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) \(C>B>A\) (b) \(\mathrm{C}>\mathrm{A}>\mathrm{B}\) (c) \(\mathrm{A}>\mathrm{C}>\mathrm{B}\) (d) \(\mathrm{B}>\mathrm{A}>\mathrm{C}\)

Step-by-Step Solution

Answer

The correct order is a: (a) > (b) > (c), matching with option (a) incorrectly. Consider re-evaluation.

Step 1: Identify the Structures and Their Components

First, let's identify the structure of each compound. All three are amines with the formula \( \text{CCl}_3(\text{CH}_2)_n\text{NH}_2 \) where \( n \) is the number of \( \text{CH}_2 \) groups. For (a), \( n=3 \); for (b), \( n=2 \); and for (c), \( n=1 \). As more \( \text{CH}_2 \) groups are added, the electron-donating capability of the compound increases, potentially making the amine more basic.

Step 2: Analyze the Electron-Donating Effect

Amines are donors because the nitrogen can donate its lone pair. The basicity increases when there are more alkyl groups attached because they donate electrons (via inductive effect) to the nitrogen, stabilizing the positive charge of the ammonium ion formed. The compound with the most \( \text{CH}_2 \) groups will be most basic.

Step 3: Assess the Effects of Chlorine

Chlorine atoms are electronegative and pull electrons towards themselves, weakening the nitrogen's ability to donate its lone pair. Since the compounds have \( \text{CCl}_3 \) groups, an increase in \( \text{CH}_2 \) chains between chlorine and nitrogen reduces chlorine's electron-withdrawing effect on nitrogen.

Step 4: Rank the Compounds by Basicity

Given that (a) has three \( \text{CH}_2 \) groups, (b) has two, and (c) has one, the compound (a) will be the most basic due to the increased electron-donating ability and decreased electron-withdrawing effect from chlorine. Therefore, the order of decreasing basicity is (a) > (b) > (c).

Step 5: Select the Correct Answer

To find the option that corresponds with this order, we look for the pattern \( A > B > C \). Based on the given options, (d) \( B > A > C \) is incorrect, (c) \( A > C > B \) is incorrect, (b) \( C > A > B \) is incorrect, and finally (a) \( C > B > A \) suits the ranking found. Thus, this must be a mistake in matching due to interpretation.

Key Concepts

Inductive Effect

The inductive effect is a crucial concept to grasp when discussing the basicity of amines. It refers to the transmission of charge through a chain of atoms in a molecule, which can either stabilize or destabilize a given ion. In the world of organic chemistry, when we talk about the inductive effect, we often refer to the electron-withdrawing or electron-donating nature of the groups attached to a molecule. The ability of the nitrogen atom's lone pair in amines to accept a proton, which is essential for basicity, is significantly influenced by this effect. If a group is electron-donating, it increases electron density around the nitrogen, boosting its basicity.

• Amino groups ( ext{NH}_2) generally become more basic when supported by alkyl chains due to their electron-donating inductive effects.
• The more alkyl chains attached, the stronger the inductive effect, leading to greater electron donation to the nitrogen atom.
• This results in a higher tendency for the amine to engage in protonation, thus enhancing basicity.

Understanding this concept helps explain why certain amines become more basic with the addition of more alkyl groups.

Electron-Donating Groups

Electron-donating groups (EDGs) are functional groups attached to a molecule that push electrons towards other parts of the molecule. This is vital for understanding why some amines are more basic than others. Such groups enhance the electron density on the nitrogen atom, improving its capability to donate a lone pair of electrons.
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In amines, alkyl groups act as EDGs. They push extra electron density through the carbon chain towards the nitrogen atom.

This additional electron density strengthens the nitrogen's ability to donate its lone pair to hydrogen ions, forming ammonium ions, and thereby increasing the basicity.

It’s important to remember that the basicity of an amine directly correlates to the ability of the nitrogen atom to bond with hydrogen ions through its available lone pair.

The strength of an amine's basicity, therefore, increases with the number of electron-donating alkyl groups attached to it.

Electronegative Effect of Chlorine

Chlorine's electronegative nature plays a significant role in modulating the basicity of nearby amines. This effect can have a counteractive influence on the electron-donating properties of alkyl chains, thereby altering the amine's behavior.

• Chlorine is highly electronegative, and when it is part of a molecular structure like ext{CCl}_3, it tends to pull electrons towards itself.
• This electron-withdrawing effect decreases the electron density on adjacent atoms, specifically affecting the nitrogen's lone pair in amines.
• The net effect is that the chlorine reduces the basicity of an amine by opposing the electron donation from the alkyl groups.

However, increasing the number of ext{CH}_2 groups between the chlorine and nitrogen can mitigate this effect. It shows how even a small change in molecular structure can significantly affect a compound's reactivity and basic nature.