Problem 139 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Introduction of a methyl group in ammonia markedly increases the basic strength of ammonia in aq. solution, but introduction of the second methyl group increases only marginally the basic strength of methyl amine in water. This is due to (a) different type of hybridization in the two amines. (b) protonated dimethyl amines are more solvated than methyl amine. (c) protonated dimethyl amine is more solvated than the protonated methyl amine. (d) protonated dimethyl amine is less stable than the protonated methyl amine.

Step-by-Step Solution

Answer

(c) protonated dimethyl amine is more solvated than the protonated methyl amine.

Step 1: Understand Basic Concept

The basic strength of an amine is measured by its ability to accept a proton, forming a protonated amine. The solvation of the protonated amine significantly affects its basic strength in aqueous solution. Solvation stabilizes the protonated amine, enhancing basicity.

Step 2: Effect of Methyl Substitution

The introduction of methyl groups to ammonia increases its basicity due to the electron-releasing effect (positive inductive effect) from the methyl group. The first methyl group significantly enhances basicity by increasing electron density on nitrogen.

Step 3: Analyze Second Methyl Group Effect

With the addition of a second methyl group, the inductive effect is present but the hydrophobic nature of the methyl groups decreases solvation of the larger protonated dimethyl amine. This marginally affects basic strength by reducing stabilization through solvation.

Step 4: Identify Key Factor

The most determining factor here is solvation. When two methyl groups are attached, the protonated dimethyl amine becomes more hindered and less efficiently solvated compared to the protonated methyl amine, impacting its stability and thus basic strength.

Key Concepts

Protonated Amine

A protonated amine is formed when an amine accepts a proton (H⁺) from an acid. This process transforms the neutral amine into a positively charged ion. The basic strength of any amine is essentially its tendency to undergo this proton-accepting transformation.
The stability of this newly formed protonated amine is critical to the basic strength of the original amine compound. If the protonated version is stable, it reflects a strong base since the original amine comfortably holds onto the incoming proton. Think of this as a magnetic attraction, where the amine readily grips the proton, signaling high base strength.

• Protonated amines are key to determining basicity.
• Stable protonated amines lead to strong basic amines.
• Instability results in lesser basic strength.

Solvation Effect

Solvation is a process where solvent molecules surround ions or molecules. In our context, when an amine is protonated, solvent molecules (like water) cradle and stabilize this protonated form. This is crucial in aqueous environments because the degree of solvation directly influences the amine's basic strength.
Water molecules are polar. This polarity enables them to interact effectively with the charged protonated amine. The more efficiently these solvent molecules can surround and stabilize the ion, the stronger the perceived basic strength. However, larger and bulkier groups attached to the amine can impede this solvation, potentially decreasing basic strength despite positive inductive effects from these groups.

• Solvation stabilizes protonated amines.
• Improved solvation increases basicity.
• Hindered solvation due to bulk could decrease basicity.

Inductive Effect

The inductive effect is all about electron distribution. In chemistry, it's the skewing of electron density along a chain of atoms, which affects how the molecule behaves. When you introduce a methyl group into an amine, the group donates electrons, which increases the electron density around the nitrogen atom.
This increased electron density makes it easier for the amine to accept protons, enhancing its basic strength. It's like giving the nitrogen atom a little boost, letting it reach for and hold a proton more effectively. Positive inductive effects amplify basicity, but as more methyl groups are added, they can also block access to solvents, offsetting the benefits they introduce.

• Methyl groups enhance basic strength via electron donation.
• They improve proton affinity at the nitrogen atom.
• Excessive substitution could impede solvation.

Methyl Substitution in Amines

Substituting hydrogen atoms in ammonia with methyl groups significantly affects how these molecules behave as bases. The first methyl substitution enhances basic strength because the methyl group provides a positive inductive effect, increasing electron density at the nitrogen and making it easier to attract protons.
However, adding a second methyl group tells a slightly different story. This additional group also adds electron density but introduces bulk. The increased size of the dimethyl amine is less effectively surrounded by solvent molecules, thus decreasing its overall stabilized state when protonated.
In conclusion, while additional methyl groups can enhance intrinsic base strength through electron donation, their physical size can reduce solvation efficiency. Hence, the second methyl group has only a marginal impact on basic strength compared to the first.

• First methyl substitution boosts basic strength greatly.
• Second methyl addition reduces solvation efficiency.
• Larger size interferes with solvent interaction.