Problem 72 - 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) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2}-\mathrm{NH}_{2}\) (c) \(\mathrm{p}-\mathrm{NO}_{2}-\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{NH}_{2}\) (d) \(\mathrm{m}-\mathrm{NO}_{2}-\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{NH}_{2}\)

Step-by-Step Solution

Answer

(b) C_{6}H_{5}CH_{2}NH_{2} (benzylamine) is the strongest base.

Step 1: Understand Base Strength

The strength of a base is usually determined by its ability to donate a lone pair of electrons. In the context of organic compounds with nitrogen, the electron-donating ability of the nitrogen's lone pair is often influenced by the surrounding structure.

Step 2: Analyze Substituent Effects

Assess how the attached groups affect the basicity of the compounds: electron-withdrawing groups (such as NO_{2}) reduce basicity by stabilizing the lone pair of electrons on N, whereas electron-donating groups increase basicity.

Step 3: Examine Each Option

- **(a) C_{6}H_{5}NH_{2} (aniline):** It has the phenyl group, which can delocalize electrons, slightly reducing basicity. - **(b) C_{6}H_{5}CH_{2}NH_{2} (benzylamine):** The benzyl group does not withdraw electrons significantly and allows free electron donation by the amino group, increasing basicity. - **(c) p-NO_{2}-C_{6}H_{4}NH_{2}:** The NO_{2} group is a strong electron-withdrawing group, greatly decreasing basicity. - **(d) m-NO_{2}-C_{6}H_{4}NH_{2}:** Similar to p-nitroaniline, it has a strong electron-withdrawing group that reduces basicity.

Step 4: Determine the Strongest Base

Among the given options, C_{6}H_{5}CH_{2}NH_{2} is benzylamine, which does not have strong electron-withdrawing groups like NO_{2}, meaning that the nitrogen's lone pair is more available to donate, making it the strongest base.

Key Concepts

Electron-Donating Groups

In organic chemistry, electron-donating groups (EDGs) are pivotal in determining the behavior of molecules. These groups donate electron density to the rest of the molecule. This often occurs via the donation of a lone pair of electrons into a conjugated system or through hyperconjugation.

Understanding the role of EDGs is crucial, especially in determining the base strength of amines. For example, in benzylamine ( CC_{6}H_{5}CH_{2}NH_{2} D), the benzyl group does not withdraw electrons significantly, allowing the lone pair on nitrogen to be more available for donation. This facilitates the base character as an electron donor, increasing basicity.

Some common examples of electron-donating groups include:

• -OH (hydroxyl)
• -OR (alkoxy)
• -NH2 (amino)
• -CH3 (methyl)

Through their ability to donate electrons, EDGs influence the reactivity and properties of organic molecules, often enhancing basicity and stabilizing positive charges.

Electron-Withdrawing Groups

On the opposite side of the spectrum, we have electron-withdrawing groups (EWGs). These groups pull electron density away from the rest of the molecule, typically through electronegativity or resonance withdrawal. This action influences the reactivity and basicity of a compound by stabilizing negative charges and deactivating certain sites for reactions.

When considering nitroanilines such as C Cp-NO_{2}-C_{6}H_{4}-NH_{2} D or m-NO_{2}-C_{6}H_{4}-NH_{2} D, the nitro group is a strong electron-withdrawing group. It significantly decreases the availability of the lone pair on the nitrogen, consequently reducing the compound’s basicity.

Commonly encountered EWGs in organic chemistry include:

• -NO2 (nitro)
• -CN (cyano)
• -COOH (carboxylic acid)
• -SO3H (sulfonic acid)

Understanding the impact of these groups is essential, as they alter the electronic environment within a molecule, influencing acid/base strength and reactivity.

Organic Chemistry

Organic chemistry is the study of carbon-containing compounds and their transformations. One of its most engaging areas is understanding the influence of various substituents on molecular properties and reactivity.

Key to this concept is how electron-donating and electron-withdrawing groups affect molecules' behavior, particularly in reaction mechanisms. For instance, amines' basicity in organic chemistry showcases the delicate balance of electronic effects posited by these groups.

With compounds such as aniline ( CC_{6}H_{5}NH_{2} D), different substituents on the benzene ring can heavily influence nitrogen's lone pair availability. Understanding these influences can determine why some compounds are stronger bases or more reactive electrophiles or nucleophiles.

In sum, organic chemistry practices necessitate a proper grasp of how various groups modify molecular frameworks, guiding reactions and molecular stability across diverse chemical environments. Mastery over these concepts opens up further insight into complex reactions and synthetic strategies.