Problem 115 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following compounds: (1) \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{NH}_{2}\) (2) \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{NH}-\mathrm{C}_{6} \mathrm{H}_{3}\) (3) \(\left(\mathrm{C}_{6} \mathrm{H}_{5}\right)_{3} \mathrm{~N}\) (4) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{NH}_{2}\) Arrange these compounds in decreasing order of their basicity. (a) \(1>2>3>4\) (b) \(2>3>1>4\) (c) \(3>2>1>4\) (d) \(4>1>2>3\)

Step-by-Step Solution

Answer

The correct order of basicity is 4>1>2>3. Select option (d).

Step 1: Understanding Basicity

Basicity refers to the ability of a compound to donate a pair of electrons. In simple terms, the more willing a nitrogen in an amine is to share its lone pair, the more basic the compound is. Basicity is influenced by the availability of the lone pair on nitrogen and the electron-donating or withdrawing effect of surrounding groups.

Step 2: Analyze Aniline's Basicity (Compound 1)

Aniline (1) (\(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{NH}_{2}\)) is a weak base because the lone pair on nitrogen is delocalized into the benzene ring through resonance. This delocalization reduces its availability for protonation.

Step 3: Assess Diphenylamine's Basicity (Compound 2)

In diphenylamine (2) (\(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{NH}-\mathrm{C}_{6} \mathrm{H}_{3}\)), the lone pair on nitrogen is even less available due to additional resonance with the second phenyl group. Thus, it is less basic than aniline.

Step 4: Consider Triphenylamine's Basicity (Compound 3)

Triphenylamine (3) (\(\left(\mathrm{C}_{6} \mathrm{H}_{5}\right)_{3} \mathrm{N}\)) has three phenyl groups, which makes the lone pair on nitrogen highly delocalized among the aromatic rings. This results in very low basicity due to limited availability of the lone pair.

Step 5: Evaluate Ethylamine's Basicity (Compound 4)

Ethylamine (4) (\(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{NH}_{2}\)) is a simple aliphatic amine. The lone pair on nitrogen is not involved in resonance and is readily available for bonding with protons, making it the most basic among these compounds.

Step 6: Compare and Order Compounds by Basicity

Taking into account all the discussions from Steps 2 through 5, the order of basicity decreases as we go from readily available lone pairs to increasingly delocalized lone pairs. Thus, the order is: 4 (ethylamine) > 1 (aniline) > 2 (diphenylamine) > 3 (triphenylamine).

Step 7: Select the Correct Answer

Comparing the determined order of basicity ( 4 > 1 > 2 > 3) with the options provided, the correct answer is option (d) 4>1>2>3.

Key Concepts

Lone Pair Delocalization

In organic chemistry, lone pair delocalization is a key concept that impacts the basicity of amines significantly. Delocalization refers to the spreading out of an electron pair across multiple atoms. This is common in systems involving conjugated π bonds such as benzene rings.
For amines like aniline, the lone pair on the nitrogen atom can participate in delocalization. Due to this, the lone pair is less available for bonding with protons, which in turn reduces the basicity of the compound. Conversely, if the lone pair is localized, it is readily available for protonation, thereby increasing the compound's basicity. Therefore, lone pair delocalization is an important factor that decreases the basicity of a compound by limiting the availability of electrons for donating.

Electron Donating Groups

Electron donating groups (EDGs) play a crucial role in affecting the basicity of a molecule. These are groups that push electron density towards a neighboring atom, thereby increasing its electron availability. In amines, EDGs attached to the nitrogen atom can enhance the basicity because they make the nitrogen's lone pair more available for bonding.
Common EDGs include alkyl groups, like methyl or ethyl groups, which are seen in aliphatic amines. These groups donate electron density through induction, a process that amplifies the electron-rich nature of the amine nitrogen. In the exercise, ethylamine, which includes an alkyl EDG, proves to be more basic than the other compounds because the lone pair on nitrogen is more accessible for protonation due to the donating effects of the alkyl group.

Resonance Effect

Resonance effect is another vital component in understanding amine basicity. The resonance effect involves the delocalization of electrons across adjacent π bonds, affecting electron availability. In compounds with aromatic rings, such as aniline and diphenylamine, resonance can lead to a significant decrease in basicity. This occurs because the lone pair on the nitrogen engages in resonance with the benzene ring, making it less available for interacting with protons.
The more resonance structures a compound can have, the more its lone pair is spread out over these structures, reducing its basicity. In the case of triphenylamine, the involvement of three phenyl groups maximizes this resonance effect, leading to very reduced basicity due to excessive lone pair delocalization.

Amines Basicity

Basicity in amines is determined by the availability of the nitrogen's lone pair to accept a proton. Several factors affect this availability, including the presence of lone pair delocalization, electron donating or withdrawing groups, and resonance effects.
Amines like ethylamine, with localized lone pairs and electron donating groups, exhibit higher basicity due to the ready availability of electrons. Conversely, aromatic amines, such as aniline and triphenylamine, demonstrate lower basicity because of delocalization, which makes their lone pairs less accessible.
To summarize, the core of an amine's basicity lies in its structural features, which either enhance or hinder the lone pair's ability to participate in electron-donation processes. This understanding helps to explain why simple aliphatic amines often prove more basic than their aromatic counterparts.