Proton affinities of saturated aliphatic methyl esters

The kinetic method was used to determine the proton affinities of methyl esters of several saturated fatty acids. Decompositions of the proton-bound dimers of the methyl esters, AHB+, were observed under different conditions with two instruments. The proton affinities (PAs) of the methyl esters increase continually with increasing carbon number in the acid. Equilibrium and initial rate experiments were performed with a Fourier transform ion cyclotron resonance mass spectrometer on the methyl ester of the C22 saturated acid (methyl behenate). These experiments give values for PA (methyl behenate) that are perhaps slightly lower than those obtained with the kinetic method. The PAs of the methyl esters of the fatty acids could be correlated with the equation: PA (ester) = (40.0 +/- 2.5)*log(n) + (784.7 +/- 3.9) kJ/mol or PA (ester) = (864 +/- 2) - (479 +/- 41)/n, where n = number of atoms in the molecule. Proton affinities of smaller sets of 1-alkylamines and 1-alkanols can be fit to similar equations.