The Thermal Unimolecular Decompositions of CH₃CD₂Cl, CD₃CD₂Cl, and CH₃CHCl₂

Abstract

The thermal decompositions of the deuterated ethyl chlorides CH3CD2C1 and CD,CD,CI and of 1,l-dichloroethane CH3CHC12 were studied in a static system in the pressure range of 0.1 -300 Torr. With a previously published rate constant for the decomposition of ethyl chloride [l] the following kinetic isotope effects were obtained for the high pressure limit in the range 710-770 K: k,(CH3CH2Cl)/k,(CH3CDzC1) = 1.096 exp( -0.4 kJ mol-'/RT), k,(CH3CH2Cl)/k,(CD3CD2Cl) = 0.513 exp(lO.O kJ mol-'/RT).The pressure dependent first order rate constants for the three reactions were compared with RRKM calculations, which used an activated complex model based on extended Huckel calculations [8].Der thermische Zerfall der deuterierten Athylchloride CH,CD,CI und CD,CD,CI sowie von 1,l-Dichlorathan, CH3CHC12, wurde in einer statischen Versuchsanordnung im Druckbereich von 0,l-300 Torr untersucht. In Verbindung mit fruher veroffentlichten Geschwindigkeitskonstanten fur den Zerfall von Athylchlorid [ 11 wurden die folgenden kinetischen Isotopieeffekte im Temperaturintervall von 710-770 K fur den Hochdruckbereich ermittelt: k,(CH3CHZCl)/k,(CH3CD,CI) = 1,096exp( -0,4 kJ mol-'/RT), k,(CH3CH2Cl)/k,(CD3CDzCl) = 0,513 exp(l0,O kJ mol-'/RT). Die Druckabhingigkeit der Geschwindigkeitskonstanten erster Ordnung wurde verglichen mit RRKM-Berechnungen, denen ein Modell fur den aktivierten Komplex aus erweiterten Hiickel-Rechnungen zugrunde liegt [S].

Unimolecular Reactions of 1,1,1-Trichloroethane, 1,1,1-Trichloropropane, and 3,3,3-Trifluoro-1,1,1-trichloropropane: Determination of Threshold Energies by Chemical Activation

“…The chemical-activation method is able to suppress the interference of the Cl atom chain reaction that makes the thermal-activation studies of 1,1,1-trichloroalkanes so difficult. These results, the thermal-activation study of CH 3 CCl 3 , and less extensive results for 1,1-dichloroalkanes − indicate a systematic decline in threshold energies when Cl atoms are bound to the C Cl atom of the transition state. This trend for CH 3 CH 2 Cl, CH 3 CHCl 2 , and CH 3 CCl 3 is supported by experimental data and by the DFT calculations.…”

“…The same trend − also exists for C 2 H 5 CH 2 Cl and C 2 H 5 CHCl 2 , although the pyrolysis experiments for C 2 H 5 CHCl 2 are not conclusive . This trend has been associated with the mesomeric effect , of the Cl atom on C Cl . The effect of the second chlorine atom on the E 0 for 1,1,1-trichloroalkanes is an important question for the development of transition-state models for these HCl elimination reactions.…”

“…However, the uncertainty in this value for E 0 must be 2–3 kcal mol –1 considering the experimental problems of the pyrolysis experiments. The threshold energies for the reactions of CH 3 CH 2 Cl , and CH 3 CHCl 2 − reactions are established as 55 ± 1 and 52 ± 1 kcal mol –1 , respectively, and the addition of a Cl atom on the carbon atom (C Cl ) that is attached to the Cl atom in the four-member ring of the transition state definitely lowers the threshold energy. The same trend − also exists for C 2 H 5 CH 2 Cl and C 2 H 5 CHCl 2 , although the pyrolysis experiments for C 2 H 5 CHCl 2 are not conclusive .…”

HCl Yield from OH + ClO: Stratospheric Model Sensitivities and Elementary Rate Theory Calculations

“…If the approximate reaction coordinate used for HCl elimination via TS3 is replaced by a higher frequency stretching or bending normal coordinate (in disagreement with the theoretical prediction), then the isotope effect from the zero-point energy difference becomes larger. For reaction 3, ethyl chloride decomposition, the measured DCl/HCl rate ratio is 0.42 at 750 K. , Using the parameters of Table , we predict the analogous ratio for (1) to be 0.73 at 750 K, a significantly different result. However, computations 42 for reaction 3, using parameters for ethyl chloride and its elimination transition state that were generated in the same way as those for HOOCl and TS2 (Table ), predict a D/H kinetic isotope effect of 0.40 at 750 K. This is consistent with the experimental measurements and lends confidence to our prediction of a significantly smaller isotope effect for reaction 1b at atmospheric temperatures.…”

“…However, ab initio calculations 42,46 show that the transition state for HCl elimination from ethyl chloride is similar to TS3, i.e., very asymmetric with a nearly dissociated chlorine atom. Also, Jonas and Heydtmann have used the out-of-plane deformation normal coordinate as the ethyl chloride reaction coordinate. This choice has implications for calculations of the H/D isotope effect, as discussed below.…”