The effect of cycloalkanes on the reaction of hexane with trifluoromethanesulfonic acid

Abstract

The chain reaction of hexane catalyzed by trifluoromethanesulfonic acid (TFMSA) under mild conditions (disproportionation and cracking) was suppressed by small amounts (1–5%) of cycloalkanes, namely methylcyclopentane (MCP), cyclohexane (CH) and cyclopentane (CP). The reaction then proceeded in the isomerization mode. The addition of ferric ions to the acid overcomes the stabilizing effect of cycloalkanes. The formation of substituted allyl cations during the induction period for cracking was also inhibited by very small amounts of MCP. Addition of MCP or 3-methylhexane had the same effect on reduction of the induction period and (minor) on the increase in rate of the reaction stabilized against cracking by CP addition. Incorporation of deuterium into the products from hexane and MCP-1-d shows that MCP acts as a hydride relay in the reaction. Hydride transfer catalysis by the reaction product is disproved for any catalyst. Both the reaction of hexane with MCP-1-d and of uniformly labeled hexane-U-d_4.3 with MCP showed H/D scrambling which is extensive among all products (2- and 3-methylpentane, cracking products and CH) and the acid catalyst, smaller in MCP, and is virtually zero in hexane. Thus, the main features of the reaction are the initiation by oxidation and the formation and hydronation of alkenes.