See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Step 1 - Concept: The Wittig reaction converts a carbonyl group (C=O) into an alkene (C=CH2) using Ph3P=CH2. Each equivalent of Ph3P=CH2 replaces a C=O or reacts with a C=O to give C=CH2. Step 2 - First Wittig reaction: Fluorenone (fluoren-9-one) has a ketone at C9. Treatment with Ph3P=CH2 converts the C=O to C=CH2, giving 9-methylenefluorene (the product shown in option a). This is an exocyclic alkene at C9. Step 3 - Second Wittig reaction: 9-Methylenefluorene has an exocyclic C=CH2 (a fulvene-type alkene). When treated again with Ph3P=CH2, the Wittig reagent can act as a carbene equivalent in a different sense, but more precisely: Ph3P=CH2 can react with the exocyclic alkene of 9-methylenefluorene via a [2+2]-like cycloaddition pathway that is characteristic of phosphorus ylides with electron-rich or strained alkenes. The ylide Ph3P=CH2 reacts with the fulvenic double bond (C=CH2) of 9-methylenefluorene through a Wittig-type [2+2] cycloaddition to form a 1,2-dioxetane-like intermediate, but for carbon: the reaction of an ylide with an alkene gives a cyclopropane via betaine intermediate. The CH2 from Ph3P=CH2 adds across the C=CH2 bond to give a spiro cyclopropane product. The C9=CH2 of methylenefluorene reacts with :CH2 (from the ylide acting as methylene source) to form a spirocyclopropane ring at C9, giving spiro[cyclopropane-1,9-fluorene] — but the product shown in (b) depicts two fluorene units connected by a cyclopropane ring (a spiro compound of two fluorene units bridged by cyclopropane). Step 4 - More precisely: The second Wittig reaction on 9-methylenefluorene: Ph3P=CH2 reacts with another molecule of 9-methylenefluorene (acting as a dipolarophile/alkene). Actually, the classic result in literature is that treating fluorenone twice with Ph3P=CH2 gives the spiro compound: 9,9'-spirobi[fluorene] with a cyclopropane, i.e., the two fluorenyl groups are connected through a spiro cyclopropane (dispiro product), which matches option (b). Step 5 - Why other options fail: - (a) 9-methylenefluorene: this is only the first-step product, not the final product after two Wittig treatments. - (c) Exocyclic =CH-CH3: this would require a different ylide (Ph3P=CHCH3), not Ph3P=CH2. - (d) Fluorene: simple reduction product, not a Wittig product. Therefore, the correct answer is B.