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Chapter 3: Problem 129

Why is pyrrole a much weaker base than pyridine?

Short Answer

Expert verified
Answer: Pyrrole is a much weaker base than pyridine due to the involvement of the nitrogen lone pair of electrons in the π-conjugated system of pyrrole, reducing its availability for accepting a proton. In contrast, the nitrogen lone pair of electrons in pyridine is not involved in the π-system, and therefore more available for accepting a proton, making pyridine a stronger base.

Step by step solution

01

Understand the concept of basicity

Basicity refers to the ability of a compound to accept a proton (H+) from an acid. The higher the basicity, the more readily a compound can accept a proton. A compound's basicity is often attributed to the presence of a lone pair of electrons on an atom, which can be donated to form a bond with an incoming proton.
02

Recognize the structures of pyrrole and pyridine

Pyrrole has a five-membered ring composed of four carbon atoms and one nitrogen atom. It also contains one double bond between adjacent carbon atoms and a lone pair of electrons on the nitrogen atom. The structure can be represented as C4H4NH. Pyridine, on the other hand, has a six-membered ring composed of five carbon atoms and one nitrogen atom. It contains two double bonds between adjacent carbon atoms and a lone pair of electrons on the nitrogen atom. The structure can be represented as C5H5N.
03

Analyze the nitrogen lone pairs in both molecules

The lone pair of electrons on the nitrogen atom in pyrrole is involved in the formation of a π-conjugated system (aromatic) with the other atoms in the ring, making it less available for accepting a proton. In the case of pyridine, the lone pair of electrons on the nitrogen atom is not involved in the π-system, meaning it is more available for accepting a proton.
04

Compare the basicities of pyrrole and pyridine

Based on the analysis of the nitrogen lone pairs in both molecules, pyrrole's lone pair of electrons is less available for accepting a proton due to its involvement in the π-conjugated system (aromatic character) of the ring. This makes pyrrole a much weaker base compared to pyridine, whose nitrogen lone pair is more available for accepting a proton as it is not involved in the π-system.
05

Conclusion

Pyrrole is a much weaker base than pyridine due to the involvement of the nitrogen lone pair of electrons in the π-conjugated system of pyrrole, reducing its availability for accepting a proton. In contrast, the nitrogen lone pair of electrons in pyridine is not involved in the π-system, and therefore more available for accepting a proton, making pyridine a stronger base.

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