[Kalibasa]

We're studying the reactions of alkenes with halogens, and while I understand the mechanism, I don't see why a bromonium ion would be more stable than a carbocation. Doesn't it have an increase in energy from ring strain?

Also, for something to undergo anti addition, does it have to add from opposite sides *every* time? Because the mechanism reminded me of the addition of HX to alkynes, where they say that you usually have a trans product, but that there are exceptions. Would this reaction count as anti addition, or do the exceptions make this not count as anti?

Thank you!

[jj74]

You should check the electronic configuration (hint: number of electrons) of all the atoms in the molecule; there’s a difference that makes the cyclic bromonium ion more stable than the classic open carbocation, even if it has of course a ring strain which is absent in the latter.

[song662309]

you are right 

[renge ishyo]

You also should consider that the bromium ion is really two carbocations stabilized by resonance while a normal carbocation is only stabilized by inductive effects. Here is a picture of the bromium ion from http://courses.chem.psu.edu/chem210/mol-gallery/bromonium/bromonium.html to show what I mean:

[Kalibasa]

Ah, that makes more sense- I might have figured it would be resonance And of course the carbocation in this case can't hyperconjugate because the bromine is too electronegative, so it can't stablilize much at all...

Thank you both!

[Molec bio]

Hi,

I have a related IB qn to Bromonium ion:

Which species react most readily with propane? A) Br2 B) .Br C) Br- D) Br+?

Although suggested answer is .Br free radical atom, but why not Br+ ion?

Wouldn't Br+ ion have a greater effective nuclear charge and less electronic repulsion, and therefore behave better as an electrophile?


Thank you for your thoughts!