BETA-HYDRIDE ELIMINATION FROM ALKYL AND CYCLOALKYL GROUPS ON A CU(100) SURFACE - RING STRAIN AND PLANARITY OF THE TRANSITION-STATE

Alkyl (C-5-C-6), cycloalkyl (C-4-C-7), and 2-bicyclo[2.2.1]heptyl (norbornyl) groups have been generated on a Cu(100) surface by the dissociative adsorption of their bromo derivatives. All these groups decompose by a beta-hydride elimination reaction upon heating the surface, and the product alkenes which are evolved from the surface were detected by mass spectrometry. Studies in which cyclohexyl-d(1) groups were generated on the surface by reacting D atoms with physisorbed cyclohexene show that the coadsorbed bromine atoms in the bromoalkane studies do not have a significant effect on the rate or mechanism of these reactions. A particularly interesting finding from these studies is that the rates of beta-hydride elimination for these structurally similar alkyl groups vary by 6-7 orders of magnitude if one extrapolates the rates to a common reaction temperature of 200 K. Assuming a common, first-order preexponential factor of 10(13) s(-1) gives the following relative rate scale (normalized to 1 for the transformation cyclohexyl --> cyclohexene): cyclobutyl (0.6) < cyclohexyl (1) < norbornyl (300) < 3-hexyl (3 x 10(4)) approximate to 3-pentyl (5 x 10(4)) < cycloheptyl = cyclopentyl (10(6)). Thermodynamic differences between these reactions, as qualitatively addressed by differences in strain energy between reactant and product, may play a role in the 20-fold difference in rate between cyclopentyl and 3-pentyl, but thermodynamics cannot account for the similar to 4 order of magnitude larger rate for 3-hexyl relative to cyclohexyl. We suggest that the dramatic rate difference between these two secondary alkyls is due to the different free energy changes required to achieve a planar geometry in the transition state, i.e., a dihedral angle of 0 degrees between the substituents on alpha- and beta-carbons.