Problem 209 - A Complete Resource Book in Chemistry for JEE Main

Question

Considering the basic strength of amines in aqueous solution, which one has the smallest \(\mathrm{pK}_{\mathrm{b}}\) value? [2014] (a) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{NH}_{2}\) (c) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}\) (d) \(\mathrm{CH}_{3} \mathrm{NH}_{2}\)

Step-by-Step Solution

Answer

(c) \((\mathrm{CH}_{3})_{2} \mathrm{NH}\) has the smallest pK_b value.

Step 1: Understanding the Question

We need to determine which of the given amines has the smallest pK_b value in an aqueous solution. The pK_b value is inversely related to the base strength. Therefore, the smallest pK_b corresponds to the strongest base.

Step 2: Analyzing the Structure of Amines

First, look at the structures of the amines:- \((\mathrm{CH}_{3})_{3} \mathrm{N}\): Trimethylamine, a tertiary amine.- \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\): Aniline, an aromatic amine.- \((\mathrm{CH}_{3})_{2} \mathrm{NH}\): Dimethylamine, a secondary amine.- \(\mathrm{CH}_{3} \mathrm{NH}_{2}\): Methylamine, a primary amine.

Step 3: Assessing Basicity of Amines

For amines, their basicity depends on the availability of the lone pair of electrons on the nitrogen atom:- Tertiary amines like trimethylamine \((\mathrm{CH}_{3})_{3} \mathrm{~N}\) have steric hindrance that can decrease basicity.- Aniline \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) has its lone pair involved in resonance with the aromatic ring, reducing its basicity.- Secondary amines like dimethylamine \((\mathrm{CH}_{3})_{2} \mathrm{NH}\) have enhanced \(\mathrm{\sigma}\) electron donation to the nitrogen, increasing basicity.- Primary amines like methylamine \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) are also relatively strong bases but less than secondary amines.

Step 4: Comparing Basicity

Dimethylamine \((\mathrm{CH}_{3})_{2} \mathrm{NH}\) typically exhibits the strongest basicity relative to the others because secondary amines are generally stronger bases than primary and tertiary amines, and much stronger than aromatic amines due to less steric hindrance and more effective electron donation.

Step 5: Conclusion: Identifying the Strongest Base

Since dimethylamine \((\mathrm{CH}_{3})_{2} \mathrm{NH}\) is the strongest base among the options, it has the smallest pK_b value. Hence, option (c) is correct.

Key Concepts

Basicity of Amines

The basicity of amines is crucial for understanding their chemical behavior in solutions. Amines are compounds that feature a nitrogen atom with a lone pair of electrons. This lone pair enables amines to accept protons, acting as bases.

Generally, the basicity of an amine depends on the availability of the lone pair for bonding with protons. The steric environment around the nitrogen and the electronic effects from substituents greatly influence this availability.

• Primary amines like CH3NH2 (methylamine) are fairly basic because there's minimal steric hindrance around the nitrogen.
• Secondary amines such as (CH3)2NH (dimethylamine) are often more basic due to better electron donation from two methyl groups.
• Tertiary amines, on the other hand, face steric hindrance issues which can make them weaker bases.

pKb value

The \( pK_b \) value is an essential parameter that indicates the basicity of amines. It is the equilibrium constant for the base dissociation reaction of an amine in water. A smaller \( pK_b \) value correlates with a stronger base.

Conversely, a higher \( pK_b \) value indicates a weaker base. The relationship between \( pK_b \) and basicity is inverse; hence, knowing the \( pK_b \) allows chemists to compare various amines effectively.

• Amines with lower \( pK_b \) values are generally more effective at accepting protons, making them stronger bases.
• Amine solutions show this property by having a higher \( -OH \) concentration when determining \( pK_b \).

Aromatic Amines

Aromatic amines like C6H5NH2 (aniline) are characterized by their structural attachment to an aromatic ring, usually benzene. This architecture notably affects their basicity.

The lone pairs on the nitrogen are involved in resonance with the aromatic system, distributing electron density over the ring. While this resonance makes the compound stable, it severely reduces the nitrogen's ability to participate in proton acceptation, thus decreasing the amine's basicity.

• Aromatic amines usually present weaker basicity compared to aliphatic amines.
• Aniline is a prime example where despite having a nitrogen with a lone pair, the resonance with the benzene ring restricts its proton affinity and overall basicity.

Steric Hindrance

Steric hindrance is a physical concept that affects the basicity of amines significantly. It refers to the physical crowding of the molecule that impairs reactions involving the nitrogen atom.

Tertiary amines exhibit steric hindrance due to bulky substituents around the nitrogen, limiting its ability to encounter and bind effectively with protons.

• In compounds like (CH3)3N (trimethylamine), steric hindrance diminishes basicity.
• The three methyl groups create a crowded environment, hindering the approach of acidic protons to the lone pair on nitrogen, thus reducing the amine's basic strength.

Electron Donation in Amines

Electron donation plays a critical role in determining the basicity of amines. The presence of electron-donating groups, such as alkyl groups, enhances the availability of nitrogen's lone pair by pushing electron density toward it.

In secondary amines, such as (CH3)2NH (dimethylamine), the electron donation is maximized, resulting in higher basicity.

• Alkyl groups, like methyl, improve the availability of the lone pair, strengthening the base.
• Dimethylamine benefits from dual alkyl group donation, showcasing superior basicity among listed amines.