Problem 68 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Give reasons for the following : (i) Dimethylamine is a stronger base than trimethylamine [19 (ii) Cyclohexylamine is a stronger base than aniline.

Step-by-Step Solution

Answer

Dimethylamine is a stronger base due to less steric hindrance, and cyclohexylamine is stronger due to lack of resonance delocalization.

Step 1: Understanding Basicity in Amines

Both dimethylamine and trimethylamine are amines, characterized by the presence of a nitrogen atom with a lone pair of electrons. Basicity refers to the ability of these amines to accept protons, which is influenced by the availability of the lone pair for accepting protons.

Step 2: Electron Donating Effect in Amines

Dimethylamine has two methyl groups attached to the nitrogen atom, which are electron-donating groups. These groups increase electron density on the nitrogen, enhancing its ability to accept a proton, thus increasing basicity.

Step 3: Steric Hindrance in Trimethylamine

Trimethylamine has three methyl groups attached to the nitrogen, compared to dimethylamine. The third methyl group introduces steric hindrance, which impedes the nitrogen's access to protons, reducing its basicity compared to dimethylamine.

Step 4: Conclusion for Dimethylamine vs. Trimethylamine

Dimethylamine is a stronger base because it has less steric hindrance and greater electron density at the nitrogen atom, compared to trimethylamine, where steric hindrance outweighs the electron-donating effect.

Step 5: Electron Delocalization in Aniline

Aniline has an aromatic benzene ring, which allows the lone pair on nitrogen to delocalize into the ring through resonance. This delocalization decreases the availability of the lone pair to accept a proton, making aniline less basic.

Step 6: Aliphatic Nature of Cyclohexylamine

Cyclohexylamine is an aliphatic amine where no resonance occurs, meaning the lone pair of electrons is localized on the nitrogen atom, making it more readily available to accept protons, thus increasing basicity compared to aniline.

Step 7: Conclusion for Cyclohexylamine vs. Aniline

Cyclohexylamine is a stronger base than aniline because it lacks resonance that would delocalize the nitrogen's lone pair, keeping it more available for protonation.

Key Concepts

Electron Donating Effect

In the world of chemistry, small changes in molecular structure can deeply influence the behavior of compounds. Amines, such as dimethylamine and trimethylamine, exemplify the Electron Donating Effect. This effect revolves around how certain atoms or groups within a molecule can push electron density toward the atom they're attached to, often enhancing chemical reactivity.
In dimethylamine, having two methyl groups attached to the nitrogen atom is significant. These methyl groups act as electron-donating groups. This means they donate electron density towards the nitrogen atom, increasing its electron cloud. As a result, the nitrogen atom in dimethylamine becomes more capable of accepting a proton.

• This increased electron density promotes the nitrogen’s basicity.
• It enhances the ability of dimethylamine to act as a base and accept protons from acids.

In contrast, trimethylamine, with one more methyl group than dimethylamine, reaches an intricate balance between donation and steric influence, eventually affecting its basicity slightly differently. Understanding the electron-donating effect is crucial in predicting and explaining the basic strength of compounds.

Steric Hindrance

Steric hindrance plays a pivotal role in determining the accessibility of reactive sites in molecules. It's all about how bulky groups attached to a molecule can physically block reactions from occurring. In trimethylamine, the presence of three methyl groups surrounding the nitrogen atom exemplifies this concept.
While methyl groups are electron-donating, their size also becomes a factor in how they influence the nitrogen's reactivity. With three methyl groups, the nitrogen finds itself surrounded and somewhat "crowded". This "crowding" is termed steric hindrance.

• Steric hindrance physically blocks the nitrogen atom from easily accessing protons.
• It reduces the ability of trimethylamine to function as an effective base compared to dimethylamine.

Despite contributing additional electrons to the nitrogen, the steric bulk of an extra methyl group in trimethylamine counteracts the basicity by preventing smooth interactions with protons. Thus, steric hindrance shows how spatial arrangements within a molecule impact its chemical properties and reactivity.

Electron Delocalization

Electron delocalization is a fascinating concept where electrons in a molecule are not fixed to a single atom or bond but instead spread across several atoms, often through resonance. Aniline showcases electron delocalization beautifully.
Unlike aliphatic amines, aniline's nitrogen is connected to a benzene ring, allowing its lone pair of electrons to spread across the ring. This phenomenon is known as resonance, and it significantly impacts aniline's basicity.

• The lone pair on nitrogen gets involved in the pi-bonding system of the benzene ring.
• This involvement decreases the availability of the lone pair for protonation.

As a result, aniline is less basic than cyclohexylamine, where no such delocalization occurs. The electrons on nitrogen in cyclohexylamine remain localized and are fully available to accept protons, enhancing its basicity. Electron delocalization, therefore, highlights the importance of the chemical environment in defining how readily an electron can be used in chemical reactions.