See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: Effervescence with sodium carbonate (Na2CO3) occurs when an acid is strong enough to liberate CO2 gas from carbonate. The reaction is: 2 HA + Na2CO3 → 2 NaA + H2O + CO2. For CO2 to be released, the acid must be stronger than carbonic acid (pKa ~6.35 for first dissociation). Step 1 - Analyze each compound's acidity: (a) C6H5CO2H (benzoic acid): pKa ~4.2. This is stronger than carbonic acid, so it will react with Na2CO3 to give CO2 effervescence. (b) Benzenesulfonic acid (C6H5SO3H): pKa ~-1. This is a strong acid, much stronger than carbonic acid, so it will react with Na2CO3 to give CO2 effervescence. (c) C6H5OH (phenol): pKa ~10. This is weaker than carbonic acid (pKa ~6.35). Therefore, phenol cannot displace CO2 from Na2CO3. Phenol does react with NaOH (a strong base) but NOT with Na2CO3 to give effervescence. (d) 2,4,6-trinitrophenol (picric acid): pKa ~0.38. The three electron-withdrawing nitro groups drastically increase the acidity of the phenolic OH, making picric acid stronger than carbonic acid. It will react with Na2CO3 to give CO2 effervescence. Step 2 - Identify which compound does NOT give effervescence: Only phenol (C6H5OH) is too weak an acid (pKa ~10) to liberate CO2 from Na2CO3. All others are sufficiently acidic. Step 3 - Why other options fail: (a) Benzoic acid — strong enough, gives effervescence. (b) Benzenesulfonic acid — strong acid, gives effervescence. (d) Picric acid — highly acidic phenol derivative, gives effervescence. Therefore, the correct answer is C.