Princeton Review Page 792

[UIC]

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I get how the halonium ion forms and that the second bromine attacks from under because the halonium ion limits on the same face. So how come that doesn't apply when its in an aqueous solution? Instead the OH and the CL are on the same side. Shouldnt they be on opposite side?

 

[PiBond]

I get how the halonium ion forms and that the second bromine attacks from under because the halonium ion limits on the same face. So how come that doesn't apply when its in an aqueous solution? Instead the OH and the CL are on the same side. Shouldnt they be on opposite side?

I don't have the book in front of me, but you're saying they're cis? if this is a halohydrin formation then the H20 should make an anti attack and result in a trans configuration

They may not be showing any stereochemistry if there were no chiral centers formed. Another possibility is that because sigma bonds rotate the product will assume its most stable configuration (being trans) and the test writers just aren't showing it.

dont worry about it too much. it sounds like you know the mechanism well enough for the mcat

 

[Danlee07]

halohydration is anti and finds most stable centers. it's not markovnikov so maybe the writers aren't showing the stereochemistry (wedged, dashed, normal)

the two sigma bonds that are formed react to the OH and the X so it's pretty much impossible that it won't rotate to form the most stable compound

 

[lesset01]

I'm pretty sure halohydrin formation is MarkoVnikov, only hydroboration-oxidation is not Markovnikov for alkene rxns.

As far as your question goes, I agree 100% with PiBond. 🙂