Kinetics of .alpha.- and .beta.-hydrogen abstraction from ethyl chloride by bromine atoms: estimate of carbon-hydrogen bond dissociation energies and heat of formation of CH3CHCl and CH2CH2Cl radicals

Abstract

The gas-phase photobromination of ethyl chloride has been investigated over an extended temperature range in the presence and absence of ethane as external competitor. The rate constant for a-hydrogen abstraction in CH3CH2C1 was redetermined relative to C2H6 between 40 and 150 "C. The abstraction of @-hydrogen was measured in the internal competition in the range 70-150 "C by use of electron capture detection. The relative rates were combined with the known rate parameters for the bromination of C2H6 [log (A/cm3 mol-' S-I) = 14.14 f 0.06, E = 13.66 f 0.14 kcal mol-I] to obtain the absolute rate constants (cm3 mol-ls-') k(a) = 10'2.71*0.06 exp[-(9.95 f 0.14)/RT] and k ( @ ) = 1012.6M0.07 exp[-(14.14 f 0.15)/RT], where the activation energies are expressed in kcal mol-'. From an assessment of thermochemical and kinetic data the following radical heats of formation and C-H bond dissociation eneFgies (kcal mol-I) in C2H5C1 have been obtained: MH,0(CH3CHC1) = 17.6 f 1, Do(CH3CHCI-H) = 96.5 f 1 and AHfo(CH2CH2CI) = 21.8 f 1, Do(CH2CICH2-H) = 100.7 f 1. These results compare satisfactorily with recent determinations by the monoenergetic electron impact method.

Reductive Chlorination and Bromination of Ketones via Trityl Hydrazones

“…[17] Theu tility of this method was further demonstrated by the reductive chlorination of the sterically encumbered hydrazone 11 j, derived from O-methyl estrone (entry 10). [18] In summary,w eh ave disclosed an ovel method for the conversion of ketones into the respective saturated alkyl chlorides.The key step involves chlorination of aketone trityl hydrazone,which upon warming fragments to give an a-halostabilized carbinyl radical that is then reduced by EtSH to furnish the alkyl chloride product. Theu nderlying concept of the reductive chlorination appeared transferrable to bromination.…”

Kinetics of the Reactions of CH3CH2, CH3CHCl, and CH3CCl2 Radicals with NO2 in the Temperature Range 221−363 K

“…In these reactions, 1-chloroethyl radicals (CH 3 CHCl) and 1,1-dichloroethyl radicals (CH 3 CCl 2 ) are the most probable products because the R-C-H bonds are the weakest carbon-hydrogen bonds of ethyl chloride and 1,1-dichloroethane. 19,20 In this paper we present results from direct time-resolved measurements of the rate coefficients of three radical reactions:…”

Reductive Chlorination and Bromination of Ketones via Trityl Hydrazones

“…Thee ffect of the different halides on selectivity appears to parallel that observed for other freeradical processes,i ncluding halogenations of alkanes.T he slightly higher selectivity seen for reductive chlorination versus bromination comports with greater stabilization of the radical accorded by chlorine over bromine,a sr eflected by CÀHb ond dissociation energies of simple haloalkanes. [18] In summary,w eh ave disclosed an ovel method for the conversion of ketones into the respective saturated alkyl chlorides.The key step involves chlorination of aketone trityl hydrazone,which upon warming fragments to give an a-halostabilized carbinyl radical that is then reduced by EtSH to furnish the alkyl chloride product. Them ethod is effective with ar ange of trityl hydrazones,a nd affords chloride products with stereoselectivities that may complement those available through ionic processes.T he basic transformation was also successfully demonstrated for the synthesis of alkyl bromides.…”