Problem 145 - 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 \([\mathbf{2 0 0 3}]\) (a) \(\mathrm{NH}_{3}<\mathrm{CH}_{3} \mathrm{NH}_{2}<\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}\) (b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}<\mathrm{NH}_{3}<\mathrm{CH}_{3} \mathrm{NH}_{2}\) (c) \(\mathrm{CH}_{3} \mathrm{NH}_{2}<\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}<\mathrm{NH}_{3}\) (d) \(\mathrm{CH}_{3} \mathrm{NH}_{2}<\mathrm{NH}_{3}<\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}\)

Step-by-Step Solution

Answer

(a) \( \mathrm{NH}_{3}<\mathrm{CH}_{3} \mathrm{NH}_{2}<\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH} \)

Step 1: Understand the Concept

The basic nature of an amine is determined by the availability of the lone pair of electrons on the nitrogen atom. This is influenced by the attached alkyl groups, which donate electron density through a +I effect.

Step 2: Analyze Each Compound

Compare the three compounds:1. Ammonia ( \( \mathrm{NH}_{3} \)) is the simplest with no alkyl groups to donate electron density to the nitrogen.2. Methylamine ( \( \mathrm{CH}_{3} \mathrm{NH}_{2} \)) has one methyl group, which donates some electron density to nitrogen.3. Dimethylamine ( \( \left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH} \)) has two methyl groups, increasing the electron density on the nitrogen further.

Step 3: Determine Increasing Basic Nature

Because electron donation increases the electron density on nitrogen, it also increases the basicity. Thus, the order of increasing basic nature is:1. \( \mathrm{NH}_{3} \) (least basic)2. \( \mathrm{CH}_{3} \mathrm{NH}_{2} \) (more basic)3. \( \left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH} \) (most basic)This corresponds to option (a): \( \mathrm{NH}_{3}<\mathrm{CH}_{3} \mathrm{NH}_{2}<\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH} \).

Key Concepts

Lone Pair of Electrons

The basicity of amines is mainly attributed to the presence of a lone pair of electrons on the nitrogen atom. This lone pair is crucial because it can potentially accept a proton ( H^{+} ), defining the amine's basic character.
Understanding the lone pair is fundamental to comprehending why different amines display varying basicities. In a simple amine like ammonia ( ext{NH}_3 ), the nitrogen atom has one lone pair of electrons available for bonding with H^{+} . This alone means it's relatively basic in nature.
However, when additional groups like methyl groups are attached, the environment around the nitrogen changes, enhancing its ability to hold or release this lone pair more effectively. Hence, when studying amines, always keep the behavior and availability of the lone pair of electrons in mind.

Electron Donating Groups

The concept of electron donating groups (EDGs) is essential in understanding the varying basicities in amines. These groups work by pushing electron density towards the nitrogen atom, which in turn makes the nitrogen more attractive to protons.
Methyl groups ( ext{CH}_3 ) are common electron donating groups attached to nitrogen in amines. They exert an electron donating effect, also known as the +I effect or inductive effect (discussed further in the next section). By increasing the electron density around nitrogen, methyl groups enhance the basicity of an amine.

• With no methyl groups as in ammonia ( ext{NH}_3 ), the nitrogen has the base level of electron density, thus the lowest basicity among these examples.
• Adding one methyl group, as seen in methylamine ( ext{CH}_3 ext{NH}_2 ), increases the electron availability on nitrogen, lending more strength to its basicity.
• Dimethylamine ( ( ext{CH}_3)_2 ext{NH} ) with two methyl groups further amplifies this effect, providing the nitrogen with the highest electron density and thus, greatest basicity among the shown options.

Inductive Effect

The inductive effect is a fundamental concept for explaining the influence that electron donating groups have on the basicity of amines. This effect describes the way electron density can be pushed through a sigma bond in a molecule.
When groups like methyl groups attach to the nitrogen in amines, they "pull" or "push" electron density through these bonds in a manner defined as the +I (positive inductive) effect.

• In ammonia ( ext{NH}_3 ), there is no inductive effect contributing to electron density on nitrogen, making it the least basic.
• For methylamine ( ext{CH}_3 ext{NH}_2 ), the single methyl group induces a slight +I effect, which boosts the electron availability to nitrogen, enhancing its ability to attract protons.
• In dimethylamine ( ( ext{CH}_3)_2 ext{NH} ), two methyl groups create a stronger inductive effect, significantly raising the electron density, thereby making dimethylamine the most basic in this series.

Understanding the inductive effect helps clarify why certain chemical modifications to amines lead to changes in their basic character. This knowledge is invaluable for predicting the behavior of amines in different chemical contexts.