Question

Electrophilic substitution reaction of pyridine, when carried out in the presence of KNO₃ and conc. H2SO₄ at 300°C, leads to the formation of:

• A. 4-Nitropyridine
• B. 3-Nitropyridine
• C. 2-Nitropyridine
• D. N-nitro pyridinium salt

Answer 1

The Correct Option is B

To determine the product of the electrophilic substitution reaction of pyridine in the presence of KNO₃ and concentrated H₂SO₄ at 300°C, we need to understand the mechanism of the nitration of pyridine.

Pyridine is aromatic and contains a nitrogen atom which is electron-withdrawing due to its higher electronegativity compared to hydrogen. As a result, the electron density of the pyridine ring is lower than that of benzene, making electrophilic substitution on pyridine more challenging.

In nitration, the nitronium ion (NO₂⁺) serves as the electrophile. Pyridine undergoes substitution reactions predominantly at the meta position relative to the nitrogen atom because:

• The ortho and para positions to the nitrogen are deactivated due to its electron-withdrawing effect, which destabilizes the cation formed during electrophilic substitution.
• The meta position is less hindered by the resonance and thus more stable for electrophilic attack.

Therefore, when KNO₃ and concentrated H₂SO₄ are used to generate the nitronium ion in the reaction with pyridine, the substitution occurs predominantly at the 3-position, leading to the formation of 3-Nitropyridine.

Thus, the correct answer to the given question is 3-Nitropyridine.

Now, let us evaluate the other options:

• 4-Nitropyridine: Substitution at the 4-position (para-position) is less favorable compared to the meta-position due to resonance instability.
• 2-Nitropyridine: Substitution at the 2-position (ortho-position) is also disfavored for the same reason as the para-position.
• N-nitro pyridinium salt: This product is not typically formed because the reaction conditions favor electrophilic substitution on the carbon atoms of the pyridine ring.