See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: The Wittig reaction converts a carbonyl compound (aldehyde or ketone) into an alkene using a phosphorus ylide. The two products are always the alkene and triphenylphosphine oxide (Ph3P=O, i.e., OPPh3). However, when the ylide is unsymmetrical (i.e., the carbanionic carbon bears two different substituents) AND the carbonyl compound is an aldehyde (which itself is unsymmetrical with H on one side), the reaction can generate E/Z isomers of the alkene product. In such a case, two alkene products (geometric isomers) are formed in addition to OPPh3. Step 1 - Analyze each option for the possibility of geometric (E/Z) isomerism in the alkene product: (a) Cyclohexanone + Ph3P=CH2: The ylide Ph3P=CH2 has two identical H substituents on the carbanionic carbon. The product alkene from cyclohexanone would be methylenecyclohexane (=CH2), which has two identical substituents on one carbon of the double bond. No E/Z isomerism is possible. Only one alkene product forms. (b) CH3CHO + Ph3P=C(CH3)2: The carbanionic carbon of the ylide bears two identical CH3 groups. One carbon of the resulting alkene will have two identical methyl groups. No E/Z isomerism is possible. Only one alkene product forms. (c) PhCHO + Ph3P=CH-Ph: Here the aldehyde is PhCHO (benzaldehyde) and the ylide is Ph3P=CH-Ph (the carbanionic carbon bears one H and one Ph). The resulting alkene is Ph-CH=CH-Ph (stilbene). One carbon has Ph and H; the other carbon also has Ph and H. Both carbons of the double bond have two different substituents, so E and Z isomers (cis- and trans-stilbene) are both possible. Two alkene products are formed. (d) HCHO + Ph3P=CH-CH3: The ylide carbanionic carbon bears H and CH3. The aldehyde is formaldehyde (HCHO), whose carbonyl carbon bears two H atoms. The resulting alkene is H2C=CH-CH3 (propene). One carbon of the double bond (from formaldehyde) has two H substituents, making E/Z isomerism impossible. Only one alkene product forms. Step 2 - Conclusion: Only option (c) produces two alkene products (cis- and trans-stilbene) in addition to OPPh3, because both carbons of the newly formed double bond bear two different substituents, enabling geometric isomerism. Why others fail: (a) and (b) have symmetric substitution on the ylide carbon; (d) has two identical H on the formaldehyde-derived carbon — none of these allow E/Z isomerism. Therefore, the correct answer is C.