Problem 183 - A Complete Resource Book in Chemistry for JEE Main

Question

The correct order of increasing basic nature for the bases \(\mathrm{NH}_{3}, \mathrm{CH}_{3} \mathrm{NH}_{2}\), and \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}\) is \([2003\) (a) \(\mathrm{CH}_{3} \mathrm{NH}_{2}<\mathrm{NH}_{3}<\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}\) (b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}<\mathrm{NH}_{3}<\mathrm{CH}_{3} \mathrm{NH}_{2}\) (c) \(\mathrm{NH}_{3}<\mathrm{CH}_{3} \mathrm{NH}_{2}<\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}\) (d) \(\mathrm{CH}_{3} \mathrm{NH}_{2}<\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}<\mathrm{NH}_{3}\)

Step-by-Step Solution

Answer

The correct order is option (c) NH₃ < CH₃NH₂ < (CH₃)₂NH.

Step 1: Understand Basicity Concept

Basicity in amines is determined by the availability of the lone pair of electrons on the nitrogen atom to accept a proton. Substituents that donate electron density to the nitrogen increase basicity, while those that withdraw it decrease basicity.

Step 2: Evaluate NH₃

In ammonia (NH₃), the nitrogen atom has a lone pair which can accept a proton. There are no alkyl groups affecting the electron density, so its basicity is moderate.

Step 3: Analyze CH₃NH₂

In methylamine (CH₃NH₂), the presence of the methyl group (CH₃-) increases the electron density on the nitrogen via inductive effect, making it more basic than NH₃.

Step 4: Examine (CH₃)₂NH

In dimethylamine ((CH₃)₂NH), there are two methyl groups donating electron density to the nitrogen, greatly increasing its basicity compared to NH₃ and CH₃NH₂.

Step 5: Order the Bases by Increasing Basicity

Based on inductive effects, NH₃ is the least basic, followed by CH₃NH₂, and then (CH₃)₂NH as the most basic. Hence, the order of increasing basic nature is NH₃ < CH₃NH₂ < (CH₃)₂NH.

Step 6: Match with Options

Comparing the derived order NH₃ < CH₃NH₂ < (CH₃)₂NH with the provided options, we find that option (c) corresponds to this order.

Key Concepts

Inductive Effect

The inductive effect is one of the key factors that influence the basicity of amines. It refers to the ability of certain groups attached to an atom to either donate or withdraw electrons through sigma bonds. This effect plays a crucial role in stabilizing charged species, like the protonated form of an amine. For amines, groups like alkyls contribute an electron-donating inductive effect.
- **Electron Donation:** When electron-donating groups, such as alkyl groups (e.g., methyl in methylamine), are attached to the nitrogen atom, they increase the electron density around it. - **Increased Basicity:** With more available electrons, the nitrogen atom can better attract and accept a proton, enhancing the amine's basicity compared to ammonia.
In dimethylamine, which has two methyl groups, the inductive effect is even more pronounced, resulting in even higher basicity.

Lone Pair Electrons

Lone pair electrons are pairs of valence electrons that are not shared with other atoms in a molecule. In amines, the nitrogen atom holds a lone pair, which is crucial for their basic nature.
- **Proton Attraction:** These lone pair electrons can attract and bond with a proton, forming a bond with the hydrogen ion (H⁺). - **Proton Acceptance:** The ability of the lone pair to form a bond with the proton largely dictates the basicity of amines.
For ammonia, the lone pair is freely available for proton acceptance. However, in methylamine or dimethylamine, where alkyl groups increase electron density, the lone pair becomes even more effective at attracting protons.

Nitrogen Atom

The nitrogen atom in amines is central to their basicity. It is characterized by its three single bonds and one lone pair of electrons.
- **Acts as a Base:** The nitrogen atom with its lone pair is responsible for making amines act as Lewis bases. - **Electron Density:** The surrounding groups can alter electron density around the nitrogen.
In ammonia, there are no additional groups altering its electron density. But in substituted amines like methylamine and dimethylamine, alkyl groups, through their inductive effect, increase the electron density around nitrogen, enhancing its ability to attract protons. This makes dimethylamine the strongest base among the comparisons because of the increased electron density from its two methyl groups.

Proton Acceptance

A proton acceptance is essential for understanding the basicity of amines. The lone pair of electrons on the nitrogen atom can pair with a proton, effectively "accepting" it. This defines the amine's role as a base.
- **Lewis Base Mechanism:** Proton acceptance is a key feature of Lewis bases like amines. The free pair of electrons on the nitrogen seeks out protons to form a new N-H bond. - **Influence of Substituents:** The ease with which an amine can accept a proton depends on the electron density of the nitrogen.
Higher electron density, as increased by inductive effects of alkyl groups, makes the nitrogen's lone pair more available, and therefore more likely to accept a proton. Thus, dimethylamine, with its higher electron density than either ammonia or methylamine, can accept a proton more readily, exhibiting stronger basicity.