THE ENOLS OF ACETIC-ACID AND METHYL ACETATE

We have determined the heat of formation of 1,1 -dimethoxyethene, Delta H-f(l) = -75.56 +/- 0.87 kcal mol(-1), by measuring the heat of hydrolysis. The heat of vaporization, 8.47 +/- 0.10 kcal mol(-1), was estimated from the boiling points at various pressures. The standard entropy of gaseous 1,1-dimethoxyethene, 82.02 +/- 2. cal deg(-1) mol(-1), was calculated using frequencies obtained by MO calculations at the 6-31G** level. The free energy of transfer, 1.02 +/- 1, was estimated by additivity with allowance for a distant polar interaction. Thus the free energy of formation in aqueous solution, Delta G(f)(aq) = -37.07 +/- 1.48 kcal mol(-1), could be calculated. The free energies of hydrolysis for enol ethers appear to be insensitive to structure; we used an average value and so calculated the values of the free energies of formation of the enols of acetic acid and methyl acetate, -67.90 +/- 1.67 and -52.48 +/- 1.53 kcal mol(-1), respectively. These values are in good agreement with other recent evaluations. This method should be applicable to related enols.