Acetic acid and Chloroacetic acid have carboxylic acid (R-COOH) as the principle functional group. The structural difference between the two is the replacement of the hydrogen in acetic acid (H-CH2-COOH) with the chloro group in chloroacetic acid (Cl-CH2-COOH). When such a carboxylic acid, is placed in an aqueous medium, it ionizes into giving proton (H+) and an anion (conjugate base). R-CO-O-H (acid) → R-COO- (conjugate base) + H + (proton)Ionization of carboxylic acid in aqueous medium The carboxylic acid strength thereby depends upon two factors- a) ease of proton (H+) liberation and b) stabilization of the anion by the inductive or the resonance effects. Ease of proton liberation- The proton can be easily liberated if the strength of the bond carrying the acidic proton (-O-H) is decreased. This can be done by introducing an electron withdrawing group such as chloro at the alpha carbon next to the carboxylic acid functional group. The chloro group by the negative Inductive effect (electron withdrawing -I effect) will pull the electron density away from the oxygen, weaken the O-H bond and easily liberate the H+.The mechanism of the proton loss can be explained by its resonance structures that are shown below. In the first step, the lone pair of an electron on the hydroxy oxygen is delocalized by resonance that further pushes the pie electrons on the carbonyl to its oxygen giving it a negative charge. Due to loss of its electrons, the hydroxy oxygen now carries a positive charge that is highly electron withdrawing in nature and unstable. It tries to pull the electron density of the O-H bond towards itself and breaks the bond. The proton is lost (in the aqueous medium H+ exists as H3O+) and stable divalent oxygen is obtained in the second step. The carboxylate anion formed is called as the conjugate base of the chloroacetic acid. The mechanism of the proton loss is same for both, the acetic acid and the chloroacetic acid but the stability of the anion is the primary factor that determines its acid strength.Anion Stability- The anion (-COO-) formed is resonance stabilized with the negative charge delocalized over the two oxygen atoms and is electron dense due to which it is electron donating in nature by the positive inductive effect (+I Effect). The excess of this negative charge is stabilized by chloro by spreading it across a larger distance. This adds to the conjugate base stability due to which chloroacetic acid exists in the stable ionized form as Cl-CH2-COO-. In case of the anion of the acetic acid, the methyl group instead of dispersing the already built up electron density, it further donates more electrons (+ I effect). This destabilizes the anion due to which it mainly exists in the more stable unionized form as CH3COOH. In addition to the resonance and the inductive stabilization of the anion, an intramolecular coulombic attraction known as the dipolar effect exists for the anion of chloroacetic acid . It is due to the presence of the C-Cl bond dipole that causes the carbon to acquire a positive charge, and it attracts the negatively charged anionic oxygen more strongly by the inductive effect further adding to the stability. Such an outcome does not exist for acetic acid. Thus, - Ease of liberation of H+ - stabilization of the negatively charged conjugate base over a larger area due to the -I effect of Chloro and - dipolar effect makes chloroacetic acid, a stronger acid over acetic acid. Remember that an acid strength is also determined by its pKa or Ka values. Thus, smaller the pKa or larger the Ka, stronger is the acid. Chloroacetic acid has lower pKa than the acetic acid.Also, note that,1) dichloro and trichloroacetic acid will be a stronger acid than chloroacetic acid (increase in the number of electron withdrawing groups) and2) changing the position of chloro from the alpha (2-chlorobutanoic acid) to the beta (3-chlorobutanoic acid) or the gamma (4-chlorobutanoic acid) position will impact the acid strength. Farther away is the electron withdrawing group, weaker is the inductive stabilization of the anion, and lower is the acid strength.