[icho2013]

Can you help me this problem
Problem 2 : Draw the mechanism for this reaction ( it's ankyne's addition). I have drawn the mechanism for this reaction but i think it not true , can you help me. Thank you very much

[orgopete]

Question: Bromine is bimolecular. For the final product, where is the other bromine atom? What effect will that have on OH^- concentration? Will there be any OH^- present?

[Kran]

you should draw the complete bromonium ion (not only the ring part, with the other carbon)
you will see OH- attacks the secondary carbon, as it can afford more of the positive charge.

Edit: bromine solution is acid becous eof the following equilibrium

Br2 + H2O   HBr + HOBr

So OH- concentration is low. You should attack with H2O, making protonated enol, and then showing desprotonation before it tautomerizes

[icho2013]

So HOBr , is attack just like Br2 , make ring with Br and OH- attack

[Kran]

Yes. the idea here is that only Br2 can make the ring, but both br2 and water can open it. So as there are much more water molecules than Br2 molecules, haloidrin formation is favoured.

[orgopete]

Yes. the idea here is that only Br₂ can make the ring, but both Br₂ and water can open it. So as there are much more water molecules than Br₂ molecules, haloidrin formation is favoured.

You could think of it that way, but I prefer to think of water as being more basic. This equilibrium shifts to the right. If water can pull a proton from HBr, I could expect it to react with a bromonium ion as well.

     HBr + H₂O  H₃O⁺ + Br^-