Read the following passage and answer the questions based on it.

Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. Sterically, the presence of two large groups in ketones hinders the attack of nucleophile to carbonyl carbon than in aldehydes. Electronically, aldehydes are more reactive than ketones because two alkyl groups reduce the electrophilicity of the carbonyl carbon more effectively than in the former.

The correct decreasing order of basic strength of following antines in aqueous solution is

$CH_3NH_2,(CH_3)_2NH,(CH_3)_3N, NH_3$

$(CH_3)_2NH>CH_3NH_2>(CH_3)_3N>NH_3$

The correct answer is Option (4) → $(CH_3)_2NH>CH_3NH_2>(CH_3)_3N>NH_3$.

Let us delve into the basic strength of amines in aqueous solution, focusing on the specific compounds mentioned: methylamine (\( \text{CH}_3\text{NH}_2 \)), dimethylamine (\( \text{(CH}_3\text{)}_2\text{NH} \)), trimethylamine (\( \text{(CH}_3\text{)}_3\text{N} \)), and ammonia (\( \text{NH}_3 \)).

Basicity refers to the ability of a compound to accept protons (H⁺ ions). In the case of amines, the basicity is largely determined by the availability of the lone pair of electrons on the nitrogen atom. The stronger the tendency of the nitrogen to share its lone pair with protons, the stronger the base.

Factors Influencing Basic Strength in Amines

Electron Donating Effects (Inductive Effect): Alkyl groups are electron-donating due to their +I effect, which means they can increase the electron density on the nitrogen atom. This makes the nitrogen more basic. More alkyl groups generally mean more electron density, enhancing the basicity.

Steric Hindrance: As the number of alkyl groups increases, steric hindrance also increases, which can inhibit the nitrogen's ability to approach and bond with a proton. This means that while electron donation from alkyl groups increases basicity, excessive steric hindrance can reduce it.

Hybridization: The hybridization state of the nitrogen atom can affect its basicity. For amines, nitrogen is typically sp³ hybridized, which is favorable for basicity.

Analysis of Each Amines

Methylamine (\( \text{CH}_3\text{NH}_2 \)):

Contains one methyl group.

Inductive Effect: The methyl group provides some electron-donating capability through the +I effect.

Steric Hindrance: Minimal, as there is only one alkyl group.

Basic Strength: Strong basicity due to a good balance of electron donation and low steric hindrance.

Dimethylamine (\( \text{(CH}_3\text{)}_2\text{NH} \)):

Contains two methyl groups.

Inductive Effect: Two methyl groups enhance the electron density on nitrogen even more compared to methylamine.

Steric Hindrance: Still manageable, allowing for good interaction with protons.

Basic Strength: Stronger than methylamine due to greater electron density

Trimethylamine (\( \text{(CH}_3\text{)}_3\text{N} \)):

Contains three methyl groups.

Inductive Effect: The three methyl groups provide a significant +I effect.

Steric Hindrance: Increased steric hindrance makes it harder for trimethylamine to approach protons effectively.

Basic Strength: Weaker than both methylamine and dimethylamine because the steric hindrance limits the nitrogen's ability to effectively bond with protons.

Ammonia (\( \text{NH}_3 \)):

No alkyl groups, just the nitrogen and three hydrogen atoms.

Inductive Effect: Lacks alkyl groups, so there's no +I effect to enhance basicity.

Steric Hindrance: None, but this is outweighed by the lack of electron-donating groups.

Basic Strength: Weaker than all the alkylated amines due to the absence of the +I effect.

Considering all these factors, the basic strength of the amines in aqueous solution can be summarized as follows:

Dimethylamine (\( \text{(CH}_3\text{)}_2\text{NH} \)): Strongest due to both high electron density and manageable steric hindrance.

Methylamine (\( \text{CH}_3\text{NH}_2 \)): Second strongest with good electron donation but slightly less than dimethylamine.

Trimethylamine (\( \text{(CH}_3\text{)}_3\text{N} \)): Weaker than the first two because of significant steric hindrance, despite high electron donation.

Ammonia (\( \text{NH}_3 \)): Weakest due to the absence of alkyl groups providing electron density.

Correct Decreasing Order of Basic Strength

Thus, the correct decreasing order of basic strength in aqueous solution is:

\(\text{(CH}_3\text{)}_2\text{NH} > \text{CH}_3\text{NH}_2 > \text{(CH}_3\text{)}_3\text{N} > \text{NH}_3\)

Conclusion

This order highlights the importance of both electron-donating effects and steric hindrance in determining the basic strength of amines. Understanding these concepts is crucial in organic chemistry, especially in the context of reactions involving amines as base