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Reviewquestions_16 - 3 What is suggested by the fact that...

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1 3) What is suggested by the fact that benzene's molar heat of hydrogenation is 36 kcal less than three times the molar heat of hydrogenation of cyclohexene? Answer: This suggests that the type of π bonding in benzene lends special stability to the molecule. Diff: 2 5) In the molecular orbital representation of benzene, how many π molecular orbitals are present? A) 1 B) 2 C) 4 D) 6 E) 8 Answer: D Diff: 1
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2 7) Provide a diagram which depicts the relative energies of the π molecular orbitals of benzene. Show which molecular orbitals are filled in benzene's ground state. Answer: Diff: 3 8) How many pairs of degenerate π molecular orbitals are found in benzene? 10) List the criteria which compounds must meet in order to be considered aromatic. Answer: 1. The structure must be cyclic. 2. Each atom in the ring must have an unhybridized p orbital. 3. The structure must be planar or nearly planar so that overlap of these p orbitals is effective. 4. The π network must contain 4n + 2 electrons (where n is a whole number), so that delocalization of the π electrons results in a lowering of the molecule's electronic energy. Diff: 2 11) Which is more stable, cyclobutadiene or 1,3-butadiene? Explain. Answer: 1,3-Butadiene is more stable. Cyclobutadiene is antiaromatic since it contains 4 (i.e., 4n) π electrons. Antiaromatic systems are less stable than their open-chain counterparts. Diff: 2 12) Aromatic molecules contain __________ π electrons.
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3 13) Is cyclooctatetraene a planar molecule? Explain. Answer: No. In addition to increasing other forms of strain, a planar conformation would make this molecule antiaromatic. Diff: 2 14) Is the all-cis form of [10]annulene aromatic? Explain. Answer: No. Planarity, which is required for aromaticity, is precluded due to excessive angle strain. Diff: 3 15) Is the [10]annulene shown below aromatic? Explain. Answer: No. The ring cannot achieve a planar conformation since such a conformation would result in severe and unfavorable steric interactions between the two H's oriented inside the ring.
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