(a) Sandmeyer's Reaction

Step 1 (Reaction Definition & Equation): [1 Mark]

Sandmeyer's reaction involves the conversion of a primary aromatic amine into an aryl halide using nitrous acid to form a diazonium salt, which is then treated with cuprous halides ($\text{CuCl}$ or $\text{CuBr}$).

$$ \text{C}_6\text{H}_5\text{NH}_2 \xrightarrow{\text{NaNO}_2 + \text{HX}, 273-278\text{K}} [\text{C}_6\text{H}_5\text{N}_2]^+ \text{X}^- \xrightarrow{\text{CuX/HX}} \text{C}_6\text{H}_5\text{X} + \text{N}_2 $$

(Where $\text{X} = \text{Cl, Br}$)

(b) Basicity of Amines in Aqueous Solution

Step 2 (Conceptual Reasoning): [1 Mark]

The basicity of amines in an aqueous solution depends on three factors: Inductive effect (+I), Solvation effect (H-bonding), and Steric hindrance.

• $(\text{CH}_3)_2\text{NH}$ (Secondary amine) has two methyl groups providing a strong +I effect and possesses one hydrogen atom available for H-bonding with water, stabilizing the conjugate acid.
• $(\text{CH}_3)_3\text{N}$ (Tertiary amine) has three methyl groups providing a stronger +I effect, but it lacks a hydrogen atom for H-bonding.

Step 3 (Final Concluded Answer): [1 Mark]

In aqueous solution, the stability of the conjugate acid formed by $(\text{CH}_3)_2\text{NH}^+$ is significantly higher due to effective solvation (H-bonding) compared to $(\text{CH}_3)_3\text{NH}^+$. The steric hindrance of the three methyl groups in the tertiary amine further destabilizes the conjugate acid. Thus, the combined effect of solvation and steric hindrance makes $(\text{CH}_3)_2\text{NH}$ more basic than $(\text{CH}_3)_3\text{N}$.

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💡 EL10 Concept Insight: Think of the amine like a person trying to hold onto a "positive charge" (the proton). The secondary amine is the "Goldilocks" choice—it has enough push from its two methyl groups to be strong, but it's still small enough to be hugged and stabilized by water molecules. The tertiary amine is too bulky and crowded, making it harder for water to help it hold onto that charge.