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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Kalibasa on October 14, 2009, 11:51:29 PM

Title: Why is a bromonium ion more stable than a carbocation?
Post by: Kalibasa on October 14, 2009, 11:51:29 PM

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!

Title: Re: Why is a bromonium ion more stable than a carbocation?
Post by: jj74 on October 15, 2009, 02:45:20 AM

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.

Title: Re: Why is a bromonium ion more stable than a carbocation?
Post by: song662309 on October 15, 2009, 06:01:58 AM

you are right  ;D ;D ;D

Title: Re: Why is a bromonium ion more stable than a carbocation?
Post by: renge ishyo on October 15, 2009, 12:17:44 PM

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:

(http://courses.chem.psu.edu/chem210/mol-gallery/bromonium/bronium2.gif)

Title: Re: Why is a bromonium ion more stable than a carbocation?
Post by: Kalibasa on October 15, 2009, 05:55:02 PM

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!

Title: Re: Why is a bromonium ion more stable than a carbocation?
Post by: Molec bio on March 28, 2017, 10:44:42 PM

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!