Download Chem 30B Winter 2004 Final Exam - Part A Solutions: Oxygen Protonation and Reaction Rates and more Exams Organic Chemistry in PDF only on Docsity! Chem 30B Winter 2004 Final Exam – Part A Solutions Page 1 Statistics: High score = 100 Average = 60.4 Low score = 06 Standard Deviation = irrelevant as it does not control grade distribution in this class. A note about exam keys: The answers presented here are usually significantly longer than expected from a student taking the exam. An exam key serves not only to reveal what was expected, but to instruct you as well. To see the final course grade cutoffs, consult the grading scale on the Chem 30B course web page. 1. Oxygen. 2. Protonation occurs most readily at the site that leads to the most stable conjugate acid. Protonation at the oxygen yields a cation with three significant resonance contributors: H2N NH2 OH H2N NH2 OH H2N NH2 OH Protonation at either nitrogen (the nitrogens are equivalent) yields a cation with just one significant resonance contributor: H2N NH3 O No additional significant contributors Everything else being equal, a greater number of resonance contributors leads to more stability (due to greater delocalization of the positive charge), so protonation at oxygen is favored over protonation at nitrogen. 3. Protonation makes the reaction faster because it makes the oxygen withdraw even more electron density from the carbonyl carbon. This amplifies the carbon’s δ+ charge. 4. H2N NH2 O H OH2 H2N NH2 OH OH2 H2N OH NH2HO H2O H H2N OH NH2HO H2O H H2N OH NH3HO H2N OH OH H2N OH O H2O H H2N OH O 5. Faster. Chem 30B Winter 2004 Final Exam – Part A Solutions Page 2 6. Resonance: The lone pair on the nitrogen atom bearing the two phenyls is delocalized onto the two benzene rings. This means it has less electron density to share with the carbonyl group. Since the carbonyl-NPh2 resonance is less than carbonyl-NH2 resonance, diphenylurea accepts a nucleophile more readily than urea. Leaving group: When diphenylamine (Ph2NH) leaves the lone pair on nitrogen is delocalized by resonance (a stabilizing feature). Ammonia does not enjoy this resonance stabilization. Therefore Ph2NH is a better leaving group than NH3, and the hydrolysis of diphenylurea is faster than the hydrolysis of urea. Steric effects: Ph2N is a larger group than NH2, so nucleophilic attack at the carbonyl carbon of diphenylurea is slower than attack on urea. If we assume the resonance is more important than steric effects, then this issue is not the most significant factor controlling the relative rates of hydrolysis. 7. NH2 O OH OH NH2O O O HO H O O 8. Faster reaction: (b). Reason #1: Acetic acid (CH3CO2H) enjoys resonance stabilization after departure, whereas ammonia does not. Therefore acetic acid is a better leaving group than ammonia, so reaction (b) is faster than reaction (a). Reason #2: Nucleophilic attack at the carbonyl carbon of urea occurs at the expense of resonance with two adjacent NH2 groups, whereas attack on the anhydride disrupts resonance with just one oxygen atom. Therefore attack on the anhydride occurs more readily than the attack on urea, so reaction (b) is faster than reaction (a). 9. (a) PhC CH 1. NaNH2 2. H2C=O 3. H3O+ PhC CCH2OH (b) O O 1. CH3MgBr (excess) 2. H3O+ CH2OH OH CH3 CH3 (c) O NaBH4 CH3OH OH (d) PhCH2Br 1. Ph3P 2. NaH 3. (CH3)2C=O Ph
