T-butyl Radicals Research Articles

The decomposition of 2,2,3,3-tetramethylbutane in KCl- and B2O3-coated vessels in the presence of oxygen

The decomposition of 2,2,3,3-tetramethylbutane (TMB) in the presence of oxygen has been studied in both KCl-coated and aged boric-acid-coated vessels. The values for k1 obtained with the two types of vessel surface are in close agreement, and combination of the values over the range 400–542 °C gives A1= 1.04 × 1017 s–1, E1= 294.7 ± 3 kJ mol–1, effectively identical with previous values using KCl-coated vessels, and thus confirming the thermochemistry suggested for the t-butyl radical: (CH3)3C—C(CH3)3→ 2(CH3)3C. (1) The observed rate constant, kobs, defined by the equation –d [TMB]/dt=kobs[TMB] increases with increasing TMB concentration with both types of vessel surface and this variation has been used to evaluate the ratio k4/k7½. The results with both types of vessel surface are in agreement, giving A4/A7½= 4.4 × 105(dm3 mol–1 s–1)½, E4–½E7= 81.7 ± 8 kJ mol–1 over the range 400–520 °C: HO2+(CH3)3C—C(CH3)3→ H2O2+(CH3)3C → C(CH3)2CH2(4) HO2+ HO2→ H2O2+ O2. (7)

Time-resolved infrared study of bimolecular reactions between t e r t-butyl radicals

The gas phase IR spectrum of t-butyl radicals near ∼3μm has been observed by the time-resolved infrared technique known as TRISP. The transient spectrum is observed to decay with second-order kinetics, and a total rate of decay kc+kd = 1010.26 liter mole−1 sec−1 is measured. From a gas chromatograph/mass spectrometer analysis of the reaction products, it is found that kd/kc = 2.9. Thus kc = 109.67 and kd = 1010.13 liter mole−1 sec−1. The t-butyl radicals were created by ruby laser photolysis of monomeric (CH3)3CNO vapor.

ChemInform Abstract: INFRARED MATRIX ISOLATION STUDIES ON THE T‐BUTYL RADICAL

Chemischer InformationsdienstVolume 12, Issue 35 Physical Organic Chemistry ChemInform Abstract: INFRARED MATRIX ISOLATION STUDIES ON THE T-BUTYL RADICAL J. PACANSKY, J. PACANSKYSearch for more papers by this authorJ. S. CHANG, J. S. CHANGSearch for more papers by this author J. PACANSKY, J. PACANSKYSearch for more papers by this authorJ. S. CHANG, J. S. CHANGSearch for more papers by this author First published: September 1, 1981 https://doi.org/10.1002/chin.198135051AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume12, Issue35September 1, 1981 RelatedInformation

Infrared matrix isolation studies on the t-butyl radical

The infrared spectrum of the t-butyl radical isolated in an argon matrix was obtained by gas phase pyrolysis of azoisobutane and 2-nitrosoisobutane. The infrared spectrum is consistent with a C3v structure for the radical. The data obtained from the matrix isolation studies were used to reassess the ΔH°f,300 for the radical.

ChemInform Abstract: ABSORPTION SPECTRA AND TERMINATION KINETICS OF TRANSIENT FREE RADICALS IN SOLUTION OBSERVED BY UV/VIS SPECTROSCOPY WITH MODULATED EXCITATION

Free radicals are generated in liquid solutions by harmonically modulated photolysis of suitable substrates. Harmonic analysis of the absorbance as functions of wavelength and modulation frequency yields the optical spectra and the decay kinetics of the transient species. The experimental technique and the analysis are described in detail. Results on t-butyl, 2-propyl and benzyl radicals generated by photolysis of the corresponding dialkyl resp. dibenzyl ketones are reported. They confirm previous spectral assignments and show that the termination reactions are diffusion controlled.

Wall-less reactor studies. Part 5.—Neopentane pyrolysis

The early stages (0.01–1%) of the pyrolysis of neopentane have been studied in a wall-less reactor between 945 and 1016 K in excess argon at 600 Torr (1 Torr = 133.3 N m–2). The radical chain mechanism deduced from conventional studies is shown to give a good account of the kinetics of formation of methane and ethane; the rate constants for chain initiation neo-C5H12→ t-C4H.9+ CH.3(1), and chain propagation by methyl radicals CH.3+ neo-C5H12→ CH4+ C5H.11(4), are obtained. The values of the latter agree with the results of conventional studies but the former are approximately ten times smaller than previous estimates. The implications of these rate constants for the thermodynamic values of the t-butyl radical are discussed. By using ΔfH(t-C4H.9) to obtain the activation energy, the Arrhenius equation for the homogeneous gas-phase rate constant k1 deduced from the mean experimental value at 983 K, log10(k1/s–1)=–2.56 ± 0.02, is k1/s–1= 1.38 × 1015 exp (–40051/T).

Primary processes in the reaction of OH·-radicals with sulphoxides

The primary processes in the OH· radical-induced oxidation mechanism of sulphoxides have been investigated by pulse radiolysis and, in particular, by an improved conductivity detection method with a time resolution of ca. 50 ns in aqueous solution. Electrophilic addition of the OH· radical to the sulphoxide group leads to a transient adduct, R2SO(OH)· which decays unimolecularly with t1/2 up to 100 ns into a sulphinic acid, RSO2H, and a radical R·. The various RSO2H have been identified by their pKa, and R·(including t-butyl and phenyl radicals) by direct optical measurement or chemical scavenging experiments. The probability of radical split-off from R2SO(OH)· for mixed sulphoxides depends on the stability of the radical leaving. Depending on the nature of the sulphoxide substituents two other OH· radical reactions compete with, and may even predominate over, the addition at the sulphoxide group. Thus hydrogen-atom abstraction readily occurs from longer chain and branched aliphatic groups and in the presence of aromatic substituents OH· radicals add to the π-system to form a hydroxycyclohexadienyl radical. The respective yields, kinetics and some physico-chemical properties of the primary species are presented and discussed.

Site selectivity in the β-scission of alkoxyphosphoranyl radicals. A reinterpretation

The rate constants for β-scission of cyclic and acyclic trigonal bipyramidal (TBP) phosphoranyl radicals [graphic omitted]Ṗ(OBut)X (A) and (EtO)2Ṗ(OBut)X (B)[X = EtO or (Me3Si)2N] to give t-butyl radicals have been measured by kinetic e.s.r. spectroscopy. The phosphoranyl radicals exist as equilibrium mixtures of isomers which differ in the occupancy of apical and equatorial sites and which interconvert much more rapidly than they undergo β-scission. Radicals for which access of the ButO group to an equatorial site is restricted undergo β-scission relatively slowly. The relative rates of t-butyl radical-forming β-scission at 233 K in hydrocarbon solution are shown to be approximately proportional to the equilibrium mole fraction of the isomer in which the ButO group occupies an equatorial site, provided that allowance is made for the retarding effect of the five-membered ring. β-Scission of TBP alkoxyphosphoranyl radicals thus involves equatorial site selectivity. Previous results for phosphoranyl radicals (A) and (B)(X = Me3SiO) are reinterpreted in terms of equatorial site selectivity rather than the apical site selectivity originally proposed. The apicophilicity of Me3SiO is almost certainly lower than that of ButO, whilst in the previous work the reverse order of apicophilicity was assumed.

ChemInform Abstract: THERMAL DECOMPOSITION OF HEXAMETHYLETHANE IN A FLOW SYSTEM

Hexamethylethane has been decomposed in a flow system in the temperature range of 700–900 K. The mechanism involves carbon–carbon bond cleavage at the most highly substituted position and rapid formation of isobutene from the t-butyl radical. The rate expression is and is completely consistent with deductions from radical buffer, shock-tube, and direct recombination studies. Of special importance is experimental evidence for large decreases of the A factor with increasing temperature and a high heat of formation for the t-butyl radical, ΔHf(tC4H9·)300 = 52.7 ± 6 kJ.

Use of t-butyldimethylchlorosilane/imidazole reagent for identification of molecular species of phospholipids by gas-liquid chromatography mass spectrometry.

The t-butyldimethylsilyl derivatives of 1,2-diakyl, 1-alk-1'-enyl-2-acyl, 1-alkyl-2-acyl and 1,2-diacyl glycerols were analysed with a gas chromatograph mass spectrometer system. The characteristic fragment ions were as follows. The molecular weight determining ion was [M-57]+, which was formed by cleavage of the t-butyl radical from the molecular ion. The nature of the alk-1'-enyl residue could be determined by the presence of ions at [RCH-CH 56]+ and [RCH = CH + 130]+ (RCH = CH = alk-1'-enyl), and the alkyl residue by the ion at [R + 130]+(R = alkyl group). Ions giving information about the acyl group, [RCO]+, [RCO + 74]+ and [M-RCH = CHO, -RO or -RCOO]+ were also observed. The mass spectra of pairs of trimethylsilyl and t-butyldimethylsilyl derivatives showed differences in several respects. The t-butyldimethylsilyl derivatives gave more effective information for elucidating the structure of phosphoglycerides.

An electron spin resonance study of radical addition to vinylphosphines

E.s.r. spectroscopy has been used to show that alkoxyl and methyl radicals add to phosphorus in the vinylic phosphines R2CC(H)PX2(X = MeO, EtO, or Me2N) to form ‘phosphoranyl’ radicals in which the unpaired electron is centred mainly on the remote carbon. These adducts are π-radicals and can be regarded as alkene radical anions which carry a phosphonium substituent, although there is significant spin density on phosphorus. The adducts R2Ċ–C(H)P(OR′)2OBut(R = H or Me) undergo fragmentation to give t-butyl radicals and the phosphonate ester R2CC(H)P(O)(OR′)2.t-Butyl radicals add to the unsubstituted carbon of the vinyl group in H2CC(H)PX2(X = RO, Me2N, or Et) to give α-phosphinoalkyl radicals ButCH2Ċ(H)PX2, the e.s.r. spectra of which exhibit relatively large splitting from 31P. There appears to be appreciable delocalisation of the unpaired electron onto phosphorus, a result which was not anticipated on the basis of previous reports of e.s.r. spectra ascribed to α-phosphinoalkyl radicals.

Radiation chemistry of cobalt(II) nitrilotriacetate in aqueous solutions

The radiolysis of cobalt(II) nitrilotriacetate (COIINTA) was studied in neutral aqueous solutions by both γ-radiolysis and pulse radiolysis methods. The OH radicals were found to abstract a hydrogen atom from the ligand with k=(1.0 ± 0.1)× 109 dm3 mol–1 s–1 and the ligand radicals decayed by disproportionation with 2k=(9.3 ± 0.9)× 104 dm3 mol–1 s–1. Ligand oxidation products were identified as iminodiacetic acid (IDA) and glyoxalic acid in equal amounts. However, O–2 oxidized the complex to CoIIINTA, probably through an intermediate binuclear species. e–aq reduces the CoIINTA to CoINTA, k=(9.6 ± 0.9)× 108 dm3 mol–1 s–1, which is an order of magnitude slower than that for the free metal ion Co2+aq. Formyl and t-butyl radicals react with CoIINTA with rate constants of (7.3 ± 0.7)× 107 and (1.1 ± 0.2)× 107 dm3 mol–1 s–1, respectively, to form transient metal–carbon bonds.

Reaction of t-butyl radicals with hydrogen and with oxygen

t-C4H9 radicals have been produced by the decomposition of 2,2,3,3-tetramethylbutane in the presence of H2 and of O2 in KCl-coated reaction vessels. From the measurement of the relative yields of i-C4H8 and i-C4H10 in the early stages of reaction over the temperature range 440–540°C, values of E10–E2= 61.9 ± 2.0 kJ mol–1 and A2/A10= 0.35 ± 0.10 have been obtained.

Formation of free radicals in the reaction of alkyllithiums with titanium tetrachloride

Alkyl radicals have been detected by electron spin resonance spectroscopy approximately 80 msec after mixing benzene solutions of alkyllithium reagents with titanium(IV) chloride. The methyl, ethyl, isopropyl, n-butyl, sec-butyl, t-butyl and cyclopentyl radicals were observed from the corresponding alkyllithium reagents. The methyl radical easily observed at 40 msec reacts to give a second paramagnetic species with ▪ 18 Gauss. It is suggested that this species is derived from (CH 3Li) 4 by hydrogen atom abstraction.

Reactions of alkyl radicals with nitrogen trifluoride

Photolysis of mixtures of nitrogen trifluoride with the appropriate ketones (in the temperature range 303–486 K) has enabled the reactions of CH3, CF3, C2H5, i-C3H7 and t-C4H9 radicals with nitrogen trifluoride to be studied. With the smaller radicals only the abstraction of difluoroamino radicals is important, but with i-C3H7 and t-butyl both difluoroamino and fluorine abstraction occur. The Arrhenius parameters are given. The disproportionation—combination ratio for methyl and t-butyl radicals was found to be 0.83 ± 0.02.

An electron spin resonance study of dialkylaminothiyl (R2NS·), dialkylaminosulphinyl (R2NSO), and alkyl(sulphinyl)aminyl [RNS(O)X] radicals. Radical addition to N-sulphinylamines

Photochemically generated t-butoxyl (or trimethylsiloxyl) radicals react with bis(dialkylamino) sulphides or sulphoxides, containing primary N-alkyl groups, to produce dialkylaminothiyl (R2NS·) or dialkylaminosulphinyl (R2NSO) radicals, respectively. The e.s.r. spectra of these radicals have been detected and show a(N) 10.7 G, g 2.0155 (R2NS·) and a(N)ca. 6.5 G, g 2.0060 (R2NSO). t-Butoxyl, trimethylsiloxyl, methyl, and t-butyl radicals (X·) add rapidly to the sulphur atom of N-sulphinylalkylamines to produce alkyl(sulphinyl)aminyl radicals [RNS(O)X], which are detected by e.s.r. spectroscopy [a(N) 8.4–10.3 G, g ca. 2.0042]. The e.s.r. spectra are discussed in terms of the electronic structure and conformation of these radicals. The 1-hydroxy-1-methylethyl radical reduces N-sulphinyl-t-butylamine, probably to give the radical ButNSOH rather than the tautomeric aminosulphinyl radical ButN(H)SO.

Kinetic Studies of Spin-trapping Reactions. I. The Trapping of the t-Butyl Radical Generated by the Photodissociation of 2-Methyl-2-nitrosopropane by Several Spin-trapping Agents

Abstract The photochemical reaction of 2-methyl-2-nitrosopropane (a spin-trapping agent) in benzene was studied at 299 K by monitoring the optical absorption intensity of the spin-trapping agent and the ESR signal intensity of the spin-adduct radical (di-t-butyl nitroxide). The observed kinetic feature was interpreted in terms of three elementary processes; the photodissociation of the spin-trapping agent giving a t-butyl radical, the spin trapping of the t-butyl radical by the trapping agent giving the spin adduct radical, and the reaction of the spin-adduct radical with the t-butyl radical giving diamagnetic products. The rate constant of the last process was found to be 10 times as large as that of the spin-trapping process, which was determined to be 3.3×106 mol−1 dm3 s−1 based on the reported rate constant for the scavenging of the t-butyl radical by tributylstannane. The relative spin-trapping rate constants toward the t-butyl radical were also determined to be 0.07, 1.0, 41, 63, and much higher than 50 for 2,4,6-tri-t-butylnitrosobenzene, 2-methyl-2-nitrosopropane, pentamethylnitrosobenzene, nitrosodurene and nitrosobenzene, respectively. C-phenyl-N-t-butylnitrone was found to be much less reactive by a factor of less than 0.003.

Pentamethylnitrosobenzene as a Spin-trapping Agent

Abstract Pentamethylnitrosobenzene synthesized readily from pentamethylbenzene was studied for utilization in a spin-trapping technique. In a benzene solution it forms a dimer with a dissociation constant of 105.26exp(−50⁄RT) in units of kJ/mol. It traps the t-butyl radical with the trapping rate constant of 1.4×108 mol−1 dm3 s−1 at 299 K. It is thus an efficient spin-trapping agent, even though only a fraction of it is reactive with the short-lived free radicals because the equilibrium lies more on the side of the dimeric form. Electron spin resonance parameters were determined for the spin-adduct radicals derived from alkyl, alkoxyl, and phenyl radicals with this trapping agent. These free radicals were found to attach exclusively to the nitrogen atom of the nitroso compound.

Vacuum Ultraviolet Photoelectron Spectroscopy of Transient Species: Part 8, the t-Butyl Radical

The vacuum ultraviolet photoelectron spectrum of the t-butyl radical produced by pyrolysis of 2,2' azoisobutane has been investigated. The band associated with the first ionization potential is the only one for which vibrational structure could be resolved and this has been analysed in terms of ionization of a vibrationally excited t-butyl molecule. From the observed Franck-Condon envelope of this band, it is concluded that the t-butyl radical has a pyramidal C3v geometry in its ground electronic state and the barrier to inversion is estimated to be (900±100) cm-1 with an out-of-plane distance of the central carbon atom of (0.30±0.05) Å. The first vertical and adiabatic ionization potentials of the t-butyl radical are estimated to be (6.90±0.01) eV and (6.58±0.01) eV respectively. The higher ionization potential region is interpreted with the aid of semiempirical molecular orbital calculations.

Electron spin resonance measurements of the termination rate constants for t-butyl radicals in solution

The rate constants, 2kt, for the mutual termination of t-butyl radicals in liquid isobutane and cyclopentane have been measured in the temperature range 170–330 K, by kinetic e.s.r. spectrometry. The results are in excellent agreement with values of 2kt reported recently for termination in a series of n-alkanes. It is concluded that the termination reaction is completely diffusion-controlled in both solvents. The value of 2kt(11 × 109 dm3 mol–1 s–1) at 298 K is in good agreement with that derived from recent gas phase measurements (2kt= 9 × 109 dm3 mol–1 s–1) at the same temperature.