Carbon Acidity. 58. Hydrogen Isotope Exchange Kinetics of Propylene with Lithium Cyclohexylamide

Article abstract of DOI:10.1021/ja00411a031

Rates for hydrogen isotope exchange with lithium cyclohexylamide (LiCHA) in cyclohexylamine-N,N-d2 (CHA-D2) and cyclohexylamine (CHA) are compared for propene and toluene.Benzyl hydrogens in toluene were found to be 6.2 times more reactive than allyl hydrogens in propene.Experimental primary isotope effects are also reported for propene: k^D(exptl)/k^T(exptl)=2.9, k^H(exptl)/k^T(exptl)=30, and k^H(exptl)/k^D(exptl)=10.2.Derived internal return values show comparable values for propene and toluene.Analysis of polydeuteration rates shows that exchange is accompanied by complete equilibration of allyl positions.The high isotope effects and comparable acidities and internal return imply a transition state for propene which resembles that of toluene.An equilibrium pK_a on the CsCHA scale of ca. 43 is deduced for propene.