Study of thermal decomposition of poly-(vinyl chloride)-type polymers with the use of model substances-V: Pyrolysis of cis-5-chloro-3-heptene and cis- and trans-5-acetoxy-3-heptene in the phase

The gas phase pyrolyses of cis-5-chloro-3-heptene (in the range 267–337°) and cis- and trans-5-acetoxy-3-heptene (300–378°) are homogeneous unimolecular first-order reactions with rate constants given respectively by: log k = (12·03 = 0·13) - (36·2 ± 0·4)/2·303 RT and (12·80 ± 0·11) - (43·0 ± 0·3)/2·303 RT. (Frequency factors in sec^?1 units, activation energies in kcal mol^?1.) No significant differences were found between the rates of decomposition of cis and trans isomers of 5-acetoxy-3-heptene. From the decomposition of these models of poly(vinyl chloride) and poly(vinyl acetate), some conclusions about the role of internal unsaturated groups in the thermal decomposition of both polymers were drawn. The possibility that groups with cis internal double bonds are the most labile structures in a poly(vinyl) chloride macromolecule is discussed.     

The Kinetics of the gas-phase isomerization of 1,trans-3,trans-5-heptatriene into the cis-5-isomer catalyzed by Nitric Oxide and the stabilization energy in the pentadienyl radical

The kinetics of the nitric oxide catalyzed, homogeneous, gas-phase isomerization of 1,trans-3,trans-5-heptatriene have been studied for temperatures ranging between 130°C and 241°C. The very clean reaction involves exclusive geometrical isomerization about the 5,6-π-bond. The observed rate constants for \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm NO} + trans - {\rm 3,}trans{\rm - 5}\stackrel{1}{\rightarrow}trans - 3,cis - 5 + {\rm NO} $\end{document} can be represented (with standard errors) by log k₁ = (7.18 ± 0.06) – (16.75 ± 0.12)/θ, where θ = 2.303 R T in kcal/mole. The consecutive-step reaction mechanism involves addition of NO to the double bond (K_{a, b} = k_a/k_b), followed by rotation of the 5,6-C? C bond in the adduct radical (k_c.) Analysis of the observed activation parameters shows, that k_c is rate-controlling and consequently k₁ = k_cK_{a, b}. Estimates of k_c and K_{a, b} lead to a value of k₁ in good agreement with experiment. Comparing our data with those previously obtained for the similar 1,3-pentadiene system results in a value for the extra stabilization energy generated in the 1,3-heptadienyl radical of 18.5 ± 1.7 kcal/mole. This value is discussed in view of comparable data in the literature.     

Study of thermal decomposition of poly(vinyl chloride) type polymers with the use of model substances—IV: Gas phase pyrolysis of 2-acetoxy-4-chloropentane

The kinetics of the gas phase pyrolysis were investigated for 2-acetoxy-4-chloropentane, the simplest model for the macromolecule of VC-VAc copolymer; both manometric and GLC method were used. It was concluded that the elimination of the first acid molecule from the model is a homogeneous unimolecular first order reaction with rate constants within the range 310–350° given by: log k₁ = (13·14 ± 0·17)—(44·9 ± 0·5)/2·303 RT for the iso and log k₁ = (13·23 + 0·15)—(44·9 ± 0·4)/2·303 RT for the syndio isomer. (Frequency factors in sec^?1 units, activation energies in kcal mol^?1.)Application of the results to the copolymer leads to the conclusion that the pronounced minimum observed by some authors in the plot of thermal stability against copolymer composition cannot be adequately explained solely in terms of the low thermal stability of an alternating unit.     

Kinetics of the iodine atom catalyzed gas phase geometrical isomerization of diiodoethylene. Rotation about a single bond

The spectrophotometric determination of the rate of iodine atom catalyzed geometrical isomerization of diiodoethylene in the gas phase from 502.8 to 609.1°K leads to a rate constant for the bimolecular reaction between I and trans-diiodoethylene of log k_t?c(M^?1 sec^?1) = 8.85 ± 0.12 ? (11.01 ± 0.30)/θ. Estimates of the entropy and enthalpy change for the addition of I atoms to trans-diiodoethylene (process a.b) lead to log K_a.b(M^?1) = ?2.99 ? 4.0/θ, and thus to log k_c (sec^?1) = log k_t?c – log K_ab = 11.8 ?7.0/θ for the rate constant for rotation about the single bond in the adduct radical. The theory for calculation of the rotation rate constant is presented and it is shown that while the exact value depends on the barrier height, a value of 6.8 kcal/mole for this quantity leads to log k (sec^?1) = 11.8 ?6.7/θ. The activation energy points to a better value of the group contribution to heat of formation of the group C -(I)₂(H)(C) than one based on bond additivity.     

Kinetics of the I2-catalyzed isomerization of allyl chloride to cis- and trans-1-chloro-1-propene. The stabilization energy of the chloroallyl radical

The I₂-catalyzed isomerization of allyl chloride to cis- and trans- l-chloro-l-propene was measured in a static system in the temperature range 225–329°C. Propylene was found as a side product, mainly at the lower temperatures. The rate constant for an abstraction of a hydrogen atom from allyl chloride by an iodine atom was found to obey the equation log [k,/M^?1 sec^?1] = (10.5 ± 0.2) ?; (18.3 ± 10.4)/θ, where θ is 2.303RT in kcal/mole. Using this activation energy together with 1 ± 1 kcal/mole for the activation energy for the reaction of HI with alkyl radicals gives DH⁰ (CH₂CHCHCl? H) = 88.6 ± 1.1 kcal/mole, and 7.4 ± 1.5 kcal/mole as the stabilization energy (SE) of the chloroallyl radical. Using the results of Abell and Adolf on allyl fluoride and allyl bromide, we conclude DH⁰ (CH₂CHCHF? H) = 88.6 ± 1.1 and DH⁰ (CH₂CHCHBr? H) = 89.4 ± 1.1 kcal/ mole; the SE of the corresponding radicals are 7.4 ± 2.2 and 7.8 ± 1.5 kcal/mole. The bond dissociation energies of the C? H bonds in the allyl halides are similar to that of propene, while the SE values are about 2 kcal/mole less than in the allyl radical, resulting perhaps more from the stabilization of alkyl radicals by α-halogen atoms than from differences in the unsaturated systems.     

Dynamics of a conformational change in aqueous 18-crown-6 by an ultrasonic absorption method

A temperature dependence study of the ultrasonic amplitudes, velocities, and relaxation times for a presumed conformational transition of noncomplexed aqueous 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) is discussed. At all temperatures a single relaxation was observed within a 15–255-MHz frequency range. The equilibrium constant for the presumed conformational transition \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CR}_1 \mathop \rightleftarrows\limits^{K_{12} } {\rm CR}_2 $\end{document} was determined to be K₂₁ = (2 ± 2) × 10^?2. The activation parameters are ΔH₂₁^≠ = 10.2 ± 1.0 kcal/mol, ΔS₂₁^≠ = 7.7 ± 0.2 cal/(mol·deg), ΔH₁₂^≠ = 7.4 ± 1.0 kcal/mol, and ΔS₁₂^≠ = 7.7 ± 0.2 cal/(mol·deg), while the thermodynamic enthalpy and entropy were found to be ?2.6 ± 1.0 kcal/mol and 0 ± 0.2 cal/(mol·deg), respectively. The rate constants at 25.0°C for the presumed conformational transition are k₂₁ = (1.0 ± 0.3) × 10⁷ sec^?1 and k₁₂ = (6.2 ± 0.2) × 10⁸ sec^?1.     

Isomerization of n-hexyl and s-octyl radicals by 1,5 and 1,4 intramolecular hydrogen atom transfer reactions

n-Hexyl and s-octyl radical isomerizations by intramolecular hydrogen atom shift have been studied in the presence of high methyl radical concentration where isomerized alkyl radicals reacted predominantly by combination and disproportionation reactions with methyl radicals. By assuming the rate coefficient of 1-hexyl radical recombination to be equal to that of ethyl self-combination, the rate coefficient of log(k₁/s^?1) = (9.5 ± 0.3) – (11.6 ± 0.3) kcal mol^?1/RT ln 10 has been derived for the 6sp isomerization of n-hexyl radicals, 1-hexyl → 2-hexyl (1). Investigation of s-octyl radical isomerization was complicated by fast interconversion between 3-octyl, 2-octyl, and 4-octyl radicals. Use of the methyl trapping technique and systematic variation of methyl radical concentration made possible the determination of log(k₂/s^?1) = (9.4 ± 0.7) ? (11.2 ± 1.0) kcal mol^?1/RT ln 10 for the 6ss isomerization of 3-octyl and the estimation of log(k₃/s^?1) = 10.5–17 kcal mol^?1/RT ln 10 for the 5ss isomerization of 2-octyl radicals, where 3-octyl → 2-octyl (2), and 2-octyl → 4-octyl (3).     

Delocalization energy of the cyanomethyl radical: Kinetics of thermal diastereoisomerization of cis- and trans-1,2-dicyano- and 1 ,2-dicyano-1-methylcyclopropanes

Arrhenius parameters for the thermal first-order geometrical isomerization of 1,2-dicyanocylopropanes(I) have been determined in naphthalene solution over the range 208.0–259.5° in both directions:

where θ = 4.575T × 10^?3 and k is in sec^?1. Since this enthalpy of activation is lower than that of the geometrical isomerization of 1,2 - dideuterocyclopropane by 17.8±0.4 kcal, it may be concluded that replacement of hydrogen by the cyano group leads to an energy lowering of 8.9$\text{kcal}\text{mol}$.Kinetic parameters have been determined in the gas-phase at two temperatures, 217.8° and 259.5°: log k_t,c = 13.73– 45.64/θ; log k_c,t =13.86–44.43/θ.The rates of cis-trans interconversion of 1,2 - dicyano - 1 - methyl - cyclopropane(II) relative to those of I have been obtained by examination of mixtures of both substances in t-butylbenzene solution at 259.5°: 1.2-dicyano, k_t,c= 1.25 and k_c,t = 3.53; 1,2 - dicyano - 1 - methyl, k_t,c = 8.09 and k_c,t = 22.35 × 10^?5 sec^?1. The rate acceleration by methyl amounts to a factor of 6.4, corresponding to ΔΔG^≠ = 1.96$\text{kcal}\text{mol}$. A preliminary examination of optically active material leads to a minimum R_A = 1.37 favoring rotation of (CN)(H) over (CN)(CH₃).     

Applications of carbon-13 resonance spectroscopy—XV : The degenerate cope-rearrangement of bullvalene

The temperature dependence of the ¹³C-NMR spectrum of bullvalene has been studied from ?67 to +128°C using fourier transform spectroscopy and ¹H broadband decoupling. Lineshape analysis based on the Anderson-Kubo-Sack theory yielded E_a=13·9±0·1 kcal/mole, log A= 14·0±0·1, ΔH3 = 13·3±0·1 kcal/mole, and ΔS3 = 3·4±0·4 e.u. The pertinent features of dynamic ¹³C-NMR spectroscopy are discussed.     

Homogeneous decomposition of vinyl ethers. The heat of formation of ethanal-2-yl

The thermal unimolecular decomposition of three vinylethers has been studied in a VLPP apparatus. The high-pressure rate constant for the retro-ene reaction of ethylvinylether was fit by log k (sec^?1) = (11.47 + 0.25) - (43.4 ± 1.0)/2.303 RT at <T> = 900 K and that of t - butylvinylether by log k (sec^?1) = (12.00 ± 0.27) - (38.4 ± 1.0)/2.303 RT at <T> = 800 K. No evidence for the competition of the higher energy homolytic bond-fission process could be obtained from the experimental data. The rate constant compatible with the C? O bond scission reaction in the case of benzylvinylether was log k (sec^?1) = (16.63 ± 0.30) - (53.74 ± 1.0)/2.303 RT at <T> = 750 K. Together with ΔH_f,300⁰(benzyl·) = 47.0 kcal/mol, the activation energy for this reaction results in ΔH_f,300⁰(CH₂CHO) = +3.0 ± 2.0 kcal/mol and a corresponding resonance stabilization energy of 3.2 ± 2.0 kcal/mol for 2-ethanalyl radical.     

Kinetics of iodination of hydrogen sulfide by iodine and the heat of formation of the SH radical

The kinetics of the gas-phase thermal iodination of hydrogen sulfide by I₂ to yield HSI and HI has been investigated in the temperature range 555–595 K. The reaction was found to proceed through an I atom and radical chain mechanism. Analysis of the kinetic data yields log k (l/mol·sec) = (11.1 ± 0.18) – (20.5 ± 0.44)/θ, where θ = 2.303 RT, in kcal/mol. Combining this result with the assumption E_?1 = 1 ± 1 kcal/mol and known values for the heat of formation of H₂S, I₂, and HI, ΔH_f,298⁰(SH) = 33.6 ± 1.1 kcal/mol is obtained. Then one can calculate the dissociation energy of the HS? H bond as 90.5 ± 1.1 kcal/mol with the well-known values for ΔH_f,298⁰ of H and H₂S.     

The nitric oxide catalyzed positional isomerization of 3-methylene-1,5,5-trimethylcyclohexene and the stabilization energy in the 3-methylenecyclohexenyl radical

The gas phase, nitric oxide catalyzed positional isomerization of 3-methylene-1,5,5-trimethylcyclohexene (MTC) into 1,3,5,5-tetramethyl-1,3-cyclohexadiene (TECD) has been studied for temperatures ranging between 296° and 425°C. The major reaction was first order with respect to nitric oxide and to MTC. The major side product, mesitylene, usually amounted to less than 10% of the TECD isomer formed. Only at high temperatures and large conversions has up to 20% been observed. Conditioned pyrex or quartz vessels coated with KCl have been used. The nitric oxide catalyzed isomerization is apparently a homogeneous process, as demonstrated by the insensitivity of the observed rate constants towards a 15-fold increase in the surface to volume ratio of the reaction vessels. However, a residual, presumably heterogeneous, thermal isomerization of the starting material could not be eliminated. Good mass balances were obtained for both NO and hydrocarbons. After correcting for the thermally induced conversion the observed rate constants for the nitric oxide catalyzed isomerization yield log k₁ (1 mole^?1 sec^?1) = (10.7 ± 0.2) – (37.3 ± 0.9)/θ where θ is 2.303 × 10^?3 RT (kcal mole^?1). Plotting log k₁ versus the ratio of the starting materials (MTC/NO)₀ it was found that for temperatures ≥ 365°C the rate constants were systematically too high. Using extrapolated values for the higher temperature range yields the more reliable corrected Arrhenius equation log k = 8.6 – 31.7/θ. The reaction mechanism is outlined and the implications with respect to the stabilization energy generated in the MTC? radical intermediate and the activation energy of the backreaction MTC? + HNO are discussed. Using for the activation energy E_?1 of the backreaction (R? + HNO) a literature value of 9.2 ± 0.9 kcal mole^?1 reported for the cyclohexadiene? 1,3? system, this yields 23.4 ± 2 kcal mole^?1 for the stabilization energy in the methylenecyclohexenyl radical, which is to be compared with the corresponding values for the allyl (10.2 ± 1.4), methallyl (12.6 ± 1) pentadienyl (15.4 ± 1) and cyclohexadienyl (24.6 ± 0.7) radicals. The pre-exponential factor agrees well with the value of (8.4 ± 0.2) reported by Shaw and co-workers for the similar reaction of NO with 1,3-cyclohexadiene. It is noteworthy that HNO, acting as sole hydrogen donor in the system, is surprisingly stable under the reaction conditions used. Nitrous oxide, HCN, H₂O and N₂ are observed in the product mixture of experiments carried out to high conversions at higher temperatures.     

Gas phase decomposition and isomerization reactions of 2-pentoxy radicals

The rate of decomposition of 2-pentoxy radical to acetaldehyde and n-propyl radical has been studied in the presence of NO in competition with nitrite formation at and above 200 kPa pressure over the temperature range of 363-413 K. The rate coefficient for the decomposition is given as log(k_la/s^?1) = (14.2 ± 0.4) - (13.8 ± 0.8) kcal mol^?1/RT ln 10. Isomerization of 2-pentoxy radical by 1,5-hydrogen shift has been investigated in the range 279–385 K in competition with the decomposition in a static system, with methyl radicals present in high concentration to ensure trapping of the isomerized free radicals. The rate coefficient for isomerization is given as log(k₃/s^?1) = (11.1 ± 0.7) - (9.5 ± 1.1) kcal mol^?1/RT ln 10. The implications of the results for atmospheric chemistry are discussed.     

Electronic spectra of the isomeric 4-benzylidene-2-phenyl-Δ2-oxazolin-5-ones and the products of their reaction with nucleophiles including α-chymotrypsin : Kinetics of the hydrolysis of the isomeric enzyme derivatives

The 4-cis- and trans-benzylidene-2-phenyl-Δ²oxazolin-5-ones have been prepared and evaluated as chromophoric reagents for the investigation of the active site of α-chymotrypsin. The UV spectra of the isomeric oxazolinones are discussed and compared with those of similar compounds, notably the 1,4-diphenyl-1,3-butadienes, A novel spectroscopic consequence of geometrical isomerism in conjugated chromophores is reported. The facile isomerization of the cis-oxazolinone has been investigated by spectrophotometry and product isolation. Both oxazolinones react rapidly with the enzyme to provide products whose UV spectra are consistent with their assignment as α-benzamido- cinnamoyl-enzymes. The uncertainties in these assignments resulting from the presence in the oxazolinones of multiple electrophilic centres are discussed. The pseudo first-order rate constants for the hydrolysis of the products of interaction of α-chymotrypsin with the isomeric oxazolinones were determined at 25·0°, I = 00·1 in the pH range 7–10–5. The pH-rate profile for the hydrolysis of the product formed by reaction of the trans-oxazolinone is consistent with this reaction being deacylation of α-benzamido-trans-cinnamoyl-α-chymotrypsin catalysed by the enzyme's electron relay system (pKa 7·8, ks= 0·159s^?1). The pH rate profile for the hydrolysis of the product formed by reaction of the cis-oxazolinone is more complex. The profile could include a component catalysed by the relay system (pKa approx 8, $\text{k}$, = 10^?3 s^?1) but the predominating reaction appears to be an unusually rapid reaction of the derivatized enzyme with hydroxide ion (k = 233 ± 10 M^?1s^?1). Possible interpretations of these pH-rate profiles are discussed.     

Triplet photochemistry of medium ring trienes

The photochemistry of the triplets of 10- and 11-membered ring 1,3,5-trienes has been studied. At ?70° cis,trans,cis-cyclodeca-1,3,5-triene goes only to the cis,cis,cis-isomer. At 25°, this latter compound is converted into cis-bicyclo[4.4.0]deca-2,4-diene via the thermally labile trans,cis,trans-cyclodeca-1,3,5-triene. At ?70° cis,trans,cis-cycloundeca-1,3,5-triene is converted to the cis,cis,cis-isomer. At 25°, this primary ptotochemical product undergoes a thermal 1,7-sigmatropic hydrogen migration to yield the trans,cis,cis, isomer. This latter triene upon sensitized irradiation yields cis-bicyclo[5.4.0]undeca-8,10-diene and trans-bicyclo[7.2.0]undeca-2,10-diene. The ratio of these latter two products changes with the temperature of the sensitized reaction. The possible mechanisims of these transformations are discussed.     

Dynamics of the isomerization of trans-stilbene in n-alcohols studied by ultraviolet picosecond absorption recovery

We have used a simple At⁺ synchronously pumped and cavity-dumped dye-laser system to generate UV picosecond pulses with energies sufficiently high for absorption recovery experiments. With these pulses, we have studied the dynamics of the isomerization of trans-stilbene in n-alcohols as a function of viscosity and temperature. It is concluded that the excited-state barrier of trans-stilbene in n-alcohols is less than 1 kcal mol^?1 and that the trans → cis isomerization yield is 0-54 ± 0.05.     

1,3‐Dipolar Cycloaddition of Diazo Compounds to Electron‐Deficient Alkenes: Kinetics and Mechanism of Formation of Dimethyl‐4,5‐dihydro‐1H‐pyrazol‐3,5‐dicarboxylate

1,3‐Dipolar cycloaddition of methyl diazoacetate to methyl acrylate was investigated by kinetic ¹Н NMR spectroscopy. It was established that the mechanism of the process includes parallel formation of trans‐ and cis‐dimethyl‐4,5‐dihydro‐3H‐pyrazol‐3,5‐dicarboxylates as a result of [3 + 2]‐cycloaddition of methyl diazoacetate to methyl acrylate; the corresponding rate constants were denoted k_1t and k_1c. The reaction rate of the isomerization of 3Н‐pyrazolines to 4,5‐dihydro‐1H‐pyrazol‐3,5‐dicarboxylate (3Н → 1Н‐pyrazoline rearrangement) was found to be sensitive to both the methyl acrylate (k_2t, k_2c) and 1Н‐pyrazoline concentrations (k_3t, k_3c). Kinetic analysis showed that the proposed scheme is valid for various reagent concentrations. The numerical solution of the system of differential equations corresponded to the reaction scheme and was used to determine the complete set of reaction rate constants (k (× 10⁵ M^–1·s^–1), 298 K; solvent, benzene‐d₆): k_1t = 2.3 ± 0.3, k_1c = 1.6 ± 0.2, k_2t = 1.1 ± 0.3, k_2c = 1.8 ± 0.5, k_3t = 1.2 ± 0.4, k_3c = 2.2 ± 0.7.     

An ab initio study of the geometry and energy of the four planar conformers of glyoxylic acid and the glyoxylate ion

The structures and energies of the four planar conformers of glyoxylic acid and the glyoxylate ion have been studied ab initio using the unscaled 4—31G basis set with full geometry optimization. Changes in the CO, OH and CO bond lengths in the conversion of the cc conformer into the ct and tt conformers, and into the tc conformer, are consistent with the formation of four-membered and five-membered hydrogen-bonded ring structures, respectively. Changes in the distances between the nearest non-bonded atoms around each C atom reveal that the internal geometry of the CHO and COOH groups is significantly affected by cis—trans isomerization with respect to the OCCO backbone, and that the geometry of the CHO group is affected by proton dissociation from the COOH group. Furthermore, the movement of the component atoms in each functional group, characterized as clockwise or anticlockwise about the C atom, results in some cases in a rotation of the functional group as a whole. Whereas experiment shows the tc conformer to be more stable than the tt by 1.2 ± 0.5 kcal mol^?1, the calculations find the tt conformer to be the most stable, separated in energy from the ct, tc and cc conformers by 0.4, 1.4 and 10.7 kcal mol^?1, respectively. Augmentation of the 4—31G basis set in several forms, and use of (9,5/4) and (9,5/4,1) basis sets, only served to decrease slightly the tt/tc energy difference, not change the sign. The calculated proton affinity of the glyoxylate ion with respect to the tt conformer is 342.7 kcal mol^?1, compared to 357.7 kcal mol^?1 for the formate ion. The expectation energy differences Δ V_nn, Δ V_ee and Δ V_en for the cis—trans isomerization of the ct and cc conformers are opposite in sign to those for the glyoxal reaction, and in magnitude they all far exceed the ΔE_T values, which shows that hydroxyl group substitution has a much greater influence than a comparison of only the ΔE_T values would suggest.