A HIGH-PRESSURE MASS-SPECTROMETRIC DETERMINATION OF THE PROTON AFFINITIES OF C-2, C-3, AND C-4 ALPHA,OMEGA-ALKANEDIOLS

Pulsed electron beam high pressure mass spectrometry has been used to obtain proton affinities for some alpha,omega-diols viz. 1,2-ethanediol (197.0 +/- 1.7 kcal mol(-1)), 1,3-propanediol (210.3 +/- .0.5 kcal mol(-1)), and 1,4-butanediol (218.3 +/- 1.0 kcal mol(-1)). The computed enthalpies of cyclization due to the formation of an intramolecular hydrogen bond in the protonated diols increases with carbon number C-2 (-6 kcal mol(-1)), C-3 (-19 kcal mol(-1)), and C-4 (-27 kcal mol(-1)). The last value shows the formation of a cyclic structure which is almost strain free. Thermodynamic data for solvation of a protonated diol by ethanol or a second molecule of the same diol shows that the second diol behaves as a bidentate ligand, but whereas the structures for 1,3-propanediol and 1,4-butanediol are bicyclic, that for 1,2-ethanediol appears to be monocyclic with two intermolecular hydrogen bonds. The thermal dehydration of protonated 1,2-ethanediol has been identified and investigated and found to be an important process. The resulting protonated acetaldehyde transfers a proton to neutral 1,2-ethanediol. The reaction scheme has been modeled to yield an activation energy of 13.3 +/- 0.4 kcal mol(-1) for the dehydration reaction. Because the ''recycling'' of the proton from acetaldehyde to diol is rapid under the experimental conditions employed, the normal equilibrium measurements of proton transfer between 1,2-ethanediol and the reference base, toluene, do not require correction for this thermal dehydration.