See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: Bronsted acid strength depends on the stability of the conjugate base formed after proton donation. A stronger acid has a more stable conjugate base. Step 1: Identify the acidic proton in each compound. - (a) Aniline (PhNH2): N-H bond; aniline is a very weak acid (pKa ~30) - (b) Cyclohexylamine (CyNH2): N-H bond; aliphatic amine is an even weaker acid (pKa ~35) because no resonance stabilization of conjugate base - (c) Cyclohexanol (CyOH): O-H bond; aliphatic alcohol (pKa ~16) - (d) Phenol (PhOH): O-H bond; aromatic alcohol (pKa ~10) Step 2: Compare O-H acids vs N-H acids. Oxygen is more electronegative than nitrogen, so O-H bonds are more acidic than N-H bonds in general. This immediately eliminates (a) and (b) as the strongest acid. Step 3: Compare phenol (d) vs cyclohexanol (c). Phenol's conjugate base is the phenoxide ion (PhO-). The negative charge on oxygen is delocalized into the aromatic ring through resonance, generating five resonance structures. This extensive resonance stabilization makes the phenoxide ion far more stable than the cyclohexoxide ion (which has no such delocalization). Cyclohexanol pKa ≈ 16; Phenol pKa ≈ 10. A lower pKa means a stronger acid. Step 4: Why other options fail. - (b) Cyclohexylamine: N-H acid, no resonance stabilization, pKa ~35 — weakest acid. - (a) Aniline: N-H acid, some resonance but nitrogen is less electronegative, pKa ~30 — still weak. - (c) Cyclohexanol: O-H acid but no resonance stabilization of conjugate base, pKa ~16. - (d) Phenol: O-H acid with resonance stabilization of phenoxide conjugate base, pKa ~10 — strongest. Therefore, the correct answer is D.