Problem 146 - A Complete Resource Book in Chemistry for JEE Main

Question

Amongst the following, the most basic compound is [2004] (a) benzylamine (b) aniline (c) acetanilide (d) p-nitroaniline

Step-by-Step Solution

Answer

The most basic compound is (a) benzylamine.

Step 1: Understanding Basicity

In organic chemistry, the basicity of a compound is often determined by the availability of a lone pair of electrons that can accept a proton. More available and less hindered lone pairs tend to contribute to greater basicity.

Step 2: Analyzing Each Compound

Let's examine each compound: - Benzylamine (a) has a primary amine group attached to a benzyl group. The nitrogen has a lone pair available for protonation. - Aniline (b) has an amine group attached directly to a benzene ring. The lone pair on nitrogen is delocalized into the aromatic ring, reducing its availability for protonation. - Acetanilide (c) has an amide linkage, with the lone pair on nitrogen delocalized through resonance with the carbonyl group, making it less basic. - p-Nitroaniline (d) has an electron-withdrawing nitro group which further reduces the basicity of the nitrogen lone pair through inductive and resonance effects.

Step 3: Comparing Basicity

Benzylamine is more basic than aniline and acetanilide because its lone pair on nitrogen is not involved in resonance with a highly electron-withdrawing group or delocalized in a resonance-stabilizing way. Aniline is less basic due to resonance with the benzene ring, acetanilide is even less basic due to strong resonance with the carbonyl group, and p-nitroaniline is significantly less basic because of the electron-withdrawing nitro group attached to the benzene ring.

Step 4: Concluding with the Most Basic Compound

Given these considerations, benzylamine (a) has the most available lone pair of electrons for protonation without significant resonance stabilization or electron-withdrawing effects. Therefore, benzylamine is the most basic compound among the given options.

Key Concepts

Lone Pair Availability

In organic chemistry, the basicity of a compound is largely determined by the availability of its lone pairs of electrons. A lone pair is a pair of valence electrons that are not shared with another atom. These electrons can be used to bond with a proton, a positive charge. For a compound to be considered basic, it generally needs lone pairs that are readily available to grab hold of a proton. More available lone pairs often mean greater basicity. If the lone pairs are readily accessible and not tied up in bonds elsewhere, the compound will typically be more basic. However, various factors can affect lone pair availability, such as where these pairs are situated in the molecular structure. If an atom's lone pairs are involved in resonance or if they are hindered by bulky groups, their ability to bond with a proton can be reduced. Understanding the position and freedom of motion of lone pairs helps predict a compound's basic characteristics.

Resonance Effect

Resonance in organic chemistry refers to the distribution of electrons across multiple structures. This involves blending multiple contributing forms to create an average structure known as a resonance hybrid. Impact on Basicity:

• When lone pairs on nitrogen or another atom are part of a resonance system, they are less available to accept a proton, reducing basicity.
• For example, in aniline, a compound examined in the exercise, the lone pair of electrons on nitrogen is delocalized within the benzene ring. This delocalization makes the nitrogen's lone pair less available for protonation, thus lowering the compound's basicity compared to benzylamine.
• In acetanilide, the resonance involving the carbonyl group further draws the lone pair away through stabilization, adversely affecting basicity even more.

The key takeaway is that resonance often ties up lone pairs in electron density spread, making them less available for reactions like protonation.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) play a crucial role in determining the basicity of compounds. These groups pull electron density away from other parts of the molecule, often making lone pairs less available for bonding. Effects of EWGs:

• When an electron-withdrawing group is present near a lone pair, it diminishes the electron density available for that pair, reducing its ability to accept a proton.
• In p-nitroaniline, the nitro group is an electron-withdrawing group which dramatically decreases the nitrogen lone pair’s availability because it attracts electron density away from the nitrogen atom both through a sigma (inductive) effect and additional resonance.
• This action makes p-nitroaniline less basic, as the lone pairs are significantly less available due to their participation in stabilizing the compound via the electron-withdrawing and resonance effects.

In summary, electron-withdrawing groups can significantly affect a molecule's basicity by decreasing the availability of lone pairs, which are essential for protonation.