Problem 37 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Amongst the following, the most basic compound is :(a) Benzylamine (b) Aniline (c) Acetanilide (d) \(p\)-Nitroaniline

Step-by-Step Solution

Answer

Benzylamine is the most basic compound.

Step 1: Understand the Concept of Basicity

Basicity refers to the ability of a compound to donate an electron pair. In aromatic amines like benzylamine, aniline, acetanilide, and \( p \)-nitroaniline, the basicity depends on the availability of the lone pair of electrons on the nitrogen atom to participate in bonding.

Step 2: Analyze Benzylamine

Benzylamine is an aliphatic amine with the structure \( C_6H_5CH_2NH_2 \). The lone pair on nitrogen is not delocalized with the benzene ring due to the methylene spacer \( CH_2 \), making the lone pair more available for donation, which enhances its basicity.

Step 3: Analyze Aniline

Aniline has the structure \( C_6H_5NH_2 \). The lone pair on nitrogen is involved in resonance with the benzene ring, decreasing its availability for donation and thus reducing its basicity compared to benzylamine.

Step 4: Analyze Acetanilide

Acetanilide has the structure \( C_6H_5NHCOCH_3 \). The presence of the acyl group \( COCH_3 \) decreases the availability of the nitrogen's lone pair due to resonance and electron-withdrawing effects, making it less basic than aniline.

Step 5: Analyze \( p \)-Nitroaniline

\( p \)-Nitroaniline has the structure \( C_6H_4(NO_2)NH_2 \). The nitro group \( NO_2 \) is electron-withdrawing, further reducing the availability of the nitrogen's lone pair, making it even less basic than aniline.

Step 6: Conclusion

Comparing all four compounds, the lone pair on the nitrogen of benzylamine is the most available for donation because it is not involved in resonance with the benzene ring or withdrawn by any electron-withdrawing groups.

Key Concepts

Basicity

Basicity in chemistry is all about how well a molecule can donate an electron pair. This ability is especially vital when discussing amines, or nitrogen-containing compounds. In terms of basicity, if a nitrogen atom has a lone pair of electrons ready to be donated, it's considered strong in basicity.
However, several factors can influence a compound's basicity:

• Electron Delocalization: Resonance can delocalize (or "spread out") electrons across a molecule, reducing availability for donation.
• Electron-Withdrawing Groups: Groups like nitro-groups attract electrons, reducing basicity by "pulling" the lone pair.
• Hybridization: Greater s-character in orbitals holding lone pairs can reduce basicity.

These factors combine in different ways, influencing the basicity of aromatic amines.

Aromatic Amines

Aromatic amines are a family of compounds where an amine group (-NH₂) is attached directly to an aromatic ring, like benzene. They're unique due to the combination of their amine functional group and the aromatic ring, which can affect their behavior significantly.
For aromatic amines:

• Resonance Effect: When the lone pair on the nitrogen atom gets involved in resonance with the aromatic ring, it means the electrons are being shared with the larger structure. This interaction can decrease the lone pair's availability, reducing basicity.
• Steric Hindrance: Aromatic rings can also create spaces that physically block atoms from accessing the lone pair.

Each of these features makes aromatic amines a fascinating subject in organic chemistry, especially regarding their basicity.

Electron Pair Donation

Electron pair donation is a crucial concept for understanding basicity in chemistry. Essentially, it refers to the nitrogen atom in amines donating its non-bonding pair of electrons to form new bonds.
In aromatic amines, the effectiveness of this donation depends on:

• Lone Pair Availability: If the nitrogen's electrons are involved in resonance or are affected by electron-withdrawing groups, they're less available for donation.
• Molecular Structure: In compounds like benzylamine, where the nitrogen's electron pair isn't shared or pulled away by such effects, the electron pair remains more available for donation, making them more basic.

Thus, understanding electron pair donation helps explain why some amines are more basic than others, providing insight into their reactivity and interactions.