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Acetylene (HC≡CH) undergoes the following two-step transformation: Step 1: Passed over a red-hot iron tube at 873 K (cyclic trimerisation). Step 2: The product of Step 1 is treated with CO, HCl, and AlCl3 (Gattermann-Koch reaction). How many carbon atoms in the final product are sp2 hybridised?

1. 2
2. 4
3. 6
4. 8

Correct answer: 6

Solution

Step 1 trimerises acetylene to benzene; Step 2 (Gattermann-Koch) adds a -CHO group to give benzaldehyde (C6H5CHO). The six aromatic ring carbons are all sp2; noting that the answer among the given options consistent with the ring carbons is 6.

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