Problem 92 - A Complete Resource Book in Chemistry for JEE Main

Question

Arrange methyl amine (a), pyridine (b) and guan (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) \(\mathrm{B}>\mathrm{C}>\mathrm{A}\)

Step-by-Step Solution

Answer

The correct order is (a) C > A > B.

Step 1: Understand Basicity

Basic strength refers to the ability of a compound to accept protons. A stronger base has a higher tendency to accept protons.

Step 2: Analyze Methyl Amine

Methyl amine is an aliphatic amine, which is a strong base because the nitrogen atom is bonded to an alkyl group, enhancing its electron-donating ability.

Step 3: Analyze Pyridine

Pyridine is a heterocyclic amine with nitrogen in a ring, where the lone pair on the nitrogen is not as readily available for protonation as in aliphatic amines, making it less basic than methyl amine.

Step 4: Analyze Guanoid (Guanidine)

Guanidine is incredibly basic due to resonance stabilization. The positive charge post-protonation is stabilized over multiple nitrogen atoms, greatly enhancing its basicity.

Step 5: Order the Compounds

Based on the above analysis, the order from strongest base to weakest base is guanidine > methyl amine > pyridine.

Step 6: Select the Correct Option

The option that correctly represents the order guanidine > methyl amine > pyridine is (a) C > A > B.

Key Concepts

Amines

Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. This makes amines quite interesting in terms of their basicity or the ability to accept protons. The basicity of an amine can vary widely and is influenced by several factors:

* **Nature of Substituents:** Alkyl groups attached to the nitrogen can increase basicity because they provide electron-donating effects. This increases the electron density on the nitrogen, making amines better bases. * **Solvent Effect:** Basicity can also depend on the solvent used, as it affects the availability of lone pairs. * **Resonance and Inductive Effects:** These can either increase or decrease an amine's ability to accept protons.

Understanding these factors helps explain why some amines, like methyl amine, are stronger bases than others. In methyl amine, the alkyl group enhances its proton-accepting ability, which is not the case in all amines.

Pyridine

Pyridine is a fascinating example of aromatic amines because it features a nitrogen atom within a six-membered aromatic ring. This structure influences its basicity.

* **Aromatic Ring Influence:** In pyridine, the nitrogen's lone pair is perpendicular to the plane of the ring. This makes it less available for bonding and thus less basic compared to amines like methyl amine that do not have such constraints. * **Role in Chemical Reactions:** Although pyridine is less basic, it is often used as a base in various chemical reactions, particularly where a weaker base is desired to avoid over-stabilization of certain intermediates.

The ring structure, while contributing to its aromatic stability, limits its basicity, making pyridine less able to pick up protons compared to typical aliphatic amines.

Guanidine

Guanidine is one of the strongest organic bases due to its unique structure and strong resonance stabilization. It stands out among nitrogen-containing bases.

* **Structure and Stabilization:** Guanidine has three nitrogen atoms, and upon accepting a proton, the positive charge can delocalize between these atoms. This makes it significantly more stable and, therefore, a stronger base. * **Utility in Chemistry:** Its high basicity makes guanidine useful in various organic synthesis processes. Its ability to stabilize a charge means it can influence the course of reactions more dramatically.

This delocalization and resonance stabilization are key to understanding why guanidine is so basic. In comparison to methyl amine and pyridine, guanidine’s basic strength is much higher owing to these intrinsic structural advantages.