Chloroethyl radicals

Fig. 63. Molecular arrangement in (a, c) plane of a mixed ethylene-chlorine binary crystal illustrating (a) radical pair formation, (b) single chain growth and (c) chain growth in the vicinity of product line. Molecules labelled 1-4 are ethylene (C2H4), chlorine, chloroethyl radical (C2H4CI) and anti 1,2-dichloroethane (C2H4CI2), respectively.

The effects of substituents in the -position to the radical center are mostly inductive in nature. Comparison of the RSE values for the ethyl radical (- 13.8 kj/mol) with those of the propyl, 2-hydroxyethyl, 2-fluoroethyl, and 2-chloroethyl radicals with RSE values of - 11.9, - 8.5, - 5.5, and - 6.7 kj/mol also indicates that electronegative substituents in the -position uniformly destabilize the radical center, the effect being larger for more electronegative substituents. Comparison of the RSE values of the 2-fluoro, 2,2-difluoro, and 2,2,2-trifluoroethyl radicals of - 5.5, + 3.0, and + 8.1 kj/mol also indicate that these effects can accumulate to yield overall destabilized radicals relative to the methyl radical. Even less favorable RSE values are found for positively charged substituents directly attached to the radical center such as - NH3+ (+ 18.3 kj/mol) or-SH2+ (+ 12.8 kj/mol) (Table 1). 

The substitution reaction of CP with methyl chloride, 2-chloroethyl radical, and allyl chloride has been treated by several different ab initio theoretical models. Depending on the method, the intrinsic barrier for the 5ivr2 process in allyl chloride is 7-11 kcalmoP higher than the barrier for the 5ivr2 reaction of methyl chloride. The reaction of CP with the 2-chloroethyl radical involves an intermediate complex, which is best described as an ethylene fragment flanked by a resonating chloride anion-chloride radical pair. There are many other points of interest. 

Alkanes. The chlorination of ethane known to produce more 1,1-dichloroethane than 1,2-dichloroethane is explained by the so-called vicinal effect.115 One study revealed285 that this observation may be explained by the precursor 1,2-dichloroethane radical (the 11 2-chloroethyl radical) thermally dissociating into ethylene and a chlorine atom [Eq. (10.54)]. Indeed, this radical is the major source of ethylene under the conditions studied. At temperatures above 300°C, the dissociation dominates over the chlorination reaction [Eq. (10.55)], resulting in a high rate of ethylene formation with little 1,2-dichloroethane ... 

Knyazev et al. have reported the decomposition of the 1-chloroethyl radical by photoionization mass spectrometry [127]. Rate coefficients, determined as a function of temperature (848-980 K) and bath gas density (3-22 x 1016 molecules cm"3) in He, Ar, and N2, were in the unimolecular falloff. The falloff behavior was modeled by a master equation analysis. 

A well-resolved spectrum of the /2-chloroethyl radical was obtained by Kawamura and coworkers in solution at various temperatures194. The negative g-shift indicated that the unpaired electron was delocalized on orbitals around the chlorine nucleus. Moreover, the two /2-protons were equivalent and exhibited a coupling (10.25 G) which was inferior to the minimum value expected by the (A + B cos26) law generally used for protons in the /2-... 

The photolysis data indicate the product ratio, 2,3-C4H8Cl2/l,3-C4H8C12 = 3.2. Since all of the chlorobutane products (including 2-C4H9Cl), are formed by appropriate combinations of H3C—CH—Cl and H2C—CH2—Cl radicals, it can be deduced from this ratio, ignoring possible differences in steric factors, that abstraction of a H atom from the a-carbon of ethyl chloride is 7.4 times more probable than abstraction from the /2-carbon. This comparatively low concentration of the -radical accounts for the failure to observe 1,4-dichlorobutane as a product in these experiments since its concentration would be (1/7.4)2 times the 2,3-dichlorobutane concentration and therefore too small to be observed by our analytical methods. The small quantity of 1-chlorobutane observed as a photolysis product can be explained similarly since it would also involve the /2-chloroethyl radical. 

Fig. 23.13. The Srn2 and Skn2 mechanisms in the reactions between Cl and (3-chloroethyl radical. The mechanism is stepwise and the structure of the intermediate is shown at the bottom of the figure.

The rate coefficients of the metathetical reactions of chloroethyl radicals with HCl and Clj have been determined by kinetic analyses of photochlorination reactions in steady or intermittent light or by competitive techniques [68, 359]. 

The photodissociation of l-bromo-2-chloroethane at 193 nm demonstrated that there were no stable bromoethyl or chloroethyl radicals formed. Dissociation of these radicals is rapid and affords ethene. 1-Chloro-1,1,3,3,3-pentafluoropropane can be made regioselectively by the photochlorination of 1,1,1,3,3-pentafluoropentane. 

All these factors make it especially difficult to deduce the structures of radicals from the coupling constants. Not long ago several groups argued that the unusually small values of a" for 2-fluoroethyl and 2-chloroethyl radical indicated that these radicals possessed distorted, bridged structures, XLII (69,70,92). The reduced values were related to the increased distance between... 

CHslsC—p—CH2—CHCI 2-ferf-Butoxy-l-chloroethyl radical... 

Chlorine elimination reactions. Chloroethyl radicals CClEt) formed by Cl or Br transfer reactions in cyclohexane can subsequently eliminate a Cl atom by reaction 34 or abstract a H atom from the solvent by reaction 35. The rate constant ratio 34/ 35 el/ H related Arrhenius parameters can be... 

There are no definitive chemical activation data or RRKM calculations for the unimolecular lifetimes of the bromo- or chloroethyl radicals. Recent work has given half-quenching pressures of 800 and 150 torr for C2H4Cl and CsHeCl formed by chlorine atom addition to ethene and pro-pene. ... 

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