SME
Organic Chemistry I · Problem type
Syn or anti addition?
Use this for “cis-But-2-ene reacts with Br₂. Is the product meso or a pair of enantiomers?”
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How do the two new groups add to the faces of the C=C?
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A reagent that adds in one step, or through a ring on one face, adds syn; a bridged ion is opened from the back, so the addition is anti.
Assumptions
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A C=C is flat, so a reagent can reach it from the top face or the bottom face.
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Syn adds both new groups to one face in one step; anti adds the second group from the face opposite the first.
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A flat cation can be reached from either face, so both arrangements form.
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On an alkyne, syn addition of H₂ gives the cis-alkene and anti addition the trans-alkene.
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