If HCl is a weaker acid than HBr, why is ClCH 2 COOH a stronger acid than BrCH 2 COOH

Chapter 2, Problem 31

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If HCl is a weaker acid than HBr, why is ClCH 2 COOH a stronger acid than BrCH 2 COOH? Solution To compare the acidities of HCl and HBr, we need to compare the stabilities of their conjugate bases, Cl- and Br - . (Notice that an HCl bond breaks in one compound and an HBr bond breaks in the other.) Because we know that size is more important than electronegativity in determining stability, we know that Br - is more stable than Cl- . Therefore, HBr is a stronger acid than HCl. In comparing the acidities of the two carboxylic acids, we again need to compare the stabilities of their conjugate bases, ClCH 2 COO - and BrCH 2 COO - . (Notice that an OiH bond breaks in both compounds.) The only way the conjugate bases differ is in the electronegativity of the atom that is drawing electrons away from the negatively charged oxygen. Because Cl is more electronegative than Br, Cl exerts greater inductive electron withdrawal. Thus, it has a greater stabilizing effect on the base that is formed when the proton leaves, so the chlorosubstituted compound is the stronger acid.