Question

Arrange methyl amine (a), pyridine (b) and guanidine (c) in decreasing order of basic strength (a) \(C>A>B\) (b) \(\mathrm{A}>\mathrm{B}>\mathrm{C}\) (c) \(\mathrm{B}>\mathrm{A}>\mathrm{C}\) (d) \(\mathrm{B}>\mathrm{C}>\mathrm{A}\)

Short answer

Guanidine > Methylamine > Pyridine; Option (a) C > A > B.

Step-by-step solution

Understand Basicity

Basicity refers to a compound's ability to accept protons. In organic chemistry, it often relates to the availability of the lone pair of electrons on the nitrogen atom.

Assess Methylamine

Methylamine (CH₃NH₂) has an easily available lone pair on the nitrogen, making it a strong base. It is a primary amine and is quite basic due to the electron-releasing nature of the methyl group.

Assess Pyridine

Pyridine (C₅H₅N) is an aromatic nitrogen compound. The lone pair on the nitrogen is part of the aromatic system, making it less available to accept protons compared to an aliphatic amine. Hence, it's less basic than methylamine.

Assess Guanidine

Guanidine (H₂N-C(=NH)-NH₂) is a very strong base due to resonance stabilization among its three nitrogen atoms and delocalization of charge in its conjugate acid form. This makes the lone pair on the nitrogen highly available.

Compare Basic Strengths

Comparing all three, guanidine, with its resonance stabilization and charge delocalization, is the strongest base. Methylamine is next due to its aliphatic nature and lone pair availability. Pyridine, with its lone pair involved in aromaticity, is the weakest base.

Order the Bases

Given the analysis, the order from strongest to weakest base is guanidine (C), methylamine (A), and pyridine (B). Therefore, the correct order is C > A > B.

Key concepts

Methylamine

Methylamine Understanding the structure of methylamine is important in assessing its basicity. Methylamine (CH₃NH₂) is a primary amine, meaning it has one carbon-containing group attached to the nitrogen atom. The lone pair of electrons on the nitrogen in methylamine is readily available to accept a proton.

• The methyl group is electron-releasing, which enhances the electron density on the nitrogen atom.
• This makes the lone pair more available, increasing its ability to accept a proton.

Due to these characteristics, methylamine is considered a strong base among amine compounds. Its basicity is largely attributed to the free availability of the lone pair, coupled with the electron-donating effect of the methyl group.

Pyridine

Pyridine (C₅H₅N) is a fascinating compound due to its aromatic nature. Its six-membered ring resembles that of benzene but contains a nitrogen atom in place of one carbon. This significantly influences its basicity properties.

• The nitrogen atom's lone pair does not participate in the aromaticity of pyridine.
• However, the lone pair is held more tightly because of the sp² hybridization of the nitrogen, reducing its availability to accept protons.

These aspects make pyridine less basic than aliphatic amines like methylamine. Despite the lone pair being free from the aromatic system, its arrangement within the aromatic ring system contributes to a lower basic strength compared to non-aromatic amines.

Guanidine

Guanidine stands out as an exceptionally strong base mainly due to its structure and resonance features.

• Its formula, H₂N-C(=NH)-NH₂, consists of three nitrogen atoms capable of sharing the positive charge when the molecule gains an extra proton.
• This charge delocalization across the nitrogen atoms is a form of resonance stabilization.

Resonance stabilization significantly enhances guanidine's basicity compared to other amines. The ability to delocalize the charge effectively makes the lone pairs on nitrogen much more available for proton acceptance. Therefore, guanidine is often considered one of the strongest organic bases.

Resonance Stabilization

Resonance stabilization is a key mechanism in understanding the basic strength of certain compounds, like guanidine. This concept involves the delocalization of electrons across multiple atoms, creating a more stable system.

• In guanidine, the resonant structures allow the positive charge to be distributed over three nitrogen atoms.
• This delocalization reduces the energy of the conjugate acid, making the compound more eager to accept protons initially.

Resonance not only provides a pathway for the stabilization of positive charges but also enhances the availability of lone pairs for proton acceptance. Thus, compounds like guanidine benefit greatly in terms of basicity due to resonance. Understanding this concept clarifies why some compounds exhibit surprisingly high basic strengths compared to others.

Aromatic System

An aromatic system refers to a ring structure characterized by a unique type of stability, known as aromaticity, which arises from a specific arrangement of electrons.

• In aromatic compounds, electrons are delocalized over the circular arrangement, creating a stable, energy-lowering effect.
• This stability impacts the compound's chemical behavior, including its basicity.

In the case of pyridine, its aromatic system includes a nitrogen atom whose lone pair is not involved in the pi-electron delocalization. However, the lone pair's involvement in the sp² hybridized state affects its basic properties, as it becomes less available to participate in protonation. This is why pyridine, despite having a nitrogen atom, exhibits lower basicity compared to aliphatic amines like methylamine.