a. Draw resonance contributors to show why pyridine-N-oxide is more reactive than pyridine toward electrophilic aromatic substitution.

검증된 단계별 안내

Identify the structure of pyridine-N-oxide. Pyridine-N-oxide is a pyridine ring with an oxygen atom bonded to the nitrogen via a coordinate covalent bond, giving the nitrogen a positive formal charge and the oxygen a negative formal charge.

Recall the concept of resonance. Resonance contributors are alternative Lewis structures that show the delocalization of electrons within a molecule. For pyridine-N-oxide, the negative charge on the oxygen can delocalize into the aromatic ring, enhancing electron density at certain positions.

Draw the resonance contributors for pyridine-N-oxide. Start by moving a lone pair of electrons from the negatively charged oxygen into the N-O bond, which pushes the π-electrons in the ring to different positions. This creates resonance structures where the negative charge is delocalized onto the carbon atoms in the ring.

Compare the resonance contributors of pyridine-N-oxide to those of pyridine. In pyridine, the nitrogen atom does not have a formal positive charge, and the electron density in the ring is lower. In contrast, the resonance contributors of pyridine-N-oxide show increased electron density at specific positions in the ring, making it more reactive toward electrophilic aromatic substitution.

Conclude that the increased electron density in pyridine-N-oxide, as shown by its resonance contributors, makes it more nucleophilic and thus more reactive toward electrophilic aromatic substitution compared to pyridine.

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주요 개념

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Resonance Structures

Resonance structures are different ways of drawing the same molecule that illustrate the delocalization of electrons. In the case of pyridine-N-oxide, resonance contributors can show how the presence of the N-oxide group affects electron density distribution, making certain positions on the aromatic ring more reactive toward electrophiles.

Electrophilic Aromatic Substitution (EAS)

Electrophilic aromatic substitution is a fundamental reaction in organic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. The reactivity of the aromatic compound is influenced by the electron-donating or withdrawing effects of substituents, which can stabilize or destabilize the intermediate formed during the reaction.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are substituents that pull electron density away from the aromatic ring, often increasing the reactivity of the ring toward electrophiles. In pyridine-N-oxide, the oxygen atom of the N-oxide group acts as an EWG, enhancing the electrophilic attack at the ortho and para positions compared to unsubstituted pyridine.