Problem 89 - A Complete Resource Book in Chemistry for JEE Main

Question

Arrange the following: I \(\mathrm{CH}_{3} \mathrm{NH}_{2} \quad \mathrm{II}\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH} \quad \mathrm{III} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) and IV \(\left(\overrightarrow{C H}_{3}\right)_{3}^{2} \mathrm{~N}\) in increasing order of basicity in aqueous medium. (a) II < I

Step-by-Step Solution

Answer

(d) III < IV < I < II

Step 1: Analyzing Basicity of Compounds

Basicity in aqueous medium depends on the willingness of the amine compound to donate its lone pair of electrons to the protons in water. The availability of this lone pair is influenced by the surrounding groups attached to the nitrogen atom.

Step 2: Understanding Inductive Effect and Steric Hindrance

Alkyl groups (like CH₃) have a +I (positive inductive) effect which increases the electron density on the nitrogen, enhancing basicity. However, steric hindrance can make it difficult for the nitrogen to interact with water, reducing basicity.

Step 3: Evaluating Compound I (2 CH₃NH₂)

In methylamine (CH₃NH₂), the lone pair on nitrogen is relatively available because there’s only one alkyl group attached, making it moderately basic. Methyl is slightly electron-donating, increasing basicity.

Step 4: Evaluating Compound II ((CH₃)₂NH)

Dimethylamine ((CH₃)₂NH) has two methyl groups, increasing the electron density more than methylamine due to the +I effect. This makes it more basic than compound I. Interaction with the solvent is still effective.

Step 5: Evaluating Compound III (C₆H₅NH₂)

Aniline (C₆H₅NH₂) has a phenyl ring attached to nitrogen, which withdraws electrons due to resonance and the -I (negative inductive) effect, reducing the availability of the lone pair. Hence, aniline is less basic.

Step 6: Evaluating Compound IV ((CH₃)₃N)

Trimethylamine ((CH₃)₃N) possesses three methyl groups, greatly increasing the electron availability on nitrogen, but steric hindrance hampers effective interaction with water, causing a slight reduction in basicity compared to dimethylamine.

Step 7: Arranging in Order of Basicity

Considering the lone pair availability and influences from attached groups, the order of basicity is: aniline < trimethylamine < methylamine < dimethylamine.

Key Concepts

Inductive Effect in Chemistry

The inductive effect is an essential concept in chemistry, especially when examining the basicity of amines. It involves the redistribution of electron density along a chain of atoms within a molecule. At its core, the inductive effect is about how an atom or group affects the electron density of nearby atoms—making them either richer or poorer in electrons.
Alkyl groups, like methyl (\( \mathrm{CH}_3 \)), possess a positive inductive effect, denoted as +I. This means they donate electron density towards the atom they are attached to, which is commonly the nitrogen atom in amines. This donation increases the overall electron density around the nitrogen, thereby enhancing its basicity.
For instance, in dimethylamine (\( (\mathrm{CH}_3)_2 \mathrm{NH} \)), the two methyl groups significantly boost the electron density around the nitrogen. However, if an amine such as aniline (\( \mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2 \)) has a phenyl ring instead, the electron density decreases due to resonance and a negative inductive effect (-I), leading to reduced basicity.

Steric Hindrance

Steric hindrance is another important factor influencing the basicity of amines. It refers to the physical obstruction caused by bulky groups around a reactive center—in this case, the nitrogen atom.
When large or numerous groups crowd the area around nitrogen, it becomes difficult for water molecules to approach closely enough to interact effectively with the lone pair of electrons on the nitrogen. This situation can lead to a decrease in the compound's basicity compared to what the inductive effect alone would suggest.
Trimethylamine ((\( \mathrm{CH}_3 \)_3 \mathrm{N})) exemplifies this phenomenon. While the three methyl groups increase electron density substantially due to their +I effect, the sheer bulk of these groups creates a crowded environment around nitrogen, hindering its interaction with protons in water. Consequently, trimethylamine exhibits lower basicity than dimethylamine, where steric hindrance is minimized.

Electron Density in Amines

Electron density plays a pivotal role in determining the basicity of amines. It is essentially about how available or concentrated the electrons are around the nitrogen atom—the site at which electrons are donated to protons in water.
High electron density typically correlates with higher basicity because more available electrons can easily engage with protons. For example, in dimethylamine (\( (\mathrm{CH}_3)_2 \mathrm{NH} \)), the two methyl groups contribute significantly to increasing electron density, thereby enhancing its basicity in aqueous solutions.
On the other hand, if the electron density is lowered, such as in aniline (\( \mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2 \)), where resonance effects draw electrons away from nitrogen, the basicity decreases. This is because the nitrogen has fewer electrons readily available to interact with protons.
This understanding of electron density helps in arranging amines in the order of their basicity, taking into account both inductive effects and steric hindrance.