The functional group in the molecule CH3COOH acetic acid responsible for its acidic properties is the carboxyl group -COOH . The carboxyl group consists of a carbonyl group C=O and a hydroxyl group O-H bonded to the same carbon atom.The mechanism of its reaction with a base involves the donation of a proton H+ from the carboxyl group to the base. This process is called proton transfer or acid-base reaction. Here's a general outline of the mechanism:1. The lone pair of electrons on the base usually a hydroxide ion, OH- or another negatively charged species attacks the hydrogen atom of the carboxyl group in the acetic acid molecule.2. The O-H bond in the carboxyl group breaks, and the electrons from this bond move to the oxygen atom, forming a negatively charged acetate ion CH3COO- .3. The base accepts the proton H+ from the acetic acid, forming a water molecule H2O or another neutral species.As a result, the acetic acid molecule loses a proton and becomes an acetate ion, while the base gains a proton and forms a neutral species. This proton transfer is the main reason for the acidic properties of acetic acid and other carboxylic acids.