0244Pb16an-10.cwk (WP)

0244Pb16an-10.cwk (WP) - a resonance-stabilized cation is...

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Answers to Problem 16, Chemistry 0244, 2010 Here are the Lewis structures: N N H N N H pyridine pyrrole imidazole All three of these molecules are heteroaromatic compounds. To find a sensible answer, it is important to examine closely the lone pair electrons on the nitrogen atoms in these compounds. In pyridine, the nitrogen lone pair electrons are not part of the (4n + 2) π system, but lie in the plane of the ring in an sp 2 orbital that is part of the σ system. Accordingly, aromaticity is not disrupted when pyridine is protonated. N H OH 2 H 2 O + N H By contrast, the nitrogen lone pair electrons are part of the aromatic sextet in pyrrole. Protonation on nitrogen would result in a loss of aromaticity, and is likely to be a very endothermic process. N H H OH 2 H 2 O + N H H When pyrrole does protonate, it does so on carbon. Although aromaticity is still lost, at least
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Unformatted text preview: a resonance-stabilized cation is formed. N H H OH 2 H 2 O + N H H H N H H H N H H H In addition, protonation at the remaining carbon (C3) would also result in a less stabilized cation. Although there are two resonance structures, the remaining C=C double bond is effectively isolated, and does not confer any added stability to the cation. NH H OH 2 H 2 O + NH H H NH H H Therefore, protonation at C2 is better than either protonation at C3 or N. The two nitrogen lone pairs of imidazole are quite different. One nitrogen lone pair is part of the six- electron system and the other is not, as it lies in the s system. Accordingly, protonation occurs on the latter lone pair as aromaticity is not lost. N N H H OH 2 H 2 O + N N H H N N H H N N H . . . . =...
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0244Pb16an-10.cwk (WP) - a resonance-stabilized cation is...

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