The major product formed in the following reaction is: · ABC26OR0195
Subject: Organic Chemistry · Chapter: Reagents, Reaction Mechanisms & Named Reactions · Topic: Named Reactions · Exam: CSIR NET JUNE 2012 · Marks: 2 · Difficulty: Medium
The major product formed in the following reaction is: 
(a)
(b)
(c)
(d)
Answer
Answer: B ✓ checked by 4AB · confidence high
Key (b) confirmed. The explanation wrongly called (d) the enantiomer of (b); (d) is the regioisomer with OMe on C-3 and Br on C-2.
Explanation
Br₂ adds to the electron-rich enol-ether C=C of 3,4-dihydro-2H-pyran, giving a bromonium ion that opens to the oxocarbenium ion stabilised by the ring oxygen. Methanol (solvent, large excess) attacks the anomeric carbon C-2, anti to bromine, so Br ends up on C-3 and OMe on C-2 with a trans relationship: trans-3-bromo-2-methoxytetrahydropyran (b). (a) is the dibromide, (c) has the cis arrangement, and (d) has the regiochemistry reversed (OMe on C-3, Br on C-2). Answer: (b)
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