Question

when the acid constants of tert-butyl alcohol and methanol are determined in the gas phase, tertiary alcohol is found to be a stronger acid. This result is opposite in aqueous medium. Consider the effects of solvation to explain these different results.

Answer 1

Answer-

The general explanation is that the larger substituents are better electron donors, which destabilize the resulting alkoxide anions. But it is completely opposite in gaseous phase. The relative acidities in the gas phase are opposite to those in aqueous solution.

The inversion of the acidities of alcohols between the gas phase and aqueous solution was pointed out by Brauman and Blair in 1968. They proposed that the ordering of acidities of alcohols in solution is predominantly due to the combination of a) polarizibility and b) solvation, and that the electron donating ability of the substituent does not play a significant role.

Polarizibility almost completely accounts for the trend in gas-phase acidities. As the size of the substituent increases, the acid becomes stronger due to the ability for the charge to be distributed over a larger volume, thereby reducing the charge density and, consequently, the Coulombic repulsion. Therefore, in the gas-phase, tert-butyl alcohol is the most acidic alcohol, more acidic than methanol. In the gas phase, tert-butyl alcohol is more acidic than methanol, which is consistent with the difference in polarizibility between a isopropyl group and a methyl group. In the absence of a solvent, the gas-phase properties reflect the instrinsic effects on the acidities.

In solution, however, the ions can be stabilized by solvation, and this is what leads to the inversion of acidity ordering. Brauman and Blair showed that smaller ions are better stabilized by solvation, which is consistent with the Born equation. Therefore, methanol is more acidic than tert-butyl alcohol because the smaller methoxide ion has a shorter radius of solvation, leading to a larger solvation energy, which overcomes the stabilization that results from polarization of the charge.