Problem 80 - A Complete Resource Book in Chemistry for JEE Main

Question

Among the amines I. \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) II. \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) III. \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}\) IV. \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}\) (a) \(\mathrm{I}<\mathrm{IV}<\mathrm{II}<\mathrm{III}\) (b) \(\mathrm{IV}<\mathrm{III}<\mathrm{II}<\mathrm{I}\) (c) \(\mathrm{I}<\mathrm{II}<\mathrm{III}<\mathrm{IV}\) (d) \(\mathrm{II}<\mathrm{III}<\mathrm{IV}<\mathrm{I}\)

Step-by-Step Solution

Answer

The answer is option (a) I < IV < II < III.

Step 1: Understanding the Problem

We need to arrange the given amines in order of their basicity. Amines are compounds that contain a nitrogen atom bonded to alkyl or aryl groups (e.g., phenyl or methyl groups in this case). The basicity of amines depends on their ability to donate an electron pair from the nitrogen atom.

Step 2: Factors Affecting Basicity

Basicity in amines is influenced by electronic effects, such as the electron-donating or withdrawing nature of substituents. Electron-donating groups increase basicity by increasing electron density on the nitrogen atom, whereas electron-withdrawing groups have the opposite effect. Steric hindrance can also affect basicity.

Step 3: Analyzing Each Amine

1. I ( ext{C}_{6} ext{H}_{5} ext{NH}_{2}): Aniline, where phenyl is electron-withdrawing, reducing basicity. 2. II ( ext{CH}_{3} ext{NH}_{2}): Methylamine, primary amine with little steric hindrance, increasing basicity. 3. III (( ext{CH}_{3})_{2} ext{NH}): Dimethylamine, secondary amine with increased electron-donating effect than methylamine. 4. IV ( ext{(CH}_{3})_{3} ext{~N}): Trimethylamine, tertiary amine with steric hindrance reducing basicity.

Step 4: Arranging the Amines by Basicity

III (Dimethylamine) is most basic because it has two electron-donating groups and minimal steric hindrance. Then, II (Methylamine) is next as it's a primary amine. IV (Trimethylamine) follows, where steric hindrance slightly reduces basicity. I (Aniline) is least basic due to the electron-withdrawing effect of the phenyl group.

Step 5: Conclusion and Answer Selection

Based on the analysis, the basicity order from highest to lowest is III > II > IV > I, which matches option (a) I

Key Concepts

Electron-Donating Effects

In the realm of chemistry, the concept of electron-donating effects is pivotal in understanding how different groups can influence the basicity of amines. Amines are nitrogen-containing compounds that can donate a pair of electrons, and their basicity is greatly influenced by any substituents present on them. When a group directly attached to the nitrogen atom donates electrons, it enhances the electron density of the nitrogen, making it a stronger base.

• Methyl Groups: Consider dimethylamine \((\text{(CH}_3\text{)}_2\text{NH})\). Here, two methyl groups are attached to the nitrogen atom. Methyl groups are known for their capacity to donate electrons, thereby increasing the electron density around the nitrogen atom and improving the molecule's ability to donate an electron pair.
• Primary Amines: In methylamine \(\text{(CH}_3\text{NH}_2)\), the single methyl group enhances electron density moderately compared to dimethylamine, which contributes to a high level of basicity among primary amines.

Thus, electron-donating groups play a crucial role in amplifying the basic character of amines.

Steric Hindrance in Amines

Steric hindrance is another critical factor in determining the basicity of amines. This refers to the crowding of atoms or groups around the nitrogen atom, which can hinder its ability to interact with other molecules or ions.

• Trimethylamine: In trimethylamine \((\text{(CH}_3\text{)}_3\text{N})\), there are three methyl groups around the nitrogen atom. This configuration creates significant steric hindrance that can obstruct the nitrogen's lone pair from engaging with protons. Consequently, despite having three electron-donating groups, the basicity is compromised due to this spatial crowding.
• Comparison with Dimethylamine: Dimethylamine, with only two methyl groups, faces less steric hindrance, allowing better access to the nitrogen’s lone pair. This explains why dimethylamine is more basic compared to trimethylamine.

Understanding steric hindrance helps in predicting how well an amine can engage in protonation.

Electron-Withdrawing Effects

Electron-withdrawing effects are characterized by the ability of certain groups to pull electron density away from the nitrogen atom, thereby reducing the basicity of the amine. This effect can drastically alter the chemical behavior of amines.

• Aniline: In aniline \(\text{(C}_6\text{H}_5\text{NH}_2)\), the phenyl group (C₆H₅-) is an electron-withdrawing group. It reduces the electron density on the nitrogen atom by pulling electrons towards itself via resonance. This makes aniline less able to donate its lone pair of electrons compared to methylamine or dimethylamine.
• Comparison Impact: When the electron density over the nitrogen diminishes, the amine's ability to act as a base drops. Consequently, aniline is the least basic among the given amines.

The electron-withdrawing effects are substantial in evaluating the relative basicity of different amines, especially when comparing aromatic to aliphatic structures.

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Key Concepts

Electron-Donating Effects

In the realm of chemistry, the concept of electron-donating effects is pivotal in understanding how different groups can influence the basicity of amines. Amines are nitrogen-containing compounds that can donate a pair of electrons, and their basicity is greatly influenced by any substituents present on them. When a group directly attached to the nitrogen atom donates electrons, it enhances the electron density of the nitrogen, making it a stronger base.

• Methyl Groups: Consider dimethylamine \((\text{(CH}_3\text{)}_2\text{NH})\). Here, two methyl groups are attached to the nitrogen atom. Methyl groups are known for their capacity to donate electrons, thereby increasing the electron density around the nitrogen atom and improving the molecule's ability to donate an electron pair.
• Primary Amines: In methylamine \(\text{(CH}_3\text{NH}_2)\), the single methyl group enhances electron density moderately compared to dimethylamine, which contributes to a high level of basicity among primary amines.

Thus, electron-donating groups play a crucial role in amplifying the basic character of amines.

Steric Hindrance in Amines

Steric hindrance is another critical factor in determining the basicity of amines. This refers to the crowding of atoms or groups around the nitrogen atom, which can hinder its ability to interact with other molecules or ions.

• Trimethylamine: In trimethylamine \((\text{(CH}_3\text{)}_3\text{N})\), there are three methyl groups around the nitrogen atom. This configuration creates significant steric hindrance that can obstruct the nitrogen's lone pair from engaging with protons. Consequently, despite having three electron-donating groups, the basicity is compromised due to this spatial crowding.
• Comparison with Dimethylamine: Dimethylamine, with only two methyl groups, faces less steric hindrance, allowing better access to the nitrogen’s lone pair. This explains why dimethylamine is more basic compared to trimethylamine.

Understanding steric hindrance helps in predicting how well an amine can engage in protonation.

Electron-Withdrawing Effects

Electron-withdrawing effects are characterized by the ability of certain groups to pull electron density away from the nitrogen atom, thereby reducing the basicity of the amine. This effect can drastically alter the chemical behavior of amines.

• Aniline: In aniline \(\text{(C}_6\text{H}_5\text{NH}_2)\), the phenyl group (C₆H₅-) is an electron-withdrawing group. It reduces the electron density on the nitrogen atom by pulling electrons towards itself via resonance. This makes aniline less able to donate its lone pair of electrons compared to methylamine or dimethylamine.
• Comparison Impact: When the electron density over the nitrogen diminishes, the amine's ability to act as a base drops. Consequently, aniline is the least basic among the given amines.

The electron-withdrawing effects are substantial in evaluating the relative basicity of different amines, especially when comparing aromatic to aliphatic structures.