Reactions of vinyl ethers with 2‐cyano‐2‐propyl and benzoyloxy radicals

tert‐Butyl, cyclohexyl, n‐propyl, and n‐dodecyl vinyl ethers have been used as comonomers with styrene and methyl methacrylate using ¹³C‐enriched samples of azobis(isobutyronitrile) and benzoyl peroxide as initiators at 60°C. Examination by ¹³C‐NMR spectroscopy of either (¹³CH₃)₂C(CN) or Ph¹³COO end‐groups in the products has shown that the vinyl ethers have low reactivities toward the 2‐cyano‐2‐propyl radical but high reactivities toward the benzoyloxy radical. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 771–777, 1999     

The rate of decarboxylation of the benzoyloxy radical as determined by CIDNP

Analysis of the CIDNP spectrum of ethylbenzoate (BzOEt) and ethylbenzene (PhEt) produced as recombination products of the decomposition of benzoylpropionyl peroxide is reported. Mathematical expressions for the yields and CIDNP intensities of both products are derived using an extension of the radical pair model. Two methods are employed to estimate the decarboxylation rate constant, k, for the benzoyloxy radical: (a) comparison of enhanced and unenhanced intensities of the PhEt quartet, and (b) comparison of the CIDNP intensities of the quartets from BzOEt and PhEt. The two methods give estimates of k agreeing within a factor of three. The lower limit to the value of k so obtained at 130°C in o-dichlorobenzene solution is 1 × 10⁸ s^?1. This is 2 to 3 orders of magnitude larger than previous estimates and implies that the lifetime of benzoyloxy radical may be considerably shorter than usually assumed.     

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.     

Polarisation nucleaire induite chimiquement: etude comparative de la decomposition thermique et photochimique de peroxydes d'acyle benzoyle

A comparative CIDNP study of the thermal and photochemical decomposition of acyl benzoyl peroxides PhCO₂O₂CR (R = methyl and isobutyl) has provided an explanation of the effect of the mode of initiation on the course of the decomposition of this class of peroxides and on the product distribution. The observed variations result from an important decrease (from 10^?8 sec to less than 10^?10 sec) in the lifetime of the benzoyloxy radical on changing from thermal to photochemical initiation.     

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.     

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.     

Reactivities of aromatic compounds towards the amino radical

The reaction between hydroxylamine and a reducing ion such as V(III) or Ti(III) was studied at the dropping mercury electrode in the pH range 0–7 in presence of a substrate as benzenesulfonic acid or phenol.A method has been developed to calculate, from measurements of catalytic currents, the competition between the reducing ion and the benzene derivative for the amino radical. The strong change in the reactivities observed around pH 3 was interpreted in terms of protonation of the amino radical.The relative reactivities of the phenol and benzenesulfonic acid towards the NH·₃⁺ and NH·₃⁺ radicals were measured; assuming for the pK_a of the NH·₃⁺ radical the literature value of 3.65 the relative rate of addition of the charged and uncharged amino radical to the same aromatic substrate was obtained. An interpretation of the experimental results in terms of inductive effect is given.     

Hydrogen atom abstraction from polypropylene and polystyrene by t-butoxy radical site of radical attack studied by spin trapping

In order to elucidate the site of radical attack on polypropylene and polystyrene, the abstractions of hydrogen atoms by t-butoxy radicals and phenyl radicals have been studied by using a spin trapping technique. The t-butoxy radical abstracted tertiary hydrogen atoms selectively from polypropylene, polystyrene and model compounds. On the other hand, the tertiary hydrogens in polypropylene and its model compounds were less reactive towards the phenyl radical than the secondary hydrogens within the same molecule and the secondary hydrogens in polystyrene were abstracted predominantly by the phenyl radical. The conformational effects on the reactivities of various hydrogens in polypropylene and model compounds were found to be similar.     

Radical reactions involving esters of fumaric acid—I. Copolymerizations

¹⁴C-labelled samples of diethyl-, ethyl n-hexyl- and di-n-hexyl fumarates have been used in studies at 60° of their radical copolymerizations with styrene and with vinyl acetate. Changing the diluent from benzene to dimethylformamide in some of the copolymerizations had no effect on the compositions of the copolymers. Monomer reactivity ratios have been calculated for most of the systems; comparisons have been made of the reactivities of the fumarate esters towards the polyvinyl acetate radical.     

Chemical ionization mass spectrometry of benzoyl peroxide: A radical aromatic substitution resulting in biphenylcarboxylic acid in the gas phase

A radical aromatic substitution resulting in biphenylcarboxylic acid is inferred for the decomposition of benzoyl peroxide from the chemical ionization and collision-induced dissociation mass spectra. The thermolysis of benzoyl peroxide gives rise to a benzoyloxy radical, which undergoes rapid decarboxylation and hydrogen abstraction leading to phenyl radical and benzoic acid, respectively. Attack of the resulting phenyl radical on the benzoic acid results in biphenylcarboxylic acid. On the other hand, the phenyl radical abstracts a hydrogen atom to yield benzene, which is then subjected to the attack of a benzoyloxy radical, affording phenyl benzoate. This substitution reaction rather than the recombination of benzoyloxy and phenyl radicals is found to be responsible for the formation of phenyl benzoate under the present conditions.     

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.     

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.     

Copolymerizations involving methyl methacrylate,methyl acrylate,stilbene and p.fluorostilbene

Homopolymerizations and copolymerizations of methyl methacrylate (MMA) and methyl acrylate (MA) have been performed in the presence of either stilbene or p.fluorostilbene, labelled with carbon-14. It has been shown that at 60°C the polyMA radical is much more reactive than the polyMMA radical towards stilbene, the velocity constants for the reactions differing by a factor of about 300; similar results are found for p.fluorostilbene. The differences between the reactivities of the radicals are associated with steric effects arising from the presence of a methyl group at the alpha position in MMA.     

Structure and reactivity of the cysteine methyl ester radical cation

The structure and reactivity of the cysteine methyl ester radical cation, CysOMe^.+, have been examined in the gas phase using a combination of experiment and density functional theory (DFT) calculations. CysOMe^.+ undergoes rapid ion–molecule reactions with dimethyl disulfide, allyl bromide, and allyl iodide, but is unreactive towards allyl chloride. These reactions proceed by radical atom or group transfer and are consistent with CysOMe^.+ possessing structure 1 , in which the radical site is located on the sulfur atom and the amino group is protonated. This contrasts with DFT calculations that predict a captodative structure 2 , in which the radical site is positioned on the α carbon and the carbonyl group is protonated, and that is more stable than 1 by 13.0 kJ mol^?1. To resolve this apparent discrepancy the gas‐phase IR spectrum of CysOMe^.+ was experimentally determined and compared with the theoretically predicted IR spectra of a range of isomers. An excellent match was obtained for 1 . DFT calculations highlight that although 1 is thermodynamically less stable than 2 , it is kinetically stable with respect to rearrangement.     

Measurement of the reactivity of OH with methyl nitrate: Implications for prediction of alkyl nitrate-OH reaction rates

The rate of the gas phase reaction of hydroxyl radical with methyl nitrate has been measured to be (3.4 ± 0.4) × 10^?14 cm³ molecule^?1 s^?1 at 298 K using flow discharge/ resonance fluorescence techniques. By means of correlation methods, this rate determination is used to predict a vertical ionization potential of 12.6 eV, a bond dissociation energy for H? CH₂ONO₂ of 101 kcal mol^?1, and a rate for O(³P) reaction with methyl nitrate of ca. 9 × 10^?17 cm³ molecule^?1 s^?1. In conjunction with previously derived relative data for reaction of alkyl nitrates with OH radical in the gas phase, a priori estimated reactivities for 1-, 2-, and 3-positionally substituted straight chain alkyl nitrates have been reexamined. Revised reactivities for OH abstraction of specific hydrogens substituted on straight chain alkyl nitrates are presented and discussed, and an atmospheric lifetime of ca. 2 yrs is estimated for methyl nitrate removal due to OH.     

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.     

The effect of substituents on the reactivity of aniline with phosphate radical

The reaction between aromatic amines and phosphate radical has been investigated. The reaction is accelerated by electron-releasing substituents and retarded by electron-withdrawing substituents pointing to an electrophilic attack by the PO₄^2xxx radical. σ⁺para values correlate the effect of substituents well. The rho value for PO₄^2xxx is more positive than that for CO₃^xxx indicating higher reactivity of phosphate radical towards aromatic amines than the carbonate radical.     

Reactions of recoil38Cl atoms with CCl4 and C6H5Cl: Effect of high electron density scavengers

Reactions of recoil³⁸Cl atoms with CCl₄ and C₆H₅Cl have been studied in presence of various high electron density scavengers. Relative reactivities of recoil³⁸Cl towards the two components of these mixtures are determined using the model proposed by Urch.     

Combination and disproportionation reactions of allylic radicals with ethyl,propyl, and t-Butyl Radicals

1-Methylallyl, 1,1-dimethylallyl, 1,2-dimethylallyl, 1,3-dimethylallyl, 1,1,2-trimethylallyl, and 1-ethylallyl radicals have been generated in the gas phase at 20 ± 1°C by addition of H atoms, formed by Hg(6³P₁) photosensitization of H₂, to appropriate dienes. Their combination reactions with ethyl radicals have been studied and the relative reactivities of the reaction centers in each allylic radical determined. Similar measurements have been made for some combination reactions of n-propyl, i-propyl, and t-butyl with 1-methylallyl and 1,1,2-trimethylallyl radicals. The more substituted reaction centers are found to be the less reactive. In addition the self-combination and disproportionation of 1-methylallyl radicals has been investigated, as has cross disproportionation of each allylic radical with ethyl. The results establish a general pattern of reactivity for these radicals, which is interpreted primarily in terms of the effects of steric interaction during reaction.