Separation of polar and steric effects on absolute rate constants and arrhenius parameters for the reaction of tert-butyl radicals with alkenes

Absolute rate constants and their temperature dependencies were measured for the reaction of tert-butyl radicals with 24 substituted ethenes and several other compounds in 2-propanol solution by time-resolved electron spin resonance. At 300 K the rate constants cover the range from 60 M^?1 s^?1 (1,2-dimethylene) over 16,500 M^?1 s^?1 (vinyl-chloride) to 460,000 M^?1 s^?1 (2-vinylpyridine). For the mono- and 1,1-disubstituted ethenes log k₃₀₀ increases and the activation energy decreases with increasing electron affinity of the olefins. The frequency factors are in the range log A/M^?1 s^?1 = 7.5 ± 1.0 as typical for addition reactions, with minor exceptions. Electron affinity (polar) and steric effects on reactivity are separated for the addition of tert-butyl to chloro- and methyl-substituted ethylenes. A comparison with rate data for methyl, ethyl, 2-propyl, and other radicals indicates both polar and steric effects on radical substitution.     

A kinetic study of the reactions of sulfate and dihydrogen phosphate radicals with epicatechin,epicatechingallate, and epigalocatechingallate

The bimolecular rate constants for the reactions of sulfate radicals with epicatechin (EC), epicatechingallate (ECG), and epigallocatechingallate (EGCG) were found to be (1.46 ± 0.06) × 10⁹, (1.20 ± 0.08) × 10⁹, and (1.04 ± 0.07) × 10⁹, respectively. The activation energy [E_A = 9 ± 3 kJ mol^?1] and preexponential factor [A = (4.8 ± 0.6) × 10¹⁰] for the reaction of EC with the sulfate radical were measured in the temperature range 288–303 K. The phenoxyl radicals of EC (λ_max = 310 nm) were obtained both by the reaction of this flavonoid with the sulfate radicals and by photoionization. The measured bimolecular rate constants for the reactions of the dihydrogen phosphate radicals with EC, ECG, and EGCG were (7.8 ± 0.9) × 10⁸, (8.5 ± 0.4) × 10⁸, and (6.8 ± 0.4) × 10⁸, respectively. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 391–396, 2010     

Different channels of ·OH radical reaction with tryptophanol in aqueous solutions: A pulse radiolysis study

The pulse radiolysis technique has been employed to study the reaction of ·OH radical with tryptophanol (TPN). Reactions of specific one-electron oxidants like Br₂· - and N₃· and ·H atom were carried out to understand the contribution of different channels of · OH radical reaction with TPN. The studies were carried out in the pH range 3 to 10. One-electron oxidation of TPN (pH 3) produced radical cation absorbing at 570 nm. However, at higher pH, deprotonation of TPN cation radical takes place from N(1) position and indolyl radical absorbing at 520 nm with a p ^K a value of 3.6 is formed. Redox titration with TMPD, ABTS^2- and MV²⁺ was performed to determine the total yield of oxidizing and reducing radicals produced during ·OH reaction.     

Kinetics of one-electron oxidation by the ClO radical

Pulse radiolysis studies were carried out to determine the rate constants for reactions of ClO radicals in aqueous solution. These radicals were produced by the reaction of OH with hypochlorite ions in N₂O saturated solutions. The rate constants for their reactions with several compounds were determined by following the build up of the product radical absorption and in several cases by competition kinetics. ClO was found to be a powerful oxidant which reacts very rapidly with phenoxide ions to form phenoxyl radicals and with dimethoxybenzenes to form the cation radicals (k = 7 × 10⁸ −2 × 10⁹ M^-1 s^-1). ClO also oxidizes ClO^-₂ and N^-₃ ions rapidly (9.4 × 10⁸ and 2.5 × 10⁸ M^-1 s^-1, respectively), but its reactions with formate and benzoate ions were too slow to measure. ClO does not oxidize carbonate but the CO^-₃ radical reacts with ClO^- slowly (k = 5.1 × 10⁵ M^-1 s^-1).     

Absolute rate constants for hydrogen abstraction from hydrocarbons by the trichloromethylperoxyl radical

Absolute rate constants have been measured for the reactions of trichloromethylperoxyl radicals with cyclohexane, cyclohexene, and hexamethylbenzene. The CCl₃O₂ radicals were produced by pulse radiolysis of air-saturated CCl₄ solutions containing various amounts of the hydrocarbons. The rate constants were determined by competition with the one-electron oxidation of metalloporphyrins, using the rate of formation of the metalloporphyrin radical cation absorption to monitor the reaction by kinetic spectrophotometry. The rate constants for hydrogen abstraction from cyclohexane, cyclohexene, and hexamethylbenzene were found to be 1 × 10³, 1.0 × 10⁵, and 7.5 × 10⁴ M^?1 s^?1, respectively.     

Single-photon ionisation of TMPD in liquid CCl4 observed by time-resolved microwave conductivity

A large microwave conductivity change is observed on laser flash photolysis (308 nm) of a solution of TMPD in CCl₄. This is ascribed to single-photon ionisation of the solute resulting in the formation of a stable TMPD⁺Cl^? ion pair. The dipole moment of the ion pair is estimated to be (8.7 ± 0.9)(φ*)^{?$\text{1}\text{2}$} where φ* is the quantum efficiency for ion-pair formation.     

Radical reactions of C84

Pulse radiolysis and steady-state radiolysis experiments describing the radical and electron transfer reactions of C₈₄ are reported here for the first time. C₈₄ reacts readily with radiolytically generated chloromethyl (^•CCl₃) and trichloromethylperoxyl (CCl₃OO^•) radicals in CCl₄. The formation of the radical adduct has been confirmed from its characteristic absorption in the UV (320 nm) and visible (480 nm). Radical-induced oxidation in 1,2-dichloroethane (1,2-DCE) resulted in a short lived transient absorbing at 920 nm. Reduction of C₈₄ in toluene/2-propanol/acetone could be conveniently followed by formation of an absorption band with an absorption maximum at 960 nm.     

Charge-transfer derivatization in fast-atom-bombardment mass spectrometry

The formation of charge-transfer complexes as derivatization reactions for fast-atom- bombardment (f.a.b.) mass spectrometry has been investigated. The donor N,N,N′,N′- tetramethyl-1,4-phenylenediamine (TMPD) and the acceptor 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) were studied. The f.a.b, spectrum of this complex in glycerol first yielded the cation radical, TMPD⁺ at m/z 164. However, with time the dominant ion becomes (TMPD+H)⁺. Results suggest that although a strong charge-transfer complex is formed, protonation of the donor molecule occurs whenever possible. In dimethylsulfoxide, initial f.a.b, spectra contain (TMPD+H)⁺, but as solvent evaporates, this ion is supplanted by the cation radical. Increased abundances of the cation radicals for charge- transfer complexes are observed in the absence of solvent or with the use of aprotic solvents. Characteristic ultraviolet/visible spectra of charge-transfer complexes clarify the competing processes of electron and proton transfer, and their time dependence. Charge- transfer derivatization is used to increase signals for the donor cation radicals of anthracene/picric acid, pyrene/picric acid, and indole/trinitrofluorenone.     

Free radical reactions of pyridoxal (vitamin B6): a pulse radiolysis study

Redox reactions of pyridoxal (P-OH) with e¯_aq, ^. OH, N ^. ₃, SO ^. ₄¯ and various organo-haloperoxyl radicals have been studied using pulse radiolysis technique. The rate constants for the reaction of P-OH or P-O¯ with the above-mentioned radicals and the transient absorption spectra have been measured. The transients formed in the reaction of hydrated electron and oxidizing radicals with pyridoxal have been assigned. An attempt has been made to find a correlation between the rate constants and Taft parameter for the reactions with the organo-haloperoxyl radicals. It has also been observed that the one-electron oxidized radical of pyridoxal is repaired by uric acid. The reduction potential for the P-OH ^.+/P-OH couple at pH 7, as measured by cyclic voltammetry, has been found to be +1.11 V vs. NHE.     

Temperature effect on quenching of CH(AΔ)

The quenching rate constants of CH(A²Δ) radicals by alcohol, alkane, O₂, and C₂H₄ molecules over the temperature range 297–653 K have been measured using laser photolysis of CHBr₃ at 266 nm to produce CH(A) radical and time-resolved fluorescence measurements. Under the simultaneous effects of multiple attractive potentials and repulsive barrier, the temperature dependence of the quenching process of CH(A²Δ) is discussed qualitatively based on a modified collision complex model.     

Philicity of Acetonyl and Benzoyl Radicals: A Comparative Experimental and Computational Study

In this work, the reactivities of acetonyl and benzoyl radicals in aromatic substitution and addition reactions have been compared in an experimental and computational study. The results show that acetonyl is more electrophilic than benzoyl, which is rather nucleophilic. A Hammett plot analysis of the addition reactions of the two radicals to substituted styrenes clearly support the nucleophilicity of benzoyl, but in the case of acetonyl, no satisfactory linear correlation with a single substituent-related parameter was found. Computational calculations helped to rationalize this effect, and a good linear correlation was found with a combination of polar parameters (σ⁺) and the radical stabilization energies of the formed intermediates. Based on the calculated philicity indices for benzoyl and acetonyl, a quantitative comparison of these two radicals with many other reported radicals is possible, which may help to predict the reactivities of other aromatic radical substitution reactions.     

Reactive oxygen and nitrogen species (RONS) scavenging reactions of o-vanillin: Pulse radiolysis and stopped flow studies

Reactions of peroxyl radicals and peroxynitrite with o-vanillin (2-hydroxy 3-methoxy benzaldehyde), a positional isomer of the well-known dietary compound vanillin, were studied to understand the mechanisms of its free radical scavenging action. Trichloromethylperoxyl radicals (CCl₃O ₂ ^· ) were used as model peroxyl radicals and their reactions with o-vanillin were studied using nanosecond pulse radiolysis technique with absorption detection. The reaction produced a transient with a bimolecular rate constant of approx. 10⁵ M⁻¹s⁻¹, having absorption in the 400–500 nm region with a maximum at 450 nm. This spectrum looked significantly different from that of phenoxyl radicals of o-vanillin produced by the one-electron oxidation by azide radicals. The spectra and decay kinetics suggest that peroxyl radical reacts with o-vanillin mainly by forming a radical adduct. Peroxynitrite reactions with o-vanillin at pH 6.8 were studied using a stopped-flow spectrophotometer. o-Vanillin reacts with peroxynitrite with a bimolecular rate constant of 3 × 10³ M⁻¹s⁻¹. The reaction produced an intermediate having absorption in the wavelength region of 300–500 nm with a absorption maximum at 420 nm, that subsequently decayed in 20 s with a first-order decay constant of 0.09 s⁻¹. The studies indicate that o-vanillin is a very efficient scavenger of peroxynitrite, but not a very good scavenger of peroxyl radical. The reactions take place through the aldehyde and the phenolic OH group and are significantly different from other phenolic compounds.     

Pulse radiolysis studies on reactions of α-hydroxyalkyl radicals with nicotinamide and 6-methyl nicotinic acid

Reactions of α-hydroxyalkyl radicals derived from 2-propanol, ethanol and methanol with nicotinamide (NICAM) and 6-methyl nicotinic acid (6-MNA) were studied at various pHs using pulse radiolysis technique. It is found that α-hydroxyalkyl radicals react with NICAM and 6-MNA at pHs when nitrogen is in the protonated state. In these reactions, radical adducts of NICAM/6-MNA with α-hydroxyalkyl radicals are formed which have absorption maxima at about 340–350 nm which subsequently decay to give pyridinyl type of radicals of NICAM and 6-MNA having λ_max at 410 nm. Rate constants for the reactions of (CH₃)₂COH, CH₃CHOH and CH₂OH radicals with NICAM and 6-MNA were found to have linear dependence on reduction potentials of corresponding α-hydroxyalkyl radicals. Adducts formed in the reactions of CH₃CHOH and CH₂OH radicals with both NICAM and 6-MNA decayed slowly compared to the decay of adduct formed in reactions with (CH₃)₂COH radicals.     

Absolute rate constants for the reactions between amino and alkyl radicals at 298 K

The absolute rate constants for the reactions of NH₂ radicals with ethyl, isopropyl, and t-butyl radicals have been measured at 298 K, using a flash photolysis–laser resonance absorption method. Radicals were generated by flashing ammonia in the presence of an olefin. A new measurement of the NH₂ extinction coefficient and oscillator strength at 597.73 nm was performed. The decay curves were simulated by adjusting the rate constants of both the reaction of NH₂ with the alkyl radical and the mutual interactions of alkyl radicals. The results are k(NH₂ + alkyl) = 2.5 (±0.5), 2.0 (±0.4), and 2.5 (±0.5) × 10¹⁰ M^?1·s^?1 for ethyl, isopropyl, and t-butyl radicals, respectively. The best simulations were obtained when taking k(alkyl + alkyl) = 1.2, 0.6, and 0.65 × 10¹⁰M^?1·s^?1 for ethyl, isopropyl, and t-butyl radicals, respectively, in good agreement with literature values.     

Rate constants for the reactions of isobutyl,neopentyl, cyclopentyl,and cyclohexyl radicals with molecular oxygen

Rate constants have been measured for the reactions of four hydrocarbon radicals with O₂ in the gas phase at room temperature. Laserflash photolysis was used to generate low concentrations of radicals. A photoinization mass spectrometer followed the radical loss as a function of time. The measured pseudo first-order decay rate of the radical and the absolute oxygen concentration were combined to give the absolute rate constants (in units of 10^?12 cm³ molec^?1 s^?1): isobutyl (2.9 ± 0.7); neopentyl (1.6 ± 0.3); cyclopentyl (17 ± 3); and cyclohexyl (14 ± 2). The cycloalkyl radicals have rate constants similar to those of other secondary radicals. However, the isobutyl and neopentyl radicals react more slowly than similar primary radicals. These new rate constants are compared in Figure 2 with the recently published correlation of reactive cross section with radical ionization potential.     

Scavenging activity of C4‐hydroxyphenyl‐ and polyhydroxyphenyl‐1,4‐dihydropyridines toward free radicals

The radical‐scavenging ability of synthesized C4‐phenolic‐substituted 1,4‐dihydropyridines (1,4‐DHPs) toward 2,2‐diphenyl‐1‐picrylhydrazyl (DPPH^?) and alkyl/alkylperoxyl ABAP‐derived radicals at pH 7.4 was assessed by UV–visible spectroscopy. Reactivity of 1,4‐DHPs toward DPPH^? was measured by following the decay of the absorption corresponding to the radical λ_max at 525 nm, permitting the calculation of EC₅₀, t_EC50, and antiradical efficiency values. Pseudo–first‐order kinetic rate constants for the reactivity between the C4‐phenolic‐substituted 1,4‐DHP compounds and alkyl/alkylperoxyl ABAP‐derived radicals were followed by the decrease in λ_max at 356 nm corresponding to 1,4‐DHP moiety. C4‐phenolic‐substituted 1,4‐DHPs were more reactive toward alkyl free radicals than the other tested radicals. The 3,4,5‐trihydroxyphenyl‐1,4‐DHP was the most reactive derivative toward this radical with a kinetic rate constant value of 513.2 s^?1. Also, this derivative was the most effective toward the DPPH^? radical with the lowest EC₅₀ value (5.08 µM). Comparative studies revealed that synthesized 1,4‐DHPs were more reactive than commercial 1,4‐DHPs. The scavenging mechanism involves the contribution of both pharmacophores, that is, hydroxyphenyl and 1,4‐DHP rings, which was supported by the identification of the reaction products. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 810–820, 2012     

Electron Transfer and Charge Transfer: Twin Themes in Unifying the Mechanisms of Organic and Organometallic Reactions

The broad varieties of organic and organometallic reactions merge into a common unifying mechanism by considering all nucleophiles and electrophiles as electron donors (D) and electron acceptors (A), respectively. Comparison of outer-sphere and inner-sphere electron transfers with the aid of Marcus theory provides the thermochemical basis for the generalized free energy relationship for electron transfer (FERET) in Equation (37) and its corollaries in Equations (43) and (44) that have wide predictive applicability to electrophilic aromatic substitutions, olefin additions, organometallic cleavages, etc. The FERET is based on the conversion of the weak nucleophile–electrophile interactions extant in the ubiquitous electron donor—acceptor (EDA) precursor complex [D, A] to the radical ion pair [D^⊕, A^?], for which the free energy change can be evaluated from the charge-transfer absorption spectra according to Mulliken theory. FERET analysis thus indicates that the charge-transfer ion pairs [D^⊕, A^?] are energetically equivalent to the transition states for nucleophile/electrophile transformations. The behavior of such ion pairs can be directly observed immediately following the irradiation of the charge-transfer bands of various EDA complexes with a 25-ps laser pulse. Such studies confirm the radical ion pair [Arene^⊕, NO₂] as a viable intermediate in electrophilic aromatic nitration, as presented in the electron-transfer mechanism between arenes and the nitryl cation (NO) electrophile.     

Type I Photosensitized Oxidation of Methionine†

Methionine (Met) is an essential sulfur‐containing amino acid, sensitive to oxidation. The oxidation of Met can occur by numerous pathways, including enzymatic modifications and oxidative stress, being able to cause relevant alterations in protein functionality. Under UV radiation, Met may be oxidized by direct absorption (below 250 nm) or by photosensitized reactions. Herein, kinetics of the reaction and identification of products during photosensitized oxidation were analyzed to elucidate the mechanism for the degradation of Met under UV‐A irradiation using pterins, pterin (Ptr) and 6‐methylpterin (Mep), as sensitizers. The process begins with an electron transfer from Met to the triplet‐excited state of the photosensitizer (Ptr or Mep), to yield the corresponding pair of radicals, Met radical cation (Met^?+) and the radical anion of the sensitizer (Sens^??). In air‐equilibrated solutions, Met^?+ incorporates one or two atoms of oxygen to yield methionine sulfoxide (MetO) and methionine sulfone (MetO₂), whereas Sens^?? reacts with O₂ to recover the photosensitizer and generate superoxide anion (O₂^??). In anaerobic conditions, further free‐radical reactions lead to the formation of the corresponding dihydropterin derivatives (H₂Ptr or H₂Mep).     

Photoinduced electron transfer reactions in the 10-methylacridinium cation–benzyltrimethylsilane system: steady-state and flash photolysis studies

The mechanism of the photoinduced reaction of the lowest excited singlet state of the 10-methylacridinium (AcrMe⁺) cation with benzyltrimethylsilane (BTMSi) in acetonitrile has been investigated by means of steady-state and time-resolved methods. A variety of stable products was found after irradiation (365 nm) of the reaction mixture under aerobic and oxygen-free conditions. The stable products were identified and analyzed using UV–Vis spectrophotometry, high performance liquid chromatography (HPLC), and mass spectrometry (MS). Based on Stern–Volmer plots of the AcrMe⁺ fluorescence quenching by BTMSi (using fluorescence intensity and lifetime measurements), the rate constants were determined to be k _q = 1.24 (± 0.02) × 10¹⁰ M⁻¹ s⁻¹ and k _q = 1.23 (± 0.02) × 10¹⁰ M⁻¹ s⁻¹, i.e., close to the diffusion-controlled limit in acetonitrile, indicating the dynamic quenching mechanism. The quenching process was shown to occur via an electron-transfer reaction leading to the formation of acridinyl radicals (AcrMe^•) and C₆H₅CH₂Si(CH₃)₃ ^•+ radical cations. Based on stationary and flash photolysis experiments, a detailed mechanism of the secondary reactions is proposed and discussed. The AcrMe^• radical was shown to decay by two processes. The fast decay, observed on the nanosecond timescale, was attributed to the back-electron transfer occurring within the initial radical ion pair. The slow decay on the microsecond timescale was explained by recombination reactions of radicals which escaped from the radical pair, including benzyl radicals formed via C–Si bond cleavage in the C₆H₅CH₂Si(CH₃)₃ ^•+ radical cation.     

Ab initio study on alkyl radical decomposition and alkyl radical addition to olefins

The β bond dissociation of alkyl radicals and their reverse reactions, the addition of alkyl radicals to olefins were studied by G3MP2 level of theory to obtain a consistent kinetic data set. Both reaction families can be classified depending on the type of radical formed by β bond scission, namely the CH₃, primary, secondary tertiary radical formed. The kinetics of the reaction classes were described by only a limited number of Arrhenius parameters. The unified A factor of 10^13.7 s⁻¹ was found for all β bond dissociations. The Arrhenius activation energies are 125, 121, 113 and 103 kJ mol⁻¹, for methyl, primary, secondary, and tertiary radicals, respectively. The activation energies of 32, 25 and 18 kJ mol⁻¹ are calculated for the terminal addition of primary (including methyl), secondary, and tertiary radicals to olefins, respectively. The biologically important nonterminal radical additions to olefins have higher barriers of 37, 31 and 35 kJ mol⁻¹, respectively. At room temperature both strongly exothermic additions can compete with H-atom abstraction. New groups for Benson’s group additivity rules were defined to describe activation parameters for the β bond dissociation reactions. The group values were calculated by using the ab initio heats of formation of transition state structures.