Problem 89 - A Complete Resource Book in Chemistry for JEE Main

Question

Arrange the following: \(\mathrm{I} \mathrm{CH}_{3} \mathrm{NH}_{2}\) \(\mathrm{II}\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}\) \(\mathrm{III} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) and IV \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}\) in increasing order of basicity in aqueous medium. (a) \(\mathrm{II}<\mathrm{I}<\mathrm{IV}<\mathrm{III}\) (b) II \(<\mathrm{III}<\mathrm{I}<\mathrm{IV}\) (c) \(\mathrm{I}<\mathrm{II}<\mathrm{III}<\mathrm{IV}\) (d) \(\mathrm{III}<\mathrm{IV}<\mathrm{I}<\mathrm{II}\)

Step-by-Step Solution

Answer

Option (d): III < IV < I < II is correct.

Step 1: Define Basicity

In an aqueous medium, basicity refers to the ability of a substance to donate a pair of electrons to a proton, forming a bond with the proton. This ability is influenced by several factors, including the availability of the lone pair on the nitrogen atom and the electron-releasing or withdrawing nature of substituents attached to the nitrogen atom.

Step 2: Analyze the Basicity of Each Compound

1. **Compound I: CH₃NH₂ (methylamine)** - Methylamine has one methyl group attached to the nitrogen atom, which releases electrons (electron-donating) and enhances basicity by making the lone pair more available. 2. **Compound II: (CH₃)₂NH (dimethylamine)** - Dimethylamine has two methyl groups, thus it is expected to be more basic than methylamine due to increased electron-donating effects. 3. **Compound III: C₆H₅NH₂ (aniline)** - In aniline, the lone pair on nitrogen is delocalized into the benzene ring, which reduces its availability. Thus, aniline is generally less basic. 4. **Compound IV: (CH₃)₃N (trimethylamine)** - Trimethylamine has three methyl groups, which makes it highly basic due to strong electron-donating effects increasing lone pair availability.

Step 3: Arrange the Compounds

Based on the analysis, we arrange the compounds from least to most basic: Aniline (Compound III) is the least basic due to lone pair delocalization. Methylamine (Compound I) comes next, followed by dimethylamine (Compound II), which has more electron-donating groups. Trimethylamine (Compound IV) is the most basic with the highest electron-donating effect. Hence, the order is: III < I < II < IV.

Step 4: Choose the Correct Option

From the options provided, the order found was III < I < II < IV. Evaluating the answer choices, none exactly match this conclusion, suggesting a re-evaluation. Upon scrutiny, realize primary and secondary amines (I, II) are often more basic than tertiary amines (IV) due to solvation effects in water, thus: III < IV < I < II. Option (d) matches: (III < IV < I < II).

Key Concepts

Electron-Donating Groups

In organic chemistry, electron-donating groups play an essential role in enhancing a compound's basicity. These groups are atoms or molecules that push or "donate" electrons towards other parts of a molecule.
When attached to a nitrogen atom in amines, electron-donating groups such as alkyl groups (e.g., methyl) increase the electron density around the nitrogen atom.
This effect enhances the ability of the nitrogen's lone pair of electrons to attract and bond with protons.

• Methyl groups are a common electron-donating group and are present in several amines like methylamine and dimethylamine.
• When more methyl groups are attached to the nitrogen, as in trimethylamine, the basicity of the compound increases due to the heightened electron-donating effects.

Thus, the presence and number of electron-donating groups directly influence the basicity of amines.

Lone Pair Availability

The lone pair of electrons on the nitrogen atom is central to the basicity of amines. The more available this lone pair is to a proton, the stronger the base. However, several factors can influence this availability.

• Electron-donating groups attached to the nitrogen can increase this lone pair's availability by providing additional negative charge.
• Conversely, if the lone pair is involved in resonance, as in aniline, it becomes less available. In aniline, the lone pair on nitrogen is delocalized into the benzene ring, reducing its ability to bond with protons.

Therefore, understanding the lone pair availability on the nitrogen atom is key to predicting the basic strength of amines.

Amines

Amines are organic compounds and functional groups that contain a nitrogen atom with a lone pair of electrons. Basicity in amines is largely due to this lone pair, which can accept a proton, forming a bond.

• Primary amines, like methylamine, have one carbon-based (alkyl) group attached to the nitrogen.
• Secondary amines, such as dimethylamine, have two carbon groups, resulting in increased basicity due to enhanced electron donation to nitrogen.
• Tertiary amines, like trimethylamine, possess three alkyl groups. While these groups increase electron donation, solvation effects in water can sometimes make them less basic compared to secondary amines.

Amines' structure and the nature of substituents significantly affect their ability to act as bases.

Nitrogen Atom Substituents

The substituents attached to the nitrogen atom in amines have a profound impact on their chemical behavior, particularly their basicity. These substituents include different types of alkyl groups, aromatic rings, or other functional groups that modify the electron density around the nitrogen.

• Methyl groups, prevalent in methylamine, dimethylamine, and trimethylamine, increase the electron density on nitrogen, enhancing basicity.
• Aniline, however, contains a phenyl ring that interacts with the nitrogen's lone pair, causing reduced basicity due to electron delocalization.

Thus, discerning the types of substituents and their electronic effects is essential for understanding how they influence the basicity of nitrogen-containing compounds.