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-ethanedio l (197.0 +/- 1.7 kcal mol(-1)), 1,3-propanediol (210.3 +/- .0.5 kcal m ol(-1)), and 1,4-butanediol (218.3 +/- 1.0 kcal mol(-1)). The computed enthalpies of cyclization due to the formation of an intramolecular h ydrogen 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 alm ost strain free. Thermodynamic data for solvation of a protonated diol by ethanol or a second molecule of the same diol shows that the secon d diol behaves as a bidentate ligand, but whereas the structures for 1 ,3-propanediol and 1,4-butanediol are bicyclic, that for 1,2-ethanedio l 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 p rotonated acetaldehyde transfers a proton to neutral 1,2-ethanediol. T he reaction scheme has been modeled to yield an activation energy of 1 3.3 +/- 0.4 kcal mol(-1) for the dehydration reaction. Because the ''r ecycling'' 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, tolu ene, do not require correction for this thermal dehydration.