See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Step 1 – Identify each species and the acidic proton involved. (A) Protonated tetrahydrofuran (cyclic oxonium ion, O+ bearing an H): The acidic proton is on an oxygen carrying a positive charge. Conjugate base is neutral THF. Loss of H+ from a positively charged oxygen is very favorable; pKa ~ 0 or slightly negative. This is a very strong acid. (D) N-methyl pyrrolidinium ion (N+ bearing H and CH3): The acidic proton is on a positively charged nitrogen. Conjugate base is N-methylpyrrolidine (tertiary amine). Protonated amines have pKa ~ 10–11. This is moderately acidic. (C) Tertiary cyclopentanol (cyclopentane with gem OH and methyl, i.e., 1-methylcyclopentan-1-ol): The acidic proton is on a neutral OH (tertiary alcohol). pKa ~ 17–19. Less acidic than the ammonium ion but more acidic than a simple N-H amine. (B) Pyrrolidine (secondary amine, N-H, neutral): The acidic proton is on a neutral nitrogen. pKa of N-H in secondary aliphatic amine ~ 35–38. This is the weakest acid among the four. Step 2 – Rank by decreasing acidity (lower pKa = stronger acid). A (oxonium O+–H, pKa ~ –1 to 0) > D (ammonium N+–H, pKa ~ 10–11) > C (alcohol O–H, pKa ~ 17–18) > B (amine N–H, pKa ~ 35) Step 3 – Match to options. The order A > D > C > B corresponds to option (c). Step 4 – Why other options fail. (a) A > C > B > D: Places alcohol above ammonium ion and amine above ammonium ion – incorrect; ammonium ions are far more acidic than neutral amines or alcohols. (b) A > D > B > C: Places neutral amine (B) above alcohol (C) – incorrect; N-H of an amine (pKa ~35) is much less acidic than O-H of an alcohol (pKa ~17). (d) D > A > C > B: Places ammonium ion above oxonium ion – incorrect; O+–H bonds are more acidic than N+–H bonds. Therefore, the correct answer is C.