HYDROGEN CHLORIDE CATALYZED ISOMERIZATION OF 1,1-DIMETHYLCYCLOPROPANE

Gaseous 1,1-dimethylcyclopropane (DMC) isomerizes in the presence of HCl at 420-476° to a nonequilibrium mixture of the three isomeric methylbutenes. The reaction is apparently homogeneous and nonradical and, for pDMC = 2-33 cm and pHCI = 2-24 cm, follows the rate law –d[DMC]/dt = ku[DMC] + kc[DMC][HCl]. The results are consistent with the occurrence of two concurrent processes: (i) a unimolecular isomerization2 (rate, ku[DMC]) mainly to 2-methylbut-2-ene (2MB2) and 3-methylbut-1-ene (3MB1), and (ii) a HCl-catalyzed isomerization (rate, kc[DMC][HCl]) mainly to 2-methylbut-1-ene (2MB1). There is subsequent isomerization among the methylbutenes toward an equilibrium mixture, this process also being HC1 catalyzed. Values of ku for process i are in good agreement with the findings of other workers.2 It is proposed that process ii is bimolecular and the formation of 2MB1 proceeds via a six-centered transition state3 which may be quasi-heterolytic. The variation of kc with temperature is expressible as kc = 2.3 × 1018 exp(–53,800/RT) sec−1 ml mole−1. There is evidence that at pressures of DMC below 20 mm, isomerization in the presence of HC1 also proceeds via a free-radical process. © 1969, American Chemical Society. All rights reserved.