Problem 35 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Among the following, the strongest base is(a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) (b) \(p-\mathrm{NO}_{2} \cdot \mathrm{C}_{6} \mathrm{H}_{4} \mathrm{NH}_{2}\) (c) \(m-\mathrm{NO}_{2} \cdot \mathrm{C}_{6} \mathrm{H}_{4} \cdot \mathrm{NH}_{2}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{NH}_{2}\)

Step-by-Step Solution

Answer

Option (d) is the strongest base: \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{NH}_{2}\).

Step 1: Understand the Structure of Aniline and its Derivatives

Aniline (\(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\)) is a base, and its basicity is affected by the electron density on the nitrogen atom. Substituents on the benzene ring affect this electron density through resonance and inductive effects.

Step 2: Evaluate Electron Withdrawing Groups

Nitro groups (\(\mathrm{NO}_{2} \)) are strong electron-withdrawing groups. They decrease the electron density on the nitrogen atom when attached to a benzene ring, reducing basicity. Hence, options (b) and (c), which have \(\mathrm{NO}_{2} \) groups, are less basic than \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\).

Step 3: Compare the Effect of Benzyl Group

The benzyl group (\(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2}\)) in \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{NH}_{2}\) is an electron releasing group and does not participate in resonance with the benzene ring, unlike the nitro groups. It increases the electron density on nitrogen compared to plain aniline, enhancing basicity.

Step 4: Determine the Strongest Base

Based on the above analysis, (d) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{NH}_{2}\) increases electron density on the nitrogen the most, making it the strongest base among the given choices.

Key Concepts

Aniline

Aniline is a compound where an amino group \(\mathrm{NH}_{2}\) is directly linked to a benzene ring, represented as \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\). The basicity of aniline stems from the lone pair of electrons on the nitrogen atom, which can accept protons (H⁺ ions), making it a base. However, the presence of the benzene ring can influence this basic nature.
Aniline's basicity is affected because the benzene ring is involved in resonance, which means that electrons can be shared between the nitrogen atom and the ring. This delocalization of electrons reduces their availability to accept protons, consequently decreasing the basicity of aniline compared to aliphatic amines. This is why aniline is a weaker base. Understanding the structure of aniline is essential in predicting how it will behave chemically, especially when substitutions occur on the benzene ring.

Electron Withdrawing Groups

Electron withdrawing groups (EWGs) are functional groups that pull electron density away from parts of the molecule they are attached to. In the context of aniline or similar compounds, such groups can significantly reduce basicity.
The nitro group (\(\mathrm{NO}_{2}\)) is one of the most common and strongest electron withdrawing groups. When attached to a benzene ring, these groups decrease the electron density on the nitrogen atom.

• This is because the nitro group can withdraw electrons through resonance and inductive effects, making the nitrogen less likely to donate its lone pair of electrons.
• This decrease in electron density means the ability of the nitrogen to act as a base (by accepting protons) is reduced.

In the given exercise, options \(b\) and \(c\), which include the \(\mathrm{NO}_{2}\) substituent, demonstrate reduced basicity resulting from these electron withdrawing effects.
Understanding how EWGs operate on aniline can provide insight into how alterations in chemical structure influence compound properties.

Benzyl Group

The benzyl group, noted as \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2}\), comprises a benzene ring attached to a methylene bridge, which is then linked to other atoms or groups. Notably, in compounds like \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{NH}_{2}\), the benzyl group plays an important role in influencing basicity.
Unlike the phenyl group attached directly to the amino group in aniline, the benzyl group does not undergo resonance with the nitrogen's lone pair.

• Instead, it acts as an electron releasing group through inductive effects. This means more electrons are available for the nitrogen atom, enhancing its ability to donate electrons and accept protons.
• This increase in electron density around the nitrogen atom makes benzylamine more basic than aniline.

In the context of the exercise, this explains why \(d\) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{NH}_{2}\) is the strongest base. Fully grasping the influence of benzyl groups helps in predicting the behavior and reactivity of similar amines in chemical reactions.