Studies on some radical transfer reactions by entrapping the radicals as polymer end groups. II. Reactions of ȮH radicals with amino acids

The reaction of ?H radicals with a number of aliphatic amino acids has been studied by entrapping the resultant radicals as end groups of poly(methyl methacrylate) that have been detected and estimated by the sensitive dye partition technique. The rate constants of the reaction (in mol^?1 L S^?1) of 7 amino acids at 25°C and at pH 1.00 have been determined as 8.33 × 10⁸ for glycine, 2.56 × 10⁹ for β-alanine, 2.01 × 10⁹ for β-alanine, 3.99 × 10⁹ for 4-amino butyric acid, 7.56 × 10⁹ for (1+) valine, 1.42 × 10¹⁰ for (1?) leucine, and 5.98 × 10¹⁰ for 6-amino caproic acid. Glycine, α-alanine, β-alanine, and 4-amino butyric acid produced radicals that underwent deamination and incorporated only carboxyl-bearing end groups in the polymer. The other amino acids, leucine, valine, and 6-amino caproic acid, produced at least two types of radicals, radicals that underwent deamination and those that remained intact, and incorporated in the polymer both carboxyl- and amine-bearing end groups but in different amounts. The latter type of radicals were about 29% from 6-amino caproic acid, 23% from leucine, and 18% from valine. The change of pH from 0.80 to 2.72 did not produce any significant change in the end group profile of the polymer obtained, indicating no appreciable change in the rate of the reaction of ?H radicals with the simplest amino acid glycine in the pH range studied.     

Studies on some radical transfer reactions by entrapping the radicals as polymer end groups. I. Reaction of hydroxyl radicals with amines

The reaction of OH radicals with a number of amines has been studied by entrapping the resultant radicals as polymer end groups which have been detected and estimated by the sensitive dye partition technique. Expressions have been developed relating the average amounts of end groups per polymer molecule to the rate constant of the radical transfer reaction, the rate constants determined for reaction with n-butyl, n-hexyl, and n-octyl amine being 1.00 × 10¹⁰, 1.31 × 10¹⁰, and 1.46 × 10¹⁰ mol^?1 L s^?1, respectively, at 25°C. The order of reactivity for amines of different classes has been found to be as primary < secondary > tertiary, the rate constants for reaction with n-butyl, dibutyl, and tributyl amine being 1.00 × 10¹⁰, 1.81 × 10¹⁰, and 1.67 × 10¹⁰ mol^?1 L s^?1, respectively, at 25°C. The change in the reactivity of the amine with chain length and amine class has been explained by activation and deactivation of the CH₂ group from which H abstraction by OH radicals occurs, respectively, by the alkyl group and by the protonated amino nitrogen under the acidic condition of the medium. Between pH 1.00 and 2.17, the rate of the reaction with n-butyl amine remains practically unchanged, but from pH 2.20 to 2.72 the rate constant increases with increasing pH, indicating that deprotonation of the positively charged nitrogen starts at about pH 2.20. The method is simple and accurate and can be applied to detect and estimate very reactive radicals.     

Studies on some radical transfer reactions. Part I. Hydrogen atom abstraction from some organic substrates by ȮH radicals

Polymerization of methyl methacrylate was carried out in aqueous and nonaqueous media in the presence of some sulfonated and carboxylic organic compounds, hydroxyl radicals generated from hydrogen peroxide being used as initiators of polymerization. The occurrence of radical transfer reactions by way of hydrogen atom abstraction from the organic substrates by the ?H radicals was demonstrated by the detection of sulfonate and carboxyl endgroups in the respective polymers. It was found that the radical transfer reactions were more favored in aqueous media than in nonaqueous systems.     

Long-lived polymer radicals. III. Synthesis of block copolymer by the reaction of living poly(N-methylacrylamide) radical with some vinyl monomers

N-Methylacrylamide (NMAAm) was polymerized quantitatively by using di-tert-butyl peroxide as photosensitizer to be, for the most part, incorporated in living poly(NMAAm) radical. The living polymer radical reacted effectively with acrylate monomers to yield block copolymer. Longer alkyl chain of the acrylate monomer caused a decrease in the conversion of the second monomer. Methacrylate monomers, such as methyl methacrylate and cyclohexyl methacrylate, showed relatively low reactivities in comparison with acrylates. Styrene exhibited a much lower conversion. The resulting block copolymers showed different thermochromic behaviors in methyl benzoate from that of poly(NMAAm). This is explained on the basis of the difference between refractive indexes of the block copolymers and poly(NMAAm).     

Products of the gas‐phase reactions of the OH radical with n‐butyl methyl ether and 2‐isopropoxyethanol: Reactions of ROC(Ȯ) < radicals

The products of the gas‐phase reactions of the OH radical with n‐butyl methyl ether and 2‐isopropoxyethanol in the presence of NO have been investigated at 298 ± 2 K and 740 Torr total pressure of air by gas chromatography and in situ atmospheric pressure ionization tandem mass spectrometry. The products observed from n‐butyl methyl ether were methyl formate, propanal, butanal, methyl butyrate, and CH₃C(O)CH₂CH₂OCH₃ and/or CH₃CH₂C(O)CH₂OCH₃, with molar formation yields of 0.51 ± 0.11, 0.43 ± 0.06, 0.045 ± 0.010, ∼0.016, and 0.19 ± 0.04, respectively. Additional products of molecular weight 118, 149 and 165 were observed by API‐MS/MS analyses, with those of molecular weight 149 and 165 being identified as organic nitrates. The products observed and quantified from 2‐isopropoxyethanol were isopropyl formate and 2‐hydroxyethyl acetate, with molar formation yields of 0.57 ± 0.05 and 0.44 ± 0.05, respectively. For both compounds, the majority of the reaction products and reaction pathways are accounted for, and detailed reaction mechanisms are presented. The results of this product study are combined with previous literature product data to investigate the tropospheric reactions of R₁R₂C(Ȯ)OR radicals formed from ethers and glycol ethers, leading to a revised estimation method for the calculation of reaction rates of alkoxy radicals. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 501–513, 1999     

Nonperfect synchronization of bond-forming and bond-rupturing processes in the reaction Ḣ + H2 → H2 + Ḣ

The simplest prototypical hydrogen transfer reaction, i.e., Ḣ+ H₂ → H₂ + Ḣ, is studied by the quantum-mechanical ab initio methods. Results reveal that during this reaction free valence which almost equals the square of the spin density develops on the migrating hydrogen atom. Bond orders are calculated using Mayer's formalism. Both the variations of bond orders and bond lengths along the reaction path are examined. Our analysis reveals that the bond formation and bond cleavage processes in this reaction are not perfectly synchronous. The bond cleavage process is slightly more advanced on the reaction path. © 1996 John Wiley & Sons, Inc.     

Disproportionation reactions between alkyl and fluoroalkyl radicals. III. A reassessment of Δ values for CFyH(3−y) radicals (y = 1,2,3) with C2H5 radicals

New determinations of the disproportionation and combination ratios between CF₂H and C₂H₅ radicals yield (the hydrogen acceptor radical is given first) Δ(CF₂H, C₂H₅) = 0.068 ± 0.008, and Δ(C₂H₅, CF₂H) = 0.37 ± 0.01. A reevaluation of the existing data on CFH₂ and CF₃ radicals leads to the following recommended values, Δ(CFH₂, C₂H₅) = 0.038 ± 0.006, and Δ(CF₃, C₂H₅) = 0.11 = ± 0.02.     

Studies on outer sphere electron transfer reactions of some surfactant-cobalt(III) complexes with ferrocyanide anion

The kinetics and mechanism of reduction of the surfactant-cobalt(III) complex ions, cis-[Co(bpy)₂(C₁₂H₂₅NH₂)₂]³⁺ and cis-[Co(phen)₂(C₁₂H₂₅NH₂)₂]³⁺ (bpy = bipyridyl, phen = 1,10-phenan-throline, C₁₂H₂₅NH₂ = dodecylamine) by Fe(CN₆)⁴⁻ in self-micelles were studied at different temperatures. Experimentally the reaction was found to be second order and the electron transfer postulated as outersphere. The rate constant for the electron transfer reaction for both the complexes was found to increase with increase in the initial concentration of the surfactant-cobalt(III) complex. This peculiar behaviour of dependence of second-order rate constant on the initial concentration of one of the reactants has been attributed to the presence of various concentration of micelles under different initial concentration of the surfactantcobalt(III) complexes in the reaction medium. The effect of inclusion of the long aliphatic chain of the surfactant complex ions into β-cyclodextrin on these reactions has also been studied.     

Rate constants for some reactions of inorganic radicals with inorganic ions. Temperature and solvent dependence

Rate constants for several reactions of inorganic radicals with inorganic anions in aqueous and aqueous/acetonitrile solutions have been measured as a function of temperature by laser flash photolysis. The reactions studied were (1) Cl₂^? + N₃^?, (2) Br₂^? + N₃^?, (3) Cl₂^? + SCN^?, (4) Br₂^? + SCN^?, (5) SO₄^? + Cl^?, (6) SO₄^? + CO₃^2?, and (7) N₃? + I^?. The rate constants were corrected for ionic strength and ranged from 10⁶ to 10⁹ L mol^?1 s^?1. The Arrhenius activation energies varied from 2 to 12 kJ mol^?1 for the first 4 reactions, were higher for reaction 6, and negative for reaction 5. The pre-exponential factors also varied considerably with log A ranging from 5 to 14. The values of k₂₉₈ decreased in most cases by more than an order of magnitude upon increasing the acetonitrile (ACN) fraction from 0 to 70%. For most reactions, this decrease in k₂₉₈ was due to changes in log A with little regularity in the small changes observed in E_a. For reaction 7, k₂₉₈ was practically unchanged due to compensating effects of the changes in E_a and log A with ACN mol fraction, giving an isokinetic relationship. An isokinetic relationship was also observed in the case of reaction 6; E_a and log A change in parallel while changing ACN mol fraction. Reaction 3 (Cl₂^? + SCN^?) was also studied in water/t-butanol and water/acetic acid mixtures. Linear correlation was found between log k and the dielectric constant of the medium for water/ACN and water/t-BuOH but the lines for the two solvent mixtures had different slopes, suggesting specific solvation effects in addition to the primary solvent polarity effects. With water/acetic acid, k decreased and then increased upon addition of acetic acid. © 1993 John Wiley & Sons, Inc.     

Iron halide mediated atom transfer radical polymerization of methyl methacrylate with N‐alkyl‐2‐pyridylmethanimine as the ligand

The controlled atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) catalyzed by iron halide/N‐(n‐hexyl)‐2‐pyridylmethanimine (NHPMI) is described. The ethyl 2‐bromoisobutyrate (EBIB)‐initiated ATRP with [MMA]₀/[EBIB]₀/[iron halide]₀/[NHPMI]₀ = 150/1/1/2 was better controlled in 2‐butanone than in p‐xylene at 90 °C. Initially added iron(III) halide improved the controllability of the reactions in terms of molecular weight control. The p‐toluenesulfonyl chloride (TsC1)‐initiated ATRP were uncontrolled with [MMA]₀/[TsC1]₀/[iron halide]₀/[NHPMI]₀ = 150/1/1/2 in 2‐butanone at 90 °C. In contrast to the EBIB‐initiated system, the initially added iron(III) halide greatly decreased the controllability of the TsC1‐initiated ATRP. The ration of iron halide to NHPMI significantly influenced the controllability of both EBIB and TsC1‐initiated ATRP systems. The ATRP with [MMA]₀/[initiator]₀/[iron halide]₀/[NHPMI]₀ = 150/1//1/2 provided polymers with PDIs ≥ 1.57, whereas those with [iron halide]₀/[NHPMI]₀ = 1 resulted in polymers with PDIs as low as 1.35. Moreover, polymers with PDIs of approximately 1.25 were obtained after their precipitation from acidified methanol. The high functionality of the halide end group in the obtained polymer was confirmed by both ¹H NMR and a chain‐extenstion reaction. Cyclic voltammetry was utilized to explain the differing catalytic behaviors of the in situ‐formed complexes by iron halide and NHPMI with different molar ratios. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4882–4894, 2004     

Reaction of thianthrene cation radical with grignard reagents : Evidence for electron transfer and trapping of alkyl radicals by the thianthrene cation radical

Reactions are reported between RMgCl and thianthrene cation radical perchlorate (Th^.+ClO^-₄) suspended in ether and tetrahydrofuran (THF). In ether solution reactions R = Bu, s-Bu, t-Bu, 5-hexenyl, and cyclopentylmethyl. Major products were the alkane, the alkene R(-H) in some cases, and, in the cases of R = Bu, 5-hexenyl, and cyclopentylmethyl, the 5-alkylthianthrenium perchlorate (ThR⁺ClO^-₄). When 5-hexenylMgCl was used a mixture of 5-(5-hexenyl)- and 5-(cyclopentylmethyl)thianthrenium per-chlorates in the ratio of approximately 2 was obtained. Since the ratio of 5-hexenyl/cyclopentylmethyl in the Grignard reagent was 10.4, it is concluded that the C₆ sulfonium ions were formed by radical trapping by Th^.+ after single electron transfer from Grignard to cation radical had occurred, thus allowing for cyclization of 5-hexenyl radical. Formation of ThBu⁺ClO^-₄ is attributed to the trapping of butyl radical by Th^·+, while formation of RH and R(-H) is in all cases also attributed to alkyl radical reactions. Reactions in THF(R = Me, i-Pr, Bu, s-Bu, t-Bu, Ph) led almost exclusively to RH and Th. Polymerization of THF was also initiated and took place slowly giving rise to low molecular weight poly(THF). By using THF-d₈, as solvent for reaction between BuMgCl and Th^.+, it was possible to find Bu groups (¹H-NMR) in the poly(THF-d₈). Polymerization of THF is attributed, in some cases (R = Me, Bu), to alkyl-cation transfer from ThR⁺ to THF. In other cases initiation of polymerization by R⁺ and THF(-H)⁺ is considered.     

α-Distonic ions as transient species in the reactions of metastable alkyl benzoate radical cations

The key intermediates to the fragmentation of metastable methyl and ethyl benzoate radical cations are α- and β-distonic isomers of the molecular ions. The α-distocic isomers are also formed by fragmentation of longer chain alkyl benzoates, but may not be long-lived, stable species. Rearrangement of the α-distonic ions prior to fragmentation can take place, but (re)formation of the benzoate molecular ions does not occur.     

Reaction rate of crosslinkings by using a metallated polymer. III. Solvent effects on rate constants of grafting and intramolecular crosslinking reactions.

Lithium-metallated (styrene-p-benzylstyrene)copolymer was reacted with chlorine-terminated polystyrene as a crosslinker polymer in a mixture of tetrahydrofuran (THF)–n-hexane at 25°C in the presence of lithium chloride(LiCl). The rate constants were estimated from the changes in the concentration of metallated polymer by photometrical measurements. As a result, the rate constant of grafting (k₁) showed a constant value in spite of a change in molecular weight of the crosslinker polymers and the addition of n-hexane. The rate constant of intramolecular crosslinkings (k_2intra) obtained in a mixed solvent (21 ～ 36 vol % of n-hexane) increased when the molecular weight of the crosslinker polymers and the extent of n-hexane were increased.     

Studies on the reactions of sylvatesmin and lantbeside with oxidizing free radicals

Sylvatesmin (SYL) and lantbeside (LAN) are two lignans, isolated from a Chinese folk medicinal herb, Lancea tibetica. Their abilities of scavenging oxidizing free radical models, OH, SO₄⁻ and N₃, were investigated in aqueous solution by pulse radiolysis techniques. The OH-adduct radicals with small amount of oxidized products were formed by the reaction of SYL or LAN with OH radical. SYL undergoes one-electron oxidation either by SO₄⁻ or N₃ radicals, LAN can only be detected to react with stronger oxidant SO₄⁻ radical anion. No reaction between LAN and N₃ radical was detectable. The relationship of structure with the abilities of scavenging free radicals was discussed. The reaction rate constants were determined by analysis of the build-up trace of the radical products.     

Structure and formation of some [C4H5O2]+ ions generated by radical losses in pseudo simple cleavage reactions

Characterization of some [C₄H₅O₂]⁺ ions in the gas phase using their collisional activation mass spectra shows that the isomeric ions \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_3 {\rm OCH = CH} - \mathop {\rm C}\limits^ + {\rm = O,} $\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm HC} \equiv {\rm C} - \mathop {{\rm C}({\rm OH}){\rm OCH}_3 }\limits^ + $\end{document} are stable for t?10^?5 s. Of these, ions of structure were generated by the site specific gas phase protonation of γ-crotonolactone with isobutane or methanol as chemical ionization reagent gases. These results and those derived from measurements on some ²H, ¹³C and ¹⁸O labelled [C₄H₅O₂]⁺ product ions, were used to study the mechanisms of unimolecular radical elimination reactions, viz. (1) loss of CH₃˙ from [trans-methyl crotonate], (2) loss of H˙ from [methyl acrylate]⁺˙, (3) loss of H˙ from [cyclopropane carboxylic acid]⁺˙ and (4) loss of CH₃˙ from [1,3-dimethoxypropyne]⁺˙. It is concluded that none of these losses occur by simple bond cleavage. Mechanisms are presented which account for the observation that the first three reactions yield product ions of structure whereas the ions generated by reaction (4) have structure \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_3 {\rm OCH = CH} - \mathop {\rm C}\limits^ + {\rm = O}{\rm .} $\end{document}. It is further proposed that a minor fraction of the [M-CH₃]⁺ ions from ionized trans-methyl crotonate is generated via a rearrangement process which yields ions of structure \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_3 {\rm OCH = CH} - \mathop {\rm C}\limits^ + {\rm = O}{\rm .} $\end{document}.     

Mechanistic Studies on the reactions of cis-diaquobisoxalatochromate(III) ion with 2,2′-dipyridyl and 1,10-phenanthroline

Kinetic studies in aqueous solutions on the replacement of the aquo ligands in cis-Cr(Ox)₂(H₂O) by 2,2′-dipyridyl (dipy) and 1,10-phenanthroline (phen) forming cis-Cr(Ox)₂(AA)^? (AA = dipy or phen) were made spectrophotometrically. The reaction in each case occured in two concurrent paths, one of which was independent of the reagent (AA) concentration (rate constant, k₀, identical for both the systems), and another which was first order with respect to it (rate constant, k_R). k₀ and k_R values have been evaluated at different temperatures (40–70°C), and from there the corresponding ΔH≠ and ΔS≠ values. The results suggest a dissociation mechanism for the reagent independent path where Cr? OH₂ bond rupture is only significant in the transition state. The value of k_R/k₀ (higher for phen compared to dipy) was of the order of 10² suggesting significant bond formation by the reagent in the transition state of the reagent dependent path. However, ΔH≠ corresponding to k_R was ca. 1.7 to 1.8 times that corresponding to k₀ indicating that in the reagent dependent path simultaneous rupture of the two Cr? OH₂ bonds in cis positions occur as expected from steric considerations for these bidendate ligands. Increase in ionic strength of the of the medium causes a slight acceleration of the reagent-dependent path only.     

Polar vs. bde effects on aldehydic hydrogen atom transfer reactions of benzaldehydes by t-butoxy radicals.

Aldehydic hydrogen atom abstractions from benzaldehydes by t-butoxy radicals from t-butylperoxide exhibit a Hammett rho of ?0.32, which is better correlated with σ⁺ than σ and rationalized in terms of the contribution of dipolar charge-separated transition state.     

Reaction of perfluoroalkanesulfonyl halide: 3. The reactions of perfluoroalkanesulfonyl bromide with α, β-unsaturated esters and chemical conversions of the products

The reactions of perfluoroalkanesulfonyl bromide with α, β-unsaturated esters were studied in detail. The reaction products were further converted to a series of perfluoroalkyl-substituted α, β-unsaturated acids or esters, α-amino acids and γ-lactones. A peculiar peak (M+15) was found to appear in the mass spectra of some perfluoroalkyl-substituted methyl esters. It was interpreted to be the result of a CH₃ group transfer to the molecular ion. Magnetic nonequivalence was observed in the ¹⁹F NMR spectra of CF₂ group linked to CH₂ in compounds 2f, g and 3f, g which showed a typical AB pattern, and was attributed to the effect of steric hindrance.