Rules of the Day
2-8-2024
Click here for a copy of the lectures notes I wrote in class
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Featured Golden Rule of Chemistry: 5. Delocalization of charge over a larger area is stabilizing.7. Delocalization of pi electron density over a larger area is stabilizing.9. Functional groups react the same in different molecules.
1. Imines can be reduced as they are made to give amines if H2/Pd or NaCNBH3 is added to the reaction.
2. Keto-enol equilibration is catalyzed by acid or base. Recall that catalysis does not change the position of equilibrium, but just accelerates the rate at which equilibrium is achieved. The keto form is almost always favored (C=O stronger bond than C=C)! In acid, the mechanism involves protonation of the carbonyl oxygen atom, followed by loss of a proton by the alpha-carbon atom.
3. The alpha-hydrogens of aldehydes and ketones are relatively acidic with a pKa around 20. Deprotonated aldehydes or ketones are resonance stabilized and are called enolates. The resonance stabilization of the enolate explains the relative acidity of aldehydes/ketones.
4. Base-catalyzed keto-enol equilibrium occurs via an enolate intermediate.
5. The enol form of a carbonyl reacts like an alkene, so even though it is only present in small amounts, some reactions characteristic of alkenes can be made to occur with aldehydes and ketones such as halogenation with X2 at the alpha position.
6. Because acid catalysis increases the rate at which keto and enol forms equilibrate, reactions that proceed through the enol form such as halogenation on the alpha carbon atom are accelerated in acid.
7. Carboxylic acids are relatively acidic because the carboxylate anion is resonance stabilized, thereby distributing the negative charge over both oxygen atoms.
8. 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.
9.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 localized charges).
10. If the pH of a solution is higher than the pKa of an acid, the acid exists predominantly in its deprotonated state. If the pH of a solution is lower than the pKa of an acid, the acid exits predominantly in its protonated state. At neutral pH, as in biological systems, carboxylic acids are deprotonated and negatively charged, which is a BIG DEAL in biochemistry.
Homework:
Read: Sections 17.5-17.9 in the ebook textbook. This text is part of the Longhorn Textbook access program.
Take the Daily Quiz 8 before 10 PM tomorrow. Click here to access the quiz. These quizzes are designed to review the important material from today's lecture.
Continue working on the Homework Problem Set 4, due at 10 PM on Monday, February 12. Click here to access the Homework Problem Set 4. Remember to do both the Aktiv learning and Gradescope portions of the homework!