Rules of the Day 2-20-08
Click here for a copy of the mechanism sheet used in lecture today.
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).
4. Grignard reagents add to CO2 to give carboxylic acids (Mechanism A).
5. Carboxylic acids are reduced by LiAlH4 but not H2/Pd or NaBH4. These differences allow you to pick which functional groups in a molecule you want to reduce.
Homework: Finish working on the practice homework problem set that is meant as a helpful recview. You will NOT turn it in. Click here to download the pdf of the practice homework. The answers have now been posted. Click here for the answers to the practice homework.
Begin studying for the exam. You are responsible for all material covered up through Friday's lecture. Keto-enol equiolibria will not be on the first exam. What we cover next week will be on the second exam. Problems: Good review problems 16.44, 16.52, 16.55 To get ready for Wednesday's lecture Read: 17.1-17.3.
For tips on how to prepare for the exam, click here
