14_all_ - 14.1 Listed below are four compounds that have...

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14.1 Listed below are four compounds that have the molecular formula C 6 H 6 . Which of these compounds would yield only one monosubstitution product, if, for example, one hydrogen were replaced by bromine? (a) H 3 CC C C CCH 3 (b) (c) (d) Answer: (a) and (b) (a) H 3 CC C C CCH 2 Br (b) Br 14.2 If benzene were 1,3,5-cyclohexatriene, the carbon-carbon bonds would be alternately long and short as indicated in the following structures. However, to consider the structures here as resonance contributors (or to connect them by a double-headed arrow) violates a basic principle of resonance theory. Explain. × Answer: Resonance structures are defined as being structures that differ only in the positions of the electrons. In the two 1,3,5-cyclotrienes shown, the carbon atoms are in different positions; therefore, they can not be resonance structures. 14.3 Use the polygon-and-circle method to outline the п molecular orbitals of cyclobutadiene and explain why, on this basis, you would not expect it to be aromatic. This molecule is predicted to have two nonbonding orbitals because it has 4 п electrons. It wound have an unpaired electron in each of the two nonbonding orbitals, such a system wound be diradical, not expected to be aromatic. 14.4 (a) Outline the п molecular orbitals of the cyclopentadienyl system by inscribing a regular pentagon in a circle and explain on this basis why the cyclopentadienyl anion is aromatic. (b) What electron distribution would you expect for the cyclopentadienyl cation? (c) Would you expect it to be aromatic? Explain your answer. (d) Would you expect the cyclopentadienyl cation to be aromatic on the basis of Huckel’s rule? (a) All п electrons are in bonding п orbitals so it is aromatic.
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(b) and (c): The cyclopentadienyl cation would be diradical, not expected to be aromatic. (c) It is not Huckel’s number because п electrons number is 4, not 4n+2. 14.5(a) Use the polygon-and-circle method to sketch the relative energies of the π molecular orbitals of the cycloheptatrienyl cation and explain why it is aromatic. (b) Would you expect the cycloheptatrienyl anion to be aromatic on the basic of the electron distribution in its π molecular orbitals? Explain. (c) Would you expect the cycloheptatrienyl anion to aromatic on the basic of H ü ckel’ rule? (a) It is a seven –member-ring with every angle about 129 o , and it is not very stable, because the most stable angle is 120 o (sp 2 ).But it has six conjugated π electrons, and it obeys H ü ckel’ rule, and it is aromatic. (b) It is not aromatic, because it has eight π electron, it is not obey H ü ckel’ rule. (c) It is not aromatic, because it has eight conjugated electron, it not suit 4n+2. 14.6 1,3,5-Cycloheptatriene (shown on page 634) is even less acidic than 1,3,5-heptatriene. Explain how this experimental observation might help to conform your answer to parts (b) and (c) of the previous problem.
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This note was uploaded on 02/23/2010 for the course CHEM 101 taught by Professor Smith during the Spring '10 term at Duke.

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14_all_ - 14.1 Listed below are four compounds that have...

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