Download Chemistry 125 Seventh Exam Answers: Reactions and Mechanisms in Organic Chemistry and more Exams Organic Chemistry in PDF only on Docsity! Chemistry 125 Seventh Examination Answers April 11, 2003 1 . (8 min) Give the principal product of each of the following reactions. (No mechanisms necessary, just draw the product) Cl O N NH2 O R O CH3 O OOH O MgBr O (CH3)3SiCl (NADH) LDA THF (-78°C) LAD Enzyme Cl OHH (S) enantiomer with enzyme, but original test omitted enzyme, so this was not required. OHH Too hindered for addition. 65% of this product and 35% of enolization. OTMS With Si enolate reacts at O rather than at C. Baeyer-Villiger oxidation. "Alkide" shift favored for group that puts less demand on electro density.O O 2 . (3 min) The equilibrium constant for the following hydration reaction is strongly dependent on the nature of the R groups. Match each pair of R groups with the appropriate equilibrium constant and briefly explain the difference. The carbonyl group is stabilized relative to the diol with a tetrahedral carbon when H substituents are replaced by alkyl groups, such a methyl. One reason for this is that C-C bonds profit more from changing sp3 to sp2 hybridization than C-H bonds do. Another is that with alkyl groups there is the possibility of hyperconjugative stabilization of the alpha C-H bonding electrons by the pi* low LUMO of the carbonyl group. R R’ K H H 4 x 10-5 CH3 CH3 10 -2 CH3 H 18 R R O + H2O R' R OH OH ' Chem 125 Seventh Exam 4 / 1 1 / 0 3 Page 2 3 . (5 min) Provide BOTH theoretical justification (in terms of orbital mixing) AND experimental support (specific facts) to justify writing the following resonance structure for a carboxylic acid: Theoretical: This structure reflects stabilization of an unshared pair of electrons on the carbonyl oxygen by mixing with the sigma* vacant orbital associated with the C-O bond. Experimental: The C=O stretching of the carboxylic acid occurs at significantly higher frequency (1760/cm) than does a ketone (1715/cm) demonstrating a stronger bond. [This occurs despite the expectation that the more “obvious” resonance structure reflecting mixing of an unshared pair of the OH group with the pi* orbital of the carbonyl group would be expected to weaken the C=O bond and lower its stretching frequency.] 4 . (6 min) An important aspect of Woodward’s cortisone synthesis was protecting a 1,2-diol as its acetonide. Use curved arrows to draw a detailed multi-step mechanism for the formation of the acetonide by reacting a 1,2-diol with acetone. O H + OH + HO OH : OH +O OHH OH O OH H + : OH2 O OH + O OH + :O O+ HO O Common deficiencies: * forgetting the acid catalyst necessary for forming a full ketal * using ROH to protonate ketone (it is some 1024 times weaker as an acid than the protonated ketone!) * using OH- as leaving group. * forgetting charges * having OH- and H+ at the same time for acid/base catalysis. Remember that the product of their concentrations is about 10-14. If you have much of one of them, the other is unavailable. * arrows from H+ instead of to it. Curved arrows denote electron pair motion. * skipping steps, or too many arrows at a time (this risks confuse you as well as the grader) * missing carbons R C O OH + _
