[saN]

Here is what I came up with. Is this correct?

[ultrashogun]

Yeah thats it, only that the attack of the double bond on Br2 forms a 3 membered bromonium ion, but attack still happens where you said it would because the more substituted carbon carries more partil charge.

[Nick.Sparrow]

Everything is simple!
First, bromine is added on double bond, second nucleophilic substitution of the corresponding bromine anion by the hydroxygroup

[Nick.Sparrow]

that is!

[ultrashogun]

Everything is simple!
First, bromine is added on double bond, second nucleophilic substitution of the corresponding bromine anion by the hydroxygroup

Are you sure about the substitution? Because I know that water can attack the bromonium ion, shouldnt the hydroxy group also be able to do so?

[Nick.Sparrow]

Are you sure about the substitution?

Yes!
Nobody've seen it, but many believe...

[Dan]

that is!

I don't agree with this mechanism, I'm with ultrashogun (see below). I think that ring closure occurs by intramolecular attack of the hydroxyl on the bromonium ion. The intramolecular opening of the bromonium ion should be faster than the intermolecular opening with bromide. Unless you are in base and can deprotonate your alcohol I expect the Sn2 on the dibromoalkane will be very, very slow.

I realise it's just a typo, but you're missing a carbon in your starting material

[cetirizine]

that is!

I don't agree with this mechanism, I'm with ultrashogun (see below). I think that ring closure occurs by intramolecular attack of the hydroxyl on the bromonium ion. The intramolecular opening of the bromonium ion should be faster than the intermolecular opening with bromide. Unless you are in base and can deprotonate your alcohol I expect the Sn2 on the dibromoalkane will be very, very slow.

I realise it's just a typo, but you're missing a carbon in your starting material

I agree with you, I think you are right

[Nick.Sparrow]

that is!

I don't agree with this mechanism, I'm with ultrashogun (see below). I think that ring closure occurs by intramolecular attack of the hydroxyl on the bromonium ion. The intramolecular opening of the bromonium ion should be faster than the intermolecular opening with bromide. Unless you are in base and can deprotonate your alcohol I expect the Sn2 on the dibromoalkane will be very, very slow.

I realise it's just a typo, but you're missing a carbon in your starting material

Yea, you are right! There should be the bromonium intermediate (J. Org. Chem., Vol. 61, No. 3, 1996), but to be absolutely correct, it must be stabilized by some additional ligands.

[RBF]

A more fundamental problem with the original mechanism is that it has the bromine attacking as a nucleophile.  Look at the direction of the arrow.  This would result in a carbanion rather than a carbocation in the intermediate.  The bromonium ion intermediate postulated by Dan and others is correct.

[saN]

The mechanism Dan drew is with a bromonium ion intermediate and than the hydroxyl group attacks to form the ring.
 
What program are you using to draw these mechanisms? I do it by hand and scan them, which can be sloppy.

[ultrashogun]

The mechanism Dan drew is with a bromonium ion intermediate and than the hydroxyl group attacks to form the ring.
 
What program are you using to draw these mechanisms? I do it by hand and scan them, which can be a PITA.

I enjoyed the scanned picture, gives it more personality!

[Nick.Sparrow]

What program are you using to draw these mechanisms? I do it by hand and scan them, which can be sloppy.

I used ISIS drow, but the best is ChemDrow from ChemOffice packet, but hand drowing looks nice on the computer screen