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Organic Chemistry II · Problem type
1,2 or 1,4 product?
Use this for “HBr adds to 1,3-butadiene at −80 °C. Is the main product the 1,2 or the 1,4 adduct?”
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Is the main product the 1,2 or the 1,4 adduct?
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Cold and short favors the faster product; warm and long lets the mixture reach the more stable one.
Assumptions
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H⁺ adds to an end carbon, giving an allylic cation whose positive charge is shared by C2 and C4; the bromide can bond at either.
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Bonding at C2, next to where H⁺ added, has the lower barrier, so the 1,2 product forms faster.
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The 1,4 product has the more substituted C=C in the middle, so it is more stable; when warm, the adducts interconvert and that one wins.
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