Lecture 11_MJD_Pyridine_Advanced Notes

Lecture 11_MJD_Pyridine_Advanced Notes - Masad J Damha Otto...

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Otto Maass Building My office (OM413A) [email protected] 1 Masad J. Damha
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Pyridine – Poor reactivity towards electrophiles Pyridine undergoes electrophilic aromatic substitution reactions, but reluctantly! WHY? C3-substitution is preferred Instead, N-substitution is observed
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N-ethyl pyridinium bromide (this is a good brominating agent) 3 N-acetyl pyridinium chloride (a good acylating agent) N-substitution maintains aromaticity of pyridine ring Pyridine – Poor reactivity towards electrophiles
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bromination of benzene with pyridinium bromide - mechanism:
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The fact that pyridine undergoes N-substitution with E + ’s explains, in part, why it is reluctant to undergo electrophilic aromatic substitution. Nitrogen is more electronegative than carbon. This results in polarization of the pi-system, resulting in decreased electron density on the ring carbons. a) b) 5 Pyridine – Poor reactivity towards electrophiles
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Orientation when electrophilic aromatic substitutions do occur: preferred
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Lecture 11_MJD_Pyridine_Advanced Notes - Masad J Damha Otto...

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