Problem 131 - A Complete Resource Book in Chemistry for JEE Main

Question

The correct order of basicity of the following compound is (1) CC(N)N (2) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}\) (3) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}\) (4) CC(N)=O (5) \(\mathrm{CH}_{3} \mathrm{CN}\) (a) \(2>1>3>4>5\) (b) \(1>3>2>4>5\) (c) \(3>1>2>5>4\) (d) \(1>3>2>5>4\).

Step-by-Step Solution

Answer

The correct order is (c) \(3 > 1 > 2 > 5 > 4\).

Step 1: Understanding Basicity

Basicity in organic compounds is often related to the availability of a lone pair of electrons. Compounds with nitrogen (usually amines) are more basic compared to those where the lone pair on nitrogen is delocalized or involved in resonance. Carbamate ions are generally less basic.

Step 2: Analyzing the Compounds

Break down the given compounds: 1. Primary amine (CC(N)N) has two alkyl groups, potentially increasing electron donation and basicity. 2. Ethyl amine (\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}\)) is a primary amine with one alkyl group.3. Diethylamine (\((\mathrm{CH}_{3})_{2} \mathrm{NH}\)) is a secondary amine, typically more basic due to two electron-donating groups.4. Amide derivative (CC(N)=O) is less basic because the lone pair on nitrogen is delocalized.5. Nitrile (\(\mathrm{CH}_{3} \mathrm{CN}\)) has nitrogen involved in a triple bond, making it less basic.

Step 3: Predicting Basicity

Rank the compounds based on their ability to donate their lone pair. Secondary amines are usually more basic than primary amines. Among primary amines, the ones with more electron-donating groups are more basic. Amides and nitriles tend to be less basic.

Step 4: Ordering the Compounds

The decreasing order of basicity is determined as follows:- Secondary amine \((\mathrm{CH}_{3})_{2} \mathrm{NH}\) is the most basic (3).- Followed by primary amine with two alkyl groups (CC(N)N) (1).- Then, primary amine with one alkyl group (\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}\)) (2).- Amide derivative (CC(N)=O) shows less basicity (4).- The nitrile (\(\mathrm{CH}_{3} \mathrm{CN}\)) is the least basic (5).

Step 5: Matching with Options

The correct order of basicity according to our analysis is Option (c) - \(3 > 1 > 2 > 5 > 4\). This matches the expected analysis of secondary amine being most basic followed by primary amines, the nitrile, and finally the amide being the least basic.

Key Concepts

Amines

Amines are organic compounds that contain nitrogen atoms with a lone pair of electrons. This lone pair is essential in determining an amine's basicity. Typically, amines are classified as primary, secondary, or tertiary depending on the number of alkyl or aryl groups attached to the nitrogen atom.

• **Primary amines**: These have one alkyl group attached to the nitrogen, which increases their ability to donate electrons and therefore their basicity. Examples include ethylamine, with the structure (ValueError: Breaking out of seamless loop.)), and the compound CC(N)N.
• **Secondary amines**: Two alkyl groups attached to the nitrogen make these amines generally more basic than primary amines due to increased electron donation. An example is diethylamine, (ValueError: Breaking out of seamless loop.)). Diethylamine is actually found to be the most basic among typical amines.
• **Tertiary amines**: They can be more complex in behavior, but are not a focus for this exercise.

The availability of the lone pair for bonding is what enhances the ability of amines to act as bases, making them able to accept protons (H⁺). In essence, secondary amines are typically more basic due to the combined electron-donating effects of two or more alkyl groups, as observed in the analysis of these compounds.

Nitriles

Nitriles are organic compounds characterized by a carbon-nitrogen triple bond. This triple bond involves significant overlap of electron orbitals, thus tightly holding onto the nitrogen atom's lone pair of electrons and making it less available for bonding. Because of this triple bond, nitriles such as (ValueError: Breaking out of seamless loop.)) are generally much less basic than other nitrogen-containing compounds like amines.

In a nitrile, the carbon linked to nitrogen uses a sp-hybridized orbital, which is why the electrons are held more tightly to the nucleus compared to sp² or sp³ hybridization in amines. Consequently, the nitrogen in a nitrile is less likely to donate its lone pair to a proton, reducing basicity to much lower levels. This accounts for the placement of nitriles low on the basicity scale compared to amines.

Amides

Amides are notably different in chemical behavior compared to amines and other nitrogen compounds. In amides, the lone pair on the nitrogen atom participates in resonance with the adjacent carbonyl group (C=O). This delocalization of electrons reduces the ability of the nitrogen to freely donate electrons, making amides much less basic than amines.
Amides can be quite versatile, but their strong resonance stabilization through structures like (ValueError: Breaking out of seamless loop.)) is key to understanding their decreased basicity. This resonance means that the lone pair isn't as available to participate in reactions as it would be in amines. Hence, in assessing basicity, amides typically rank lower, often even below nitriles due to enhanced resonance effects.

Electron Donation

Electron donation refers to the ability of atoms or groups attached to a nitrogen atom to push electrons toward it, enhancing the nitrogen's ability to donate its lone pair. This effect plays a vital role in determining the basicity of nitrogen compounds.

• In **amines**, particularly secondary amines such as diethylamine, two alkyl groups increase electron donation, raising basicity.
• Primary amines, like ethylamine, also feature electron donation, but to a lesser extent, thus are less basic than secondary amines.

In amides, the electron donation is largely negated by resonance, while in nitriles, electron donation occurs very minimally due to the robust triple bond holding electron pairs tightly close to the centers of the bonded atoms. Consequently, it significantly affects where these compounds place on the scale of basicity.

Resonance in Nitrogen Compounds

Resonance is a key concept in understanding the behavior of nitrogen-bearing compounds, particularly amides. In resonance, electron pairs are delocalized over multiple atoms, creating a hybrid of possible structures and resulting in high stability for the molecule.
This concept significantly impacts basicity when, for example, a lone pair on a nitrogen is involved in resonance with an adjacent carbonyl, such as in amides where the structure can be visualized with interaction between nitrogen's lone pair and the carbonyl. This decreases the electron availability significantly.

• In **amides**, this leads to lower basicity, contrary to compounds where electron donation predominates.
• **Nitriles** do not experience resonance between nitrogen's lone pairs due to the nature of their triple bond, impacting basicity differently.

Understanding these interactions is crucial to predicting and rationalizing the basic nature of different nitrogen compounds and their chemical behavior.