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 (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

Option (a) \(\mathrm{NH}_3 < \mathrm{CH}_3\mathrm{NH}_2 < (\mathrm{CH}_3)_2\mathrm{NH}\) shows the correct order.

Step 1: Understand Basic Nature

The basic nature of molecules depends on their ability to donate a lone pair of electrons. In amines, the presence of alkyl groups (like methyl groups, \(\mathrm{CH}_3\)) can increase the electron density on the nitrogen atom through an inductive effect, making the amine more basic.

Step 2: Analyze NH3

Ammonia (\(\mathrm{NH}_3\)) is the simplest molecule here, with nitrogen bonded to hydrogen atoms. It has a lone pair of electrons on nitrogen, making it a base. However, there are no alkyl groups to enhance its basicity.

Step 3: Examine CH3NH2

Methylamine (\(\mathrm{CH}_3\mathrm{NH}_2\)) contains one methyl group attached to the nitrogen, which increases the electron density due to the +I inductive effect of \(\mathrm{CH}_3\). This makes \(\mathrm{CH}_3\mathrm{NH}_2\) more basic than \(\mathrm{NH}_3\).

Step 4: Assess (CH3)2NH

Dimethylamine \((\mathrm{CH}_3)_2\mathrm{NH}\) has two methyl groups attached to nitrogen, further increasing electron density at the nitrogen via inductive effects, thereby enhancing its basicity compared to \(\mathrm{NH}_3\) and \(\mathrm{CH}_3\mathrm{NH}_2\).

Step 5: Compare Basic Nature

Comparing the basic nature based on electron density and number of alkyl groups: \(\mathrm{NH}_3 < \mathrm{CH}_3\mathrm{NH}_2 < (\mathrm{CH}_3)_2\mathrm{NH}\). The order reflects increasing basicity due to the increasing number of alkyl groups attached.

Key Concepts

Amines

Amines are organic compounds derived from ammonia (\( \mathrm{NH}_3 \)) by substituting one or more hydrogen atoms with alkyl groups or aryl groups. The basic nature of amines comes from the lone pair of electrons on the nitrogen atom, which can accept a proton (\( \mathrm{H}^+ \)). As a result, amines are considered bases. Here are some key points about amines:

• The presence of nitrogen in amines leads to a characteristic behavior of forming hydrogen bonds with water, making them soluble to some extent.
• The classification of amines is based on the number of carbon-containing groups bonded to the nitrogen: primary amines have one, secondary amines have two, and tertiary amines have three alkyl or aryl groups.
• Their basicity is influenced by the presence of electron-donating or withdrawing groups attached to the nitrogen atom.

This basicity is critical in determining how amines react with acids and form salts—important reactions, especially in organic synthesis.

Electron Donor Ability

The electron donor ability of a molecule refers to its capability to donate its lone pair of electrons during chemical interactions. This ability strongly influences its basicity and reactivity. For amines, the nitrogen's lone pair is the source of this electron donor potential. The more electron density on the nitrogen atom, the stronger its ability to donate electrons.

• The higher electron donor ability enhances basicity, as the molecule can more easily accept protons.
• The donor strength rises with the addition of electron-releasing groups like alkyl groups to the nitrogen atom, contributing to a higher overall electron density.
• This property is pivotal in mediating various chemical reactions, especially acid-base reactions and other processes requiring nucleophilic activity.

Thus, enhancements in electron donor ability directly translate into increased basicity, which is why amines attached with electron-donating alkyl groups exhibit greater basicity than those without.

Inductive Effect

The inductive effect is a concept critical for understanding how atoms or groups of atoms affect molecules. It involves the transfer of electron density through the bonds of a molecule, usually from more electronegative to less electronegative elements. Within amines, the inductive effect plays a significant role in determining basicity:

• Alkyl groups, such as methyl \((\mathrm{CH}_3)\), exhibit a +I effect, meaning they tend to donate electrons through sigma bonds, increasing electron density on the nitrogen atom.
• This increased electron density makes the nitrogen atom more ready to share its electron pair, enhancing the basic strength of amines.
• The more alkyl groups inducing the +I effect, the stronger the inductive influence and thus the greater the basicity.

Because of the inductive effect, the basic nature of amines can often be substantiated by the presence and number of substituents that donate electrons toward the nitrogen, as seen in methylamine and dimethylamine.

Alkyl Groups

Alkyl groups are chains or branches of carbon and hydrogen atoms attached to molecules, playing a pivotal role in organic chemistry. In the context of amines, alkyl groups impact the molecule's basic nature significantly:

• They are known for their electron-donating properties, which increase electron density on the nitrogen atom when attached to amines.
• Their ability to influence molecular behavior extends to enhancing the basic nature of amines, with more alkyl groups correlating with stronger bases.
• Alkyl groups can vary in size and structure, impacting how they influence electronic structures and contribute to the overall chemical characteristics of the amine.

Thus, the presence of alkyl groups transforms a simple amine into a more robust base, as evident in the increased basicity from ammonia to methylamine and further to dimethylamine. Their capable electron donation explains why amines with such groups are more reactive in certain conditions.