Test: Heterocyclic Level - 1 - Chemistry MCQ

Option 2 : lone pair on nitrogen is delocalized with p-orbital electrons of carbon

Pyrole is most reactive towards the electrophiles because in this case it is carrying one lone pair of electron .and it is attached with one hydrogen atom , resulting less resonance stabilize than that of pyridine one the another hand thiophene is more reactive because it's carry 2 lone pair of electrons at last pyrole is more reactive than thiophene because nitrogen is smaller in size than that of sulphur so the availability of electrons cloud is maximum than sulphur ... consequently it is highly reactive towards electrophiles

There is a relatively less partial positive charge on meta positions of Pyridine. Hence, when an electrophile attacks it attacks on meta position.
Hence D is correct.

• Pyridine is more basic than pyrrole because lone pair of electrons on N in pyridine and pyrrole are different in nature. These form a part of aromatic sextet in pyrrole, while not in pyridine.
• Pyrrole, C₄H₄NH (in which N contributes a lone pair) has a pKa−3.8 but pyridine (where N is part of the ring's double bond) has a pKa 5.14. Electron pair availability indicates the strength of basicity. In this case, pyridine is the stronger base.
• In pyrrole, the lone pair electrons of the nitrogen atom gets involved with the 2 carbon-carbon double bonds in the 5-member ring to form a conjugated system of pi electrons, leading to greater stability of the molecule.
• Pyridine, on the other hand, already has a stable conjugated system of 3 double bonds in the aromatic hexagonal ring, like benzene. Hence, the lone pair electrons on the N atom in pyridine can be easily donated to a H+ ion or a Lewis acid.

• In the case of pyrrole coupling reactions, dilute acids such as dilute HCl or dilute sulfuric acid (H₂SO₄) are often used.
• Pyrrole is sensitive to strong acids (like concentrated acids), which can lead to protonation of the nitrogen atom and polymerization or degradation of the molecule.
• Dilute acid provides mild reaction conditions to facilitate coupling without breaking the pyrrole structure.

Thus, dilute acid is used as the reagent.

The CH₂O/HCl reagent is suitable for chloromethylation of various polycyclic aromatic hydrocarbons as well as heterocyclic aromatic compounds. Thiophene gives 2-(chloromethyl)thiophene in 40-41% isolated yield, and 4-methylimidazole gives the 5-chloromethyl derivative (51-68%).