In trimethylamine $[(\text{CH}_3)_3\text{N}]$, the nitrogen atom is $sp^3$ hybridized with three bond pairs and one lone pair. Due to lone pair-bond pair repulsion, the molecular geometry is trigonal pyramidal (with $\text{C}-\text{N}-\text{C}$ bond angle of approx. $108^\circ$).

Benzene diazonium ion ($[\text{C}_6\text{H}_5\text{N}_2]^+$) is a relatively weak electrophile and can only attack strongly activated aromatic rings containing electron-rich groups such as $-\text{OH}$ (phenol) or $-\text{NH}_2$ (aniline, o-toluidine). Nitrobenzene contains a strongly deactivating $-\text{NO}_2$ group which depletes electron density from the ring, completely preventing azo coupling.

Direct nitration of aniline using nitrating mixture at $298\text{ K}$ yields:

• p-Nitroaniline: $51\%$
• m-Nitroaniline: $47\%$
• o-Nitroaniline: $2\%$

In strongly acidic medium, aniline undergoes protonation to form anilinium ion ($\text{C}_6\text{H}_5\text{NH}_3^+$). The $-\text{NH}_3^+$ group is strongly electron-withdrawing and meta-directing, leading to a surprisingly large yield ($47\%$) of the meta-isomer.

In aqueous solution, the basicity of ethyl-substituted aliphatic amines depends on inductive effect, solvation energy, and steric hindrance, following the order: $2^\circ \gt 3^\circ \gt 1^\circ$.

• $(C_2H_5)_2NH$ ($2^\circ$) is the strongest base.
• $(C_2H_5)_3N$ ($3^\circ$) comes next.
• $C_2H_5NH_2$ ($1^\circ$) comes after.
• Aniline ($C_6H_5NH_2$) is the weakest base because the nitrogen lone pair is delocalised into the aromatic $\pi$-system.

Increasing order: $\text{C}_6\text{H}_5\text{NH}_2 \lt \text{C}_2\text{H}_5\text{NH}_2 \lt (\text{C}_2\text{H}_5)_3\text{N} \lt (\text{C}_2\text{H}_5)_2\text{NH}$.