Proton-transfer and electron-transfer processes in reaction of p-nitrotoluene with bases. A spectrophotometric study

ERWIN BUNCEL and ALLEN W. ZABEL. Can. J . Chem. 59, 3177 (1981). Kinetic data have been evaluated for hydrogen exchange in 1,3-dinitrobenzene (DNB), occurring at the 2-position, in dimethylformamide (DMF) -D,O mixtures containing deuteroxide ion. The pseudo first order rate constants for exchange, k,,,. show inverse dependence on the initial DNB concentration, which can be quantitatively related to the extent of o-complex formation in these systems. The profile fork,,,, as function of medium compositio~l exhibits a maximum at ca. 70 mol% DMF, and E ,~, for o-complex formation reaches a plateau at about the same medium composition. The equilibrium constants for o-complex formation, K,, are used to calculate the free. or uncomplexed, deuteroxide ion concentrations, which then allow one to calculate k 2 . the second order rate constants for exchange. The k, values show a uniformly increasing tendency with increased DMF content. Correlations with medium basicity are examined, and the nature of the log k, and log K, vs. H-plots afford insight into the origin of the medium dependence of proton exchange. The results of the present study are consistent with the cr-complex being an unreactive form of the substrate towards exchange, which occurs via uncomplexed DNB present in small concentration. The study also rules out the possibility that the colored species present in these systems is the carbanion formed on deprotonation of DNB.ERWIN BUNCEL et ALLEN W . ZABEL. Can. J. Chem. 59. 3177 (1981). O n a evalue les donnees cinetiques de l'echange d'hydrogene se produisant en position 2 du dinitro-1,3 benzene (DNB) dans des melanges de dimethylformamide (DMF) -D,O contenant I'ion deuteroxyde. L a constante de vitesse d e pseudo ordre un de l'echange k,,,, revele un effet inverse de la concentration initiale de DNB qui peut etre relie quantitativement aux taux deformation du complexe o dans ces systemes. 1.e profil de k,,, en fonction de la composition du milieu montre un maximum pour environ 70 molR de DMF et le E, , , de la formation du complexeo atteint un plateau pour environ la meme composition du milieu. On a utilise la constante d'equilibre d e la formation du complexe o K,, pour calculer les concentrations de I'ion deuteroxyde libre. ou non complexe qui permettent de calculer k, la constante d e vitesse d'ordre deux de I'echange. Les valeurs de k2 montrent une tendance d'augmentation uniforme avec la teneur en DMF. On a examine les correlations avec la basicite du milieu; la nature des courbes de log k, et log K, en fonction de H-fournit des renseignements sur I'origine de l'effet du milieu lors de I'echange de proton. Les resultats de la presente etude sont en accord avec un complexe o qui ne donnerait pas de reaction d'echange celui-ci se produisant par I'intermediaire du DNB non complexe present a une concentration faible. L'etude ecarte egalement la possibilite que l'espece coloree presente dans ces systemes soit le carbanion forme par la protonation du DNB.[Traduit par le journal]

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confidence: 99%

Hydrogen exchange studies. XIV. Kinetics and mechanism of base-catalyzed hydrogen exchange in 1,3-dinitrobenzene in aqueous dimethylformamide and relationship with σ-complex formation

Buncel¹,

Zabel²

1981

“…Recently Buncel and co-workers working in aprotic media have generated relatively stable solutions of nitrobenzyl carbanions, through toluene deprotonation (15) and by the thermal…”

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confidence: 99%

Pronounced solvent effect on the absorption spectra of the photochemically produced 2,4-dinitrobenzyl carbanion

McClelland¹,

Steenken²

1987

The photochromism of 2,4-dinitrotoluene in aqueous solution has been attributed to formation of the 2,4-dinitrobenzyl carbanion (J. Chem. Phys. 39, 1218 (1963)). The spectrum of the colored transient is, however, significantly different from that of ground state carbanion when formed in aprotic solvents through thermal decarboxylation of potassium 2,4-dinitro-phenylacetate (J. Org. Chem. 49, 413 (1984)). In this paper absorption spectra obtained upon 248 nm laser photolysis are reported, for solutions in water, acetonitrile, and their mixtures. The spectrum in acetonitrile does closely resemble that of the carbanion formed by decarboxylation. It is suggested that the same ion is formed in water, and a solvent effect is responsible for the significantly different spectrum. Spectra in the mixed solvents suggest that there is an equilibrium between differently solvated forms of the anion, the extremes being an unspecifically solvated form with λmax 400 and 640 nm as in acetonitrile and a form in water that is hydrogen bonded through the nitro group with λmax 350 and 500 nm. Experiments with conductivity detection provide corroborative evidence for carbanion formation. In acetonitrile a non-conducting transient is produced as a precursor to the anion. This is suggested to be the conjugate acid of the aci-nitro anion, formed by intramolecular hydrogen abstraction in the excited nitrotoluene.

“…In a typical experiment, 100 mg of the substrate is dissolved in aqueous CH3CN (100 mL; 10-30% CH3CN; pH [1][2][3][4][5][6][7][8][9][10][11][12][13][14] and saturated with oxygen-free argon. Photolyses were carried out using quartz vessels in a Rayonet photochemical reactor (300 nm lamps).…”

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confidence: 99%

“…The formation of p,pr-dinitrobibenzyl as a product is evidence for a p-nitrobenzyl carbanion intermediate on photolysis, since Russell and Buncel and their respective co-workers have shown in earlier work (10-13) that this compound is formed via the p-nitrobenzyl carbanion generated under more strongly basic conditions. The mechanism for the formation of p,p1-dinitrobibenzyl from the p-nitrobenzyl carbanion has been studied in some detail (10)(11)(12) in which the first step is believed to involve the ejection of an electron from the carbanion to give p-nitrobenzyl radical. The electron acceptor may be a molecule of substrate or p -nitrotoluene product (10)(11)(12) In addition, p-nitrotoluene is photostable under the reaction conditions.…”

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confidence: 99%

“…The mechanism for the formation of p,p1-dinitrobibenzyl from the p-nitrobenzyl carbanion has been studied in some detail (10)(11)(12) in which the first step is believed to involve the ejection of an electron from the carbanion to give p-nitrobenzyl radical. The electron acceptor may be a molecule of substrate or p -nitrotoluene product (10)(11)(12) In addition, p-nitrotoluene is photostable under the reaction conditions. Thus the p-nitrobenzyl carbanion is generated photochemically and reacts in a similar manner as the species generated under much more basic conditions.…”

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confidence: 99%

See 1 more Smart Citation

Photochemical retro-aldol type reactions of nitrobenzyl derivatives. Mechanistic variations in the elimination of nitrobenzyl carbanions from nitrobenzyl derivatives on photolysis

Wan¹,

Muralidharan²

1986

PETER WAN and S . MURALIDHARAN. Can. J. Chem. 64, 1949Chem. 64, (1986. The photochemical retro-aldol type reactions of several nitrobenzyl derivatives are reported. The reactions are observed only in aqueous solution with quantum efficiencies being pH dependent for several derivatives, consistent with the existence of several mechanistic pathways for reaction. The primary photochemical event is believed to involve the generation of a nitrobenzyl carbanion, which acts as a photolabile leaving group in these reactions, and the generation of an oxocarbocation (or oxocarbocation-derived) fragment.PETER WAN et S . MURALIDHARAN. Can. J . Chem. 64, 1949Chem. 64, (1986. On a CtudiC les rCactions photochimiques de type rCtro-aldol de plusieurs dCrivCs nitrobenzyles. On n'a observC ces rCactions qu'en solution aqueuse et les rendements quantiques dkpendent souvent du pH. Ceci tCmoigne de l'existence de divers chemins rkactionnels. On pense que la premi5re rCaction photochimique implique la formation d'un carbanion nitrobenzyle, agissant comme groupe photolabile ainsi que celle d'un fragment oxocarbocation (ou d'un dCrivC d'oxocarbocation).[Traduit par la revue]The photochemistry of nitroaromatic compounds has been a topic of continued interest and several reviews (1-4) summarize the photochemical behaviour of this functional group in solution. Many photoreactions of nitro compounds are known to proceed via radical intermediates which in many examples (e.g., photoreduction) are formed via initial hydrogen transfer to the photoexcited nitro group, consistent with the notion of an electrophilic n, T * reactive state of the nitro group (5). In this work, we report several closely related photochemical reactions of appropriately substituted nitroaromatic compounds which are observed only in aqueous solution and are best explained by proposing the formation of nitrobenzyl carbanion and oxocarbocation (or oxocarbocation-derived) intermediates, via an excited state configuration other than the usual n , r * . One of these reactions appears suited for direct photogeneration of a variety of dioxocarbocations and hemi-orthoesters in aqueous solution, which is amenable for flash-photolysis study. These results provide new insights into the intriguing photobehaviour of nitroaromatic compounds in aqueous solution (6-9).