T-butyl Radicals Research Articles

Thermolysis of azoalkanes in a stirred-flow system

The gas-phase thermal decomposition of azoethane, ethaneazo-1′-methylethane, and azo-2′-methylpropane have been investigated in a stirred-flow system using the toluene carrier technique. The order of the reaction for the disappearance of the azoalkanes has been found to be nearly unity in each case. The temperature dependence of the rate coefficients is given by the Arrhenius equations: azoethane, log k= 14.2 ± 0.2 –(186 ± 2 kJ mol–1)/2.30RT; ethaneazo-1′-methylethane, log k= 16.5 ± 0.5 –(206 ± 3 kJ mol–1)/2.30RT, and azo-2′-methylpropane, log k= 15.6 ± 0.3 –(170 ± 3 kJ mol–1)/2.30RT. No evidence was found for hydrogen atom abstraction by alkyl radicals from the toluene carrier. An average value of 0.14 ± 0.03 was obtained for the disproportionation : combination ratio of the ethyl radicals in the temperature range 280–400 °C. For the t-butyl radicals this ratio had a value of 2.9 ± 0.2 between 210 and 260 °C.

The geometry of t-butyl radicals: An ab initio study

Ab initio calculations for the t-butyl radical, (CH3)3C· suggest that it is planar, and that it has a greater resistance to out-of-plane bending than has the methyl radical. The calculated isotropic hyperfine coupling to 13C is +36·8 G for the rigid molecule, together with +32 G for the zero-point vibratory contribution, giving a predicted total of +68·8 G. The experimental isotropic coupling of 46–51 G cannot, therefore, be taken as providing evidence for a pyramidal ground state for this radical.

Homolytic organometallic reactions : XII. Homolytic dealkylation of di-n-butyl-t-butyltin chloride: preferential displacement of the n-butyl radical

t-Butoxyl radicals react with di-n-butyl-t-butyltin chloride to show the ESR spectrum of the n-butyl rather than the t-butyl radical. Cumyloxyl radicals behave similarly, but trimethylsiloxyl and benzoyloxyl radicals react to give both n-butyl and t-butyl radicals. The unexpected preferential formation of the less stable n-alkyl radical is tentatively ascribed to steric constraints in a 5-coordinate intermediate.

Study of the spectra and recombination kinetics of alkyl radicals by molecular modulation spectrometry. Part 2.—The recombination of ethyl, isopropyl and t-butyl radicals at room temperature and t-butyl radicals between 250 and 450 K

Using the technique of molecular modulation spectrometry, we have found transient ultraviolet absorptions which we assign to ethyl, isopropyl and t-butyl radicals. The spectra are broad continua with maxima near 250, 233 and 230 nm, respectively. From the rate constants of their mutual reactions, measured by monitoring the radicals in the ultraviolet, and the disproportionation/recombination ratios found from separate experiments, the rates of recombination are measured at room temperature to be: ethyl, (1.3 ± 0.3)× 10–11 cm3 molecule–1 s–1; isopropyl, (8.3 ± 2.0)× 10–12 cm3 molecule–1 s–1; t-butyl, (4.0 ± 1.0)× 10–12 cm3 molecule–1 s–1. The t-butyl rate falls by a factor of 2 as the temperature is increased to 423 K. These results are compared with other recent gas- and liquid-phase determinations, and we conclude that the currently accepted thermochemistry of larger alkyl radicals and our results are incompatible.

Studies of Short-lived Radicals in the γ-irradiated Aqueous Solution of Uridine-5′-monophosphate by the Spin-trapping Method and the Liquid Chromatography

An aerated aqueous solution of uridine-5'-monophosphate was gamma-irradiated with 2-methyl-2-nitrosopropane as a spin-trapping reagent. Liquid chromatography was applied to separate the stable nitroxide radicals in the irradiated solution. The radicals were detected by U.V. and e.s.r. spectrometry. The e.s.r. detection showed four peaks in the chromatogram. The orcinol method for detection of the residual sugar moieties was applied before and after reduction of the base to determine the existence of the 5,6-double bond for the molecules in each fraction. From the combined results of the e.s.r. and orcinol methods, the short-lived radicals which were trapped by 2-methyl-2-nitrosopropane were identified as radicals of N-1 and C-6 positions of the base moiety and t-butyl radical which was the radiolytic product of the trapping reagent.

Quantitative chemically induced nuclear polarization (CIDNP) study of the kinetics of the photolysis of pivalophenone in various solutions

A simple mechanistic study of the photolysis of pivalophenone in a series of solvents showed that under suitable conditions a unique reaction mechanism obtains. In all the solvents investigated benzoyl and t-butyl radicals are produced which undergo two competitive cage reactions, recombination and disproportionation. In chloroform at 310 K 15 % of the radicals recombine and 85 % disproportionate; in the fluorocarbon PP9 with added tetrachloromethane the figures at 310 K are 17 % and 83 % respectively and the activation energies of the two processes differ by 16.3 ± 2.0 kJ mol–1. This observation is used to rationalise unusual behaviour in the electron polarization phenomena of the radicals.The overall pseudo first-order rate constant for the scavenging of t-butyl radicals in neat chloroform was measured directly by flash-photolysis e.s.r. techniques as 5.44 ± 0.13 × 103 s–1; CIDNP exposed that two scavenging routes occur: hydrogen abstraction with a rate constant k*H= 2.54 ± 0.07 × 102 dm3 mol–1 s–1 and chlorine abstraction with a rate constant k*C1= 1.84 ± 0.07 × 102 dm3 mol–1 s–1. In PP9 the second order rate constant for reaction of t-butyl radicals with tetrachloromethane is 4.9 × 104 dm3 mol–1 s–1, with an activation energy of 13.8 ± 4.2 kJ mol–1. These kinetic results are compared with literature values and shown to be consistent with previous observations of the t-butyl radical.The spin-lattice relaxation time of the protons in this radical is 2.2 ± 0.7 × 10–3 s in chloroform solution and its diffusional correlation time is 4 ± 1 × 10–11 s; this is consistent with theoretical predictions of the time required to produce polarization in a solution having the viscosity of chloroform. The whole provides a stringent test of kinetic CIDNP theory as applied to radical reactions in solution.

Chain initiation of neopentane pyrolysis and a suggested reconciliation of the thermochemically calculated and measured rate constants for the recombination of t-butyl radicals

The rates of initiation of neopentane pyrolysis over the temperature range 756–845 K have been measured at reactant pressures in the range 100–200 Torr, by processing of data relating to the formation of the termination product, ethane, using an exact algebraic procedure. Thus, the data are free of the ambiguities introduced by the empirical extrapolation procedures used in previous direct determinations of the initial rates and the derived Arrhenius parameters of neo–C5H12→ CH3+ t–C4H9. (1) The result obtained is log (k1/s–1)= 16.1 ± 1.0 –(330 ± 15 kJ mol–1)/2.303 RT. Values of k1 are in agreement with previously measured values but the Arrhenius parameters are inconsistent with the presently accepted thermochemistry of reaction (1). A modified thermochemistry for the t-butyl radical is proposed leaving the heat capacity unchanged but using the new values for 298 K, ΔHf/kJ mol–1= 39, S(1 mol cm–3)/J mol–1 K–1= 210.8. These, and the presently derived k1 values allow the calculation for 800 K of log (k–1/cm3 mol–1S–1)= 12.9 with essentially zero activation energy and hence, via the geometric mean rule, lead to log (k9/cm3 mol–1 s–1)= 11.9 for the mutual recombination reaction, 2t–C4H9→ 2,2,3,3-tetramethylbutane. (9)The new thermochemistry suggested for t-butyl reconciles most thermochemically calculated values of k9 with directly measured values over a wide temperature range. However, further experimental and theoretical justification is necessary before the new thermochemistry can be unequivocally accepted.Concurrently evaluated with k1 is the result log (k3k6/k5/cm3 mol–1 s–1)= 13.2 ± 1.5 –(127 ± 12 kJ mol–1)/2.303 RT CH3+ neo–C5H12→ CH4+ C5H11(3), CH3+ i–C4H8→ CH4+ C4H7(6), 2CH3→ C2H6(5) which is shown to be in excellent agreement with that calculated from the independent results of other workers.

Photolysis of di-t-butyl peroxide under acid conditions

Photolysis of di-t-butyl peroxide in the presence of trifluoroacetic acid gives a species which can fragment to give a methyl radical, or can add to an alkene; this species is tentatively identified as the t-butyl alcohol radical cation, But[graphic omitted].

An electron spin resonance study of phosphoranyl radicals derived from tetra-alkyl esters of phosphorous phosphoric anhydride (RO)2POP(O)(OR)2

A series of phosphoranyl radicals with the general structure (RO)2P(X)OṖ(OR)3(X = O or S) have been studied in solution using e.s.r. spectroscopy. The phosphoranyl radicals were produced by addition of photochemically generated t-butoxyl radicals to (RO)2P(X)OP(OR)2(X = O or S) or diethoxyphosphoryloxyl radicals (EtO)2P(O)O to (RO)3P (X = O). The e.s.r. spectra of these phosphoranyl radicals are consistent with a trigonal bipyramidal structure, and the apicophilicity of (RO)2P(O)O is greater than that of RO. The spectrum of the cyclic radical [graphic omitted](O)(OEt)2 shows that apical and equatorial endocyclic alkoxy-substituents undergo rapid exchange. The fβ-scission of (EtO)2ButOPOP(O)(OEt)2at –60° to give t-butyl radicals is 40 times slower than that of (EtO)3ṖOBut.

The ultraviolet absorption spectrum of tert-butyl radicals and the rate constant for their recombination

Using the technique of molecular modulation spectrometry, an ultraviolet absorption spectrum which we assign to t-butyl radicals has been obtained during the photolysis of azoisobutane and nitrogen mixtures and di-tert-butyl peroxide and isobutane mixtures. The spectrum is a broad absorption band about 20 nm wide centered at 230 nm lying on top of a weaker continuum that extends to 270 nm. At the peak (λ ≈ 229 nm) the cross section is 5 × 10 −18 cm 2. From the rate constant of their mutual reaction and the recombination/disproportionation ratio measured in separate experiments, the rate of the reaction 2(CH 3) 3C → (CH 3) 3CC(CH 3) 3 has been measured as (2.0 ± 0.3) × 10 −12 cm 3 molecule −1 s −1 at 25°C (1.2 × 10 9 g mol −1 s −1). This result is compared with other recent gas- and liquid-phase determinations.

Reactions of 2,4,6-Tri-t-butylnitrosobenzene with Grignard Reagents

Abstract Reactions of 2,4,6-tri-t-butylnitrosobenzene (1) with alkylmagnesium halides afforded 6-alkyl-1-hydroxy-imino-2,4,6-tri-t-butyl-2,4-cyclohexadienes (2), 4-alkyl-1-hydroxyimino-2,4,6-tri-t-butyl-2,5-cyclohexadienes (3), and N-alkyl-2,4,6-tri-t-butylanilines (4) depending on the Grignard reagents used. The distribution of the reaction products indicated that the oximes were formed via an attack of the anionic species on the ortho- and the para-carbons, respectively, to the nitroso group, but that the N-alkylanilines via a free radical mechanism involving an electron transfer from the Grignard reagent to 1. The free radical mechanism was confirmed in the case of t-butylmagnesium chloride by trapping the t-butyl radicals as 1-t-butoxyimino-2,4,4,6-tetra-t-butyl-2,5-cyclohexadiene (8). Such a mechanistic difference has been explained in terms of the difference in electron affinities of the free radicals derived from Grignard reagents.

Termination rate constants of t-butyl radicals, in solution: interference by cis-azoisobutane

The rate constant for the mutual termination reaction of t-butyl radicals in solution has been measured by kinetic ESR spectroscopy and is (11.1 ± 3.4) × 10 9 M −1 sec −1 over the temperature range from 220 to 330°K. Azoisobutane was shown to be unsuitable as a photo-initiator in the experiments because the formation of the thermally unstable cis-isomer during photolysis led to erroneous values of the rate constant.

Nucleophilic character of alkyl radicals—X : Polar and steric effects in the alkylation of 3-substituted pyridines by t-butyl radical

Homolytic t-butylation of 3-substituted pyridines occurs with complete selectivity m position 6. Steric and polar effects contribute to determine the exceptional selectivity. The relative rates correlate with the Hammett σ p constants (ϱ = 5·5). The results are discussed in terms of a transition state similar to a charge-transfer complex.

A probe for homolytic reactions in solution. Part VIII. An electron spin resonance study of the rates of fragmentation and spin trapping of t-butoxycarbonyl radicals and acyl radicals

From a study of the competition between spin trapping and decarboxylation, the rate constant for addition of the t-butoxycarbonyl radical to 2-methyl-2-nitrosopropane (nitrosobutane) has been determined to be 1·1 × 106 I mol–1s–1 at 40° in di-t-butyl peroxide solvent. The rate constant in benzene must be very similar. Competitive scavenging and decarbonylation of a series of acyl radicals (RĊO) shows that the ease of loss of carbon monoxide increases along the series R = Me < Pri < adamantyl < But < PhCH2. An absolute rate constant (3·9 × 103s–1 at 40°) for the decarbonylation of PriĊO is obtained by assuming that this radical adds to nitrosobutane at the same rate as t-butoxycarbonyl. This rate constant is considerably smaller than that obtained previously in the gas phase and a possible source of error in the gas-phase value is identified. The formation of t-butoxy t-butyl nitroxide by addition of t-butoxyl radicals to nitrosobutane is reversible at ambient temperatures. Fragmentation of this nitroxide to give t-butoxyl radicals occurs with a rate constant of ca. 0·14 s–1 at 40°, whilst fragmentation with loss of t-butyl radicals is shown to occur by N–C, and not O–C, fission with a rate constant of ca. 10–3 s–1 at 40°.

The reaction of t-butoxyl radicals with diborane

The reaction of butoxyl radicals with diborane has been shown by e.s.r. spectroscopy to yield t-butyl radicals. The rate constant for this process is given by k= 108·4× 102·9/θ l mol–1 s–1 where θ= 2·303 RT in kcal mol–1. No intermediate 4-co-ordinate boron radical could be detected. Analogies are drawn with t-butoxyl–trialkyl phosphite reaction and with the oxidation of trialkylboranes by alkaline hydrogen peroxide and by amine oxides. t-Butyl radicals are not formed in detectable quantities in either the t-butoxyl–alkyldiborane or in the t-butoxyl–phosphine reactions.

The reaction of trifluoromethyl radicals with neopentane. An S H2 reaction at a saturated aliphatic carbon atom.

When hexafluoroacetone is photolysed in the presence of neopentane at 300°, isobutane is formed as a primary product, and 1,1,1-trifluoroethane is formed in amounts much greater than would be expected from combination of methyl and trifluoromethyl radicals. These results strongly support the occurrence of a homolytic bimolecular substitution reaction (SH2) by a trifluoromethyl radical at a methyl carbon atom, with the displacement of a t-butyl radical.

Nucleophilic character of alkyl radicals—VII : Substituent effects on the homolytic alkylation of protonated heteroaromatic bases with methyl, primary, secondary and tertiary alkyl radicals

The relative rates of the homolytic alkylation of protonated 4-substituted pyridines with methyl, n-propyl, n-butyl, sec-butyl and t-butyl radicals, obtained by the silver-catalyzed decarboxylation of acids with ammonium peroxydisulphate, are reported. This is the first time that the reactivities of the main alkyl radicals have been compared using the same reagent model in a homolytic aromatic alkylation. The alkylation takes place exclusively in the 2 position and the chemical reactivity parameters correlate well with the chemical shifts, but not with the Hammett σ m constants because of an enhanced conjugation of the electron-releasing substituents. The influence of the polar characteristics on the t-butyl radical is very striking. The results are discussed in terms of a π-complex transition state.

Unstable intermediates. Part 120.—Electron spin resonance study of γ-irradiated t-butyl chloride: coupling to β-chlorine atoms

Two radicals were detected by e.s.r. spectroscopy after exposure of t-butyl chloride (1H and 2H) to γ-rays at 77 K. One, the t-butyl radical, is well known, but the other, Me2Ċ—CH2Cl, probably formed by loss of a methyl hydrogen atom followed by a 1,2-chlorine shift, is novel and shows the following features. These results suggest not only that the radical is locked in a conformation in which maximum overlap between the carbon 2p-orbital of the radical and the C—Cl σ-bond is achieved, but also that considerable distortion involving movement of the chlorine atom towards the radical centre has occurred.

The electron spin resonance spectra and decarboxylation of alkoxycarbonyl radicals

The e.s.r. spectrum of the t-butoxycarbonyl σ-radical, generated by u.v. irradiation of a liquid mixture of di-t-butyl peroxide and t-butyl formate, has been observed. Decarboxylation of this radical has been monitored by kinetic e.s.r. spectroscopy and the unimolecular rate constant for the reaction But·ĊO → But·+ CO2 is given by log (k/s–1)= 13·4 – 12·1/θ where θ= 2·303 RT/kcal mol–1.These kinetic parameters differ from those reported in an earlier communication because it was assumed that the rate constants for reaction of a t-butyl radical with a t-butyl radical or with a t-butoxycarbonyl radical had the same value. This assumption has been tested experimentally and shown to be incorrect.Abstraction of hydrogen from the alkoxy-groups of alkyl carbonates gives radicals which do not decompose to form alkoxycarbonyl radicals.

The mercury (3 P 1)-photosensitized decomposition of keten and some reactions of triplet methylene

The mercury-photosensitized reaction of keten has been investigated in the presence and absence of neopentane and other gases (O2, N2, and CF4). For the pure system a mechanism is suggested, based upon product analyses at low conversion. A key reaction in this mechanism is the combination of two triplet methylene species (3CH2). In the presence of neopentane, most of the products are explained by radical combination processes, some of them involving 3CH2. The observation of products derived from t-butyl radicals supports the specific case of neopentyl–3CH2 cross combination. A portion of the 2,2-dimethylbutane (neohexane) product, unaccounted for by radical reactions, is attributed to an insertion reaction by single methylene.