Question

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

Short answer

Correct answer is (a) C > A > B.

Step-by-step solution

Understand the Concept of Basic Strength

The basic strength of a compound depends on its ability to donate a lone pair of electrons. Compounds with higher electron density on the nitrogen atom tend to be stronger bases.

Examine Methyl Amine (A)

Methylamine is an aliphatic amine and its nitrogen has an available lone pair that is only influenced by the methyl group, which is electron donating. This makes methylamine fairly basic.

Examine Pyridine (B)

Pyridine is an aromatic amine where the lone pair on the nitrogen is not part of the aromatic sextet, allowing it to participate in electron donation. However, the aromatic ring's electron-withdrawing nature somewhat reduces the basicity compared to aliphatic amines.

Examine Guanidine (C)

Guanidine has a strongly electron-donating effect due to resonance with multiple amino groups. This resonance stabilization of the guanidinium ion makes it one of the strongest known bases.

Arrange in Order of Basic Strength

Based on the analysis, guanidine (C) is the strongest base, followed by methyl amine (A), and finally pyridine (B). Therefore, the order of decreasing basic strength is C > A > B.

Key concepts

Methyl Amine

Methyl amine is a simple aliphatic amine which features a nitrogen atom bonded to a single methyl group (CH₃). This structure makes it part of a class known as primary amines. But what makes methyl amine quite interesting is its underlying electron dynamics. The lone pair of electrons on the nitrogen plays a crucial role in guiding its basicity.

Being an aliphatic amine , methyl amine benefits from the electron-donating properties of the methyl group. This means that the methyl group pushes electron density towards the nitrogen atom. This increase in electron density enhances the availability of the lone pair for donation, strengthening its basic character.

Consider these key points about methyl amine:

• It is an aliphatic compound meaning it's based on a chain structure, which limits electronic interactions often present in cyclic structures.
• The electron-donating methyl group boosts basicity.
• Methyl amine is a stronger base than aromatic amines like pyridine due to its straight chain structure and electron donation.

These characteristics make methyl amine a moderately strong base.

Pyridine

Pyridine is an aromatic heterocyclic compound featuring a nitrogen atom within a six-membered ring. Its nitrogen atom carries a lone electron pair which contributes to its basicity, but pyridine's aromaticity plays a significant role in determining its overall behavior as a base.

Although the lone pair on nitrogen is not participating in the aromaticity of the ring, pyridine's aromatic system introduces some intriguing effects. The aromatic ring inherently exhibits electron-withdrawing characteristics due to resonance, which decreases the electron density available on the nitrogen atom.

Here's how pyridine shapes up in terms of basicity:

• The electron-withdrawing nature of the aromatic ring diminishes basic strength, making pyridine less basic than aliphatic amines.
• The lone pair is available for bonding, which enables it to act as a base, but it's less available than in non-aromatic amines.
• Despite the decreased basicity, pyridine can still become protonated, reflecting its ability to function as a weak base.

While pyridine is less basic compared to methyl amine, it still holds its ground as a useful base in various chemical contexts.

Guanidine

Guanidine is an exceptionally fascinating compound when it comes to basicity, being one of the strongest known organic bases. It possesses a unique structural setup, with a nitrogen central atom connected to three nitrogen-containing groups, allowing remarkable electron donation and resonance stabilization.

This nitrogen-centered structure of guanidine accommodates extensive resonance. In resonance, multiple structures share electron distribution, and in guanidine this sharing leads to a highly stable guanidinium ion once it donates a proton.

What makes guanidine a standout base includes:

• The presence of multiple nitrogen atoms fostering a high level of electron donation.
• Resonance effects that effectively stabilize the resulting guanidinium ion.
• Superior basic strength making it a far more potent base than both methyl amine and pyridine.

Thanks to these properties, guanidine is often employed in laboratory settings where strong basic conditions are necessary, reflecting its impressive electron-donating capabilities and stability when acting as a base.