The gas-phase acidities of the alkanes

The gas-phase acidities of 15 simple alkanes have been determined in a flowing afterglow-selected ion flow tube (FA-SIFT) by a kinetic method in which alkyltrimethylsilanes were allowed to react with hydroxide ions to produce a mixture of trimethylsiloxide ions by loss of alkane and alkyldimethylsiloxide ions by loss of methane. The reaction is proposed to proceed by addition of hydroxide ion to the silane to form a pentacoordinate siliconate ion intermediate which decomposes through two transition states, one in which negative charge is placed on a methyl group and the other in which negative charge is placed on the alkyl group. The ratio of siloxide ions produced is proposed to be correlated to the relative basicity of the methyl and alkyl anions. The method is calibrated by making use of the known acidities of methane (/Delta/H/sub acid/) = 416.6 kcal/mol and benzene (/Delta/H/degree//sub acid/ = 400.7 kcal/mol). In general, methyl substitution is found to stabilize alkyl anions in the gas phase except that the ethyl anion is found to be more basic than the methyl anion. By combining the gas-phase acidities with the bond dissociation energies, the electron affinities (EA) of the corresponding alkyl radicals can be calculated. Many simplemore » alkyl radicals are found to have negative EA's. The results for the alkyl groups studied are given.« less