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Rules of the Day 2nd midterm

Rules of the Day 2nd midterm - Rules of the Day Click here...

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Rules of the Day 2-18-08 Click here for a copy of the mechanism sheet used in lecture today. 1. 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! In acid, the mechanism involves protonation of the carbonyl oxygen atom, followed by loss of a proton by the α -carbon atom. 2. 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. 3. Base-catalyzed keto-enol equilibrium occurs via an enolate intermediate . 4. 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 X 2 at the alpha position . 5. 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 . 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.
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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 CO 2 to give carboxylic acids (Mechanism A). 5. Carboxylic acids are reduced by LiAlH 4 but not H 2 /Pd or NaBH 4 . 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.
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