Hydrogen Abstraction by Chlorine Atoms

Hydrogen Abstraction by Chlorine Atoms. Absolute rate constants and Arrhenius parameters for the chlorination of a series of dilorine bstituted methanes and their deuteriated analogues have been obtained by Oyne and Walker, using mass spectrometric analysis of molecular reactant consumption in excess dilorine atoms. Uncertainties in the kinetic parameters for Cl + H2, the reference reaction in competitive dilorinations, are discussed. 

Rate-constant ratios of kcnjkco — (0.70 0.04) exp(3.2 0.1 kJ taol IRT) and Achj cia/Arcm cij = (0.97 0.05) exp(1.4 0.1 kJ mol /RT) over a range of temperatures are in agreement with predictions of the semi-empirical BEBO method. 

Oyne and Walker studied reaction (26) at elevated temperatures (300—686 K) 

303 RT, is in agreement with the results of mass-spectrometric kinetic measurements at 298 K. However, a recent measuronent by Watson and Davis, using flash photolysis and resonance fluorescence in the temperature range 218—401 K, gives the result logio 26 = -1U2 -(10.20 0.42) kJmol / 

303 RT. The lower activation energy found by these workers would indicate a rate constant for reaction (26) that is almost a factor of two greater than that predicted from the results of Clyne and Walker at the stratospherically important temperature of 220 K. 

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Molecular mechanics calculations on transition states for hydrogen abstraction by chlorine atoms attached to iodine of various iodoaryl substrates help in designing effective systems for chlorination at the desired position [40], The chloro-deriva-tives are normally further dehydrochlorinated in order to generate unsaturated products. This elimination, although sometimes spontaneous, was generally difficult to achieve. To overcome this problem, the substrates were brominated by adding an... 

Another example of a solvent-dependent atom-transfer reaction is hydrogen abstraction by chlorine atoms during the photochemical chlorination of hydrocarbons with molecular chlorine for an excellent review, see reference [571]. Russel reported that in the photochlorination of 2,3-dimethylbutane, according to reaction scheme (5-68), certain solvents do not have any effect on the selectivity of the reaction as measured by the rate ratio whereas other solvents increase this ratio significantly (c/. 

When methane is allowed to react with an equimolar mixture of chlorine and bromine, only hydrogen abstraction by chlorine atoms is observed. Explain. 

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