Problem 19 - A Complete Resource Book in Chemistry for JEE Main

Question

Among the following compound \(\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{NH}_{2}, \mathrm{NH}_{3}\), \(\mathrm{CH}_{3} \mathrm{NH}_{2}, \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NH}_{2}\) and \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) the least basic com- pound is (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) (b) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NH}_{2}\) (c) \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) (d) \(\mathrm{NH}_{3}\)

Step-by-Step Solution

Answer

The least basic compound is \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2} \) (aniline).

Step 1: Understanding Basicity

Basicity in amines is determined by the ability of the nitrogen atom to donate its lone pair of electrons. Aromatic amines like phenylamine (\( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2} \)) are less basic due to delocalization of the lone pair into the aromatic ring.

Step 2: Identifying Structures

The compounds given are: \(\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{NH}_{2} \) (n-propylamine), \( \mathrm{NH}_{3} \) (ammonia), \( \mathrm{CH}_{3} \mathrm{NH}_{2} \) (methylamine), \( \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NH}_{2} \) (ethylamine), and \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2} \) (aniline or phenylamine).

Step 3: Assessing Aliphatic Amines

The aliphatic amines \( \mathrm{CH}_{3} \mathrm{NH}_{2} \), \( \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NH}_{2} \), and \( \mathrm{C}_{3} \mathrm{H}_{7} \mathrm{NH}_{2} \) are generally more basic than ammonia due to the electron-donating effect of the alkyl groups.

Step 4: Comparing Ammonia and Aniline

Ammonia (\( \mathrm{NH}_{3} \)) is more basic than aniline (\( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2} \)) because the lone pair on aniline's nitrogen is involved in resonance with the benzene ring, whereas ammonia has its lone pair free to donate.

Step 5: Conclusion

Among the given compounds, aniline (\( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2} \)) is the least basic because of the delocalization of the nitrogen's lone pair into the aromatic ring.

Key Concepts

Aromatic Amines

Aromatic amines are organic compounds where an amino group is attached to an aromatic ring like benzene. The most common example of this is aniline, which is phenylamine (C_{6}H_{5}NH_{2}). These amines are distinct from their aliphatic counterparts because the nitrogen's lone pair of electrons can participate in resonance with the aromatic ring. This involvement in electron delocalization significantly decreases the basicity of aromatic amines.

For instance, in aniline, the lone pair on the nitrogen atom interacts with the c-electron system of the benzene ring. This resonance stabilization of the aromatic system leads to less availability of the lone pair for protonation. Hence, when comparing the basicity using a pKa scale, aromatic amines typically have much higher pKa values (indicating weaker bases) than aliphatic amines. In essence, the lone pair is "busy" being part of the larger c-system, so it can't easily be donated to a proton, which is necessary for basicity.

Aliphatic Amines

Aliphatic amines are amines in which the nitrogen atom is bonded to an alkyl group, such as methylamine (CH_{3}NH_{2}), ethylamine (C_{2}H_{5}NH_{2}), or propylamine (C_{3}H_{7}NH_{2}). In these compounds, the nitrogen's lone pair is not involved in resonance like it is in aromatic amines.

This situation allows the lone pair to be much more available for bonding with protons, which enhances their basicity. Aliphatic amines are generally stronger bases than both ammonia and aromatic amines. The electron-donating effect of the alkyl groups intensifies this basicity.

• Alkyl groups release electron density towards the nitrogen, further helping it attract protons due to the inductive effect.
• Therefore, as the chain length of the alkyl groups increases, the basic character can also increase, depending on the structure and other substituents.

Electron Delocalization

Electron delocalization is a concept wherein electrons are spread over several atoms, creating resonance structures in a molecule. This phenomenon is particularly crucial in aromatic amines, where electron delocalization over the aromatic ring can significantly affect basicity.

In the case of aniline, the nitrogen's lone pair of electrons participates in the electron cloud of the benzene ring, resulting in a delocalized pi-electron system. This sharing across the benzene ring decreases the availability of the lone pair to react with hydrogen ions.

• This effect leads to reduced basicity because the lone pair is less available to accept a proton.
• Delocalization also helps in the stabilization of the aniline's structure but at the cost of increased reluctance to engage in basic behavior.

Overall, electron delocalization causes a reduction in the basicity of compounds like aniline, making them significantly less basic than their aliphatic counterparts.