Rules of the Day 2-21-07

Mechanism Sheet for Today's Lecture

Featured Golden Rule of Chemistry: 4. Nature hates localized charges. If a molecule must have a charge, it is better to have the charge distributed over as many atoms as possible through resonance, inductive effects, and hyperconjugation. In addition, when given the choice, it is better to have more negative charge on a more electronegative atom (e.g. O), and more positive charge on a less electronegative atom (e.g. C).

1. Carboxylic acids are relatively acidic because the carboxylate anion is resonance stabilized, thereby distributing the negative charge over both oxygen atoms.

2. Carboxylic acids exist in solution as hydrogen bonded dimers. A good way to think about hydrogen bonds is as attraction between regions of molecules with partial positive charge (the H atom of an OH or NH bond) and the partial negative charge of a lone pair of electrons, usually on an O or N atom.

3. The stronger the acid, the lower the pKa and the more stable the conjugate base. Distribution of the negative charge over more atoms leads to a more stable conjugate base (Nature hates charges).

Homework: Study for the exam. You are responsible for all material covered up through last Friday's lecture. What we cover this week will be on the second exam.