Problem 66 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following is most basic? (a) Nc1ccccc1 (b) Cc1ccccc1N (c) Cc1cccc(C)c1N (d) Cc1cc(C)cc(N)c1

Step-by-Step Solution

Answer

Option (b) is the most basic due to direct electron donation from the methyl group to nitrogen.

Step 1: Identify the Basic Site

The basic site in these molecular structures is the nitrogen atom. In organic chemistry, a nitrogen atom is considered basic if it has a lone pair of electrons that can accept a proton (H⁺). For all options, the nitrogen atom is part of an amine group, connecting through a C-N single bond, making nitrogen the site of basicity.

Step 2: Analyze Electron Donation

Check how the surrounding structure affects the nitrogen's ability to donate its lone pairs. Electron-donating groups near the nitrogen increase basicity, as they enhance the availability of the lone pair. Here, options (b), (c), and (d) contain alkyl groups which are electron-donating and influence the basicity.

Step 3: Determine Position and Influence of Alkyl Groups

The position of alkyl groups affects nitrogen's basicity. Compare each molecule: - (a) has no alkyl group affecting nitrogen directly. - (b) contains a methyl group attached to nitrogen, increasing its electron density directly. - (c) and (d) have methyl groups on the benzene ring, but farther from nitrogen, reducing the direct impact compared to (b).

Step 4: Conclude the Most Basic Structure

Based on the contribution of the alkyl groups, option (b) is more basic than the others. The methyl group directly connected to nitrogen significantly increases the electron density around it, making the nitrogen's lone pair more available for protonation compared to the others.

Key Concepts

Nitrogen lone pair impact on basicity

In organic chemistry, the basicity of a molecule often revolves around the nitrogen atom and specifically its lone pair of electrons. A lone pair is a pair of valence electrons that are not shared with another atom and are found in the outer shell of an atom. For nitrogen, which is commonly found in amines, these lone pairs are ready to accept protons, or hydrogen ions (H⁺). The availability of this lone pair for bonding directly relates to how "basic" a compound is.
An accessible and available lone pair can readily interact with protons, thus increasing the compound's basicity. On the contrary, if the lone pair is involved in other interactions, such as pi-bonding or resonance, its availability is hindered, reducing basicity.
This is why in our exercise, identifying where the nitrogen's lone pair resides and its accessibility is the primary step in determining which molecule is most basic.

How electron donation affects basicity

Electron donation is a crucial concept when discussing the basicity of a nitrogen atom in a molecule. When a group attached to a nitrogen atom donates electrons, it increases the electron density around the nitrogen. This makes the nitrogen's lone pair more available for bonding with protons. Such groups are known as electron-donating groups.
In our specific case, some of the molecules have alkyl groups, like methyl groups, near the nitrogen. These groups are classic examples of electron-donating groups. They're effective because they push electron density through the sigma bonds between carbon and nitrogen, making the nitrogen more likely to share its lone pair with an incoming hydrogen ion.
Therefore, in molecules where nitrogen has electron-donating alkyl groups attached, or in proximity, the nitrogen's lone pair is more accessible, increasing the overall basicity of the compound.

Impact of alkyl groups on basicity

Alkyl groups play a significant role in the basicity of nitrogen-containing organic compounds. These groups, such as methyl (-CH₃) and ethyl (-C₂H₅), influence how readily the nitrogen's lone pair can attract and hold onto a proton.
They do this by increasing the electron density around the nitrogen. When alkyl groups are positioned close to the nitrogen, they enhance the compound's basicity by making the lone pair on nitrogen more attracted to the incoming proton. This means that the basicity is directly affected by how close these alkyl groups are positioned relative to the nitrogen atom.

• If an alkyl group is directly attached to the nitrogen atom, as seen in option (b) of the exercise, the basicity is significantly enhanced.
• In contrast, if the alkyl groups are on other parts of the molecule, like farther down a benzene ring, their effect is less pronounced, as seen in options (c) and (d).
• Option (a) with no alkyl group directly affecting the nitrogen exhibits lesser basicity compared to option (b).

Thus, the proximity and presence of alkyl groups greatly dictate how strongly nitrogen's basicity is impacted, which is evidenced by the direct attachment of methyl groups in option (b), making it the most basic among the given choices.