Effect of pH on kinetics of the decay of the radicals formed during photolysis of 2,2,4,6-tetramethyl-1,2-dihydroquinoline in aqueous and micellar solutions

Nekipelova, T. D.; Pirogov, N. O.

doi:10.1007/bf02495193

Cited by 4 publications

(14 citation statements)

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“…It should be mentioned here that photolysis in organic solvents, which resulted in the formation of aminyl radicals, is a monophotonic process. Based on the results obtained in this study, we can revise our previous results on the photolysis of 1 in aqueous and micellar (SDS) media [1,14]. In those works, it was assumed that the transient species generated in these media were aminyl radicals and radical cations.…”

Section: The Reaction Time (Min): 0 (1) 15 (2) 30 (3) 45 (4) 60 (mentioning

confidence: 58%

“…In the pH range from 3 to 10, the first-order rate constant increases linearly with the concentration of 1 in micelles indicative of the direct reaction of 1 with the transient cation in micelles [1,14]. This reaction is parallel to the reaction of water with this transient, and it becomes predominant with concentrating of 1 in micelles.…”

Section: The Reaction Time (Min): 0 (1) 15 (2) 30 (3) 45 (4) 60 (mentioning

confidence: 92%

“…When the reaction was accomplished, the reaction mixture was analyzed by TLC on silufole using a hexane: ether mixture 3 : 1 as an eluent and was compared with the mixture obtained upon photolysis. Flash photolysis experiments were carried out as described elsewhere [1].…”

Section: Methodsmentioning

confidence: 99%

“…The rate constants obtained from these curves depending on pH are from one to two orders of magnitude lower than those obtained for methanol. That is why, in this case, the first reaction is no longer a limiting step and manifests itself only in the very beginning as a deviation in the higher direction from the exponential law [1]. The measured first-order rate constant is the rate constant for the second step.…”

Section: The Reaction Time (Min): 0 (1) 15 (2) 30 (3) 45 (4) 60 (mentioning

confidence: 96%

“…The polarity of a medium and hydrogen bonding between a reagent and a solvent play the most important role in this phenomenon. We have found earlier that the nature of a solvent affects the direction and kinetics of photolysis of 2,2,4,6-tetramethyl-1,2-dihydroquinoline (1) [1]. In this work, based on the identification of the product of the photolysis in water and methanol, we report the results of the kinetic examination of the reaction in methanol, which allows us to better understand the results obtained earlier for the photolysis of 1 in water and micellar solutions.…”

Section: Introductionmentioning

confidence: 99%

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Phototransformations of non‐toxic antioxidants, the derivatives of 1,2‐dihydroquinolines, in homogeneous and micellar solutions

Nekipelova

1999

International Journal of Photoenergy

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Abstract. Reactions of transient species photogenerated from 6-R-2,2,4-trimethyl-1,2-dihydroquinolines (TMDQ) are very sensitive to medium variation. In anhydrous organic solvents, aminyl radicals were generated. They decay in the reaction of dimerization with the second-order rate constant decreasing in a row heptane>benzene>2-propanol. When passing from organic solvents to water, methanol, and water-alcohol solutions, the kinetics and the direction of the reaction crucially change. As a result of the photolysis, the product of the addition of a solvent to the double bond of heterocycle, 4-hydroxy-or 4-methoxy-6-R-2,2,4-tetramethyl-1,2,3,4-tetrahydroquinoline is formed in water and methanol, respectively. The transformation is a complex reaction, and the formation of excited transient species is followed by a sequence of first-order and pseudo-first-order reactions. Unlike the photolysis in anhydrous organic solvents, the reaction in water and methanol does not involve aminyl radicals. In aqueous solutions, the first-order rate constants for the decay of transient species are higher in acidic and neutral solutions. At the pH close to pK a of the transient species, it drops, indicating that the neutral form is less reactive. The same product is formed over the whole range of pH. For the anionic surfactant (SDS) in acidic and alkaline solutions, the apparent rate constant in the micellar solutions is lower than that in the aqueous (negative micellar catalysis). At the medium pH, a positive micellar catalysis is observed, and the rate constant of the decay depends linearly on the concentration of TMDQ in the micelles, indicative of the direct reaction between TMDQ and the cationic transient species.

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Section: The Reaction Time (Min): 0 (1) 15 (2) 30 (3) 45 (4) 60 (mentioning

confidence: 58%

Section: The Reaction Time (Min): 0 (1) 15 (2) 30 (3) 45 (4) 60 (mentioning

confidence: 92%

Section: Methodsmentioning

confidence: 99%

Section: The Reaction Time (Min): 0 (1) 15 (2) 30 (3) 45 (4) 60 (mentioning

confidence: 96%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Phototransformations of non‐toxic antioxidants, the derivatives of 1,2‐dihydroquinolines, in homogeneous and micellar solutions

Nekipelova

1999

International Journal of Photoenergy

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Photoaddition of water and methanol to 2,2,4,6-tetramethyl-1,2-dihydroquinoline

et al. 1999

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Products of the steady-state photolysis of 2,2,4,6-tetramethyl-1,2-dihydroquinoline (1) in water, methanol, and water--ethanol were isolated for the first time and identified by IH and J3C NMR spectroscopy, IR spectroscopy, and mass spectrometry. As a result of the photolysis, the molecule of the solvent is added to the double bond of the heterocycle with formation of 4-hydroxy-(2) or 4-methoxy-2,2,4,6-tetramethyl-1,2,3,4-tetrahydroquinoline (3) in water and methanol, respectively. Compounds 2 and 3 are converted gradually into 1 in the dark. The rate of the back reaction depends on the solvent and the concentration of the product.Comparison of the products of the photolysis in methanol and hexane at 45 ~ and of the azoisobutyronitrile-initiated oxidation of 1 at the same temperature has shown that unlike the photolysis in hexane, aminyl radicals are not precursors of the product of the photolysis in methanol. The reaction proceeds via an excited singlet state.Key words: t,2-dihydroquinolines, photoaddition to the double bond, steady-state photolysis, effect of a solvent, lH and 13C NMR.Effect of the medium on the mechanism and kinetics of chemical transformations is one of the key problems of modern physical chemistry. The polarity of medium and hydrogen bonding between a reagent and a solvent play the most important role in this phenomenon. We have found earlier that the nature of the solvent affects the direction and the kinetics of photolysis of 2,2,4,6-tetramethyl-1,2-dihydroquinoline (1). t 6-Substituted and partly hydrogenated quinolines of general formula A, especially ethoxyquin (R I = EtO and R 2 = H), are widely used as antioxidants and antiozonants for rubbers, 2 as stabilizers for vitamin A and 13-carotene in feed, and as a post-harvest dip for apples. 3,4 Upon photolysis of these compounds in organic solvents (alkanes, benzene, toluene, and propan-2-ol), the hydrogen atom is split out from the molecule of dihydroquinoline and an aminyl radical is formed, which decaysat the ambient temperature predominantly in recombination reaction s giving a l-8"-dimer B. 6,7 The rate constants of the recombination of the transient radicals measured by flash-photolysis s and by ESR 7 are close and vary from 106 to 107 L tool -1 s -l depending on the substituent in position 6 and the solvent character. If the parent 1,2-dihydroquinoline has a substituent R 2 ~ H in position 8, the rate constant for radical decay dramatically decreases and becomes equal to 102 L tool -I s-I. 7The photolysis of aqueous solutions of compound I (A with R 1 = Me and R 2 = H) gives a product the absorption spectrum of which significantly differs from that of the dimer. 1 The kinetics of the decay of the transient species changes crucially: it follows the firstorder law with the rate constant depending on pH and varying from 25 s -t in acidic and neutral solutions to 3.9 s -~ in alkaline solutions i at the pH range corresponding to the value of pK a 10.4 measured for the transient species. ~ These experiments pose a number of questions con...

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10.1007/s10975-008-2008-z

2000

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The kinetics of the photolysis of substituted 1,2-dihydroquinolines (DHQ) in micellar solutions was studied by steady-state and flash photolysis. The photolysis mechanism depends dramatically on the location of DHQ molecules in micelles, which is governed by the surfactant nature. In micellar solutions of the anionic surfactant sodium dodecyl sulfate (SDS), where the DHQ molecules are located in the Stern layer, the intermediate species decay kinetics follows a first-order law. When DHQ is in neutral form (pH 4-12), the rate constant of the intermediate carbocation decay increases from 25 to 198 s -1 with an increasing concentration of DHQ in micelles. The positive micellar catalysis is caused by the acceleration of the final product formation with the DHQ molecule via proton abstraction from the intermediate cation. The formation of several types of intermediate species-carbocations in the aqueous phase and aminyl radicals in micelles-is observed in micellar solutions of the cationic surfactant cetyltrimethylammonium bromide (CTAB) due to the preferential location of DHQ molecules in the micellar core. The carbocation decays via a pseudofirst-order reaction with a rate constant close to that in the aqueous solution. The lifetime of the DHQ aminyl radicals in the micellar solutions is longer by several orders of magnitude than the lifetime observed for homogeneous solutions of hydrocarbons and alcohols.

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Effect of pH on kinetics of the decay of the radicals formed during photolysis of 2,2,4,6-tetramethyl-1,2-dihydroquinoline in aqueous and micellar solutions

Cited by 4 publications

References 12 publications

Phototransformations of non‐toxic antioxidants, the derivatives of 1,2‐dihydroquinolines, in homogeneous and micellar solutions

Phototransformations of non‐toxic antioxidants, the derivatives of 1,2‐dihydroquinolines, in homogeneous and micellar solutions

Photoaddition of water and methanol to 2,2,4,6-tetramethyl-1,2-dihydroquinoline

10.1007/s10975-008-2008-z

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