Cumyl Radicals Research Articles

Insights into the mechanism of cumene catalytic oxidation using ionic liquid [Bmim]OH

Experiment indicates that [Bmim]OH can effectively accelerate the oxidation reaction rate of cumene and improve the conversion of cumene. To investigate the catalytic effect of [Bmim]OH, the mechanism of molecular oxygen-activated [Bmim]OH-catalyzed oxidation reaction of cumene was proposed using the dispersion-corrected DFT-D3 calculation method. The nature of the catalytic effect of [Bmim]OH ionic liquid and the catalytic active site were investigated. The [Bmim]OH carbine-water structure plays an obvious catalytic role in the oxidation of cumene, which can induce the OO homolytic cleavage in cumyl hydroperoxide (CHP), leading to the formation of cumylhydroxy radicals (RO•) and carbene-OH radical-like intermediates, both of which are beneficial to the reactant with H of benzylic CH bond to accelerate the formation of cumyl radicals, involving in the propagation reaction that resulting in the formation of hydroperoxide and increasing the conversion of cumene. The overall reaction rate control step is the OO activation of CHP by [Bmim]OH carbine-water structure, which needs to overcome the energy barrier of 20.2 kcal/mol, lower than the auto-decomposition reaction energy barrier of CHP. The present results provide some new insights and understanding for the mechanism of the cumene oxidation catalyzed by ionic liquids.

The Excited Triplet State of Azoalkanes: Electron Spin Polarization and Magnetic Field Effects During Triplet-Sensitized Photolysis of trans-Azocumene in Solution

The triplet-sensitized photodecomposition of azocumene into nitrogen and cumyl radicals is investigated by time-resolved electron paramagnetic resonance and absorption spectroscopy. The radicals are found to be created spin polarized with a yield depending on the strength of the applied magnetic field. The phenomenon arises because in triplet azocumene, the decay into radicals competes with a fast triplet-sublevel selective intersystem crossing back to the azocumene ground state. The size of the initial spin polarization of the radicals and the magnetic field effect on their yield are determined in solvents of different viscosities. Data analysis yields rate constants for the intersystem crossing and the cleavage reaction of triplet azocumene as well as its zero-field splitting DZFS. At room temperature in nonpolar solvents, the most probable values are: kx = ky = 1.2 × 1011 s−1 and kz = 1.9 × 1010 s−1 for the intersystem crossing from the energetically lower and upper triplet substates, respectively, kp = 1.6 × 109 s−1 for the cleavage reaction and for the zero-field splitting DZFS = −3.4 × 1010 s−1 (0.18 cm−1).

Antioxidative Activity of Carbon Nanotube and Nanofiber

Carbon nanotubes (CNTs) and carbon nanofibers (CNFs) have electron affinities similar to those of fullerenes C60 and C70 and they are therefore capable of acting as radical scavengers in free radical chain reactions, including polym- erisation and the thermo-oxidative degradation of polymers. It is assumed that the CNTs and CNFs used as integral part of polymer composites are able to exhibit an antioxidant effect in these materials because of their radical accepting capacity. To examine this presumption the antioxidative activity of original and purified commercial multiwall carbon nanotube MWCNT and carbon nanofibre of platelet structure CNF-PL has been studied by means of a model oxidation reaction of cumene initiated (2,2'-azobisisobutyronitrile, AIBN) in liquid phase. This model reaction was designed to simulate the thermo-oxidative processes in carbon-chain polymers and allows comparison and transfer of obtained results to a polymer system. Kinetic measurements of oxidation rates showed that the effect of inhibition for the model oxidative reaction in the pres- ence of the original and purified MWCNT and CNF-PL strongly depends on the presence of metals (Co, Fe) in the nanoparticles. Rates of oxidation Wo2 (CNT;CNF) observed for the unrefined samples are result of the two competing rates - rate of inhibition Winh.(CNT; CNF) caused by structures of the CNT or CNF and the rates of initiation Wi(M) due to the follow- ing interaction: ROOH + M (Co;Fe) i.e, Wo2 ~ W inh (CNT;CNF) + Wi(M). The effective rate constants for the addition of cumyl radicals (R� ) to MWCNT and CNF-PL have been determined. These constants reduced to the same concentration (0.5wt.%) and temperature (60 o C) units have magnitudes: k1(MWCNT) (MWCNT) = (2.8 ± 0.3) � 10 4 s -1 and k1(CNF) (CNF) = (6.0 ± 1.0) � 10 3 s -1 . Thus, the effective rate constant, reflecting the antioxidative activity for the CNT, is five times higher than that for the CNF, is about equal to the rate constant for HAS Chimassorb 2020: k1(Chim.2020)(Chim. 2020) = (2.2 ± 0.3) � 10 4 s -1 , is ten times less than that for the HAS Chimassorb 119FL: k1(Chim.119FL)(Chim. 119FL) = (2.8 ± 0.3) � 10 5 s -1 and is about forty times less than that for the case of fullerene C60: k1(C60)(C60)(353K) = (1.2 ± 0.2) � 10 6 s -1 .

Radical scavenging efficiency of different fullerenes C 60–C 70 and fullerene soot

This investigation was undertaken to determine the antioxidant activity of a range of fullerenes C 60 and C 70 in order to rank them according to their comparative efficiency. The model reaction of initiated (2,2′- azobisisobutyronitrile, AIBN) cumene oxidation was used to determine rate constants for addition of radicals to fullerenes. Measurements of oxidation rates in the presence of different fullerenes showed that the antioxidant activity as well as the mechanism and mode of inhibition were different for fullerenes C 60 and C 70 and fullerene soot. All fullerenes – C 60 of gold grade, C 60/C 70 (93/7, mix 1), C 60/C 70 (80 ± 5/20 ± 5, mix 2) and C 70 operated as alkyl radical acceptora, whereas fullerene soot surprisingly retarded the model reaction by a dual mode similar to that for the fullerenes and with an induction period like many of the sterically hindered phenolic and amine antioxidants. For the C 60 and C 70 the oxidation rates were found to depend linearly on the reciprocal square root of the concentration over a sufficiently wide range thereby fitting the mechanism for the addition of cumylalkyl radicals to the fullerene core. This is consistent with literature data on the more ready and rapid addition of alkyl and alkoxy radicals to the fullerenes compared with peroxy radicals. Rate constants for the addition of cumyl radicals to the fullerenes were determined to be k (333K) = (1.9 ± 0.2) × 10 8 (C 60); (2.3 ± 0.2) × 10 8 (C 60/C 70, mix 1); (2.7 ± 0.2) × 10 8 (C 60/C 70, mix 2); (3.0 ± 0.3) × 10 8 (C 70), M −1 s −1. The increasing C 70 constituent in the fullerenes leads to a corresponding increase in the rate constant. The fullerene soot inhibits the model reaction according to the mechanism of trapping of peroxy radicals; the oxidation proceeds with a pronounced induction period and kinetic curves are linear in semi-logarithmic coordinates. For the first time the effective concentration of inhibiting centres and inhibition rate constants for the fullerene soot have been determined to be fn[C 60−soot] = (2.0 ± 0.1) × 10 −4 mol g −1 and k inh = (6.5 ± 1.5) × 10 3 M −1 s −1 respectively. The kinetic data obtained specify the level of antioxidant activity for the commercial fullerenes and scope for their rational use in different composites. The results may be helpful for designing an optimal profile of composites containing fullerenes.

Mechanism and selectivity of 2,3-dimethyl-2,3-diphenylbutane mediated addition of vinyltriethoxysilane to polyethylene

The thermolysis of 2,3-dimethyl-2,3-diphenylbutane (bicumene) at temperatures ranging from 220 to 310 °C is used to initiate the radical-mediated graft addition of vinyltriethoxysilane (VTEOS) to polyethylene. Model hydrocarbon studies indicate that the cumyl radicals generated by the slow decomposition of bicumene are capable of direct hydrogen atom abstraction at levels that are sufficient to sustain a graft propagation sequence of high kinetic chain length. The interaction of O 2 with cumyl radicals can lead to oxidation of the initiator and the hydrocarbon substrate, thereby enhancing the macroradical population and improving grafting rates and yields. In addition to providing remarkable kinetic chain lengths for VTEOS additions, high-temperature bicumene-based processes can induce HDPE and LDPE fragmentation such that the effects of radical combination on melt viscosity are counteracted. As a result, alkoxysilane-modified polymers that moisture-cure efficiently can be produced without incurring the undesirable increases in molecular weight that accompany conventional grafting processes.

Study of the coupling reactions between electrochemically generated aromatic radical anions and methyl, alkyl and benzyl radicals

Alkyl radicals produced in the indirect reduction of alkyl halides or alkyldimethylsulfonium salts by electrochemically generated aromatic radical anions couple fast with the latter and alkylated or dialkylated dihydro compounds are formed. Rate constants measured for the coupling reaction between on one hand methyl, primary, secondary and tertiary alkyl radicals as well as benzyl and cumyl radicals and on the other hand a wide spectrum of electrochemically generated aromatic radical anions are found to be about 1×10 9 M −1 s −1. Previous measurements of coupling rate constants for primary alkyl radicals have been re-evaluated since they were affected by the presence of an S N2 reaction occurring between the alkyl halides used as radical precursors and the aromatic radical anions. New experiments are also included using alkyldimethylsulfonium salts as precursors in order to prevent such S N2 artefacts. It is concluded that sterical hindrance does not play a significant role for the radical–radical anion coupling reactions. In general the rate constants for the coupling reactions are all close to 10 9 M −1 s −1.

2,3-Dimethyl-2,3-diphenylbutane mediated grafting of vinyltriethoxysilane to polyethylene: a novel radical initiation system

The melt grafting of vinyltriethoxysilane (VTEOS) to polyethylene using 2,3-dimethyl-2,3-diphenylbutane (bicumene) as a radical initiator is demonstrated and compared to conventional peroxide-initiated processes. A bicumene-initiated modification of high density polyethylene at 290 °C provided no benefits in terms of selectivity when compared to a standard peroxide-based process operating at 180 °C. However, the selectivity of linear low-density polyethylene modification was influenced by chain scission, which counteracted the molecular weight effects of macroradical combination. Reactive extrusion data as well as studies of bicumene decomposition kinetics suggest that initiation occurs through an auto-oxidation mechanism that is facilitated by the interaction of cumyl radicals with oxygen.

Magnetic field dependence of the deactivation rates of triplet azocumene in solution

The triplet sensitized photo-decomposition of azocumene into nitrogen and cumyl radicals is investigated by time resolved EPR and optical absorption spectroscopy. It is found that the cumyl radicals carry an initial spin polarization and are formed with a yield that depends on both the solution viscosity and the strength of an external magnetic field. The phenomenon is interpreted in terms of a depopulation-type triplet mechanism, i.e. a competition between decay into radicals and fast, triplet sublevel selective intersystem crossing (ISC) back to the azocumene ground state. Analysis of the data yields relative rate constants for the ISC processes and the cleavage reaction of triplet azocumene. The energetically lower zero field triplet substate is depopulated by ISC about seven times faster than the upper one and about two orders of magnitude faster than depopulation by cleavage occurs. Cleavage probably takes place on the nanosecond time scale, while the ISC must proceed on the picosecond scale, as at elevated viscosity it becomes faster than the rotational Brownian motion of the molecule.

Fullerene C 60 as an antioxidant for polymers

The antioxidative activity of fullerene C 60 has been studied by model reaction of the initiated oxidation of cumene and also in accelerated tests of the mixtures with polystyrene and polydimethylsiloxane rubber. It was established that the initiation rate of the model oxidation is substantially reduced in the presence of C 60. The rate constant for the addition of cumyl radicals to C 60 was determined as k (333–353 K)=(2.0 ±0.8)·10 8 M −1 s −1. In accelerated tests fullerene showed a stabilizing effect comparable with the influence of the sterically-hindered phenol Irganox1076.

The Importance of Spin Polarization in Electronic Substituent Effects of the Zero-Field EPR D Parameter in 1,3-Diarylcyclopentane-1,3-diyl Triplet Diradicals

The zero-field-splitting D parameter of 11 meta,para-disubstituted 1,3-diarylcyclopentane-1,3-diyl triplet diradicals has been determined in a 2-MTHF glass matrix at 77 K by EPR spectroscopy. The experimental ΔDexp value, conveniently defined as DH − DX, is a sensitive function of the delocalizing property of the X substituent on the phenyl ring. In contrast to the previously observed additivity for a large variety of para and meta substituents (cf. Adam, W.; Harrer, H. M.; Kita, F.; Nau, W. M. Adv. Photochem. 1998, 24, 205−254), such a simple relation no longer upholds for meta,para disubstitution. For the latter, the higher degree of spin delocalization is accounted for in terms of spin polarization and electronic field effects. For the first time also the increased spin density at the cumyl position caused by meta substituents has been satisfactorily explained by means of spin polarization. The spin density distribution in the cumyl radicals, computed by density functional theory (B3LYP/6-31g*), substa...

Cleavage Reactions of Radical Anions that Range from Homolytic to Heterolytic within the Same Family of Compounds

Properties of the radical anions of three α-nitrocumenes (α-nitrocumene, p-cyano-α-nitrocumene, and p-nitro-α-nitrocumene, 1a−c) have been determined by electrochemical methods. In particular, the standard potentials of the neutral/radical anion couples were found to be −2.20, −2.04, and −1.43 V with respect to the ferrocenium ion/ferrocene potential and the rate constants for expulsion of nitrite from the radical anions were 3 × 106, 5 × 106, and 240 s-1 for 1a−c, respectively. Comparison of these potentials with those of related compounds demonstrates that reduction of the nitroalkyl portion of the molecule occurs in 1a and 1b while in 1c the electron is added to the nitrophenyl group. Thus, using previously defined terminology, the cleavage of the radical anions of 1a and 1b to give nitrite and the corresponding cumyl radicals are examples of homolytic cleavage reactions but the cleavage of the radical anion of 1c is heterolytic. The driving force for the three cleavage reactions has been estimated and it is concluded that the large decrease in magnitude of the cleavage rate constants on going from 1a and 1b to 1c is mainly due to the much larger driving force for the first two.

The radical-induced decomposition of 2-methoxyphenol

The thermolysis of 2-methoxyphenol has been studied between 680 and 790 K, applying cumene as a radical scavenger. Two pathways were observed: a homolytic route involving the cleavage of the methoxyl O–C bond, leading to methane and 1,2-dihydroxybenzene, obeying kuni (s–1) = 1015.2 ± 0.2 exp(–239 ± 8 (kJ mol–1)/RT), and an induced route starting with abstraction of the phenolic hydrogen by cumyl radicals. After intramolecular hydrogen transfer a cascade of reactions yields phenol, 2-hydroxybenzaldehyde, and 2-hydroxybenzyl alcohol. The latter compound decomposes instantaneously into o-quinone methide (o-QM) which is reduced to o-cresol.

ChemInform Abstract: Additivity of the Electronic meta‐Substituent Effect in 3,5‐Disubstituted Cumyl Radicals Assessed by the EPR D Parameter of 1,3‐Arylcyclopentane‐1,3‐diyl Triplet Diradicals.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Additivity of the Electronic Meta-Substituent Effect in 3,5-Disubstituted Cumyl Radicals Assessed by the EPR D Parameter of 1,3-Arylcyclopentane-1,3-diyl Triplet Diradicals.

The D parameter (EPR zero-field splitting) of the 3,3',5,5'-tetrasubstituted triplet diradicals 6 (X = X' = H, NO(2), CH(3), OAc, OCH(3), NH(2), and OH) were determined in a MTHF matrix at 77 K and serves as a spectroscopic tool for the determination of electronic substituent effects in multiply-substituted benzyl-type monoradicals through its spin density dependence. The linear correlation (m = 2.00 +/- 0.01, r(2) = 0.974) of the experimental D values of these meta-disubstituted triplet diradicals 6 versus the 3,3'-disubstituted triplet diradicals 5 demonstrates the additivity of the electronic meta-substituent effect in the corresponding 3,5-disubstituted cumyl monoradicals 4 and is corroborated by theoretical (PM3-AUHF) spin density calculations for the latter. Thus, the combined use of the experimental D parameter and semiempirically calculated alpha spin density has provided for the first time the unambiguous demonstration of the additivity of electronic effects exerted by meta-substituents in the cumyl monoradicals 4.

Cumyl phenyl sulfide forms bicumyl instead of cumyllithium upon reductive lithiation. Thiophenoxide as a leaving group in nucleophilic substitution by SET

Upon reduction with aromatic radical-anions, cumyl phenyl sulfide ( 1 S ) yields mainly bicumyl ( 2) rather than the expected cumyllithium ( 3), despite a literature report, herein revealed to be in error, that anion 3 is produced. Related examples from the literature are compiled. A suggested mechanism involves a single electron transfer from the generated cumyllithium ( 3) to the cumyl phenyl sulfide ( 1 S ) leading to thiophenoxide anion and two cumyl radicals, which mainly couple in a solvent cage. Such a thiophenoxide displacement by anion 3 has been demonstrated experimentally. Para tert-butyl groups, either on the phenyl or cumyl ring, increase the ratio of the cumyllithium to the bicumyl, presumably by sterically inhibiting the pi complexation thought to be necessary for electron transfer.

Quantitative time-resolved EPR CIDEP study of the photodecomposition oftrans-azocumene in solution

The cumyl radical system, which is created after laser flash irradiation oftrans-azocumene in benzene solution at room temperature, is investigated using time-resolved EPR spectroscopy. From the quantitative analysis of EPR time-profiles at different microwave powers the spin relaxation timesT 1=3.5±0.3 μs andT 2=2.5±0.1 μs are evaluated as well as the magnitude of the chemically induced electron polarization (CIDEP), which is generated by the radical pair mechanism (RPM). The geminate RPM polarization is found to be considerably smaller than the F-pair one, 32±2 and 48±5 in units of the Boltzmann polarization, respectively. This is attributed to an initial radical separation in the geminate pair, caused by the cleavage reaction. Besides cleavage, the photoexcitedtrans-azocumene also decays via isomerization to the thermally unstablecis-isomer, the lifetime of which is found to be 14±3 μs at 293 K in benzene, three times longer than in cyclohexane. The quantum yield of free radicals, escaping from the primary cage, is determined as 0.28±0.06 for the decay of the excitedtrans-azocumene and 0.18±0.04 for the thermal cleavage of thecis-isomer. The self-termination of cumyl radicals proceeds with a rate constant 2k t=7±1)·108 M−1s−1 in benzene at RT.

New Method in Free Radical Chemistry Using 2,4-Diphenyl-4-methyl-1-pentene as Radical Trapping Agent

The thermal decomposition of four dialkyl peroxides was carried out in 2,4-diphenyl-4-methyl-1-pentene (α-methylstyrene dimer, MSD) at 140°C. Free radicals generated from the peroxides efficiently added to the double bond of MSD, and the adduct radicals underwent fragmentation to give olefinic compounds (1) and cumyl radicals. The characterization of 1 provided useful information about the free radical species that added to the styrenic double bond of MSD. The present method using MSD as a radical trapping agent is useful for investigating the initiation mechanisms of organic peroxides even at high temperature.

Electronic Effects of para- and meta-Substituents on the EPR D Parameter in 1,3-Arylcyclopentane-1,3-diyl Triplet Diradicals. A New Spectroscopic Measure of α Spin Densities and Radical Stabilization Energies in Benzyl-Type Monoradicals

The zero-field splitting D parameter was determined in a 2-MTHF glass matrix at 77 K for a large set (35 derivatives) of para- and meta-substituted 1,3-arylcyclopentane-1,3-diyl triplet diradicals 6. The D values are a sensitive function of electronic substituent effects; for convenience, the ΔD scale was defined as the difference DH − DX. Spin acceptors decrease while spin donors increase the D value relative to the unsubstituted reference system (DH). Theoretical (PM3-AUHF) α spin densities (ρα) for the corresponding cumyl monoradicals 7 display a good linear dependence (r2 = 0.963) when plotted against the D parameters of the triplet diradicals 6. The radical stabilization energies (RSE) of the cumyl radicals 7 were semiempirically calculated as the energy difference between in-plane (full conjugation) and perpendicular (no conjugation) conformations of the aryl groups and shown to correlate linearly (r2 = 0.947) against the experimental D parameter for the corresponding triplet diradicals 6. These lin...

Localized triplet diradicals as a probe for electronic substituent effects in benzyl-type radicals: The ΔD scale

Abstract

ChemInform Abstract: Absolute Rate Constants for the Addition of Benzyl (PhC.H2) and Cumyl ( PhC.Me2) Radicals to Alkenes in Solution.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.