Ch_322b_14.07 - means there is no direct interaction The...

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Heterocyclic Aromatic Compounds Cyclic organic compounds where a ring carbon is replaced by an atom such as nitrogen, oxygen or sulfur are called heterocycles. When these compounds show unusual stability and reactivity because of ! " electronic features as found in aromatic structures, they are called heterocyclic aromatic compounds. Four important examples are shown below as Kekule' structures. N . . N . . H O . . . . S . . . . pyridine pyrrole furan thiophene The above heterocycles have ! " electronic structures analogous to previously discussed "aromatics" when the heteroatom is sp 2 hybridized.
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Pyridine has a sextet of ! " electrons (one from each of the 5 carbon and single nitrogen atoms) with the nonbonding electron pair on nitrogen residing in an sp 2 orbital in the plane of the ring. Note that the 90 o angle between the ! " electrons and the nonbonding electrons
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Unformatted text preview: means there is no direct interaction. The nonbonding electrons are localized on the nitrogen. N : The pyridine ! " electrons are stabilized, as in benzene, because they reside in a closed shell structure. N . . pyridine In, pyrrole , furan and thiophene , the four ring carbons contribute four electrons to the ! " electronic structure while the heteroatom (N,O,S) contributes a nonbonding pair to give a sextet of ! " electrons . The "aromaticity" of these five-membered ring heterocycles results from a closed shell electronic structure as is found in the cyclopentadiene anion. Note again the heteroatom is sp 2 hybridized. N H N-H . . pyrrole O : O . . : furan S : S . . : thiophene six ! " electron systems These heterocyclic aromatic compounds are widely found in biological structures....
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