Problem 20 - Chemistry in 30 days

Question

Which of the following is least basic? (a) \(\mathrm{FCH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}\) (c) \(\mathrm{ClCH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\) (d) All are equally basic

Step-by-Step Solution

Answer

(a) \(\mathrm{FCH}_2 \mathrm{CH}_2 \mathrm{NH}_2\) is the least basic.

Step 1: Understanding Basicity

Basicity refers to the ability of a compound to accept protons. In organic amines, basicity is influenced by the electronic environment around the nitrogen atom. Electronegative atoms near the amine group can withdraw electron density, reducing the basicity.

Step 2: Analyzing the Structures

Compare the structures:(a) \(\mathrm{FCH}_2 \mathrm{CH}_2 \mathrm{NH}_2 \\) contains a fluorine atom, which is highly electronegative.(b) \(\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{NH}_2 \\) has only hydrogen and carbon adjacent to the amine group.(c) \(\mathrm{ClCH}_2 \mathrm{CH}_2 \mathrm{NH}_2 \\) contains a chlorine atom, also electronegative but less than fluorine.

Step 3: Evaluating Electron-Withdrawing Effects

Fluorine is more electronegative than chlorine, so it withdraws more electron density from the nitrogen in compound (a) than chlorine in compound (c). Compound (b) doesn't have an electronegative atom pulling away electron density, so it should be more basic than (a) and (c).

Step 4: Determining Least Basic Compound

Since (a) has the highly electronegative fluorine, it withdraws the most electron density from the nitrogen, making it the least capable of accepting a proton. Thus, \(\mathrm{FCH}_2 \mathrm{CH}_2 \mathrm{NH}_2\) is the least basic.

Key Concepts

Electron-Withdrawing Effects

Electron-withdrawing effects play a significant role in determining the basicity of organic compounds, especially amines. When an electronegative atom is close to an amino group, it pulls electron density away from the nitrogen atom. This decreases the ability of the nitrogen to accept a proton, thus lowering its basicity.
Some key points to remember about electron-withdrawing groups are:

• They attract electrons towards themselves.
• This results in decreased electron density around the nitrogen atom in amines.
• Fluorine and chlorine are common examples of such atoms that significantly affect basicity.
• Fluorine is more effective as an electron-withdrawing atom than chlorine due to its higher electronegativity.

Understanding these effects highlights why some amines are more basic than others, making it crucial to consider the surrounding atoms when evaluating basicity.

Electronegativity in Organic Compounds

Electronegativity is the tendency of an atom to attract shared electrons in a chemical bond. In organic chemistry, this property is vital in determining how atoms within molecules interact.
Electronegative atoms like fluorine (\( ext{F}\)) and chlorine (\( ext{Cl}\)) are frequently used as indicators of where electron density shifts within a molecule. When these atoms are adjacent to an amine group, they pull electron density away from the nitrogen, consequently reducing its basicity.

• Fluorine, with an electronegativity of about 4.0, is more electronegative than chlorine, with a value of about 3.0.
• Even a slight shift in electron density can impact the basic properties of an organic compound.
• Such changes can make the compound less eager to participate in chemical reactions involving proton acceptance.

Considering electronegativity helps predict the behavior of molecules, especially in reactions involving amines.

Amines and Basicity

Amines are organic compounds derived from ammonia, distinguished by the presence of a nitrogen atom bonded to hydrogen and carbon chains or rings. They exhibit basic characteristics because they can accept protons with their lone pair of electrons on nitrogen.
Basicity in amines depends largely on the local electronic environment, which is affected by several factors like electron-withdrawing effects mentioned earlier.

• Primary amines like \( ext{CH}_3 ext{CH}_2 ext{NH}_2\) are more basic than halogenated counterparts like \( ext{FCH}_2 ext{CH}_2 ext{NH}_2\).
• The presence of electronegative atoms decreases the electron density of the nitrogen atom.
• These changes hinder the ability of nitrogen to accept protons, making such amines less basic.

Understanding the factors impacting amine basicity helps in predicting their behavior in chemical reactions and in designing compounds with desired properties.