Substituent effects on the photochemistry of diphenylketyl radicals: elucidation of the rate constants and quantum yields of relaxation processes from the excited radicals

1992 ◽  
Vol 114 (25) ◽  
pp. 9768-9773 ◽  
Author(s):  
Robert W. Redmond ◽  
J. C. Scaiano ◽  
Linda J. Johnston
Keyword(s):  
Rate Constants ◽  
Substituent Effects ◽  
Quantum Yields ◽  
Relaxation Processes

Substituent effects, Arrhenius activation parameters, and rate constants for the photo–Claisen rearrangement of allyl aryl ethers

2008 ◽  
Vol 86 (7) ◽  
pp. 686-690 ◽  
Author(s):  
Carlos M Gonzalez ◽  
James A Pincock
Keyword(s):  
Rate Constants ◽  
Claisen Rearrangement ◽  
Ion Pairs ◽  
Substituent Effects ◽  
Activation Parameters ◽  
Quantum Yields ◽  
Intermediate Radical ◽  
Fluorescence Quantum Yields ◽  
Aryl Ethers ◽  
Electron Donating Groups

The temperature-dependence of fluorescence quantum yields in both methanol and methylcyclohexane has been used to obtain the rate constants of reaction for the activated process that converts the singlet excited state S1 of a set of ring-substituted aryl allyl ethers to an intermediate radical pair in the photo–Claisen rearrangement. These rate constants are correlated with the O–H bond dissociation energy of the corresponding ring-substituted phenols; that is, electron-donating groups (CH3, OCH3) accelerate the reaction relative to electron-withdrawing groups (CF3, CN). The rate constants obtained span two orders of magnitude, from 5.4 × 107 s–1 for X = 3–CN to 800 × 107 s–1 for X = 4–OCH3, in methylcylcohexane. Moreover, the rate constants are similar in the two solvents, methanol and methcyclohexane, indicating that radical pairs, not ion pairs, are the reactive intermediates, as expected on the basis of previous mechanistic proposals for the photo–Claisen rearrangement. Finally, the rate constants obtained by this temperature-dependent method are in good agreement with those previously reported from a method using the corresponding unreactive anisoles as a model.Key words: allyl aryl ethers, photo–Claisen rearrangement, substituent effects, activation parameters.

Kinetic study of hydrolysis of benzoates. part xxvii. ortho substituent effect in alkaline hydrolysis of phenyl esters of substituted benzoic acids in aqueous Bu4NBr

2009 ◽  
Vol 74 (1) ◽  
pp. 29-42 ◽  
Author(s):  
Vilve Nummert ◽  
Mare Piirsalu ◽  
Signe Vahur ◽  
Oksana Travnikova ◽  
Ilmar A. Koppel
Keyword(s):  
Alkaline Hydrolysis ◽  
Rate Constants ◽  
Pure Water ◽  
Substituent Effects ◽  
Benzoic Acids ◽  
Resonance Effects ◽  
The Media ◽  
Substituted Benzoic Acids ◽  
Hydrolysis Of ◽  
Ortho Substituent

The second-order rate constants k (in dm3 mol–1 s–1) for alkaline hydrolysis of phenyl esters of meta-, para- and ortho-substituted benzoic acids, X-C6H4CO2C6H5, have been measured spectrophotometrically in aqueous 0.5 and 2.25 M Bu4NBr at 25 °C. The substituent effects for para and meta derivatives were described using the Hammett relationship. For the ortho derivatives the Charton equation was used. For ortho-substituted esters two steric scales were involved: the EsB and the Charton steric (υ) constants. When going from pure water to aqueous 0.5 and 2.25 M Bu4NBr, the meta and para polar effects, the ortho inductive and resonance effects in alkaline hydrolysis of phenyl esters of substituted benzoic acids, became stronger nearly to the same extent as found for alkaline hydrolysis of C6H5CO2C6H4-X. The steric term of ortho-substituted esters was almost independent of the media considered. The rate constants of alkaline hydrolysis of ortho-, meta- and para-substituted phenyl benzoates (X-C6H4CO2C6H5, C6H5CO2C6H4-X) and alkyl benzoates, C6H5CO2R, in water, 0.5 and 2.25 M Bu4NBr were correlated with the corresponding IR stretching frequencies of carbonyl group, (ΔνCO)X.

Kinetic Study of Hydrolysis of Benzoates. Part XXVI. Variation of the Substituent Effect with Solvent in Alkaline Hydrolysis of Substituted Alkyl Benzoates

2006 ◽  
Vol 71 (11-12) ◽  
pp. 1557-1570 ◽  
Author(s):  
Vilve Nummert ◽  
Mare Piirsalu ◽  
Ilmar A. Koppel
Keyword(s):  
Kinetic Study ◽  
Alkaline Hydrolysis ◽  
Rate Constants ◽  
Substituent Effect ◽  
Substituent Effects ◽  
Alkyl Substituent ◽  
Solvent Parameters ◽  
Substituent Constants ◽  
Hydrolysis Of ◽  
Main Factor

The second-order rate constants k2 (dm3 mol-1 s-1) for the alkaline hydrolysis of substituted alkyl benzoates C6H5CO2R have been measured spectrophotometrically in aqueous 0.5 M Bu4NBr at 50 and 25 °C (R = CH3, CH2Cl, CH2CN, CH2C≡CH, CH2C6H5, CH2CH2Cl, CH2CH2OCH3, CH2CH3) and in aqueous 5.3 M NaClO4 at 25 °C (R = CH3, CH2Cl, CH2CN, CH2C≡CH). The dependence of the alkyl substituent effects on different solvent parameters was studied using the following equations:      ∆ log k = c0 + c1σI + c2EsB + c3∆E + c4∆Y + c5∆P + c6∆EσI + c7∆YσI + c8∆PσI     ∆ log k = c0 + c1σ* + c2EsB + c3∆E + c4∆Y + c5∆P + c6∆Eσ* + c7∆Yσ* + c8∆Pσ* .  ∆ log k = log kR - log kCH3. σI and σ* are the Taft inductive and polar substituent constants. E, Y and P are the solvent electrophilicity, polarity and polarizability parameters, respectively. In the data treatment ∆E = ES - EH2O , ∆Y = YS - YH2O , ∆P = PS - PH2O were used. The solvent electrophilicity, E, was found to be the main factor responsible for changes in alkyl substituent effects with medium. When σI constants were used, variation of the polar term of alkyl substituents with the solvent electrophilicity E was found to be similar to that observed earlier for meta and para substituents, but twice less when σ* constants were used. The steric term for alkyl substituents was approximately independent of the solvent parameters.

Solvent and substituent effects on rearrangement of 4-(4-X-phenyl)-2,7-dioxa-3-azabicyclo[3,3,0]oct-3-enes

1985 ◽  
Vol 50 (9) ◽  
pp. 1971-1981 ◽  
Author(s):  
Lubor Fišera ◽  
Marta Konopíková ◽  
Ladislav Štibrányi ◽  
Hans-Joachim Timpe
Keyword(s):  
Substituent Effects ◽  
Quantum Yields ◽  
Polar Solvents ◽  
Aromatic Residue ◽  
Product Ratio ◽  
Oxazoline Derivatives ◽  
Oxazine Derivatives

Preparation of the title compounds V is described. They give, on irradiation, the 2,3-dihydro-6H-1,3-oxazine derivatives VI as the main products besides the tetrahydrofuro[3,4-d]oxazoline derivatives VII. The VI to VII product ratio depends on the substituent bound to the aromatic residue. Polar solvents favour formation of the VI derivatives in the order Cl >H > CH3. In non-polar solvents the proportion of VII is increased. The quantum yields of the photoreaction vary within the limits from 0.006 to 0.04 (H >F > Cl > CH3 > OCH3).

Mechanistic studies of aromatic ketone-sensitized photoreduction of bis(acetylacetonato)copper(II)

1983 ◽  
Vol 61 (5) ◽  
pp. 801-808 ◽  
Author(s):  
Yuan L. Chow ◽  
Gonzalo E. Buono-Core ◽  
Bronislaw Marciniak ◽  
Carol Beddard
Keyword(s):  
Rate Constant ◽  
Rate Constants ◽  
Flash Photolysis ◽  
Decay Rates ◽  
Polynuclear Aromatic Hydrocarbons ◽  
Aromatic Ketone ◽  
Quantum Yields ◽  
Triplet States ◽  
Quenching Rate ◽  
Triplet Energy

Bis(acetylacetonato)copper(II), Cu(acac)2, quenches triplet excited states of ketones and polynuclear aromatic hydrocarbons efficiently, but only aromatic ketones with high triplet energy successfully sensitize photoreduction of Cu(acac)2 in alcohols under nitrogen to give derivatives of aeetylacetonatocopper(I), Cu(acac). For the triplet state benzophenone-sensitized photoreduction of Cu(acac)2, the quantum yields of photoreduction (ΦC) and those of benzophenone disappearance (ΦB) were determined in methanol with various concentrations of Cu(acac)2. The values of the quenching rate constant, kq, determined from these two types of monitors on the basis of the proposed mechanism were in good agreement (6.89 ~ 7.35 × 109 M−1 s−1). This value was higher, by a factor of about two, than that obtained from the monitor of the benzophenone triplet decay rates generated by flash photolysis in the presence of Cu(acac)2. The quenching rate constants of various aromatic ketone and hydrocarbon triplet states by Cu(acac)2 were determined by flash photolysis to be in the order of the diffusion rate constant and the quantum yields of these photoreductions were found to be far from unity. Paramagnetic quenching, with contributions of electron exchange and charge transfer, was proposed as a possible quenching mechanism. For a series of aromatic ketone sensitizers with higher triplet energy, this mechanism was used to rationalize the observed high quenching rate constants in contrast to the low quantum yields of photoreduction.

scholarly journals SUBSTITUENT EFFECTS ON THE PHOTOCURRENT QUANTUM YIELDS OF PARA-SUBSTITUTED TETRAPHENYLPORPHYRINS

1980 ◽  
Vol 9 (6) ◽  
pp. 627-628 ◽  
Author(s):  
Kazuo Yamashita ◽  
Keiko Miyoshi ◽  
Atsuhiro Osuka ◽  
Hitomi Suzuki
Keyword(s):  
Substituent Effects ◽  
Quantum Yields

Wavelength controlled production of SO4- radicals in the ultraviolet photolysis of ceric sulfate solutions

1984 ◽  
Vol 37 (3) ◽  
pp. 475 ◽  
Author(s):  
RW Matthews
Keyword(s):  
Rate Constants ◽  
Photochemical Reaction ◽  
Transfer Reaction ◽  
Quantum Yields ◽  
Oh Radical ◽  
Transfer Energy ◽  
Fluorescence Measurements ◽  
Sulfate Solutions ◽  
Ultraviolet Photolysis ◽  
Primary Photochemical Reaction

Solutions of cerium(III)/(IV) and formic acid in 0.4 M sulfuric acid have been photolysed under 254 nm and 365 nm light. Marked differences in the reaction kinetics and quantum yields are observed at the two different wavelengths. At 365 nm, the reactions leading to cerium(IV) reduction are caused almost exclusively by the SO4- radical. The ratio of rate constants, k(SO4- + CeIII)/ k(SO4- + HCOOH), is 116 � 11 and the quantum yield of sulfate radicals, ф(SO4-), is 0.023 � 0.002. At 254 nm, the reactions leading to cerium(IV) reduction are caused mainly by the OH radical, but approximately 35% of the oxidizing radicals formed in the primary photochemical reaction are SO4-. Cerium(III) species, excited at 254 nm, transfer energy to cerium(IV) and this results in an additional yield of OH and SO4- radicals. Fluorescence measurements confirmed the efficiency of the energy transfer reaction. The ratio of rate constants, k(OH+CeIII)/k(OH+HCOOH), is 2.22 � 0.18 and ф(CeIV*) and ф(CelIII*) giving oxidizing radicals are 0.116 � 0.010 and 0.0083 � 0.0008 respectively. Thus about 5 times more total oxidizing radicals are produced from excited cerium(IV) species at 254 nm than at 365 nm.

Substituent effects of rate constants for thermal [4π + 2π] cycloreversion of spiro-fused β-lactam oxadiazolines

1993 ◽  
Vol 71 (6) ◽  
pp. 907-911 ◽  
Author(s):  
Michel Zoghbi ◽  
John Warkentin
Keyword(s):  
Thermal Decomposition ◽  
Rate Constant ◽  
Rate Constants ◽  
Transition States ◽  
Substituent Effects ◽  
Ground States ◽  
Phenyl Substituent ◽  
First Order ◽  
Average Value ◽  
Carbonyl Ylide

Twelve Δ3-1,3,4-oxadiazolines in which C-2 is also C-4 of a β-lactam moiety (spiro-fused β-lactam oxadiazoline system) were thermolyzed as solutions in benzene. Substituents in the β-lactam portion affect the rate constant for thermal decomposition of the oxadiazolines to N2, acetone, and a β-lactam-4-ylidene. The total spread of first-order rate constants at 100 °C was 47-fold and the average value was 6.7 × 10−4 s−1. A phenyl substituent at N-1 or at C-3 was found to be rate enhancing, relative to methyl. At C-3, H and Cl were also rate enhancing, relative to methyl. The data are interpreted in terms of the differential effects of substituents on the stabilities of the ground states, and on the stabilities of corresponding transition states for concerted, suprafacial, [4π + 2π] cycloreversion. The first products, presumably formed irreversibly, are N2 and a carbonyl ylide. The latter subsequently fragments to form acetone (quantitative) and a β-lactam-4-ylidene.

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