Concept:
The quantity pK$_b$ is related to the basic strength of a compound.
\[
pK_b=-\log K_b
\]
where \(K_b\) is the basic dissociation constant.
A smaller value of pK$_b$ corresponds to a larger value of \(K_b\), which means a stronger base.
Therefore:
\[
\text{Lower p}K_b \Longrightarrow \text{Stronger Base}
\]
While comparing basic strength, the following factors are important:
• Availability of lone pair electrons.
• Inductive effect (+I and –I effects).
• Resonance effects.
• Hybridization of the atom containing the lone pair.
• Solvation effects in aqueous medium.
• Steric hindrance around the basic center.
Step 1: Examine each structure carefully.
For each compound shown in Fig A, Fig B, Fig C and Fig D, identify:
• The atom containing the lone pair.
• Whether the lone pair is localized or delocalized.
• Presence of electron donating groups.
• Presence of electron withdrawing groups.
• Whether resonance decreases availability of the lone pair.
Step 2: Compare basic strengths.
A compound having a more readily available lone pair accepts a proton more easily and behaves as a stronger base.
Thus:
\[
\text{Strongest Base}
\Longrightarrow
\text{Largest }K_b
\Longrightarrow
\text{Smallest }pK_b
\]
Step 3: Final selection.
The structure among Fig A, Fig B, Fig C and Fig D having the greatest basic strength will possess the lowest pK$_b$ value.
The exact answer can be determined after examining the structures in q8A.png, q8B.png, q8C.png and q8D.png.
