[souro10]

Is this mechanism correct? Does such a Bromine migration really occur?
Could there be specific examples?

If Bromine does not migrate, why doesn't it?
Is the case not exactly similar to addition of Br-Cl to the corresponding alkene?

[Dan]

I would also predict the outcome you describe (1-bromo-2-chloro-2-methylpropane), but I am unaware of any experimental evidence.

The intermediate bromonium is indeed the same as that generated in the treatment of 2-methylpropene with BrCl - and the major product of that reaction is 1-bromo-2-chloro-2-methylpropane.

[souro10]

But I have read in a book, in a different context, when Bromonium ion executes NGP, the bond with which it donates lone-pair is weaker than the bond with the carbon atom to which it was attached. In that case, Bromine shouldn't migrate, should it?

I think experimental examples will clear up the confusion. But I couldn't find any.

[Dan]

But I have read in a book, in a different context, when Bromonium ion executes NGP, the bond with which it donates lone-pair is weaker than the bond with the carbon atom to which it was attached.

Can you post the example cited in the book?

If a bromonium is formed, then I would expect chloride to break the weakest C-Br bond. Depending on the substrate, this may or may not be the bond involving the C to which the leaving group was originally attached. If the mechanism involves a bromonium then there is no way for the Br to remember which C it was attached to.

[souro10]

Actually, the book adopts dotted bonds to show weaker bonds. Now, the book is March's Advanced Organic Chemistry, 6th edition ( Smith & March ) Page number : 446.

They are showing the basic steps of any N.G.P. mechanism on a 2-carbon substrate, with 4 "R" groups , a "Z" group which shows N.G.P. and " X" , the leaving group. After X was lost, the new bond that "Z" forms with the supposedly non-classical carbocation is shown in dotted lines, while the original C-Z bond is shown in normal line.

That made this question crop up in my mind.

[orgopete]

But I have read in a book, in a different context, when Bromonium ion executes NGP, the bond with which it donates lone-pair is weaker than the bond with the carbon atom to which it was attached. In that case, Bromine shouldn't migrate, should it?

I think experimental examples will clear up the confusion. But I couldn't find any.

I am in agreement with this. However, that does not describe the example given. In acid, loss of water can generate a hypothetical carbocation adjacent to a tertiary bromide. Donation of electrons from the bromine will give the indicated bromonium ion. The "bonds" of the bromonium ion will not be equal. Tertiary carbons are more electron rich resulting in a weaker C-Br and more easily broken bond. If it were the stronger bond, then it would not break and no migration would have occurred. (This similar to tertiary bonds being weaker and react faster under SN1 conditions.)

This is the essence of the similar epoxide opening question. I was arguing that if the reaction were driven by nucleophilic attack, then attack should occur on the least substituted carbon. If it does not occur there, then bond breakage must be playing a greater role in the reaction. Again, when isobutylene oxide were reacted, apparently bond breakage must have become the greater factor.

I view these two examples as similar in nature. I don't necessarily consider chloride to be a strong driving force for attack per se. The charges are balanced, but one also needs to consider bond breakage as part of an analysis. Here I like using examples to help me judge how great one factor might be compared with another.

If you are reading between the lines, then you might consider that in SN1 reactions bond cleavage occurs before bond formation and in SN2 reactions attack occurs before blond cleavage, but in actual examples, reactions occur between these two extremes. Our job is to recognize how that might be true. Just an opinion.

[souro10]

But I have read in a book, in a different context, when Bromonium ion executes NGP, the bond with which it donates lone-pair is weaker than the bond with the carbon atom to which it was attached. In that case, Bromine shouldn't migrate, should it?

I think experimental examples will clear up the confusion. But I couldn't find any.

I am in agreement with this. However, that does not describe the example given. In acid, loss of water can generate a hypothetical carbocation adjacent to a tertiary bromide. Donation of electrons from the bromine will give the indicated bromonium ion. The "bonds" of the bromonium ion will not be equal. Tertiary carbons are more electron rich resulting in a weaker C-Br and more easily broken bond. If it were the stronger bond, then it would not break and no migration would have occurred. (This similar to tertiary bonds being weaker and react faster under SN1 conditions.)

This is the essence of the similar epoxide opening question. I was arguing that if the reaction were driven by nucleophilic attack, then attack should occur on the least substituted carbon. If it does not occur there, then bond breakage must be playing a greater role in the reaction. Again, when isobutylene oxide were reacted, apparently bond breakage must have become the greater factor.

I view these two examples as similar in nature. I don't necessarily consider chloride to be a strong driving force for attack per se. The charges are balanced, but one also needs to consider bond breakage as part of an analysis. Here I like using examples to help me judge how great one factor might be compared with another.

If you are reading between the lines, then you might consider that in SN1 reactions bond cleavage occurs before bond formation and in SN2 reactions attack occurs before blond cleavage, but in actual examples, reactions occur between these two extremes. Our job is to recognize how that might be true. Just an opinion.

But nucleophile definitely plays a key role. Consider the acid-catalyzed ring of opening of Propylene oxide in water. Water attacks at hindered site; Chloride attacks a less-hindered site- substrate same, solvent- both protic. Then?

Edit: Orgopete, let us continue our epoxide discussion in that thread.

About this example, I am surprised why we cannot find specific examples of such Bromine migration!
I am also not conclusive yet about whether the situation is same as the Bromonium-ion during Br-Cl addition, or not. As I said, examples/data is the best way to clear this confusion.

I have found two articles which show direct experimental evidence that the product of addition of Br-Cl or I-Cl to propene produces products with Chlorine at 2-position, rather than one- which is quite different from the reaction of epoxides with HCl.

http://www.researchgate.net/publication/232746411_The_Reaction_of_Propene_with_Iodine_Bromide