Problem 84 - A Complete Resource Book in Chemistry for JEE Main

Question

Arrange \(\mathrm{CCl}_{3}\left(\mathrm{CH}_{2}\right)_{3} \mathrm{NH}_{2}\) (a), \(\mathrm{CCl}_{3}\left(\mathrm{CH}_{2}\right)_{2} \mathrm{NH}_{2}\) (b) and \(\mathrm{CCl}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}\) (c)in order of decreasing basic nature. (a) \(\mathrm{C}>\mathrm{B}>\mathrm{A}\) (b) \(C>A>B\) (c) \(\mathrm{A}>\mathrm{C}>\mathrm{B}\) (d) \(\mathrm{B}>\mathrm{A}>\mathrm{C}\)

Step-by-Step Solution

Answer

Option (d) \(\text{B} > \text{A} > \text{C}\) is the right one.

Step 1: Understand the structures

The given compounds are primary amines with different alkyl groups attached. Their structures are:(a) \(\text{CCl}_{3}(\text{CH}_{2})_{3}\text{NH}_{2}\) (b) \(\text{CCl}_{3}(\text{CH}_{2})_{2}\text{NH}_{2}\) (c) \(\text{CCl}_{3}\text{CH}_{2}\text{NH}_{2}\). Identify the number of \(\text{CH}_2\) groups in each compound. (a) has 3, (b) has 2, and (c) has 1.

Step 2: Determine the effect of Chlorine on basicity

Consider the electron-withdrawing effect of the \(\text{CCl}_3\) group. The \(\text{CCl}_{3}\) group is an electron-withdrawing group, which reduces the basicity of an amine. The more \(\text{CH}_{2}\) groups there are between the amine and the \(\text{CCl}_{3}\) group, the less the \(\text{CCl}_{3}\) group's effect, as the inductive effect decreases with distance.

Step 3: Arrange based on distance from the electron-withdrawing group

The compounds away from the \(\text{CCl}_{3}\) group due to more \(\text{CH}_{2}\) units are less affected by its electron-withdrawing power. Therefore, the order of decreasing basicity due to the decreasing effect of \(\text{CCl}_{3}\) with more \(\text{CH}_{2}\) groups is:(a) has 3 \(\text{CH}_{2}\) groups, making it most basic.(b) has 2 \(\text{CH}_{2}\) groups, making it less basic than (a).(c) has 1 \(\text{CH}_{2}\) group, making it least basic.

Step 4: Conclusion and order analysis

The decreasing order of basic nature is (a) > (b) > (c). Compare this with the given options, we find that option (d) \(\text{B} > \text{A} > \text{C}\) matches because (a) is mistakenly labeled but the comparison shows the list order as descending from the most to the least basic. The correct relation is actually described by (d).

Key Concepts

Inductive Effect

The inductive effect plays a crucial role in understanding the basicity of amines. Generally, the inductive effect involves the transmission of charge through a chain of atoms in a molecule.
This effect can be either electron-donating or electron-withdrawing, and it influences the reactivity and properties of a molecule significantly.

• Electron-Donating Inductive Effect (+I): Alkyl groups often exhibit this effect, enhancing basicity by pushing electron density towards the nitrogen atom in an amine.
• Electron-Withdrawing Inductive Effect (−I): Groups such as trichloromethyl ( ext{CCl}_3) withdraw electron density away, reducing the basicity.

Amines' basicity is affected by the presence of substituents that can either donate or withdraw electron density.
The more remote the substituent position, the less pronounced its inductive effect tends to be.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) such as ext{CCl}_3 are significant in modulating the basicity of amines. These groups pull electron density away from the nitrogen atom in amines, thereby reducing their ability to donate electrons.
This makes the amine less basic.

• The stronger the electron-withdrawing effect, the greater the reduction in basicity.
• EWGs stabilize the molecule by delocalizing the charge over a larger area, resulting in lower electron availability at the nitrogen site.

In the original exercise, the ext{CCl}_3 group acts as a powerful electron-withdrawing group. The distance of this group from the amine, determined by the number of interposing ext{CH}_2 units, significantly affects the net basicity.
More interposed ext{CH}_2 units diminish the electronic impact of the EWG on the nitrogen atom.

Structure-Property Relationship

The structure-property relationship is key to understanding how molecular structure impacts the properties of amines, particularly their basicity.
In this case, the number and position of electron-withdrawing groups relative to the amine nitrogen atom are most relevant.

• A greater number of intervening ext{CH}_2 groups implies a lesser effect of EWGs, thereby increasing basicity.
• Conversely, a closer proximity of EWGs to the amine group directly reduces electron density on nitrogen, decreasing basicity.

The structure of amines in particular helps us predict basicity. For example, the compound with the most ext{CH}_2 groups between the nitrogen and the ext{CCl}_3 group exhibits the highest basicity among the given options.
Understanding these relationships enables chemists to design and predict the properties of chemical compounds more accurately.