Problem 99 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

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

Step 1: Understanding Base Strength

Base strength refers to the ability of a compound to donate an electron pair. In aromatic amines, this depends on the availability of the lone pair on the nitrogen atom. Electron-donating groups increase base strength by making electrons more available, while electron-withdrawing groups decrease it.

Step 2: Analyzing Options Without Substituents

Consider \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\), aniline, which has a lone pair on nitrogen available for donation. The benzene ring slightly delocalizes this lone pair, making it a moderate base.

Step 3: Substituent Effects on Base Strength

The effects of the substituents (\(\mathrm{p}-\mathrm{NO}_{2}\) and \(\mathrm{m}-\mathrm{NO}_{2}\)) in \(\mathrm{p}-\mathrm{NO}_{2}-\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{NH}_{2}\) and \(\mathrm{m}-\mathrm{NO}_{2}-\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{NH}_{2}\) are significant. The nitro group (\(\mathrm{NO}_{2}\)) is a strong electron-withdrawing group and reduces lone pair availability, decreasing base strength.

Step 4: Evaluating Electron-Withdrawing Effects

In \(\mathrm{p}-\mathrm{NO}_{2}\), the nitro group is positioned such that it greatly decreases base strength by withdrawing electrons through resonance and inductive effects. For \(\mathrm{m}-\mathrm{NO}_{2}\), the effect is lesser as it cannot participate in resonance as effectively.

Step 5: Consideration of Alkyl Groups

In \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{NH}_{2}\), the presence of \(\mathrm{CH}_{2}\) group directly attached to \(\mathrm{NH}_{2}\) provides more electron density to the nitrogen, enhancing base strength compared to the others.

Step 6: Conclusion

Given the analysis, the compound \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{NH}_{2}\) is the strongest base due to the electron-releasing effect of the alkyl group, which outweighs the aryl group’s influence.

Key Concepts

Base Strength

Base strength is a measure of how readily a compound donates an electron pair to form a new bond. This concept is essential in understanding reactions in chemistry, especially for bases like ammonia or amines. In simple terms, the stronger a base, the more willing it is to give away its electrons. This ability depends heavily on the availability of this electron pair.
The easier it is for electrons from the nitrogen atom to be transferred or shared, the stronger the base will be.
If other atoms or groups make electrons readily available, they enhance the base’s strength. However, if they pull electrons away, they weaken it. Understanding this concept helps in predicting how different chemicals will react.

Aromatic Amines

Aromatic amines are compounds where an amine group (-NH2) is directly attached to an aromatic ring like benzene. These types of amines, such as aniline (C_{6} H_{5} NH_{2}), offer a fascinating look into how structure affects chemical properties.
The lone pair on the nitrogen in these compounds can participate in resonance with the aromatic ring, which affects its reactivity.
The resonance with the benzene ring generally results in a lower availability of the lone pair for donation, compared to aliphatic amines, making aromatic amines moderately strong bases.

• This resonance participation tends to stabilize the molecule, leading to decreased basicity.
• It is also influenced by the nature and position of substituents on the aromatic ring.

Aromatic amines are crucial in the synthesis of dyes, rubber, and other industrial chemicals.

Electron-Donating Groups

Electron-donating groups (EDGs) are substituents that release or push electrons towards the rest of the molecule.
Common examples are alkyl groups like -CH3, which can increase the electron density around an atom like nitrogen, enhancing its basicity.
When present in aromatic amines, EDGs facilitate the donation of the lone pair on the nitrogen atom.

• The more easily the lone pair can interact with protons (H+ ions), the stronger the base.
• Such groups operate mainly through the inductive effect, which involves the shift of electron densities along bonds.

EDGs generally boost the base strength by making electrons more available for forming new bonds with protons. They play a significant role in synthetic chemistry, as they can significantly alter the reactivity and stability of organic molecules.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are atoms or groups that pull electron density away from other parts of the molecule.
They often contain atoms that are highly electronegative, such as halogens or nitro groups (NO_{2}).
When these groups are attached to aromatic amines, they reduce the availability of the lone pair on the nitrogen atom, thus reducing base strength.

• EWGs make the nitrogen less nucleophilic, meaning it is less likely to donate electrons.
• This decrease in electron availability happens mainly through resonance and inductive effects, retracting electron density from the nitrogen atom.

In the evaluation of base strength, EWGs are key factors in decreasing the proton affinity of bases by hindering the electron donation capability of nitrogen.