The Gas-Phase Acidities of the Alkanes

Article abstract of DOI:10.1021/ja00188a003

The gas-phase acidities of 15 simply alkanes have been determined in a flowing afterglow-selected ion flow tube (FA-SIFT) by a kinetic method in which alkyltrimethylsilanes are 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 (ΔH⁰_acid=416.6 kcal/mol) and benzene (ΔH⁰_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 that 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 simple alkyl radicals are found to have negative EA's.The results for the alkyl groups studied are as follows, where the first number is the ΔH ⁰_acid (kcal/mol) of the corresponding alkane and the second number (in parentheses) is the EA (kcal/mol) of the alkyl radical: ethyl 420.1 (-6.4), isopropyl 419.4 (-9.5), cyclobutyl 417.4 (-7.5), cyclopentyl 416.1 (-7.0), sec-butyl 415.7 (-5.8), n-propyl 415.6 (-1.9), tert-butyl 413.1 (-5.9), isobutyl 412.9 (0.8), 3-butenyl 412.0 (1.7), cyclopropyl 411.5 (8.4), cyclopropylmethyl 410.5 (3.2), 1-methylcyclopropyl 409.2 (8.0), neopentyl 408.9 (4.8), vinyl 407.5 (16.1), 2-propenyl 405.8 (15.8).