Structure of Protonated Formic Acid. 1. Conformation.

The carbonyl activity of carbxylic acids does not suffice to cause the addition of neutral nucleophiles. Therefore, in the first step of the Fischer esterification the carboxylic acid is protonated at the carbonyl-O to increase the carbonyl activity. The structure shown is not tbe most stable structure of protonated formic acid; this structure is 4.8 kcal/mol higher in energy. The proton affinity of formic acid is greater than the proton affinity of water and the protonation of formic acid by hydronium is about 1.2 kcal/mol exothermic.

Figure. The MP2(full)/6-31G* optimized structure of protonated formic acid. In this conformation, the O-H bonds are aligned.