Anchimeric assistance in the gas phase dehydrohalogenation of 5‐chloro‐2‐methylpent‐2‐ene

Abstract

5-Chloro-2-methylpent-2-ene decomposes a t temperatures of 370-420°C with initial pressures from 61 to 158 torr to yield hydrogen chloride and a mixture of methylpentadiene isomers. In a static system, with seasoned vessel and propene inhibitor, the reaction is homogeneous, unimolecular, and of the first order. The rate coefficient is expressed by the following Arrhenius equation: log k (sec-') = (13.43 f 0.30) -(215.0 f 3.7) kJlmo1/2.303RT. The result of the present work additionally supports the participation of the neighboring aliphatic olefinic double bond in the rate of HC1 elimination of alkenyl chlorides in the gas phase. Moreover, it also confirms the three-membered conformation as the most favored structure for anchimeric assistance. IntroductionThe polar nature of the transition state in the gas phase pyrolysis of alkyl halides [l] has been found to be an essential condition for neighboring double bonds to assist the rate of HC1 elimination in alkenyl chlorides [2]. The three-membered conformation was shown to be the most favored structure for anchimeric assistance to occur. The results described in these pyrolysis works showed a close similarity of relative rates in the 7r-bond participation of compounds of analog systems in reaction solvolysis. That is, the rate of decomposition of 4-chlorobut-1-ene is by a factor of 4.7 faster than for n-butyl chloride, whereas in 98% formic acid, 3-buten-1-yl tosylate solvolyses faster than n-butyl tosylate by a factor of 3.7 [3].A classical study of a neighboring aliphatic olefinic double-bond participation is the acetolysis of 4-methyl-3-penten-1-yl tosylate [4]:

Stereochemistry of the four-centered gas-phase dehydrohalogenation reaction

“…This view is consistent with the ion-pair model of Benson and co-workers,37"39 the decrease in the Arrhenius activation energy upon alkyl substitution at the a-or /3-carbons, and the anchimeric assistance found for alkyl chlorides with a double bond in the /3-position. 40 Molecular orbital theory was used by Hiberty9 to calculate potential energy surfaces corresponding to syn elimination and planar and nonplanar anti elimination of HC1 from ethyl chloride. The syn mechanism having an activation barrier 9.8 kcal/mol lower than the planar anti energy was favored.…”

Pyrolysis of 5‐chloropentan‐2‐one and 4‐chloro‐1‐phenylbutan‐1‐one. Participation of the carbonyl group

“…(5). (5) suggests a modification of the highly restricted rotation of the transition state as an intimate ion pair for the reported gas-phase elimination of alkenyl [2,3]. The ketoalkene product 1-penten-4-one in the presence of gaseous HC1 and a propene inhibitor, under the experimental conditions used, isomerizes in very high yield (~9 0 % ) to the conjugated 2-penten-4-one.…”

“…However, a seven-membered cyclic transition state in the gas phase is unlikely. An olefinic double bond at the y position of alkenyl halides has been found to assist the rate of dehydrohalogenation of these types of compounds [2,3]. Consequently it is possible to believe that the mechanism for the haloketone decomposition may well be as described in eq.…”

“…These observations favor the choice of the alternate mechanisms described above, namely, that the dehydrochlorination process of these chloroketones proceeds through participation of the carbonyl group [eqs. (3) and (4)]. …”