See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Step 1 - Concept: Acid strength of alcohols and phenols depends on the stability of the conjugate base (phenoxide or alkoxide ion). Greater stabilization of the negative charge after proton loss means stronger acid. Electron-withdrawing groups (EWG) stabilize the conjugate base and increase acidity; electron-donating groups destabilize it and decrease acidity. Resonance stabilization is more effective than inductive stabilization. Step 2 - Ranking Compound 2 (cyclohexanol): This is a simple aliphatic alcohol. Its conjugate base (alkoxide) has no resonance stabilization and the negative charge is localized on oxygen. pKa ~ 16-17. Least acidic overall. Step 3 - Ranking Compound 4 (phenol): The phenoxide conjugate base is stabilized by resonance delocalization into the aromatic ring. pKa ~ 10. More acidic than cyclohexanol. Step 4 - Ranking Compound 1 (3-nitrophenol, meta): The NO2 group at the meta position stabilizes the phenoxide primarily through the inductive/field effect (electron withdrawal through sigma bonds). Resonance donation of the negative charge into the NO2 group is not possible from the meta position. pKa ~ 8.3. Step 5 - Ranking Compound 3 (4-nitrophenol, para): The NO2 group at the para position stabilizes the phenoxide both through the inductive effect AND through direct resonance delocalization - the negative charge can be delocalized all the way onto the oxygen atoms of the NO2 group via resonance through the ring. This is more effective stabilization than the meta isomer. pKa ~ 7.1. Most acidic of the four. Step 6 - Final order (most acidic to least acidic): Compound 3 (para-nitrophenol, pKa ~7.1) > Compound 1 (meta-nitrophenol, pKa ~8.3) > Compound 4 (phenol, pKa ~10) > Compound 2 (cyclohexanol, pKa ~16-17). This gives 3 > 1 > 4 > 2. Why other options fail: - (a) 2 > 4 > 1 > 3: Puts cyclohexanol as most acidic, which is wrong since aliphatic alcohols are less acidic than phenols. - (b) 1 > 3 > 4 > 2: Places meta-nitrophenol above para-nitrophenol, ignoring that para resonance stabilization is more effective. - (c) 3 > 1 > 2 > 4: Places cyclohexanol above phenol, which is incorrect since phenol is more acidic than cyclohexanol due to resonance stabilization of phenoxide. Therefore, the correct answer is D.