Problem 150 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is the strongest base in Aqueous solution? (a) Trimethylamine (b) Aniline (c) Dimethylamine (d) Methyl amine

Step-by-Step Solution

Answer

Dimethylamine (c) is the strongest base in aqueous solution.

Step 1: Understanding the Basics of Basicity

To determine the strongest base, we should first understand that a base's strength in an aqueous solution is determined by its ability to accept protons (H⁺ ions). Aliphatic amines (those with alkyl groups) are typically stronger bases than aromatic amines (those with a benzene ring) because alkyl groups donate electron density, increasing the availability of the lone pair on the nitrogen atom for proton acceptance.

Step 2: Analyze Aromatic vs. Aliphatic Amines

Aniline (b) is an aromatic amine, which means its nitrogen's lone pair is delocalized into the benzene ring, making it less available to bond with H⁺ ions. Therefore, aniline is a weaker base compared to aliphatic amines.

Step 3: Comparing Aliphatic Amines

Trimethylamine (a), Dimethylamine (c), and Methyl amine (d) are aliphatic amines with varying numbers of methyl groups attached to the nitrogen. The electron-donating nature of these methyl groups generally increases the basicity, but sterics and solvation effects play a role.

Step 4: Examining the Steric Effects

Trimethylamine has three bulky methyl groups, which can hinder its ability to accept protons due to steric hindrance. Hence, despite its high electron donation, it is not the strongest base among the options due to this steric effect.

Step 5: Identifying the Strongest Base

Dimethylamine (c) has two methyl groups, providing a good balance: sufficient electron donation without excessive steric hindrance. This makes it a stronger base than both trimethylamine and methylamine in aqueous solution.

Key Concepts

Aromatic vs. Aliphatic Amines

When discussing amines, their classification into aromatic and aliphatic is key. Aromatic amines contain a benzene ring in their structure, which has significant effects on their chemical behavior. A prime example is aniline, where the nitrogen's lone pair of electrons is partially involved in the aromatic system due to delocalization into the benzene ring. This reduces the nitrogen's ability to donate its lone pair to a proton, making aromatic amines generally weaker bases compared to their aliphatic counterparts.
Aromatic structure stabilizes at the cost of reduced basicity.
In contrast, aliphatic amines are made up of alkyl groups (like methyl or ethyl) attached to the nitrogen. These alkyl groups enhance the electron density on the nitrogen atom, thereby increasing its availability to accept protons and neutralize the charge. As a result, aliphatic amines are typically stronger bases in aqueous solutions than aromatic amines.

• Aromatic amines: Lone pair less available due to delocalization.
• Aliphatic amines: Electron donation increases basicity.

Proton Acceptance

The essence of a base's strength in an aqueous solution hinges on its ability to accept protons, commonly denoted as H⁺ ions. This ability is primarily determined by the availability and readiness of the nitrogen's lone pair of electrons to form a bond with the H⁺ ion.
Aliphatic amines, in particular, excel in this proton acceptance role. Due to the inductive effect of attached alkyl groups, which push electron density towards the nitrogen atom, these compounds often have a more pronounced tendency to gain protons than their aromatic counterparts.

• Aliphatic amines: Better proton acceptors.
• Aromatic amines: Less effective due to electron delocalization.

However, factors such as steric hindrance affect this ability. In a compound like trimethylamine, the bulky methyl groups can physically impede the approach of protons, causing a decreased ability to accept them, even though electron donation is high. This is why understanding both electronic and steric factors is crucial in assessing basicity.

Electron Donation in Amines

Electron donation plays a critical role in deciding the basicity of amines. Amines are nitrogen-containing compounds where the nitrogen atom has a lone pair of electrons available for bonding. In aliphatic amines, alkyl groups contribute to electron density around the nitrogen, making them more reactive towards accepting protons.
This phenomenon is known as the inductive effect. It's more pronounced in aliphatic amines like dimethylamine, where two methyl groups enhance electron density and basicity effectively.

• Alkyl groups: Enhance electron density via inductive effect.
• Trimethylamine: Faces steric hindrance despite high electron donation.

On the other hand, aromatic amines like aniline have a lone pair that is less available for protonation because it is delocalized across the aromatic system. This reduces the electron donation towards the nitrogen and consequently decreases basicity. Therefore, alongside electron donation, structural factors such as steric hindrance and electron delocalization must also be considered.