Radical-induced aromatic substitution between benzoyl peroxide and benzenediols in the gas phase characterized by mass spectrometry

A radical-induced aromatic substitution mechanism for the reaction between benzoyl peroxide and benzenediols in the gas phase was characterized by mass spectrometry. The benzoyloxy radical produced from the homolysis of benzoyl peroxide associates at its carbonyl group with the phenolic hydroxyl group. The pairing tendency of the unpaired electron on the oxygen of the radical induces electron transfer along the hydrogen bond, which results in the rupture of the O? H bond of the phenol and aromatic substitution at the ortho position of the benzoyloxy radical. Supporting evidence for the mechanism was obtained by isotope labelling.     

Non-ideality in vinyl polymerization—VI: Polymerization of methyl methacrylate initiated by benzoyl peroxide

The polymerization of methyl methacrylate in benzene was initiated by benzoyl peroxide and examined by kinetic analysis particularly from the point of view of primary radical termination. It is concluded that the velocity constant for dissociation of the benzoyloxy radical to give the phenyl radical is affected by the nature of the medium.     

Reaction of Acenaphthylene with the Benzoyloxy Radical

Acenaphthylene (ACN) has been polymerized at 60° C using benzoyl peroxide labeled with carbon-14 and tritium as the Lnitiator. End-group analyses show a very high proportion of benzoate groups among the incorporated initiator fragments. It is deduced that ACN is much more reactive than most other monomers toward the benzoyloxy radical. There is evidence that transfer to benzoyl peroxide is significant during the polymerization of ACN.     

Further studies of end groups derived from benzoyl peroxide

There has been a suggestion that, for polymerizations initiated by benzoyl peroxide, the use of benzene as diluent may cause uncertainties about the conclusions reached from studies of the end groups in the resulting polymers. It is now shown that no substantial changes are caused by the replacement of benzene by other solvents. This finding contributes to the conclusion that any capture of benzoyloxy radicals by benzene is only transitory and does not invalidate the deductions made from the results of analyses of polymers for benzoate and phenyl end groups derived from the peroxide. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 2955–2960, 1997     

Unravelling the ultrafast photodecomposition mechanism of dibenzoyl peroxide in solution by time-resolved IR spectroscopy

The ultrafast photo-fragmentation of dibenzoyl peroxide (DBPO) is studied using femtosecond UV excitation at 266 nm and mid-infrared broadband probe pulses to elucidate the dissociation mechanism. With the help of (13)C-labeled DBPO it was possible to unambiguously assign transient IR bands in the fingerprint region to the benzoyloxy radical. Our experiments show that the fragmentation is controlled by the S(1)-lifetime of DBPO and within 0.4 +/- 0.2 ps leads to a benzoyloxy/phenyl radical pair plus CO(2)via concerted bond breakage of the O-O and the phenyl-C(carbonyl) bond. 20% of the radical pairs geminately recombine to phenyl benzoate on a timescale of 70 ps.     

Chemisorbed benzoate-to-benzene conversion via phenyl radicals on Cu(110): kinetic observation of conformational effects

The adsorption and decomposition of benzoic acid on the Cu(110) surface has been investigated using temperature-programmed reaction (TPR) spectroscopy and scanning tunneling microscopy (STM). The benzoate species is found to exist in two conformations: a phase containing upright species at monolayer saturation and a phase containing many tilted species at lower coverages. Thermal decomposition begins to occur near 500 K, yielding benzene and CO2. It is found that phenyl radicals, generated preferentially from the tilted benzoate species, efficiently abstract H atoms from undecomposed benzoate species to produce benzene in a rate-controlling process with an activation energy of about 29 kcal/mol. Using deuterium atom substitution at the 4-C position on the benzoate ring, it is found that the hydrogen abstraction reaction is selective for 2-,3- and 5-,6-C-H bonds. This observation indicates that the mobile phenyl radical is surface bound and preferentially attacks C-H bonds which are nearest the Cu surface binding the benzoate species, either as an upright species or as a tilted species.     

The reactions of 2-vinylnaphthalene and 4-vinylbiphenyl with the benzoyloxy radical

Benzoyl peroxide, labelled with carbon-14 and tritium, has been used to initiate polymerizations of 2-vinylnaphthalene and 4-vinylbiphenyl. The numbers of benzoate and phenyl end-groups in the polymers have been compared. The results have been used for study of the competition between decarboxylation of the benzoyloxy radical and its reaction with monomer. Both monomers are appreciably more reactive than styrene towards the radical.     

The interaction between polymerization initiators and inhibitors in solutions—II. The reaction between benzoyl peroxide and p-benzoquinone in concentrated solutions; Some observations on the induced decomposition of the peroxide

The reaction between benzoyl peroxide and p-benzoquinone in concentrated solutions in a wide variety of solvents has been investigated by isolation and identification of the reaction products. Despite the high efficiency of p-benzoquinone as a trap for benzoyloxy radicals, partial decarboxylation to phenyl radicals usually occurs. Complete suppression of decarboxylation is achieved only when p-benzoquinone is present at such a high concentration that it is effectively the solvent for the reaction.The benzoyloxy- and phenyl semiquinones show marked differences in reactivity, the former tend to combine to form dibenzoyloxy dibenzoquinone while disproportionation is favoured by the latter to form quinhydrone of monophenylbenzoquinone.At lower quinone ratio, the peroxide undergoes induced decomposition by phenyl radicals both in “reactive” and “unreactive” solvents. The induced decomposition involves the formation of radical intermediates which undergo disproportionation, but not intramolecular rearrangement, to form p-phenylbenzoyloxy radicals. The latter can be captured, before undergoing decarboxylation, by the benzoyloxysemiquinones formed in the reaction.A correlation between the electron donating property of a radical and its capability to induce the decomposition of the peroxide was developed.     

Chemically induced dynamic nuclear polarization. Decarboxylation of photochemically generated benzoyloxy radicals

CIDNP has been studied during thermal decomposition, photolysis, and sensitized photolysis of benzoyl chloroacetyl peroxide. The ratio of the CIDNP intensities for the recombination products benzyl chloride and chloromethyl benzoate is dependent on the mode of decomposition, reflecting the extent of rapid decarboxylation of the primary formed benzoyloxy radicals.     

The benzoyl cation: The participation of isolated electronic excited states in the dissociation of molecular ions of the form [C6H5COX]+˙

The heat of formation of the benzoyl cation generated from [C₆H₅COX]⁺˙ is found to depend on X, while the heat of formation of the phenyl ion produced therefrom is, with one exception, independent of X. The excess energy of the benzoyl cation can be accounted for by an electronic excited state of the ion in the mass spectra of benzoic acid, benzaldehyde, benzamide, methyl benzoate and possibly benzophenone; the benzoyl cation is not excited in the mass spectra of acetophenone and benzoyl chloride.     

Two-photon chemistry in the laser jet: generation of radical intermidiates and their photochemical transformations

The novel laser jet technique provides sufficiently high photon densities to permit the observation of the photochemistry of photochemically generated radicals (two-photon chemistry) in the liquid phase. Four recent applications of this novel photochemically useful method are presented: these include the photochemistry of hydroxydiphenylmethyl, 9-hydroxyxanthenyl, diphenylmethyl, and benzoyl radicals under laser jet and normal photolysis conditions.

The regioselectivity of cross-coupling reactions of hydroxydiphenylmethyl or 9-hydroxyxanthenyl radicals with solvent-derived radicals changes when these species are electronically excited,i.e. under the high intensity conditions of the laser jet, cross-coupling at the para position (head-to-tail combination) is significantly enhanced relative to the normal coupling mode at the hydroxy-bearing radical site (head-to-head combination). Semiempirical calculations of the spin density distributions for the ground and first excited states of the radicals confirm the change in spin density from the hydroxy-bearing carbon atom to the conjugating benzene rings in these radical species on photoexcitation.

For the diphenylmethyl radical, two reaction pathways have been observed under the high photon densities of the laser jet: the electronically excited diphenylmethyl radical can either abstract a chlorine atom from carbon tetrachloride through an electron transfer process or can be photoionized on further photoexcitation (multiphoton chemistry). The resulting benzhydryl cation was trapped by methanol as the corresponding ether product, which unequivocally demonstrates that carbene formation by photoejection of a hydrogen atom does not take place under laser jet photolysis conditions.

An advantage of the high photon densities produced in laser jet photolysis is the high steady state concentration of short-lived transients that are generated, which enable unprecedented intermolecular reactions to be observed. Thus, about a millimolar concentration of tert-butoxy radicals can be obtained in the laser jet photocleavage of tert-butyl peroxide. When the tert-butoxy radicals are produced in the presence of benzaldehyde, the main product is tert-butyl benzoate. If carbon tetrachloride is also present, chlorobenzene can be detected. This is rationalized as the product derived from chlorine abstraction by phenyl radicals, which are presumably produced by the photodecarbonylation of benzoyl radicals.

An alternative method of obtaining benzoyl radicals is the two-photon cleavage of benzil. The laser jet photolysis of benzil in tert-butyl peroxide yields mainly tert-butyl benzoate, whereas in carbon tetrachloride, benzoyl chloride, chlorobenzene and ,,-trichloroacetophenone are observed. The first two products result from chlorine atom abstraction by the photochemically generated benzoyl and phenyl radicals, and the last product from in-cage cross-coupling between benzoyl and trichloromethyl radicals.

Such product studies provide detailed mechanistic information on the photochemical behaviour of electronically excited, short-lived transients which complements nicely the kinetic and spectral data of time-resolved laser flash studies. Consequently, the laser jet technique constitutes a valuable tool for determining the mechanism of two- photon reactions.     

Phenyl species formation and preferential hydrogen abstraction in the decomposition of chemisorbed benzoate on Cu(110)

The adsorption and decomposition of benzoic acid on the Cu(110) surface has been investigated using temperature-programmed reaction (TPR) spectroscopy and scanning tunneling microscopy (STM). The benzoate species is found to exist in two conformations--a phase containing upright species at monolayer saturation and a phase containing many lying-down species at lower coverages. Thermal decomposition begins to occur near 500 K, yielding benzene and CO(2). It is found that phenyl species, generated preferentially from the lying-down benzoate species, efficiently abstract H atoms from undecomposed benzoate species to produce benzene in a rate-controlling process with an activation energy of about 29 kcal/mol. Using deuterium-atom substitution at the 4-C position on the benzoate ring it is found that the hydrogen-abstraction reaction is selective for 2,3 and 5,6 C-H bonds. This observation indicates that the mobile phenyl species is surface bound and preferentially attacks C-H bonds which are nearest the Cu surface and bind the benzoate species as either an upright species or a tilted species.     

Transfer to benzoyl peroxide during the polymerization of p-methoxystyrene

Transfer to benzoyl peroxide is pronounced in the polymerization of p-methoxystyrene (MOS) at 60°. The process is responsible for the low molecular weights of polymers formed when the peroxide is used as initiator; there is no evidence for a non-radical polymerization of the type found with N-vinylcarbazole and the peroxide. Data on the reactivity of MOS towards the benzoyloxy radical and the copolymerization of MOS with methyl methacrylate are presented.     

The involvement of 2-vinylthiophene and 2-styrylthiophene in the radical polymerization of methyl methacrylate

The reactivity of 2-vinylthiophene (2VT) towards the 2-cyano-2-propyl radical and that of 2-styrylthiophene (2ST) towards the benzoyloxy radical have been assessed by methods depending upon examination by NMR of ¹³C-enriched end-groups in polymers and copolymers. At 60°C, 2VT is about 5 times as reactive as methyl methacrylate (MMA) towards (CH₃)₂C(CN); 2ST is nearly 300 times as reactive as MMA towards C₆H₅·CO·O. When MMA is polymerized with benzoyl peroxide in the presence of 2ST at quite low concentration, very little 2ST is incorporated within the polymer chains but many of the end-groups consist of benzoate groups attached to 2ST units.     

Die photolytische α-spaltung von benzoinalkyläthern in sauerstoffhaltigem methanol

The primary step in the photolytical α-cleavage of benzoin alkyl ether in oxygen saturated methanol at room temperature is the formation of a benzoyl (1) and an α-alkoxybenzyl radical (2), which react via their peroxi radicals 3 und 4 to the final products yielding perbenzoic acid (5), alkyl benzoate (7), benzaldehyde (11), benzaldehyde dimethylacetal (10) and benzil to 100%. Product analysis of the final products of the photolysis of specifically deuterated benzoin methyl ethers shows that methyl benzoate, benzaldehyde and benzaldehyde dimethylacetal are formed via the α-methoxybenzyl radical (2) while the perbenzoic acid results from the benzoyl radical (1). Both the peroxi radicals 3 and 4 have an independent reaction pathway to the final products. Hydrogen abstraction of 3 and 4 from the solvent methanol give rise to hydroxy methyl radicals which yields formic acid, hydrogen peroxide, performic acid and formaldehyde via their hydroxy methyl peroxi radicals. The reaction pathway of the primary radicals is discussed.     

Analysis of diacyl peroxides by Ag+ coordination ionspray tandem mass spectrometry: Free radical pathways of complex decomposition

Organic peroxides have significance in organic synthesis and biological processes. Characterization of these compounds with weak O-O bonds is sometimes difficult due to their thermal instability and sensitivity to acid or base. Coordination of diacyl peroxides with AgBF4 provides a means for analysis of these compounds by coordination ionspray tandem mass spectrometry (CIS-MS/MS). Precursor ion (Q1) scans of acetyl benzoyl peroxide give two Ag+ adducts, [M + Ag + solvent]+ and [M + Ag + M]+. These silver ion adducts can be selectively dissociated (CID) to give unique structural information about the analyte. Decomposition of the [M + Ag + solvent]+ adduct generates fragmentation products due to apparent homolytic cleavage of the O-O bond followed by decarboxylation of the resultant radicals. The bis-diacylperoxide complex, [M + Ag + M]+ gives CID pathways that involve homolysis of the (O-O bond and free radical cross-coupling of the two diacyl peroxides coordinated to the silver ion, i.e. formation of dibenzoyl peroxide, phenyl benzoate, and biphenyl from acetyl benzoyl peroxide. The observation of free radical CID modes is uncommon in mass spectrometry but these pathways are consistent with well-known solution and gas phase processes for peroxide compounds. The proposed fragmentation pathways have been supported by experiments with (18)O and deuterated substrates. This technique can be applied to analyze diacyl peroxides with different substituents as well.     

Biochemical Studies on Hemoglobin Modified with Reactive Oxygen Species (ROS)

Hemoglobin is the iron-containing oxygen transporting metalloprotein in the red cells of blood in mammals and other animals. Hemoprotein-mediated oxidative stress is thought to play a major role in pathophysiology of cerebral hemorrhage, blast pressure injury, crush injury, myocardial ischemia reperfusion injury. Hemoglobin undergoes oxidation-reduction reactions that lead to both generation and consumption of highly reactive oxygen and nitrogen species. In the present study, hemoglobin molecule was treated with hydrogen peroxide and the modification so incurred was analyzed by UV spectra, sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and detection of carbonyl content. Our observations suggest that carbonyl content increases with increase in concentration of hydrogen peroxide. Production of hydroxyl radical was assessed by using benzoate degradation analysis. Our results was in tandem with the fact that hemoglobin on treatment with hydrogen peroxide rapidly generates free-radical species that can degrade benzoate to thiobarbituric acid reactive material which on reacting with thiobarbituric acid gives color. The increase in absorbance of ROS-modified hemoglobin at 532 nm shows the increase in benzoate degradation, which is a parameter of hydroxyl radical formation with increase in concentration of hydrogen peroxide. Modified hemoglobin was treated with catalase, mannitol, thiourea, glutathion, sodium benzoate and their effect were detected by spectroscopy and SDS-PAGE (12%). Substantial scavenging effect of aforementioned antioxidants reiterates the formation of hydroxyl radical. Catalase shows the maximum scavenging effect followed by thiourea and mannitol.     

Chemical polarization of nuclei. PMR spectra of decomposition products of dimethyl peroxydiphthalate

Chemical polarization of protons was observed in methyl benzoate formed during the thermal decomposition of dimethyl ester of peroxydiphthalic acid. The polarization pattern of methyl benzoate aromatic protons was very different in this case from that observed during the thermal decomposition of acetyl benzoyl peroxide. The unpolarized products formed from the methoxy radical, CH₂O and CH₃OH, were found in the mixture of decomposition products of this peroxide and were identified by means of PMR spectroscopy.     

Homolytic reactions of polyfluoroaromatic compounds Part VI. The mechanism of the phenylation of hexafluorobenzene

The decomposition of phenylazotriphenylmethane in hexafluorobenzene gives very little 2,3,4,5,6-pentafluorobiphenyl, since no mechanism exists for the defluorination of the radical intermediate. The yield of biaryl is raised by added benzoic acid, although this also introduces a competition with the substrate for the radicals and their precursor. The decomposition of benzoyl peroxide in hexafluorobenzene gives pentafluorobiphenyl, the yield of which increases considerably when the reaction is caried out in the presence of p-fluorobenzoic acid; in the latter case, 2,3,4,4′,5,6-hexafluorobiphenyl is also found.A mechanism incorporating these observations is suggested.     

Some hydrogen abstractions involving the benzoyloxy radical

The reactivities of triphenylmethane and various penta-arylethanes towards hydrogen abstraction by the benzoyloxy radical are compared. The procedure involved study of the effects of these hydrocarbons upon the yields of carbon dioxide during the decomposition in benzene ofbenzoyl peroxide suitably labelled with carbon-14. Triphenylmethane is less reactive than pentaphenylethane towards the benzoyloxy radical but the reactivity difference is less than might have been predicted from the behaviour of the hydrocarbons with the polystyrene radical. The reactivities of the penta-arylethanes are correlated with the Taft σ^* constants for the substituents.