section: Resultsmentioning

confidence: 99%

Development of a Novel Environmentally Friendly Electrolytic System by Using Recyclable Solid‐Supported Bases for In Situ Generation of a Supporting Electrolyte from Methanol as a Solvent: Application for Anodic Methoxylation of Organic Compounds

Tajima

2005

Chemistry A European J

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“…Concerning the speed of the ionization step, in acetic acid, an indirect estimate of the rate constant k \ is available from the analysis of proton exchange rates. [10][11][12] The minimum value for k{ is thus found to be in the range 1 X 108 to 3 X 109 sec-1 for a series of bases ranging from methylamine to p-fluoroaniline, for which the ionization constants K\ (=ki/k-•,) in acetic acid range from 8 X 105 to 5. However, according to the most plausible reaction mechanism, k , should be well above 109 sec-1 and might be as high as 1012-1014 sec-1, i.e., of the characteristic magnitude for exoergic proton tunneling.10 Relaxation Time for Ionization and Electric Dipole Moment.…”

mentioning

confidence: 83%

Ionization of anilines in carboxylic acid solvents. Equilibrium constants, stoichiometry, solvent and substituent effects, electric dipole moments, and proton exchange

Eustace¹,

Grunwald²

1974

J. Am. Chem. Soc.

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On the basis of electric dipole moments, base ionization of substituted anilines and tertiary amines (B) in octanoic acid (HA) is best represented by the chemical equation B . HA + HA F! BH+. A--HA and appears not to be ultrafast.Equilibrium constants (Ki) for the ionization of a series of N,N-dialkylanilines were measured spectrophotometrically in acetic, propionic, and octanoic acid. Substituent effects on log Ki were approximately equal in these three solvents. Solvent effects on log Ki for several anilines conformed fairly well to a dipole-solvation model.According to Eigen,2 the kinetics of Brqhsted acid-base reactions is usefully discussed in terms of the three-step scheme in eq 1-3. k1 k2 HA + B B.HA (hydrogen-bond association) (1) ki k-i. B-HA === BH'. A-(ionization) (2 1 k3 k4 BH+.A-BH' + A-(ion-pair dissociation) (3)Eigen also suggested that many exoergic acid-base reactions are diffusion controlled, i.e., k , > k2. This would imply that the rate constant for ionization ( k , ) is greater than -1O'O sec-I. This is possible because the proton may tunnel through, rather than surmount, the potential barrier that separates the states B . HA and BH+ -A-.3 The actual proton tunneling frequency may be as high as 1012-1014 ~8 c -I .~ Proton transfer at such rates of speed has been called ultrafast.In the present paper we investigate the speed of the ionization step in the ionization of anilines in carboxylic acid solvents by a combination of dipole moment and spectrophotometric measurements. Recent work by others5-' has demonstrated that electric dipole moments of hydrogenbonded acid-base complexes are particularly informative about the nature of the ionization step. We hope to demonstrate that, when the degree of ionization is measured by an independent method, the electric dipole moments can also tell us whether proton transfer is ultrafast. Carboxylic Acid Solvents. In terms of the general scheme in eq 1-3, the acid-base reactions of anilines in carboxylic acid solvents of low dielectric constant, such as acetic acid, are relatively simple because the concentrations of the "free" base B in (1) and of the dissociated ions BH+ in (3) are stoichiometrically insignificant. The ionization step (eq 2) is therefore isolated. That the unprotonated aniline exists largely in the form of hydrogen-bonded complexes is indicated by the optical absorption in the region of the n -+ T * transition, which closely resembles that in methanol and clearly differs from that in c y c l~h e x a n e .~.~ That the anilinium ion exists largely in the form of ion pairs is indicated by the small magnitude of the ion-pair dissociation con~t a n t .~ Concerning the speed of the ionization step, in acetic acid, an indirect estimate of the rate constant ki is available from the analysis of proton exchange rates.I0-l2 The minimum value for ki is thus found to be in the range 1 X lo8 to 3 X IO9 sec-I for a series of bases ranging from methylamine to p-fluoroaniline, for which the ionization constants K , ( = k i / k -, ) in acetic acid range f...

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“…Therefore, the equilibrium of Equation (2) shifts to the left, and the resulting proton mobility is much lower than those of the other bases. [12] Anodic acetoxylation of 3 was successfully carried out 10 times after recycling of the silica gel supported morpholine. In this recycling process, silica gel supported morpholine is easily separated and recovered by simple filtration (Figure 2).…”

Section: Methodsmentioning

confidence: 99%

An Electrolytic System That Uses Solid‐Supported Bases for In Situ Generation of a Supporting Electrolyte from Acetic Acid Solvent

Tajima

Fuchigami

2005

Angew Chem Int Ed

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Ionization and Proton Exchange of Amines in Acetic Acid. II. Effect of Structure on Reactivity and the Reaction Mechanism

Cited by 19 publications

References 2 publications

Development of a Novel Environmentally Friendly Electrolytic System by Using Recyclable Solid‐Supported Bases for In Situ Generation of a Supporting Electrolyte from Methanol as a Solvent: Application for Anodic Methoxylation of Organic Compounds

Development of a Novel Environmentally Friendly Electrolytic System by Using Recyclable Solid‐Supported Bases for In Situ Generation of a Supporting Electrolyte from Methanol as a Solvent: Application for Anodic Methoxylation of Organic Compounds

Ionization of anilines in carboxylic acid solvents. Equilibrium constants, stoichiometry, solvent and substituent effects, electric dipole moments, and proton exchange

An Electrolytic System That Uses Solid‐Supported Bases for In Situ Generation of a Supporting Electrolyte from Acetic Acid Solvent

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