Question

Pyridine is less basic than triethylamine because: (a) Pyridine has aromatic character (b) Nitrogen in pyridine is sp \(^{2}\) hybridized (c) Pyridine is a cyclic system (d) In pyridine, lone pair of nitrogen is delocalized.

Short answer

Pyridine is less basic than triethylamine due to the delocalization of the nitrogen's lone pair (option d).

Step-by-step solution

Understand Pyridine and Basicity

Pyridine is an aromatic heterocyclic compound containing a nitrogen atom in its ring. Basicity in nitrogen-containing compounds depends on the availability of the lone pair of electrons on the nitrogen atom to accept protons.

Compare Hybridization

In pyridine, the nitrogen atom is sp² hybridized, meaning its lone pair of electrons are in an sp² orbital. In contrast, in triethylamine, nitrogen is sp³ hybridized, which allows for easier donation of the lone pair of electrons since this lone pair is in an sp³ orbital.

Examine Lone Pair Delocalization

In pyridine, the lone pair on nitrogen is part of the aromatic system and is delocalized over the ring. This delocalization reduces the availability of the lone pair for protonation, thus reducing its basicity.

Analyze Influences of Aromatic Character

The aromatic character of pyridine makes its electron structure more stable due to delocalization, which also contributes to the lower availability of the lone pair for protonation, compared to aliphatic amines like triethylamine.

Identify the Key Factor

While sp² hybridization and aromatic character both influence basicity, the key factor making pyridine less basic than triethylamine is the delocalization of the nitrogen's lone pair of electrons within the aromatic system.

Key concepts

Hybridization in Pyridine

Pyridine is a fascinating molecule because it is an aromatic heterocycle composed of a six-membered ring with five carbon atoms and one nitrogen atom. A key attribute of nitrogen in pyridine is its hybridization. The nitrogen atom in pyridine is sp² hybridized. This means that one of its orbitals is a hybrid of one s and two p orbitals. As a result, the lone pair of electrons on the nitrogen is located in an sp² orbital rather than the unhybridized p orbital.
This sp² hybridization affects the nitrogen atom's ability to donate its lone pair of electrons. Why does this matter? Well, when a nitrogen atom is sp² hybridized, it becomes part of a planar structure that allows effective overlap with other p orbitals in the ring, contributing to the molecule's aromatic character.
In contrast, a nitrogen atom in a structure like triethylamine is sp³ hybridized, where the orbitals are shaped differently, resembling more of a tetrahedral configuration. This sp³ hybridization typically makes it easier for the nitrogen to donate its lone pair of electrons, hence triethylamine is more basic than pyridine.

Lone Pair Delocalization

The lone pair of electrons on the nitrogen atom in pyridine plays a significant role in determining its chemical behavior, especially its basicity. In pyridine, this lone pair is not just sitting idle on the nitrogen atom, ready to jump at the next proton. Instead, it is delocalized over the aromatic ring structure.
Delocalization means that the lone pair is shared across the entire pi-electron cloud of the ring, contributing to the molecule's stable aromatic nature. This sharing of electrons stabilizes the molecule but limits the lone pair's availability to bond with protons. Consequently, pyridine's lone pair is less available for proton donation relative to aliphatic amines' lone pairs, where such delocalization does not occur.
In summary, the delocalization of the lone pair reduces pyridine's basicity because the electrons are "busy" stabilizing the aromatic system rather than binding to protons. This characteristic is a key reason why pyridine is less basic than its aliphatic cousin, triethylamine.

Aromatic Compounds

Aromatic compounds are a special class of molecules known for their unique electron configuration and stability. Pyridine is an example of an aromatic compound due to the arrangement of electrons in its ring structure. Aromaticity involves an electron cloud above and below the plane of the atoms, which in pyridine includes six pi electrons (one from each carbon and one from nitrogen) contributing to a continuous overlap.
The requirement for aromaticity is governed by Hückel's rule, which requires \(4n+2\) pi electrons in a cyclic continuous overlap. Pyridine fits this rule, marking it an aromatic compound. The aromatic character augments the stability of pyridine, as the electrons are delocalized across the ring, offering a unique electronic structure.
This aromatic stability, combined with lone pair delocalization, makes any potential reaction involving its lone pair less favorable. This reluctance diminishes pyridine's basicity compared to non-aromatic amines, which do not benefit from such stabilization.