Discussion
The major product formed in the following reaction is :
- A.
- B.
- C.
- D.
Answer: A
The product is the oxygen-bridged adduct in A, a methyl-substituted 1,4-dihydro-1,4-epoxynaphthalene.
Sodium amide in liquid ammonia removes H-Br from the p-bromotoluene (the proton ortho to Br goes with Br), making a benzyne, 4-methylbenzyne, by elimination. Benzyne is a very strained, reactive dienophile; furan, a diene, traps it in a Diels-Alder reaction. The two new bonds join the benzyne carbons to C-2 and C-5 of furan, so the oxygen becomes a one-atom bridge and the former C-3=C-4 of furan remains as a C=C in the new six-membered ring, exactly as in A.
B has no C=C in the bridged ring, which would need two extra hydrogens. C and D alter the benzene ring (it is no longer aromatic), which the reaction cannot do.
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