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Consider the following reaction sequence: 2CHCl3 + 6Ag --heat--> P, then P --Fe, hot tube--> Q. What is the degree of unsaturation (index of hydrogen deficiency) of compound Q?

1. 2
2. 3
3. 4
4. 5

Correct answer: 4

Solution

2CHCl3 + 6Ag reacts via dehalogenation (similar to Wurtz reaction for gem-dihalides). Each CHCl3 loses 3 Cl atoms giving carbene units that combine to form HC≡CH (acetylene = P) with 6AgCl as byproduct. Acetylene over Fe at ~300 deg C undergoes Berthelot trimerization: 3 HC≡CH → C6H6 (benzene = Q). Degree of unsaturation of benzene (C6H6) = (2*6 + 2 - 6)/2 = (14 - 6)/2 = 8/2 = 4.

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