P967: Kinetics of cyclohexenyl cation formation: An ideal experiment for physical chemistry or advanced organic chemistry lab
Carbocation intermediates typically have half-lives of microseconds and shorter, however cycloalkenyl carbocations are non-typical, exhibiting nearly indefinite existence in highly acidic media. This stability is attributed to resonance stabilization and reduced ring-strain due to planar configuration of the conjugated pi-system. In addition, cycloalkenyl cations exhibit a strong UV absorbance band (lambda-max ~300 nm) and are easily formed by introduction of cyclohexanol at ~0.1 mM in 80-96% (w/w) H2SO4. Whereas acid-catalyzed dehydration of cyclohexanol at concentration used in undergraduate organic lab results in formation of cyclohexene end product, at low concentration (~0.1 mM) the high acidity – relative to cyclohexanol – supports cyclohexanyl carbocation intermediate which abstracts hydride ion from a saturated alpha-carbon on unreacted cyclohexene forming the highly stable cyclohexenyl cation (see Figure). The combination of long residence time, strong UV absorbance band, and ease of reaction makes formation of the cyclohexenyl cation from cyclohexanol highly amenable to kinetic study and therefore well-suited for an undergraduate laboratory experiment in physical chemistry illustrating kinetic treatment of a reaction, or in advanced organic chemistry focusing on formation of a non-typical carbocation. At 60 oC and 89% (w/w) H2SO4, reaction with cyclohexanol is complete (equilibrium reached) in 20 minutes and shows first-order dependence with rate constant on the order of 0.0010 s-1.