Problem 47 - A Complete Resource Book in Chemistry for JEE Main

Question

Pyridine is less basic than triethylamine because (a) pyridine has aromatic character (b) nitrogen in pyridine is sp^{2} hybridised (c) pyridine is a cyclic system (d) in pyridine, lone pair of nitrogen is delocalised.

Step-by-Step Solution

Answer

(d) in pyridine, lone pair of nitrogen is delocalised.

Step 1: Identify the Basicity Factors

The basicity of a compound refers to its ability to donate a pair of electrons, which in the case of nitrogen-based compounds like pyridine and triethylamine, comes from the lone pair on the nitrogen atom. The basicity is influenced by factors such as hybridization, aromatic character, cyclic nature, and delocalization of electrons.

Step 2: Evaluate Aromatic Character

Pyridine is an aromatic compound with a structure similar to benzene and a nitrogen atom as part of the ring. Aromaticity usually does not directly weaken or strengthen basicity; instead, it provides stabilization that doesn't interfere significantly with the lone pair of electrons. Basicity is not primarily influenced by aromatic character, making option (a) less plausible.

Step 3: Analyze Hybridization Effect

In pyridine, the nitrogen atom is sp² hybridized. Sp² hybridization involves a greater s-character compared to sp³, meaning the electron pair is held closer to the nitrogen nucleus, lowering its availability for donation. However, sp² hybridization alone isn't a definitive reason for reduced basicity, so option (b) may not be the main factor.

Step 4: Consider Cyclic Structure Influence

While pyridine is cyclic, cyclic nature by itself does not inherently decrease basicity unless it affects the lone pair of electrons or hybridization of nitrogen. Thus, the cyclic nature of pyridine is not the main reason for its lower basicity compared to triethylamine.

Step 5: Assess Lone Pair Delocalization

In pyridine, the lone pair of electrons on nitrogen is not part of the aromatic pi system; however, it is somewhat involved in resonance stabilization of the ring structure via delocalization, reducing its availability to act as a base. This delocalization of the lone pair significantly reduces its basicity, making option (d) the most plausible reason for pyridine being less basic than triethylamine.

Key Concepts

Hybridization and Basicity

The hybridization of the nitrogen atom plays a crucial role in determining the basicity of a compound. In general, basicity is the ability of a compound to donate a lone pair of electrons. In the case of pyridine, the nitrogen atom is sp² hybridized. This means that the nitrogen atom has a higher s-character, causing the lone pair of electrons to be held more tightly to the nitrogen nucleus. This tight hold reduces the availability of the lone pair to be donated, thus affecting the basicity. For comparison, in triethylamine, the nitrogen is sp³ hybridized. Sp³ hybridization involves a lower s-character, allowing the lone pair of electrons to be more freely available for donation. Therefore, the greater the s-character in the hybrid orbitals, the lower the basicity because the tighter hold decreases electron pair availability.

Aromaticity and Basicity

Aromaticity can influence the properties of a compound in significant ways. While aromaticity itself typically refers to structural stability, it can also have indirect effects on the basicity of a compound. Pyridine is an aromatic compound, meaning it has a cyclic, planar structure with conjugated pi electrons that follow Hückel's rule for aromaticity. The aromatic ring in pyridine does not directly interfere with the lone pair of electrons on the nitrogen atom. As a result, aromaticity in itself isn't a direct reason for the lower basicity of pyridine compared to non-aromatic amines like triethylamine. However, aromaticity can contribute to the overall stability of the molecule, which can, in turn, affect how freely the lone pair of electrons on nitrogen can be shared. Ultimately, although aromaticity stabilizes the molecule, it's not the primary driver behind the basicity differences in aromatic and non-aromatic systems.

Delocalization of Electrons

Delocalization of electrons is one of the most significant factors affecting the basicity of pyridine. Though the lone pair of electrons on the nitrogen atom in pyridine is not directly part of the aromatic pi system, it is affected by resonance. This delocalization occurs because the lone pair can participate marginally in the resonance stabilization of the aromatic ring. The partial involvement of the lone pair in resonance means they are less freely available to bond with a proton, reducing the basicity of pyridine compared to compounds like triethylamine, where the lone pair is fully available for proton bonding. Delocalization stabilizes the nitrogen, making it less eager to donate its electron pair. This diminished willingness to donate electrons is the reason why pyridine exhibits a lower basicity compared to other non-aromatic amines.