See image — Aldehydes Ketones and Carboxylic Acids Chemistry Question

💡 Solution & Explanation

Concept: The reaction of cyclopentadiene with a ketone under basic conditions (EtONa/EtOH) with heat is a Knoevenagel-type condensation (aldol-type condensation). Cyclopentadiene is acidic at the sp3 CH2 position (C5, pKa ~16) due to the formation of a cyclopentadienyl anion upon deprotonation, which is aromatic (6 pi electrons). Step 1 - Deprotonation: EtONa (a strong base) deprotonates cyclopentadiene at the sp3 methylene position (C5) to form the cyclopentadienyl anion (aromatic, stabilized). Step 2 - Nucleophilic addition: The cyclopentadienyl anion attacks the carbonyl carbon of acetone (CH3COCH3) in an aldol-type addition to form an alkoxide intermediate. Step 3 - Elimination (dehydration) with heat: Under heating conditions, the beta-hydroxy intermediate undergoes elimination of water to form the thermodynamically stable exocyclic alkene product. The product is 6,6-dimethylfulvene: cyclopentadiene ring with an exocyclic C=C(CH3)2 double bond at C1. This is a fulvene (cross-conjugated system: the exocyclic double bond is conjugated with the diene system of the ring). Why option (a) fails: Option (a) shows two cyclopentadienyl rings connected through a C(CH3)2 group with sp3 carbon - this would require reaction of two equivalents of cyclopentadiene with acetone in a different manner and does not represent the simple condensation product. Why option (b) fails: Option (b) is the aldol addition product (before elimination). Since heat is applied, elimination of water occurs to give the alkene, not the alcohol. Why option (d) fails: Option (d) shows a C(CH3)2 group bearing two cyclopentadienyl rings attached via sp3 carbons - this is not the product of a simple condensation. Why option (c) is correct: Option (c) shows the cyclopentadiene ring with an exocyclic double bond to C(CH3)2, i.e., cyclopentadienylidene=CMe2, which is 6,6-dimethylfulvene. This is the direct product of Knoevenagel condensation (aldol condensation + dehydration) of cyclopentadiene with acetone under basic conditions and heat. Therefore, the correct answer is C.