ALBERT

Notizen: In recent publications from this laboratory, we have shown that the fragmentation of photoexcited olefinic molecules in the vacuum UV region leads mainly to the cleavage of a C - C bond located in the ß position relative to the double bond. The allyl fragment bears away part of the excess energy of the photon. At low pressure, this excited radical is capable of undergoing further decomposition. From the pressure effect, we were able to measure the first order rate constant for this secondary fragmentation. In this paper we shall use RRKM calculations in order to get a better idea on how the energy is distributed among the primary fragments. In cases where α- and β;-methallyl radicals were involved, the results show that an important part of the excess energy is located in the methallyl fragment in the 7.1 and 7.6 eV photolysis of 3-methyl-1-butene, 2-methyl-1-butene, and cis-2-pentene.

Notizen: A chain mechanism is proposed to account for the very rapid termination reactions observed between alkyl peroxy radicals containing α-C - H bonds which are from 104 to 106 faster than the termination of tertiary alkyl peroxy radicals. The new mechanism is with termination by. \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm R}\overline {{\rm CHOO}} $\end{document} is the zwitterion originally postulated by Criegee to account for the chemistry of O3-olefin addition. Heats of formation are estimated for \documentclass{article}\pagestyle{empty}\begin{document}$ \overline {{\rm CH}_2 {\rm OO,}} {\rm }\overline {{\rm RCHOO}} $\end{document}, and \documentclass{article}\pagestyle{empty}\begin{document}$ ({\rm C}\overline {{\rm H}_3 )_2 {\rm COO}} $\end{document} and it is shown that all steps in the mechanism are exothermic. The second step can account for (1Δ)O2 which has been observed. k1 is estimated to be 109-2/θ liter/M sec where θ = 2.303RT in kcal/mole. The second and third steps constitute a chain termination process where chain length is estimated at from 2 to 10. This mechanism for the first time accounts for minor products such as acid and ROOH found in termination reactions. Trioxide (step 3) is shown to be important below 30°C or in very short time observations (〈10 s at 30°C). Solvent effects are also shown to be compatible with the new mechanism.

Notizen: Flash photolysis technique has been used to obtain the rate and thermodynamic parameters of the reversible dimerization reactions of a range of ten phenoxy radicals (I-X) in a toluene-dibutylphthalate mixture (0.6 cP ≤η≤18.4 cP): \documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm R}^{.} + {\rm R}^{.} {\mathop{{\buildrel{-\!\!\longrightarrow}\over{\longleftarrow}}}\limits_{k_{-1}}^{k_1}}{\rm D} $$\end{document} The main reason for the difference in the k1 values are the different steric hindrances in radicals. It has been found that the values of k1 for 2,6-diphenyl-4-methoxy- (I), 2-phenyl-(III), and 2-methoxyphenoxy (IV) radicals are 3-5 times smaller than the respective diffusion constants calculated according to the Debye formula with regard to the spin-statistical factor: \documentclass{article}\pagestyle{empty}\begin{document}$$ k_{diff} = \sigma \frac{{8{\rm RT}}}{{3000{\rm \eta }}} $$\end{document} The resultant ΔH1≠values for these radicals in toluene and dibutylphthalate are close to the activation energies of the viscous flow of the solvents B. Linear relationships with a slope equal to unity are observed between log k1 and log(T/η). The recombination of radicals I, III, and IV is limited by translational diffusion. The k1 values for 2,6-diphenyl- (VII), 2,6-di-tert-butyl- (IX), and 2,6-di-tert-butyl-4-methylphenoxy (X) radicals are 10-60 times smaller than kdiff and Δ H≠ B. In the case of radical X in toluene ΔH1≠ 0. The recombination of these three radicals includes an intermediate step of complex formation: \documentclass{article}\pagestyle{empty}\begin{document}$${{\rm R}^\cdot+{\rm R}^\cdot}{\mathop {{\scriptstyle\longleftarrow}^{\hskip-13pt\longrightarrow}}}{\rm R^\cdot}\ldots {\rm R}^\cdot \rightarrow {\rm D}$$ \end{document} For 4-phenyl- (II), 2,6- dimethoxy- (V), 2,4-diphenyl- (VI), and radicals VII, IX, and X the linear relationships between log k1 and log (T/η) have a slope of from 0.5 ± 0.05 to 0.8 ± 0.05. The k1-1 versus η relationships for these radicals are not straight lines. The recombination of these six radicals is limited by translational and rotational diffusion. With the aid of theoretical models, the k1 versus η relationships have been used to derive the steric factor f in radical recombination and the angle θ between the axis and the solid angle generatrix. The solid angle defines the reaction spot on the radical-sphere surface. The recombination of the 2,6-diphenyl-4-diphenylmethylphenoxy radical (VIII) takes place in the region intermediate between the diffusion and the kinetic ones, and the relationship between log k1 and log (T/η) for this radical has a plateau portion. The log k-1 versus log (T/η) relationships have precisely the same form as the corresponding k1 relationships, which is quite in line with the theory of diffusion-controlled reversible recombination reactions.

Notizen: Di-tert-butylnitroxide (DTBN) is the simplest of the stable nitroxide radicals and is only consumed at temperatures higher than 90°C or in the presence of very reactive substrates. The pyrolysis of DTBN in solution gives, at least at low conversion, 2-methyl-2-nitrosopropane and di-tert-butylnitroxide-tert-butyl ether. The reaction involves, as the rate-limiting step, the cleavage of the C—N bond. This reaction takes place with an activation energy of 33 kcal/mol. DTBN is stable in the presence of styrene, aldehydes, hydrogen peroxide, α-methyl-N-ethyl nitrone, phenol, and triphenylmethane. On the other hand, it reacts readily with diethylhydroxylamine, ascorbid acid, ethanethiol, and hexanethiol. For the two former compounds the reaction involves a hydrogen transfer as the rate-determining step, and the reaction proceeds, a low conversion, with simple second-order kinetics. The reaction with the thiols is complex and shows a clear inductiontime.

Notizen: The rate of the reverse reaction of the system has been measured in the range of 584-604 K from a study of the azomethane sensitized pyrolysis of isobutane. Assuming the published value for the rate constant of recombination of t-butyl we obtain \documentclass{article}\pagestyle{empty}\begin{document}$$ \log k_{{\rm - 1}} (\sec ^{- 1}) = 14.67 - 39.4\,{\rm kcal}/{\rm mol}/(2.3{\rm RT}) $$\end{document} Combination with our published data for k1 permits the evaluation \documentclass{article}\pagestyle{empty}\begin{document}$$ \log K_1 ({\rm atm}^{ - 1}) = 7.94\,\,{\rm at}\,\,600{\rm K} $$\end{document}We have modified a previously published structural model of t-butyl by the inclusion of a barrier to free rotation of the methyl groups in order to calculate values of the entropy and enthalpy of t-butyl as a function of temperature. Using standard data for H and for i-C4H8 we obtain \documentclass{article}\pagestyle{empty}\begin{document}$$ \Delta H_ f^\circ(t - {\rm butyl},\,300\,{\rm K})({\rm kcal}/{\rm mol}) = 10.6 \pm 0.5 $$\end{document}We have obtained other, independent values of this quantity by a reworking of published data using our new calculations of the entropy and enthalpy of t-butyl. There is substantial agreement between the different values with one exception, namely, that derived from published data on the equilibrium \documentclass{article}\pagestyle{empty}\begin{document}$$ i - {\rm C}_{\rm 4} {\rm H}_{{\rm 10}} + {\rm I}\rightleftharpoons t{-} {\rm C}_4 {\rm H}_9 + {\rm HI} $$\end{document} which is significantly lower than the other values.We conclude that the value \documentclass{article}\pagestyle{empty}\begin{document}$$ \Delta H_ f^\circ(t - {\rm butyl},\,300\,{\rm K})({\rm kcal}/{\rm mol}) = 10.5 \pm 1.0 $$\end{document}obtained from the present work and a reworking of published data which involves the use of experimental data on t-butyl recombination is incompatible with the result based on iodination data.

Notizen: The kinetics of the reaction have been investigated in H2SO4 medium under different conditions. The observed bimolecular rate constant kobs, has been found to depend on [H+]-0.55 and to increase with the initial concentration ratio of the reactants R0 = [H2O2]0/[U (IV)]0 above 0.49. The activation energy of the overall reaction has been determined as 13.79 and 14.3 kcal/mol at R0 = 1 and 0.35, respectively. Consistent with experimental data, a detailed reaction mechanism has been proposed where the hydrolytic reaction (4) followed by the rate-controlling reaction (10) and subsequent fast reactions of U (V) and OH radicals are involved: A kinetic expression has been derived from which a graphical evaluation of (kK4)-1 and k-1 has been made at R0 = 1 as (12.30 ± 0.09) × 10-3 M min, (6.23 ± 2.19) × 10-4 M min; and at R0 = 0.35 as (12.63 ± 2.13) × 10-3 M min, (8.32 ± 6.62) × 10-4 M min, respectively. Indications of some participation of a chain reactionat R0 = 1 have been obtained without affecting thesecond-order kinetics as observed.

Notizen: The kinetics of thermal decomposition of ethyl, isopropyl, and t-butyl trifluoroacetates have been studied in the gas phase. In each case initial decomposition follows the normal ester route to give an olefin and trifluoroacetic acid, and elimination of hydrogen fluoride does not occur. However, trifluoroacetic acid is thermally unstable at ethyl and isopropyl ester decomposition temperatures, and further products result, including those from the difluorocarbene produced by decomposing trifluoroacetic acid. Placing a CF3 group at an ester's γ carbon increases the polarity of its transition state and decreases its thermal stability. The activation energies of the ethyl and isopropyl esters are lowered by 3.8 and 4.7 kcal/mol compared to the corresponding acetates, and the primary decomposition kinetics, which are homogeneous and of the first order, are expressed by α-Methylation enhances the reactivity of the trifluoroacetates, and the t-butyl ester, the transition state for which is sufficiently polar for heterogeneous decomposition to occur, shows signs of thermal instability at room temperature. The equilibrium was also investigated and gave ΔH° = +13,580 cal/mol and ΔS° = +31.07 gibbs/mol in the forward direction. The results obtained extend and support the known structure-rate correlations in the gas-phase elimination of esters.

Notizen: The thermal unimolecular decomposition of ethylbenzene, isopropylbenzene, and tert-butylbenzene was studied using the very-low-pressure pyrolysis (VLPP) technique. Each reactant decomposed by way of β C—C bond homolysis, producing methyl radicals and benzyl or benzylic-type radicals. RRKM calculations show that the observed rate constants, when combined with thermochemical estimates, are consistent with the following high-pressure rate expressions: \documentclass{article}\pagestyle{empty}\begin{document}$ \log k(\sec ^{ - 1}) = 15.3 - (72.7/{\rm \theta)} $\end{document} for ethylbenzene between 1053 and 1234 K, \documentclass{article}\pagestyle{empty}\begin{document}$ \log k(\sec ^{ - 1}) = 15.8 - (71.3/{\rm \theta)} $\end{document} for isopropylbenzene between 971 and 1151 K, and \documentclass{article}\pagestyle{empty}\begin{document}$ \log k(\sec ^{ - 1}) = 15.9 - (69.1/{\rm \theta)} $\end{document} for tert-butylbenzene between 929 and 1157 K, where θ (kcal/mol) = 2.303RT. Resulting activation energies combined with heat capacity and heat of formation data led to the following dissociation enthalpies and enthalpies of formation at 298 K: DH° (øCH(CH3)—CH3) = 73.8 kcal/mol, ΔHf° (øÇCH(CH3)) = 39.6 kcal/mol, DH° (øC(CH3)2—CH3) = 72.9 kcal/mol, and ΔHf° (øÇ(CH3)2) = 32.4 kcal/mol. Derived high-pressure rate constants are in good accord with results of lower temperature toluene- and aniline-carrier experiments.