In simple carboxylic acids, both oxygen atoms are equivalent due to resonance, and their electron density is similar, making it difficult to determine which has a more negative charge. During esterification in an acidic environment, the proton attacks the double-bonded oxygen, leading to a protonated carboxylic group that loses its symmetry and resonance. The protonation of either oxygen occurs randomly, but this does not affect the final product due to the free rotation around the carbon bond. The key intermediate in the reaction is a doubly protonated carboxyl group, where both oxygens are protonated, creating a carbocation with a significant partial positive charge. This carbocation is susceptible to nucleophilic attack, typically by the oxygen of a hydroxyl group, facilitating the esterification process.
