Decide which of the following acids is strongest and explain why: CF3COOH, CCl3COOH, CBr3COOH

Question

Decide which of the following acids is strongest and explain why: CF3COOH, CCl3COOH, CBr3COOH

Answer 1

The strength of these three acids are: CF₃ COOH > CCl₃COOH > CBr₃COOH. This is because the electronegative halogen atoms delocalizes the negative charge on the carboxylate anion, which stabilizes the conjugate base, and the more electronegative the halogen the greater this stabilization becomes. Since the electronegativity decreases from F>Cl>Br, this results in the order of acid strength shown.

Answer 2

Acid strength from strongest to weakest:

CF₃COOH > CCl₃COOH > CBr₃COOH

Acid strength depends on the ability of the substance to easily donate a proton (H⁺). The electronegativity of an atom (or atoms) on the alpha carbon adjacent to the carboxylic acid functional group can affect acid strength. The greater the electronegativity, the stronger the acid. In all 3 acids, the alpha carbon contains halogen atoms. F is the most electronegative, then Cl and finally Br.

The dissociation reaction of the acid can be written as:

CX₃COOH ⇔ H⁺ + CX₃COO^- (where X = F, Cl or Br)

Since the halogens are electron withdrawing, they increase delocalization of the negative charge (electrons) in the conjugate base (CX₃COO^-) thereby increasing its stability. This in turn, increases the acid strength.