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

Option (c) \(3>1>2>5>4\) is the correct order of basicity.

Step 1: Identify Functional Groups

First, identify the compound structure from their SMILES or chemical formulas:1. (1) CC(=N)N: Acetamidine, containing an imine (C=N) group and an amine group (NH).2. (2) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}\): Ethylamine, a primary amine.3. (3) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}\): Dimethylamine, a secondary amine.4. (4) CC(N)=O: Acetamide, which has an amide group.5. (5) \(\mathrm{CH}_{3} \mathrm{CN}\): Acetonitrile, which contains a nitrile group (C≡N).

Step 2: Analyze Basicity of Each Compound

Assess the basicity of each compound: 1. Acetamidine is hydrogen-bonded and stabilizes the positive charge best among the nitrogen compounds. 2. Ethylamine is a primary amine, making it quite basic since the lone pair is not delocalized. 3. Dimethylamine is more basic than primary amines because of electron-donating methyl groups. 4. Acetamide has a lone pair conjugated with the carbonyl, reducing basicity. 5. Acetonitrile is least basic as the lone pair is involved in a strong triple bond with carbon.

Step 3: Arrange Compounds by Basicity

Comparing the basicity: - Dimethylamine is the most basic due to the electron-donating effect. - Acetamidine comes next as the resonance stabilizes the charge adequately. - Ethylamine follows due to less electron donation than dimethylamine. - Acetonitrile has very little basic characteristics, higher than amides because involvement of electrons is limited. - Acetamide is least basic due to resonance delocalization of lone pair.

Step 4: Match the Order to Given Choices

The correct order determined from analysis is \(3>1>2>5>4\): 1. Dimethylamine \(\left(3\right)\)2. Acetamidine \(\left(1\right)\)3. Ethylamine \(\left(2\right)\)4. Acetonitrile \(\left(5\right)\)5. Acetamide \(\left(4\right)\)Thus, from the options given, option (c) matches this order.

Key Concepts

Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. They play a crucial role in organic chemistry because they determine the properties and reactivity of the compound. For instance, a molecule containing a hydroxyl group will behave differently than one containing an amine group.

Identifying functional groups is the first step in understanding how a compound will react. Functional groups like amines, amides, nitriles, and others have unique structures and define many properties such as solubility, boiling point, and acidity or basicity. In the exercise, recognizing the different functional groups helps in predicting which compound is more basic or acidic. Let's dive into some important functional groups relevant to this exercise.

Amine Basicity

Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. The basicity of amines is primarily due to the presence of this lone pair of electrons, which can accept a proton (H⁺).

The basicity of an amine can be affected by alkyl groups attached to the nitrogen. More alkyl groups can increase basicity because they donate electrons to the nitrogen, enhancing its ability to attract protons.

• Primary amines, such as ethylamine ((\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}\))), have one alkyl group.
• Secondary amines, like dimethylamine ((\(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}\))), have two alkyl groups, which often increases their basicity over primary amines.
• Tertiary amines have three alkyl groups, potentially making them even more basic depending on the solvation effect and steric hindrance.

Amines are key players in many chemical reactions due to their ability to donate their lone pair to form a bond with protons or other electrophiles.

Resonance Effect

The resonance effect is a crucial concept in chemistry that describes the delocalization of electrons in molecules. This effect impacts how stable a molecule is and influences its reactivity.

Delocalization occurs when the electrons can move around in different positions, often across a backbone of alternating single and double bonds or through participation with lone pairs. This movement can stabilize positive or negative charges.

• Resonance can decrease basicity when it involves the lone pair of a base, as observed in amides, because the electrons are not freely available to interact with protons.
• In molecules like acetamidine (CC(=N)N), resonance can actually enhance basicity by stabilizing the conjugate acid formed when the molecule accepts a proton.

Understanding resonance is vital as it affects not just the basicity, but overall molecular stability and reactivity.

Nitrile Group

The nitrile group is characterized by a carbon atom triple-bonded to a nitrogen atom ((\(\mathrm{C} \equiv \mathrm{N}\))). This strong bond significantly influences the properties of nitriles.

Nitriles, like acetonitrile ((\(\mathrm{CH}_{3} \mathrm{CN}\))), have very low basicity. The reason is the lone pair on nitrogen is partially involved in the strong nitrile bond, reducing its availability to accept protons.

Because of the triple bond, nitriles are less reactive as bases compared to amines. However, they're often used as solvents and in organic synthesis due to their stability and ability to engage in other types of reactions.

Amide Group

Amides are compounds that contain a carbonyl group (C=O) bonded to a nitrogen atom. This functional group is prevalent in proteins, where it forms the peptide bonds connecting amino acids.

Amides, such as acetamide (CC(N)=O), are typically poor bases. This is due to the resonance effect, where the lone pair on the nitrogen is involved in resonance with the carbonyl group, making it less available to react with protons.

• In this resonance form, the electron pair from the nitrogen contributes to the stability of the carbonyl, making them poor proton acceptors compared to other nitrogen-containing groups such as amines.

The understanding of amide's basicity is critical in fields such as pharmaceuticals, where the amide linkages play a small role in drug design and how molecules interact in biological systems.