Concept:The term "least pH" designates the strongest acid (highest \( [\text{H}^+] \)). The strength of halogenated acids depends on both the number of halogen atoms and their proximity to the carboxyl group.
Solution:- Options A and D are unsubstituted, weak aliphatic acids.
- Option C (3-chloropropanoic acid) has one chlorine atom on the far \( \beta \)-carbon. The inductive effect diminishes rapidly with distance.
- Option B (2,2-dichloropropanoic acid) has two highly electronegative chlorine atoms, and they are located on the \( \alpha \)-carbon, directly adjacent to the carboxyl group.
- This creates massive stabilization for the resulting anion, making Option B undeniably the strongest acid, hence producing the lowest (least) pH.
Why other options are incorrect:They either lack halogens entirely or the halogens are too few and too distant to compete with 2,2-dichloropropanoic acid.
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