Reactions of unsymmetrically substituted allyl radicals with methyl radicals

The Hg(6³P₁) photosensitized decompositions of 3-methyl-1-butene, 2-methyl-2-butene, 3,3-dimethyl-1-butene, and 2,3-dimethyl-1-butene have been used to generate 1-methylallyl, 1,2-dimethylallyl, 1,1-dimethylallyl, and 1,1,2-trimethylallyl radicals in the gas phase at 24 ± 1°C. From a study of the relative yields of the CH₃ combination products, the relative reactivities of the reaction centers in each of these unsymmetrically substituted ambident radicals have been determined. The more substituted centers are found to be the less reactive, and this is ascribed primarily to greater steric interaction at these centers during reaction. Measurement of the ratio of trans- to cis-2-pentene formed from the 1-methylallyl radical, combined with published values for this ratio at higher temperatures, enabled the differences in entropy and heat of formation of the trans- and cis-forms of this radical to be calculated as 0.62 ± 0.85 J mol^?1 K^?1 and - 0.63 ± 0.25 kJ mol^?1, respectively, at 298K. Approximate values of the disproportionation/combination ratios for reaction of CH₃ with 1,1-dimethylallyl and 1-methylallyl have been estimated and used to compute rate constants for the recombinations of tert-butyl and isopropyl radicals that are in agreement with recently published data.     

The Combination and Disproportionation of Ethyl,n-Propyl,and sec-Propyl Radicals: A DFT Study

Density functional theory (B3LYP) has been applied to the problem of free radical combination and disproportionation reactions. These have been examined for the ethyl, n-propyl, and sec-propyl radicals. For the combination reactions, the enthalpy of activation fell in the range of 5 to 7 kcal/mol, which is within the limits of computational error. The transition structure of disporportionation reaction each displayed a bent C...H...C angle for the transfer of the hydrogen in contrast to earlier reports done at the semiempirical level. The activation enthalpies for the hydrogen transfer process increased with the number of attached methyl groups on the donor radical.     

S′H type reactions of substituted allylic compounds

S′_H reactions of allyl sulfides and halides with phenyl radicals are reported. Thermal decomposition of phenylazotriphenylmethane with allyl sulfides and bromide has been shown to give allylbenzene. This apparent substitution reaction involves attack of a phenyl radical on the terminal unsaturated carbon atom of the allyl sulfide; the reaction in α,α-dimethylallyl ethyl sulfide produced 2-methyl-4-phenylbutene-2. To estimate the relative reactivities of allylic substrates towards phenyl radicals, competitive reactions of phenyl radicals with allylic compounds and carbon tetrachloride were investigated. The data indicate that the radical formed by addition of a phenyl radical to the allylic sulfide looses thiyl radicals almost quantitatively.     

Quantum chemical investigation of the reaction of O(3 P 2) with certain hydrocarbon radicals

The reaction of ground-state atomic oxygen [O(³ P ₂)] with methyl, ethyl, n-propyl and isopropyl radicals has been studied using the density functional method and the complete basis set model. The energies of the reactants, products, reaction intermediates and various transition states as well as the reaction enthalpies have been computed. The possible product channels and the reaction pathways are identified in each case. In the case of methyl radical the minimum energy reaction pathway leads to the products CO + H₂ + H. In the case of ethyl radical the most facile pathway leads to the products, methanal + CH₃ radical. For propyl radical (n- and iso-), the minimum energy reaction pathways would lead to the channel containing ethanal + methyl radical.     

Disproportionation reactions between alkyl and fluoroalkyl radicals. VII. Difluoromethyl with perfluoro-n-propyl and methyl radicals

Mixtures of 1,1,3,3-tetrafluoroacetone and perfluorodi-n-propyl ketone have been photolyzed together over the temperature range 50° to 200°C, and the disproportionation/combination ratio for n-C₃F₇ and CF₂H radicals has been determined to be Δ(n-C₃F₇, CF₂H) = 0.072 ± 0.003. A reevaluation of existing data on CH₃ and CF₂H radicals leads to a value of Δ(CH₃, CF₂H) = 0.35. The large variations in Δ for the reactions of alkyl and perfluoroalkyl radicals with CF₂H radicals are discussed. © John Wiley & Sons, Inc.     

Cis-trans isomerization of chemically activated 1-methylallyl radical and fate of the resulting 2-buten-1-peroxy radical

The cis-trans isomerization of chemically activated 1-methylallyl is investigated using RRKM/Master Equation methods for a range of pressures and temperatures. This system is a prototype for a large range of allylic radicals formed from highly exothermic (～35 kcal/mol) OH + alkene reactions. Energies, vibrational frequencies, anharmonic constants, and the torsional potential of the methyl group are computed with density functional theory for both isomers and the transition state connecting them. Chemically activated radicals are found to undergo rapid cis-trans isomerization leading to stabilization of significant amounts of both isomers. In addition, the thermal rate constant for trans → cis isomerization of 1-methylallyl is computed to be high enough to dominate reaction with O(2) in 10 atm of air at 700 K, so models of the chemistry of the (more abundant and more commonly studied) trans-alkenes may need to be modified to include the cis isomers of the corresponding allylic radicals. Addition of molecular oxygen to 1-methylallyl radical can form 2-butene-1-peroxy radical (CH(3)CH═CHCH(2)OO(?)), and quantum chemistry is used to thoroughly explore the possible unimolecular reactions of the cis and trans isomers of this radical. The cis isomer of the 2-butene-1-peroxy radical has the lowest barrier (via 1,6 H-shift) to further reaction, but this barrier appears to be too high to compete with loss of O(2).     

Isomerization of n-hexyl and s-octyl radicals by 1,5 and 1,4 intramolecular hydrogen atom transfer reactions

n-Hexyl and s-octyl radical isomerizations by intramolecular hydrogen atom shift have been studied in the presence of high methyl radical concentration where isomerized alkyl radicals reacted predominantly by combination and disproportionation reactions with methyl radicals. By assuming the rate coefficient of 1-hexyl radical recombination to be equal to that of ethyl self-combination, the rate coefficient of log(k₁/s^?1) = (9.5 ± 0.3) – (11.6 ± 0.3) kcal mol^?1/RT ln 10 has been derived for the 6sp isomerization of n-hexyl radicals, 1-hexyl → 2-hexyl (1). Investigation of s-octyl radical isomerization was complicated by fast interconversion between 3-octyl, 2-octyl, and 4-octyl radicals. Use of the methyl trapping technique and systematic variation of methyl radical concentration made possible the determination of log(k₂/s^?1) = (9.4 ± 0.7) ? (11.2 ± 1.0) kcal mol^?1/RT ln 10 for the 6ss isomerization of 3-octyl and the estimation of log(k₃/s^?1) = 10.5–17 kcal mol^?1/RT ln 10 for the 5ss isomerization of 2-octyl radicals, where 3-octyl → 2-octyl (2), and 2-octyl → 4-octyl (3).     

Disproportionation reactions between alkyl and fluoroalkyl radicals. V. Perfluoro-n-propyl and ethyl radicals revisited

A redetermination of the disproportionation/combination ratio for n–C₃F₇ and C₂H₅ radicals gives a value of Δ(n–C₃F₇, C₂H₅) = 0.13 ± 0.01, independent of the temperature. The radicals were produced by the photolysis of n–C₃F₇COC₂H₅. The previous determinations of this ratio are discussed and are found to be largely incorrect. The values for Δ(CF₃, C₂H₅) and Δ(C₂F₅, C₂H₅) are also re-evaluated, and the recommended values are 0.10 ± 0.02 and 0.12 ± 0.02, respectively. Systems involving perfluoroalkyl and ethyl radicals are complicated due to rapid perfluororadical addition to the ethylene formed in the disproportionation process. The extent of this reaction, and its consequences, are discussed and evaluated. The role of the propionyl (C₂H₅CO) radical in the room temperature photolysis is also assessed. However, it is found that the Δ values determined by the intercept method used in this work are not affected by the secondary reactions that occur. It is concluded that high cross-combination ratios are general to perfluoroalkyl-alkyl radical interactions. For C₃F₇ and C₂H₅ radicals the ratio is 2.7–2.8. Above 100°C ratios exceed 3 due to secondary reactions.     

The mechanism and rate parameters for the pyrolysis of n-hexane in the range 723–823 K

The pyrolysis of n-hexane has been investigated in the ranges 723–823 K and 10–100 Torr at up to 3% decomposition. The reaction is homogeneous and free from the self-inhibition by olefin products observed for several other alkanes. The products of the reaction are hydrogen, methane, ethane, ethene, propene, but-1-ene, and pent-1-ene, with smaller amounts of propane. It is shown that the results are in quantitative agreement with a conventional Rice-Herzfeld chain mechanism terminated by the combination and disproportionation of ethyl radicals, but with the mechanism extended so as to include the unimolecular isomerizations via a six-membered cyclic transition state between 1-hexyl and 2-hexyl (1-methylpentyl) radicals. The overall rate constant of initiation is estimated to be given by The rate constant for the reaction is given by which when combined with published data gives an Arrhenius plot curved upwards at low values of 1/T as has been observed for several other hydrogen abstraction reactions of methyl and of ethyl. Estimates are made of rate constants and ratios of rate constants for several reactions of the free radicals involved in the reaction. It is suggested that the minor product propane arises mainly from a hydrogen abstraction by 1-propyl from hexane with a contribution from a minor termination process involving ethyl and methyl.     

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 2-propyl radicals in the liquid phase in the presence and absence of oxygen

2-Propyl radicals have been generated from the photolysis of solutions of 2,2-azopropane and 2,4-dimethyl-3-pentanone in decane in a glass and a metal cell. The time course of their reactions in the presence and absence of oxygen has been monitored between 323 and 373 K. The primary process involves the formation of solvent-caged radical pairs, two 2-propyl radicals and a 2-propyl and a 2-methylpropanoyl radical from the azo and ketone precursors, respectively. Subsequently these radicals are partitioned between cage escape and dimerization and disproportionation within the cage. In oxygenated solution the free 2-propyl radicals are effectively trapped as 2-propylperoxyl radicals. However, oxygen does not react with the solvent-caged radicals. This leads to a major difference in the hydrocarbon products from the two precursors. 2,2′-Azopropane gives propane, propene, and 2,3-dimethylbutane from the start of the reaction whereas the ketone only gives propene. Following the depletion of oxygen or in the absence of oxygen, both precursors behave analogously and give all three hydrocarbons. The 2-propylperoxyl radicals undergo self-reaction and hydrogen abstraction from the solvent to give 2-propanol, propanone, and 2-propyl hydroperoxide and, under conditions of low oxygen concentration, by reaction with 2-propyl radicals they give 2,2′-dipropylperoxide. Although the two cells lead to different overall rates of reaction, the relative rates and product distributions are unaffected by the cell design. A unified mechanism is described and the known and best estimates of rate constants for the individual steps are used to simulate the time dependence of the product yields from the photolysis of both precursors. © 1996 John Wiley & Sons, Inc.     

Structure,thermal stability and decomposition of bis-allyl-zinc compounds

The structure, thermal stability and decomposition of solutions of diallylzinc (I), bis(2-methylallyl)zinc (II), bis(3-methylallyl)zinc (III) and bis(3,3-dimethylallyl)zinc (IV) in deuterated solvents, have been investigated by¹H NMR and by kinetic measurements at temperatures between ?125 and +180°C. At room temperature I, II, III and IV are dynamic systems and are best described as being rapidly equilibrating mixtures of all isomeric σ-allyl forms; the NMR spectra are averages weighted according to the relative concentrations of the respective forms. I displays a¹H NMR spectrum of a static σ-allyl system only below ?125°C and II only below ?115°C. At temperatures above 100°C the thermal decomposition of I–IV results in coupling of the allyl groups, decomposition via radicals being the major process. The coupled products exhibit CIDNP, in which the multiplet polarisations confirm a decomposition via randomly diffusing allyl radicals. In the allyl radicals CH₂CR¹CR²R³ an alternating spin density was proved experimentally. The thermal stability decreases in the order I > II > III > IV.     

Nachweis von π → σ-Umlagerungen bei Ligandenverdrängungsreaktionen von π-Allyl-π-cyclopentadienyl-Palladiumkomplexen

It has been proved by NMR. measurements at low temperatures that the ligand displacement reactions of (π-all)Pd(π-C₅H₅) and Lewis bases L yielding PdL₄ proceed by a π → σ rearrangement of the allylic group as the primary step. The organic reaction product is the 1-isomer of the corresponding allylcyclopentadiene but in the reactions of (π-1,1,2-Me₃C₃H₂)Pd(π-C₅H₅) with L besides the isomeric allylcyclopentadienes also 2,3-dimethylbutadiene and cyclopentadiene are formed. The reaction mechanism will be discussed.     

A calculation of thermal degradation initiated by random scission,unsteady radical concentration

Changes in molecular weight distribution and in sample volume were calculated for thermal degradation of a polymer. The thermal degradation scheme consists of random scission initiation, depropagation and disproportionation termination reactions. An unsteady radical concentration was considered. There are two parameters, normalized zip length z/x₀ and radical number per initial chain length z′x₀, describing the thermal degradation scheme with an unsteady radical concentration. The effects of the initial number-average molecular weight and order of the disproportionation termination reaction on changes in molecular weight, the sample volume and polydispersity are not significant as long as these two parameters have the same value for each polymer sample. Molecular weights of a degrading sample calculated from the steady state radical concentration tend to be over-estimated and sample volumes tend to be underestimated compared to those calculated with an unsteady radical concentration. The validity of approximations used in the calculation assuming a steady state radical concentration is examined by comparing with results calculated with an unsteady radical concentration for various values of the two parameters. An unrealistically large build-up of monomer radicals is found for both calculations based on the steady state and the unsteady radical concentrations. Two special treatments of monomer radicals can dissipate the build-up of monomer radicals: (1) their immediate vaporization, or (2) an enhanced rate of the termination reaction for the monomer radicals. As a guide, the model based on an unsteady radical concentration is preferred, if the value of z′x₀ exceeds 0.1.     

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.     

Thermal degradation of poly(alkyl acrylates). II. Primary esters: Ethyl,n-propyl,n-butyl,and 2-ethylhexyl

Quantitative analyses of the products of thermal degradation of poly(ethyl acrylate), poly(n-propyl acrylate), poly(n-butyl acrylate) and poly(2-ethylhexyl acrylate) have been made, principally by the combined application of GLC and mass and infrared spectroscopy. Data are recorded in mass balance tables. The major gaseous products are carbon dioxide and the olefin corresponding to the ester group. The minor gaseous products include the corresponding alkane, the alkane/olefin ratio being of the order of 10^?2–10^?3, and traces of carbon monoxide and hydrogen. The alcohol corresponding to the alkyl group is the major liquid product but there are also traces of monomer and the corresponding methacrylate. Alcohol production exhibits autocatalytic properties. The chain fragment fractions of the products are colored yellow and have average chain lengths of 3.2, 3.3, 3.6, and 5.6 for the ethyl, n-propyl, n-butyl and 2-ethylhexyl esters, respectively. The infrared spectra are similar to those of the parent polymers but with well defined differences. Insolubility develops in the ethyl, n-propyl, and n-butyl esters, but the residual material from poly(2-ethylhexyl acrylate) remains soluble even at very advanced stages of degradation. All of these products and reaction characteristics are accounted for in terms of radical reactions with a unique initiation step.     

Transition states for deactivation reactions in the modeled 2,2,6,6‐tetramethyl‐1‐piperidinyloxy‐mediated free‐radical polymerization of acrylonitrile

The geometries and energetics of transition states (TS) for radical deactivation reactions, including competitive combination and disproportionation reactions, have been studied for the modeled 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO)‐mediated free‐radical polymerization of acrylonitrile with quantum mechanical calculations at the DFT/UB3‐LYP/6‐311+G(3df,2p)//(U)AM1 level of theory (where DFT is density functional theory, AM1 is Austin model 1, and UAM1 is unrestricted Austin model 1). A method providing reasonable starting geometries for an effective search for TS between the TEMPO radical and 1‐cyanopropyl radical mimicking the growing polyacrylonitrile macroradical is shown. For the hydrogen atom abstraction reaction by the TEMPO radical from the 1‐cyanopropyl radical, practically one TS has been found, whereas for the combination reaction of the radicals, several TS have been found, mainly differing in out‐of‐plane angle α of the N? O bond in the TEMPO structure. α in the TS is correlated with the activation energy, ΔE, determined from the single‐point calculation at the DFT UB3‐LYP/6‐311+G(3df, 2p)//UAM1 level for the combination reaction of CH₃AN· with the TEMPO radical. The theoretical activation energy for the coupling reaction from DFT UB3‐LYP/6‐311+G(3df, 2p)//UAM1 calculations has been estimated to be 11.6 kcal mol^?1, that is, only about 4.5 times smaller than ΔE for the disproportionation reaction obtained with the DFT UB3‐LYP/6‐311+G(3df, 2p)//(U)AM1 approach. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 914–927, 2006     

Substituent effects on the disproportionation—combination rate constant ratios for gas‐phase halocarbon radicals,Part 5: Reactions of CF3 + CF3CH2CHCH3 and CF3CH2CHCH3 + CF3CH2CHCH3

Rate constant ratios, k_d/k_c for the disproportionation/combination reaction have been measured as 0.07 ± 0.02 when an H is removed from the CH₂ position of the CF₃CH₂CHCH₃ radical and as 0.24 ± 0.03 when the H is removed from the CH₃ position in the reaction with the CF₃ radical. For the self‐reaction between two CF₃CH₂CHCH₃ radicals, k_d/k_c has been measured as 0.27 ± 0.03 when the H is removed from the CH₂ position and as 0.47 ± 0.04 when the H is removed from the CH₃ position. The branching fraction, corrected for the number of hydrogens at each site, is 0.70 favoring the methyl position when the acceptor radical is CF₃ and 0.54 when CF₃CH₂CHCH₃ is the acceptor radical. Branching fraction results show that the CF₃ substituent on the CF₃CH₂CHCH₃ radical hinders disproportionation when CF₃ is the acceptor radical. When the accepting radical is CF₃CH₂CHCH₃ the CF₃ substituent may slightly impede the disproportionation reaction, but the branching ratio is nearly statistical. The effect of substituents on the donor radical, CF₃CH₂CHX, will be discussed for the series X = H, CF₃, Cl, and CH₃ when the acceptor radical is CF₃. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 549–557, 2001     

Branch selective allylation of acetylacetone catalyzed by Water-soluble rhodium complex catalyst

[RhCl(1,5-cod)]₂/PTA is highly active branch-selective catalyst for C-allylation of acetylacetone by allylic carbonates in water/ethyl acetate biphasic system, where either crotyl or 1-methylallyl carbonates gives sterically congested branched allylation product, suggesting formation of a common allylrhodium intermediate.     

Alkyl halide/tertiary amine as novel initiators for free radical polymerizations of methyl methacrylate,methyl acrylate and styrene

A series of combinations of alkyl halide with tertiary amine such as ethyl α-bromophenylacetate/tris[2-(dimethylamino)ethyl)]amine (αEBP/Me₆TREN), ethyl 2-bromoisobutyrate/triethylamine (EBiB/TEA), and ethyl 2-chloropropionate/N,N,N′,N′,N′′-pentamethyldiethylenetriamine (ECP/PMDETA) have been developed as novel free radical initiators and used for the polymerizations of methyl acrylate (MA), methyl methacrylate (MMA) and styrene (St). The effects of the structure of alkyl halide and tertiary amine on the polymerization of MA were investigated. Gel permeation chromatograph (GPC) and proton nuclear magnetic resonance (¹H NMR) have been utilized to analyze the end group of the obtained poly(methyl acrylate). Electron spin resonance (ESR) spectroscopy was employed to identify the structure of the radicals produced by αEBP/Me₆TREN, and the results indicated that αEBP reacted with Me₆TREN via a single electron transfer (SET) nucleophilic mechanism to produce corresponding ethyl α-phenylacetate radicals which subsequently initiated the polymerization of MA. As both alkyl halide and tertiary amine are commercially available at low cost, non-explosive, and ease of use and storage in comparison with conventional azo, peroxide or persulfate initiators, the combination of alkyl halide and tertiary amine as a free radical initiator is promising for large-scale practical applications.