Energetics and mechanism of alkyl cleavage transition states: Relative gas-phase acidities of alkanes

The product branching ratios from the infrared multiple-photon dissociation (IRMPD) of a series of tertiary alkoxides are examined and an overall orde r for the relative acidities of several alkyl groups is inferred. Compariso n with the order of acidities derived from a similar silane system reveals an overall similarity, although small differences exist. A model system, th e reaction of phenyltrimethylsilane plus hydroxide to give either benzene o r methane, is studied more closely. Analysis of the energetics of the alkox ide system and the silane system using statistical reaction rate theory (RR K and RRKM) reveals differences between the mechanisms of decomposition for the systems. These discrepancies between the known, experimental product r atio and the energetics suggest that the transition state of pentacoordinat e hydroxysilane anion decomposition has concerted, albeit polar character. Decomposition of the pentacoordinate adduct formed by the reaction of metho xide plus phenyltrimethylsilane is consistent with this suggestion. Caution should be used when inferring the relative acidities of alkyl groups from kinetic methods when assumptions of the decomposition mechanism have not be en validated.