The Base Strengths and Ultraviolet Absorption Spectra of the 2- and 3-Monohalopyridines<sup>1,2</sup>

Brown, Herbert C.; McDaniel, Darl H.

doi:10.1021/ja01619a022

Cited by 76 publications

(21 citation statements)

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“…After formation and before reactions, the protonated and unprotonated radicals may be able to reach a new equilibrium controlled by their basicities. Since 4-dehydropyridine has a PA greater by about one kcal/mol than its 3-isomer (Table 7), it almost certainly also has a greater pKa (e.g., for 2-bromopyridine, PA = 216.3 kcal/mol 48a and pKa = 0.9, 48b while for 3-bromopyridine, the corresponding values are 217.5 kcal/mol and 2.84, respectively; the same situation is also true for 2- and 3-chloropyridines 48 ). Hence, even in this situation, more protonated 4-dehydropydidines should remain.…”

Section: Resultsmentioning

confidence: 99%

Direct Comparison of Solution and Gas-Phase Reactions of the Three Distonic Isomers of the Pyridine Radical Cation with Methanol

et al. 2012

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In order to directly compare the reactivity of positively charged carbon-centered aromatic σ-radicals toward methanol in solution and in the gas phase, the 2-, 3-, and 4-dehydropyridinium cations (distonic isomers of the pyridine radical cation) were generated by ultraviolet photolysis of the corresponding iodo-precursors in a mixture of water and methanol at varying pH. The reaction mixtures were analyzed by using liquid chromatography/mass spectrometry. Hydrogen atom abstraction was the only reaction observed for the 3- and 4-dehydropyridinium cations (and –pyridines) in solution. This also was the major reaction observed earlier in the gas phase. Depending on the pH, the hydrogen atom can be abstracted from different molecules (i.e., methanol or water) and from different sites (in methanol) by the 3- and 4-dehydropyridinium cations/-pyridines in solution. In the pH range of 1 – 4, the methyl group of methanol is the main hydrogen atom donor site for both 3- and 4-dehydropyridinium cations (just like in the gas phase). At higher pH, the hydroxyl groups of water and methanol also act as hydrogen atom donors. This finding is rationalized by a greater abundance of the unprotonated radicals that preferentially abstract hydrogen atoms from the polar hydroxyl groups. The percentage yield of hydrogen atom abstraction by these radicals was found to increase with lowering the pH in the pH range of 1.0-3.2. This pH effect is rationalized by polar effects: the lower the pH, the greater the fraction of protonated (more polar) radicals in the solution. This finding is consistent with previous results obtained in the gas phase and suggests that gas-phase studies can be used to predict solution reactivity, but only as long as the same reactive species is studied in both experiments. This was found not to be the case for the 2-iodopyridinium cation. Photolysis of this precursor in solution resulted in the formation of two major addition products, 2-hydroxy- and 2-methoxypyridinium cations, in addition to the hydrogen atom abstraction product. These addition products were not observed in the earlier gas-phase studies on 2-dehydropyridinium cation. Their observation in solution is explained by the formation of another reactive intermediate, the 2-pyridyl cation, upon photolysis of 2-iodopyridinium cation (and 2-iodopyridine). The same intermediate was observed in the gas phase but it was removed before examining the reactions of the desired radical, 2-dehydropyridinium cation (which cannot be done in solution).

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Section: Resultsmentioning

confidence: 99%

Direct Comparison of Solution and Gas-Phase Reactions of the Three Distonic Isomers of the Pyridine Radical Cation with Methanol

et al. 2012

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“…Introduction of fluorine into the pyridine ring reduces the basicity of the latter [396]. 1 kPa), is obtained by the reaction of 2,6-dichloropyridine with potassium fluoride in the absence of solvent.…”

Section: Ring-fluorinated Pyridinesmentioning

confidence: 99%

Fluorine Compounds, Organic

Siegemund

Schwertfeger

Feiring

et al. 2016

Ullmann's Encyclopedia of Industrial Chemistry

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“…This reagent has the following advantages: 1) a stable crystalline compound without deliquescent properties; 2) significantly fast rate for the formation of Tf 2 C=CH 2 13 in several organic solvents; and 3) low boiling point of 2-fluoropyridine, which was formed as a side product during the 2,2-bis(triflyl) ethylation reaction, and its remarkably weak basicity (pK aH in H 2 O=−0.44). 48) Recently, Alcaide and Almendros have reported interesting cycloaddition chemistry using our 2-fluoropyridinium 14a (Chart 28). For example, Tf 2 C=CH 2 13 generated in situ from 14a smoothly reacted with the internal alkyne to give gem-bis(triflyl) cyclobutene 27.…”

Section: -Fluoropyridinium: An Effective Reagent For In-situ Generatmentioning

confidence: 99%

Chemistry of Fluorinated Carbon Acids: Synthesis, Physicochemical Properties, and Catalysis

Yanai

2015

CHEMICAL & PHARMACEUTICAL BULLETIN

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The bis[(trifluoromethyl)sulfonyl]methyl (Tf 2 CH; Tf SO 2 CF 3 ) group is known to be one of the strongest carbon acid functionalities. The acidity of such carbon acids in the gas phase is stronger than that of sulfuric acid. Our recent investigations have demonstrated that this type of carbon acids work as novel acid catalysts. In this paper, recent achievements in carbon acid chemistry by our research group, including synthesis, physicochemical properties, and catalysis, are summarized.

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The Base Strengths and Ultraviolet Absorption Spectra of the 2- and 3-Monohalopyridines^1,2

Cited by 76 publications

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Direct Comparison of Solution and Gas-Phase Reactions of the Three Distonic Isomers of the Pyridine Radical Cation with Methanol

Direct Comparison of Solution and Gas-Phase Reactions of the Three Distonic Isomers of the Pyridine Radical Cation with Methanol

Fluorine Compounds, Organic

Chemistry of Fluorinated Carbon Acids: Synthesis, Physicochemical Properties, and Catalysis

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The Base Strengths and Ultraviolet Absorption Spectra of the 2- and 3-Monohalopyridines1,2

Cited by 76 publications

References 0 publications

Direct Comparison of Solution and Gas-Phase Reactions of the Three Distonic Isomers of the Pyridine Radical Cation with Methanol

Direct Comparison of Solution and Gas-Phase Reactions of the Three Distonic Isomers of the Pyridine Radical Cation with Methanol

Fluorine Compounds, Organic

Chemistry of Fluorinated Carbon Acids: Synthesis, Physicochemical Properties, and Catalysis

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The Base Strengths and Ultraviolet Absorption Spectra of the 2- and 3-Monohalopyridines^1,2