Understanding Basic Strength in Aqueous Solution

The basic strength of an amine or ammonia in aqueous solution is determined by its ability to accept a proton (H⁺) from water. This ability depends on the availability of the lone pair of electrons on the nitrogen atom. Several factors influence this availability and the stability of the resulting conjugate acid (ammonium ion):

1. Inductive Effect: Alkyl groups (like methyl, $- \text{CH}_3$) are electron-donating (+I effect). They push electron density towards the nitrogen atom, increasing the electron density on nitrogen and making the lone pair more available for donation. This generally increases basic strength. The more alkyl groups, the stronger the inductive effect.
2. Resonance Effect: If the lone pair on nitrogen can be delocalized into a pi system (like a benzene ring), it becomes less available for protonation, decreasing basic strength.
3. Solvation Effect: In aqueous solution, the conjugate acid formed after protonation (RNH₃⁺, R₂NH₂⁺, R₃NH⁺, or NH₄⁺) is stabilized by hydrogen bonding with water molecules. The more hydrogen atoms attached to the positively charged nitrogen, the greater the stabilization through solvation, which favors the formation of the conjugate acid and thus increases basic strength.
4. Steric Hindrance: Bulky groups around the nitrogen atom can hinder the approach of a proton (H⁺) and also reduce the extent of solvation of the conjugate acid.

Analyzing the Basic Strength of Each Compound

Let's analyze each given compound (A-E) based on the factors affecting basic strength:

• (D) $\text{NH}_3$ (Ammonia): This serves as a baseline. It has no alkyl groups for +I effect but its conjugate acid, $\text{NH}_4^+$, has four hydrogen atoms and can be extensively stabilized by hydrogen bonding with water molecules.
• (A) $\text{CH}_3\text{NH}_2$ (Methylamine - Primary Amine): Has one methyl group. The +I effect of the methyl group increases electron density on nitrogen compared to $\text{NH}_3$. Its conjugate acid, $\text{CH}_3\text{NH}_3^+$, has three hydrogen atoms available for hydrogen bonding with water, providing good solvation stabilization.
• (C) $(\text{CH}_3)_2\text{NH}$ (Dimethylamine - Secondary Amine): Has two methyl groups. The +I effect from two methyl groups is stronger than that from one. Its conjugate acid, $(\text{CH}_3)_2\text{NH}_2^+$, has two hydrogen atoms available for hydrogen bonding with water, providing moderate solvation stabilization.
• (B) $(\text{CH}_3)_3\text{N}$ (Trimethylamine - Tertiary Amine): Has three methyl groups. The +I effect from three methyl groups is the strongest among the methylamines. However, its conjugate acid, $(\text{CH}_3)_3\text{NH}^+$, has only one hydrogen atom available for hydrogen bonding, resulting in poor solvation stabilization. Steric hindrance from the three methyl groups can also slightly impede protonation and solvation.
• (E) $\text{C}_6\text{H}_5\text{NH}_2$ (Aniline - Aromatic Amine): The lone pair on the nitrogen atom is delocalized into the benzene ring through resonance. This significantly reduces the availability of the lone pair for protonation, making aniline a much weaker base compared to ammonia and aliphatic amines.

Combining Inductive and Solvation Effects in Aqueous Solution

In aqueous solution, the basic strength of aliphatic amines is determined by a balance between the +I effect (increasing basicity) and solvation of the conjugate acid (increasing basicity by stabilizing the conjugate acid). For methylamines, the observed order of basic strength in water is typically Secondary amine > Primary amine > Tertiary amine > Ammonia.

• Secondary amine $((\text{CH}_3)_2\text{NH})$ benefits significantly from the +I effect of two methyl groups and still has reasonable solvation of its conjugate acid.
• Primary amine $(\text{CH}_3\text{NH}_2)$ has a good +I effect from one methyl group and excellent solvation of its conjugate acid.
• Tertiary amine $((\text{CH}_3)_3\text{N})$ has the strongest +I effect but suffers from poor solvation of its conjugate acid and some steric hindrance.
• Ammonia $(\text{NH}_3)$ lacks the +I effect but has excellent solvation.
• Aniline $(\text{C}_6\text{H}_5\text{NH}_2)$ is the weakest due to resonance.

Determining the Decreasing Order of Basic Strength

Based on the analysis, the decreasing order of basic strength in aqueous solution is:

1. $(\text{CH}_3)_2\text{NH}$ (C) - Strongest basicity among methylamines due to optimal balance of +I and solvation.
2. $\text{CH}_3\text{NH}_2$ (A) - More basic than tertiary amine and ammonia due to good +I effect and solvation.
3. $(\text{CH}_3)_3\text{N}$ (B) - Less basic than primary and secondary amines due to poor solvation despite strong +I effect.
4. $\text{NH}_3$ (D) - Less basic than aliphatic amines but more basic than aniline.
5. $\text{C}_6\text{H}_5\text{NH}_2$ (E) - Least basic due to resonance stabilization of the lone pair.

So, the decreasing order is (C) > (A) > (B) > (D) > (E).

Comparing the relative strengths and combining the effects, the order (C) > (A) > (B) > (D) > (E) emerges as the correct decreasing order of basic strength in aqueous solution.

Revision Table: Basic Strength Concepts

Additional Information: Basic Strength in Gas Phase vs. Aqueous Solution

It is important to note the difference in basic strength order between the gas phase and aqueous solution. In the gas phase, only the inductive effect and potentially steric effects matter, as there is no solvation. The order is typically based purely on the number of alkyl groups (due to +I effect):

Gas Phase Basicity Order: Tertiary amine > Secondary amine > Primary amine > Ammonia

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

In aqueous solution, the solvation effect becomes significant. The excellent solvation of primary amine conjugate acids (RNH₃⁺) and ammonia conjugate acid (NH₄⁺) is due to the presence of multiple $\text{N-H}$ bonds capable of forming hydrogen bonds with water. Tertiary amine conjugate acids (R₃NH⁺) have only one $\text{N-H}$ bond and are poorly solvated. This interplay leads to the specific order observed for methylamines in water:

Aqueous Solution Basicity Order (Methylamines): Secondary amine > Primary amine > Tertiary amine > Ammonia > Aniline

$((\text{CH}_3)_2\text{NH}) > (\text{CH}_3\text{NH}_2) > ((\text{CH}_3)_3\text{N}) > (\text{NH}_3) > (\text{C}_6\text{H}_5\text{NH}_2)$

This question specifically asks for the order in aqueous solution, which is influenced by both inductive effect and solvation.