ACIDITY OF HYDROCARBONS .32. KINETIC ACIDITIES OF HYDROGENS IN TRIPTYCENE TOWARD CESIUM CYCLOHEXYLAMIDE

Bridgehead tritium exchange from triptycene with cesium cyclohexylamide in cyclohexylamine at 25° is 0.24 as fast as that of benzene. The primary isotope effect, kD/kT = 2.2, and the relative rates of m-methyl (0.51) and p-methyl (0.34) ring substituents indicate that the transition state for exchange has a substantially broken C-H bond and high carbanion character with essentially no internal return. The kinetic acidity of the bridgehead position is accounted for quantitatively by the electron-attracting polar effect of the aromatic carbons and by the s character of the C-H bond. Of the aryl hydrogens, the kinetic acidity of the 1-hydrogen is 7.3 times that of the 2-hy-drogen; this difference is interpreted on the basis of increased p character of the bonding orbitals to the bridgehead carbons. © 1969, American Chemical Society. All rights reserved.