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A (Most Stable) > B > C > D CHEMISTRY 314-81 MIDTERM # 2 – ANSWER KEY July 10, 2002 Statistics: · Average: 47 pts (67%); · Highest: 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 [4+2] 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) Cyclooctatetraene is an antiaromatic molecule; · (F) Ethylene is aromatic since it has 2 p-electrons; · (T) A molecule is UV-active if it has a conjugated system; 2. Circle 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 Hückel’s Rule a system is antiaromatic if: a. It is not conjugated; b. It is not planar; c. It has 4n p-electrons; d. It is not cyclic; C. (2 pts) Conjugated 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; C. 1,2,6-heptatriene; D. 1,2,3-heptatriene; A (most stable) > B > C > D 4. (5 pts) Circle the molecules, which would be expected to exhibit UV-activity (i.e. have observable UV signals at wavelengths above 200 nm): O OH HO O O 5. (5 pts) Cyclopentadiene is a highly acidic compound with a pKa of 16. Cycloheptatriene on the other hand has a pKa of 36, which means that it is much less acidic. Write the equations for deprotonation of both cyclopentadiene and cycloheptatriene by sodium + - ethoxide (Na OC2H5). 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. H H H Na+ :OC H 2 5 Aromatic, thus stabilized. Deprotonation is facilitated. pKa is relatively low. H H H Na+ :OC H 2 5 Antiaromatic, thus destabilized. Deprotonation is very unfavorable. pKa is very high. 6. (5 pts) The three compounds below exhibit very different reactivity in SN1 reactions. Suggest an explanation for this supported by relevant chemical equations and structures (Hint: What is the first step in every SN1 reaction?). Br Br Br least reactive most reactive S 1 - Br N - Br SN1 - Br SN1 Antiaromatic, destabilized, Secondary alkyl carbocation. Allyl carbocation. Stabilized difficult to obtain. Moderate stability through resonance. Reaction Reaction will be very slow. will be fast. 7. (5 pts) Label each of the following systems as aromatic, antiaromatic or nonaromatic. Do not explain! N N H N S N S antiaromatic aromatic aromatic nonaromatic 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. Br Br Br + + + Br2 Br Br Br kinetic control kinetic control thermodynamic control 9. (5 pts) Provide structures for the following compounds: CH3 COOH OH NH2 O2N NO2 O2N NO2 other isomers possible Br Br NO2 Cl Cl para-chlorobenzoic acid 3-methyl-2,4-hexadiene 2,4,6-trinitrotoluene 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: O H O O + O O O H O O O OCH OCH3 3 O OCH3 + H H + H OCH3 O OCH 3 OCH3 Br Br Br + Br2 + Br Br Br + Br2 11. (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. OH OH + H3O + H3O OH2 - H2O - H2O OH2 Br Br Br Br 12. (5 pts) Outline a synthetic sequence for the preparation of the molecule below, using cyclopentene as starting material and any other necessary reagents. O Br O O NBS Na+ CCl4 NaOH HO 13. (5 pts) Dimethyl acetylenedicarboxylate reacts with a certain diene to give a Diels-Alder product with a molecular formula C14H20O4. The 1H NMR spectrum of the product is shown below. Suggest structures for the Diels-Alder product and the starting diene. O OCH3 C C C H3CO O dimethyl acetylenedicarboxylate CH3 O OCH3 O C H3C CH3 CH3 H C OCH3 + CH3 C H OCH3 CH H C CH3 3 H3CO O 3 O diene Diels - Alder product 6 CH3 3 1 CH3 H 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 .
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