Product [A]: From PhNH₂, steps (1) NaNO₂ + HCl forms diazonium salt, (2) CuCN gives benzonitrile, (3) H₃O⁺/Δ hydrolyzes to benzoic acid (PhCOOH), pKa ≈ 4.2.
Product [B]: From CH₃CH₂CHO, Tollens' oxidation with [Ag(NH3)2]+ produces acetic acid (CH₃COOH), pKa ≈ 4.8.
Product [C]: From CH₄ + O₂ with heat and Na₂Cr₂O₇/H⁺ produces formic acid (HCOOH), pKa ≈ 3.8 (strongest acid).
Product [D]: From PhCH₂MgBr + CO₂, the Grignard reaction produces phenylacetic acid (PhCH₂COOH), pKa ≈ 4.3.
Acidic strength order (lower pKa = stronger): C (3.8) > A (4.2) > D (4.3) > B (4.8).
