No abstract

A general mechanism for atomic halogenation reactions is considered, which is supposed to comprise all possible reactions steps in a great variety of cases. The kinetic analysis of this mechanism leads to the well‐known equations showing that the rate of these reactions may depend on the power 1/2, 1 or 3/2 of the halogen concentration. Rough estimates of activation energies and entropies of the elementary steps permit one to foresee the rate equation which must be expected in most specific cases and what changes may be caused by varying the experimental conditions (temperature, partial pressure). Similarity, the rules governing the competition of addition and substitution are given. Further it is shown that in many cases a halogen substitution reaction may undergo a fundamental change in mechanism, a sensitized dehydrohalogenation replacing the substitution reaction at a higher temperature and a lower halogen concentration.

La spCcificitC cles lialog6nat.ions atomiyues 111. La photochlorirration chi t6trachlorCthyl6ne pa.r J. ADAM (I), P . GOLDFINGER et P . A. GOSSELAIN (l) (Bruxelles) SUMMARY :According to the general discussion on atomic halogenation reactions, two distinct mechanisms must be expected for chlorine addition reactions, one at low, another a t high temperature. This is confirmed in the case of the photochlorination of gaseous C,Cl,, and permits one to determine the dissociation energy D~.,~i,--ci of the C,CI, radical (22,s Kcal/mole) and the steric factor (10-4y5) of the reaction step C,CI, + C1, -+ C,C1, + C1.

SUMMARYThe thermal reaction of N-chlorosuccinimide with toluene has been investigated and an atomic and free radical mechanism for the reactions of N-haloimides in non-polar media proposed. Because of the small steric factors of reactions between polyatomic particles, these reagents do not react directly, but act as a source of molecular halogen, which by an atomic mechanisni leads to the observed reactions. This explains the long induction period, followed by a rapid reaction, the accelerating effect of HC1 and H,O and the inhibition by 0,. It accounts also for the well-known specificity of compounds of this type t o substitute in a to a double bond.

SUMMARY :From 1300C, the photoahlorination of gaseous C,HCl, results in the formation of C,Cl according to the equationThe kinetic study of the transition betv e m the low temperature substitution reaction and the high temperature dehydrochlorination reaction permits one to confirm the values found previously for DC,CI,-CI and for the steric factor of the step