CHEMISTRY MIDTERM # 2 ANSWER KEY July 10, 2002

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1 EMISTRY MIDTERM # 2 ANSWER KEY July 10, 2002 Statistics: Average: 47 pts (67%); ighest: 69 pts (99%); Lowest: 26 pts (37%) Number of students performing at or above average: 12 (46%) 1. (7 pts) Mark as true (T) or false (F) the following statements. Do not explain! (T) The Diels-Alder reaction is a [42] cycloaddition process; (T) Molecular orbitals with equal number of nodes are degenerate; (T) Molecular orbitals with equal energies are degenerate; (F) The Diels-Alder reaction occurs faster with electron-deficient dienes; (F) yclooctatetraene is an antiaromatic molecule; (F) Ethylene is aromatic since it has 2 π-electrons; (T) A molecule is UV-active if it has a conjugated system; 2. ircle all that apply: A. (2 pts) We say that benzene is stabilized because: a. It has higher than expected heat of hydrogenation; b. It has lower reactivity; c. It has low boiling point; d. It has large resonance energy; B. (2 pts) According to ückel s Rule a system is antiaromatic if: a. It is not conjugated; b. It is not planar; c. It has 4n π-electrons; d. It is not cyclic;. (2 pts) onjugated Dienes: a. Are more stable than the corresponding dienes with isolated double bonds; b. Are less stable than the corresponding dienes with cumulated double bonds; c. Are UV-active; d. Give predominantly 1,2-addition products at low temperatures ; D. (2 pts) The Diels-Alder reaction: a. Is stereospecific ; b. Is regiospecific ; c. Requires an s-cis conformation of the dienophile; d. Requires an s-trans conformation of the diene; 3. (3 pts) Arrange the following molecules in order of increasing stability: A. 1,3,5-heptatriene; B. 1,3,6-heptatriene;. 1,2,6-heptatriene; D. 1,2,3-heptatriene; A (most stable) > B > > D 4. (5 pts) ircle the molecules, which would be expected to exhibit UV-activity (i.e. have observable UV signals at wavelengths above 200 nm): 5. (5 pts) yclopentadiene is a highly acidic compound with a pk a of 16. ycloheptatriene on the other hand has a pk a of 36, which means that it is much less acidic. Write the equations for deprotonation of both cyclopentadiene and cycloheptatriene by sodium

2 ethoxide (Na ). Try to account for the acidity difference of the two compounds on the basis of the structures and expected properties of the products of deprotonation. Na : 2 5 Aromatic, thus stabilized. Deprotonation is facilitated. pk a is relatively low. Na : 2 5 Antiaromatic, thus destabilized. Deprotonation is very unfavorable. pk a is very high. 6. (5 pts) The three compounds below exhibit very different reactivity in S N 1 reactions. Suggest an explanation for this supported by relevant chemical equations and structures (int: What is the first step in every S N 1 reaction?). least reactive most reactive S N 1 S N 1 S N Antiaromatic, destabilized, difficult to obtain. Reaction will be very slow. Secondary alkyl carbocation. Moderate stability Allyl carbocation. Stabilized through resonance. Reaction will be fast. 7. (5 pts) Label each of the following systems as aromatic, antiaromatic or nonaromatic. Do not explain! N N S N N S nonaromatic aromatic antiaromatic aromatic aromatic 8. (3 pts) The addition of bromine to 2-methyl-1,3-butadiene yields a mixture of products, as shown below. Identify the product(s) of kinetic and thermodynamic control. 2 kinetic control kinetic control thermodynamic control

3 9. (5 pts) Provide structures for the following compounds: 2 N 3 N 2 N 2 2 N N 2 other isomers possible N 2 l l 3-methyl-2,4-hexadiene 2,4,6-trinitrotoluene para-chlorobenzoic acid 3,5-dibromoaniline 4-chloro-2,6-dinitrophenol 10. (8 pts) Draw the expected product(s) from each of the following reactions. Provide explicit stereochemical and regiochemical assignment, wherever applicable: (6 pts) Treatment of either 2-buten-1-ol or 3-buten-2-ol gives the same mixture of 1-bromo -2-butene and 3-bromo -1-butene. Account for this by writing the detailed mechanisms of the two reactions

4 12. (5 pts) utline a synthetic sequence for the preparation of the molecule below, using cyclopentene as starting material and any other necessary reagents. NBS Na l 4 Na 13. (5 pts) Dimethyl acetylenedicarboxylate reacts with a certain diene to give a Diels-Alder product with a molecular formula The 1 NMR spectrum of the product is shown below. Suggest structures for the Diels-Alder product and the starting diene. 3 3 dimethyl acetylenedicarboxylate diene Diels - Alder product

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