Problem 27 - A Complete Resource Book in Chemistry for JEE Main

Question

Which is most basic? (a) \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) (b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}\) (c) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NH}_{2}\) (d) \(\left(\mathrm{C}_{6} \mathrm{H}_{5}\right)_{2} \mathrm{NH}\)

Step-by-Step Solution

Answer

(b) \((\mathrm{CH}_3)_2\mathrm{NH}\) is the most basic because it has two electron-donating methyl groups.

Step 1: Understanding Basicity

Basicity refers to the ability of a substance to accept protons (H+ ions). The more easily a molecule can accept a proton, the more basic it is.

Step 2: Analyze Each Compound's Structure

Examine each compound structure:(a) \(\mathrm{CH}_3\mathrm{NH}_2\): Methylamine, with one methyl group attached to the nitrogen atom.(b) \((\mathrm{CH}_3)_2\mathrm{NH}\): Dimethylamine, with two methyl groups enhancing electron density on nitrogen.(c) \(\mathrm{C}_2\mathrm{H}_5\mathrm{NH}_2\): Ethylamine, with one ethyl group attached.(d) \((\mathrm{C}_6\mathrm{H}_5)_2\mathrm{NH}\): Diphenylamine, with two phenyl groups, which decrease electron density due to aromaticity.

Step 3: Consider Electron Donating Ability

Methyl (\(\mathrm{CH}_3\)) and ethyl (\(\mathrm{C}_2\mathrm{H}_5\)) groups donate electron density to nitrogen, making (b) \((\mathrm{CH}_3)_2\mathrm{NH}\) potentially most basic due to two electron-donating groups. In contrast, phenyl groups in (d) withdraw electrons, reducing basicity.

Step 4: Finalizing the Answer

Based on electron donation, dimethylamine, which has two electron donating groups, is more basic than the other compounds. Phenyl groups in (d) actually make \((\mathrm{C}_6\mathrm{H}_5)_2\mathrm{NH}\) less basic due to their electron-withdrawing nature.

Key Concepts

Electron Donation

Electron donation is a key factor when determining the basicity of molecules. Basicity is all about how well a molecule accepts protons (H+ ions), and electron donation plays an important role here. When a group donates electron density to an atom like nitrogen within a molecule, it increases the availability of electrons that can interact with protons.

Let's look at the methyl group (\(\mathrm{CH}_3\)) and the ethyl group (\(\mathrm{C}_2\mathrm{H}_5\)). Both are known to donate electrons to nitrogen effectively, which enhances the nitrogen's ability to accept an additional hydrogen ion. Therefore, molecules such as dimethylamine (\((\mathrm{CH}_3)_2\mathrm{NH}\)), with two methyl groups, have increased electron density at the nitrogen atom, making them more basic.

These electron-donating groups increase the molecule's basicity compared to other groups that may not increase nitrogen's electron density as effectively or might even withdraw electron density, making a molecule less basic.

Proton Acceptance

Proton acceptance is a measure of a molecule's basicity. It revolves around the ability of a molecule, specifically atoms like nitrogen in this context, to accept protons. The concept can be beautifully illustrated by comparing how different groups attached to nitrogen impact this ability.

The greater electron density a nitrogen atom has, the easier it is for it to attract and accept a proton. Thus, the presence of electron-donating groups, such as methyl and ethyl groups, enhances nitrogen's electron density. This increase generally translates to stronger basicity. For example, in the compound dimethylamine (\((\mathrm{CH}_3)_2\mathrm{NH}\)), the presence of two methyl groups significantly spikes up the electron density around nitrogen, facilitating easier proton acceptance.

In contrast, compounds like diphenylamine (\((\mathrm{C}_6\mathrm{H}_5)_2\mathrm{NH}\)) harbor phenyl groups which tend to withdraw electron density. This electron-withdrawing nature decreases its ability to accept protons, reducing the compound's basic nature.

• More electron density = easier proton acceptance.
• Reduction in electron density = decreased proton acceptance ability.

Molecular Structure Analysis

Analyzing molecular structures helps understand their basicity by examining how atomic arrangement and attached groups affect electron distribution and proton interaction. With our compounds in focus, this analysis involves closely observing the groups attached to the nitrogen atom.

Let's start with methylamine (\(\mathrm{CH}_3\mathrm{NH}_2\)) and ethylamine (\(\mathrm{C}_2\mathrm{H}_5\mathrm{NH}_2\)). Here, methyl and ethyl groups push electron density towards the nitrogen atom, impacting the molecule's ability to act as a base favorably. When we consider dimethylamine (\((\mathrm{CH}_3)_2\mathrm{NH}\)), it even further bumps up the basicity by having an extra methyl group to double the electron-donating action.

On the other hand, when we look at diphenylamine (\((\mathrm{C}_6\mathrm{H}_5)_2\mathrm{NH}\)), phenyl groups are present. Phenyl groups are unique due to their aromatic nature, which tends to pull electron density away from nitrogen, making the structure less capable of accepting protons.

Therefore, the composition and arrangement of groups around the nitrogen atom in different molecular structures dictate their basic nature. This electron interplay within structures is pivotal when determining how basic a molecule is.