Discussion
The major product formed in the following reaction is :
- A.
- B.
- C.
- D.
Answer: A
The keyed product is 9,10-dibromo-9,10-dihydrophenanthrene, option A.
The 9,10 bond of phenanthrene has much more double-bond character than any other bond, because both its carbons belong to only one of the three rings and breaking it leaves two intact benzene rings. Br2 in CCl4 therefore adds across it like an alkene (via a bromonium ion) rather than substituting on a benzene ring; no Lewis acid such as Fe or AlCl3 is present, so ring substitution (B and D, at C-3 and C-2) is not expected.
9-Bromophenanthrene (C) is what the dibromide gives on heating, by loss of HBr, and is the product when the reaction is run hot; the stem names no heating, so the addition product is the answer the key takes.
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