[Organishe]

Greetings,

A discussion came up about the mechanism of the addition of Br₂ to ethene. I contend that there is no carbocation formed, that the reactants go directly from alkene+Br-Br to the bromonium + Br^-.  This must be the case, because of the observed stereochemical outcome of the reaction, as well as the fact that carbocation rearrangements do not occur with this reaction.  The antagonist here contends that a carbocation is first formed, then "it does the triangle thingy". I am unsure of what other evidence to give to him, and its unfortunate because this discussion is on a forum, helping an ochem student to understand the reaction, and this other guys is teaching him incorrectly! 

So my questions to you:

1. Is it true that I am correct here, that there is NO discrete carbocation formed during this reaction?

2. Is there any other evidence, or does anyone know of a website that specifically addresses this topic? i've linked a few different webpages to him, yet to no avail. 

sites I have linked include: http://courses.cm.utexas.edu/bpagenkopf/ch310m/fall2004/Calendar/Nifty%20Stuff/13%20Alkene%20Reactions%20Bromination.htm
http://www.fsj.ualberta.ca/chimie/chem161/bALKENE2/sld032.htm
http://www.fsj.ualberta.ca/chimie/chem161/bALKENE2/sld033.htm

Frankly I feel that the above 3 sites explain this in a perfectly clear manner, but they have been hand-waved off as "unprofessional, and not to be trusted as accurate information sources"

Thanks for your time, guys and gals.

[plu]

The intermediate in this reaction is best described using both the bromonium ion intermediate and possible carbocation intermediates (a set of resonance structures, in fact).  However, in some cases, one of these two intermediate forms will predominate over the other.  In the case of primary and secondary carbons, the bromonium ion intermediate will dominate (the primary and secondary carbocations being unstable in relation).  On the other hand, in the case of tertiary and benzyllic carbons, carbocation resonance forms do play a substantial part in the description of the bromine-addition intermediate and allow for both S_N1 and S_N2 addition of the second bromine atom to these structures.

[movies]

You are right.  The bromonium ion is much lower in energy than the carbocation form.

This book agrees with you.  The explanation it gives is based on orbital interactions.  The interaction must be between the  sigma anti-bonding orbital of the bromine molecule and the pi bonding orbital of the alkene.  The bulk of the electron density in the pi bond is between the two atoms, not at one of the carbons, so that is the part that will interact with bromine.  His explanation would require the pi bond to break and from a carbanion on one carbon and a carbocation on the other carbon prior to attacking the bromine; essentially you would have to promote the electrons in the pi bonding orbital to the pi anti-bonding orbital.

You can also think of it in terms of resonance structures.  Your mechanism reacts from the lowest energy resonance structure, his would go through the higher energy, charge separated resonance structure.

[Yggdrasil]

The existence of a bromonium ion in the mechanism of the bromination of an alkene with Br₂ can be seen experimentally in three experiments:

1)  Ethylene can be brominated to yield 1,2-dibromoethane.  If this proceeded through a mechanism which included the formation of a carbocation, this would be a very unfavorable reaction because it involves the formation of a primary carbocation.

2)  The bromination of cyclohexene or some other cycloalkene yields only trans-brominated products.  If the reaction proceeded through a carbocation, the reaction would produce a mixture of trans- and cis-brominated products.  The diastereoselectivity of this reaction suggests a bridged intermediate.

3)  In brominations, no carbocation rearrangements are observed.  For example, the bromination of 3-methyl-1-butene yields 1,2-dibromo-3-methylbutane and not 1,3-dibromo-3-methybutane.

[AWK]

Bromonium cation was experimentally found in a crystal structure of:
bis(Adamantylideneadamantanebromonium) hydroxonium
tris(trifluoromethanesulfonate) dichloromethane solvate
JACS 116,2448 (1994)

[Organishe]

Thanks for the information guys and gals, much appreciated. However, he has now directed me to this link: http://www.chemguide.co.uk/mechanisms/eladd/symbr2.html , which he is trying to say proves he is right. But...BUT, this completely ignores the stereochemical outcome of the reaction!  As was mentioned by plu, there will be cases where a carbocation will be formed, but not in the majority of cases. So, with that I give up...this guy is beyond hope of enlightenment to the world of organic chemistry, and trying further to correct him is just going to be frustrating.

[movies]

That's ridiculous!!  That link proves that you are right!  It says explicitly that the direct bromonium formation is more accurate than the stepwise one!!!

The summary for that link: his mechanism is the one you learn if you don't really care and you can get away with it; your mechanism is the right one which you should learn if you aren't lazy.

Kudos to you, Organishe, for fighting the good fight.