8.6 Molecular Structure of Acids and Bases

When an acid HA gives up H⁺, it leaves behind A⁻, which carries the electron pair from the old bond. The more easily A⁻ can hold that negative charge, the more readily HA gives up its proton, and the stronger the acid.

Look at the anions left behind by the strong acids HCl, HBr, HI, HNO₃, HClO₄ and H₂SO₄: Cl⁻, Br⁻, I⁻, NO₃⁻, ClO₄⁻ and HSO₄⁻. Each one holds its negative charge comfortably, thanks to a large or electronegative atom, resonance over several oxygens, or both. That makes them extremely weak bases: they almost never take a proton back from water. (HSO₄⁻ can still give up its own proton, so it acts as a weak acid, but it doesn't act as a base.) Likewise, strong bases have very weak conjugate acids.

Electronegative atoms near the acidic hydrogen pull electron density toward themselves through the bonds. This is called the inductive effect. It weakens the O–H bond and spreads out the negative charge in the conjugate base, making the acid stronger.

Example: chloroacetic acid, ClCH₂COOH (pKa about 2.9), is much stronger than acetic acid, CH₃COOH (pKa 4.74), because the Cl atom pulls electron density away from the COOH group. Adding more Cl atoms makes the acid stronger still, and the effect weakens the farther the electronegative atom is from the acidic group.

The same idea ranks oxyacids with the same structure. In HOCl, HOBr and HOI, the more electronegative the halogen, the stronger the acid: HOCl > HOBr > HOI. Adding more oxygen atoms to the central atom also strengthens the acid: HClO < HClO₂ < HClO₃ < HClO₄.

If the negative charge on the conjugate base can be spread over several atoms by resonance, the base is more stable and the acid is stronger.

In a carboxylic acid, R–COOH, losing H⁺ gives a carboxylate ion, R–COO⁻, whose negative charge is shared equally between two oxygen atoms. In ethanol, CH₃CH₂OH, the conjugate base has its charge stuck on one oxygen. That's why acetic acid (pKa 4.74) is enormously more acidic than ethanol (pKa about 16), even though both lose H⁺ from an O–H group. Only the H in the –COOH group of a carboxylic acid is acidic; the C–H hydrogens are not.

Nitrogen compounds with a lone pair, such as ammonia (NH₃) and amines (like CH₃NH₂), are weak bases: the lone pair on N accepts a proton from water, forming NH₄⁺ or CH₃NH₃⁺ plus OH⁻.

Carboxylate ions, such as acetate (CH₃COO⁻), are weak bases too, because they're the conjugate bases of weak acids.

The weaker the acid, the stronger its conjugate base, and the reverse (topic 8.3).

HF is a weak acid even though fluorine is the most electronegative element. Among the hydrogen halides, bond strength matters more than electronegativity: the H–F bond is much stronger than H–Cl, H–Br or H–I, so H⁺ is harder to remove.