Problem 47 - A Complete Resource Book in Chemistry for JEE Main

Question

Arrange the following in increasing basicity order: I. \(\mathrm{Cl}_{2} \mathrm{CHCH}_{2} \mathrm{NH}_{2}\) II. \(\mathrm{Cl}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}\) III. NC1CCCCC1 IV. \(\mathrm{CF}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}\) (a) \(\mathrm{I}<\mathrm{III}<\mathrm{II}<\mathrm{IV}\) (b) \(\mathrm{II}<\mathbb{I V}<\mathrm{III}<\mathrm{I}\) (c) \(\mathrm{IV}<\mathrm{II}<\mathrm{I}<\mathrm{III}\) (d) \(\mathrm{I}<\mathrm{II}<\mathrm{IV}<\mathrm{III}\)

Step-by-Step Solution

Answer

(d) I < II < IV < III

Step 1: Understand Basicity Concept

Basicity is a measure of how well a compound can donate an electron pair or accept a proton (H+). Amines are among the basic compounds due to the presence of a lone pair on the nitrogen atom.

Step 2: Analyze Structure I (Cl2CHCH2NH2)

Structure I has a chlorine substituent on a carbon adjacent to the amine group. Chlorine is an electron-withdrawing group, which decreases the electron density on the nitrogen, reducing its basicity.

Step 3: Analyze Structure II (Cl3CH2NH2)

Structure II has three chlorine atoms attached to the carbon adjacent to the amine. This strong electron-withdrawing effect greatly reduces the basicity of the amine compared to Structure I.

Step 4: Analyze Structure III (Cyclohexylamine NC1CCCCC1)

Structure III is cyclohexylamine. The cyclic alkyl group does not significantly affect the basicity as much as electron-withdrawing groups do. It remains relatively basic compared to chlorine-substituted amines.

Step 5: Analyze Structure IV (CF3CH2NH2)

Structure IV contains a trifluoromethyl group, which is a very strong electron-withdrawing group. This significantly reduces the electron density on the nitrogen and decreases basicity compared to Structure I and Structure III.

Step 6: Compare and Order Basicity

Comparing all structures: - Structure III is the most basic due to minimal electron-withdrawing effects. - Structure I is next as it has only two chlorines influencing its basicity. - Structure II is less basic than Structure I due to enhanced electron-withdrawing by three chlorines. - Structure IV is the least basic due to the strong electron-withdrawing effect of the CF3 group.

Key Concepts

Electron-withdrawing groups

In organic chemistry, electron-withdrawing groups (EWGs) play a significant role in determining the reactivity and properties of compounds. These groups, such as chlorine or trifluoromethyl (\(\mathrm{CF}_3\)), contain atoms that are more electronegative and effectively "pull" electron density from nearby atoms or functional groups.
This electron-withdrawing effect lowers the electron density required for the nitrogen atom in amines to donate electrons, thereby reducing the basicity of the compound.

For example:

• In \(\mathrm{Cl}_{3}\mathrm{CH}_2\mathrm{NH}_2\) (Structure II), three chlorine atoms strip electrons more efficiently than just one or two chlorines, drastically lowering its basicity.
• The compound \(\mathrm{CF}_3\mathrm{CH}_2\mathrm{NH}_2\) (Structure IV) with a \(\mathrm{CF}_3\) group is even less basic due to the intense electron-withdrawing effect of the trifluoromethyl group.
• EWGs can be strategically utilized in synthetic chemistry to modify chemical behavior according to specific needs!

Amines

Amines are organic compounds that contain a nitrogen atom with a lone pair of electrons. This lone pair makes amines capable of acting as bases, where they can donate an electron pair to accept a proton (\(\mathrm{H}^+\)).
Amines vary in basicity depending on their structure and substituents. Simple alkylamines are generally quite basic because alkyl groups can donate electron density toward the nitrogen, enhancing its ability to donate electrons.

Consider cyclohexylamine (Structure III):

• This structure has no electron-withdrawing groups unlike other examples, allowing it to retain a good degree of basicity due to the support from its alkyl ring.
• Conversely, amines with electron-withdrawing groups attached show reduced basicity. The further the electron-withdrawing effect, the less basic the amine becomes.

The versatility of amines makes them useful in various applications, from serving as intermediates in chemical syntheses to functioning as active components in pharmaceuticals.

Structure-activity relationship

The structure-activity relationship (SAR) in organic chemistry involves understanding how different chemical structures impact the reactivity and function of a compound. This concept is crucial in pharmaceutical chemistry, where slight alterations in a molecule's structure can dramatically influence its biological activity and function.

In the context of basicity, the SAR focuses on how functional groups and molecular geometry affect a compound's ability to donate electrons.

• For instance, in the exercise at hand, comparing \(\mathrm{Cl}_{3}\mathrm{CH}_2\mathrm{NH}_2\) with cyclohexylamine, it is evident that the presence of three chlorines significantly lowers the nitrogen's basicity due to electron withdrawal.
• Understanding SAR helps chemists design better molecules by recognizing patterns of how electron-withdrawing or donating groups modify a compound's basicity.

Ultimately, by grasping SAR, we can predict and manipulate the behavior of chemical compounds, paving the way for various technological and therapeutic advances.