The mechanism of the thermal decomposition of a series of alkyl trimethyl ammonium hydroxides ([RMe3N][OH], R=Et, n-Pr, i-Bu, PhCH2, Me3CCH2) was studied using TGA, evolved gas analysis and NMR spectroscopy due to the importance of these and related ions in anion exchange fuel cell membranes. Isotopic labeling with deuterium showed that deprotonation of the methyl groups of the ammonium ions by deuteroxide establishes a rapid equilibrium between the tetraalkyl ammonium ions and the nitrogen ylide species and water that scrambles the deuterium with the proton on the methyl groups. The products of the thermal decomposition when R= Et, n-Pr, i-Bu are predominately olefins arising from Hoffmann elimination, while the neopentyl substituted ammonium ion gives only neopentyl trimethyl amine and methanol, the products of SN2 attack of hydroxide on the methyl groups.  DFT studies of these reactions confirm the relative activation barriers that are observed in the experimental decomposition studies.

Decomposition pathways of tetraalkylammonium hydroxides: Experimental and DFT studies and their implications for alkaline exchange fuel cell membranes

243rd National Meeting (2012)

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