(picture) reaction between Br(sub)2 and ethylene?

[devenpatel2O]

Hey everyone. Having some trouble understanding why this reaction mechanism would work this way and not by another mechanism. I've heard from a UA that molecules with double bonds generally act as electrophiles. But in this mechanism, the double bond in ethylene acts like a nucleophile. Electrons leave the C-C double bond and go to the Bromine molecule. In the end the carbons in the "ethylane" wouldn't even have an octet anymore. Shouldn't the electrons move from the bromine molecule to the ethylene (the opposite way)? Furthermore, the pi bond between the carbons is vulnerable to attack from the bromine molecule...so why would the pi bond attack the bromine, then? Attached is a picture of the mechanism.

Thanks everyone. And sorry if it seems like I'm posting a lot tonight. I'm working with a few others and we are collectively stuck :/

[Christine Pham 1K]

It may have something to do with molecular orbitals. The animations from the link might help you visualize the reaction better.

http://www.chemtube3d.com/Electrophilic ... omine.html


Also, in our course reader (page 79), it says that the neutral Br2 is polarized, with the Br^(delta+) reacting as an electrophile.

[Chem_Mod]

Hi Deven, a double bond can act as a nucleophile OR an electrophile depending on the situation.

In this case, the driving force for the pi electrons attacking the Br2 lie in that Br- is a good leaving group. Additionally, the intermediate is stabilized by the bromine atom, as you can see in this image:

http://upload.wikimedia.org/wikipedia/c ... eletal.png

Through this "bromine bridge" arrangement, the bromine atom bears MOST of the positive charge. This is more stable than having positive charge on carbon (unstable carbocation) because bromine's large electron cloud is flexible. Also in this way, both carbons DO get to have an octet! After we have created this intermediate, the Br- floating around is now a good nucleophile due to its lone pairs, and attacks the intermediate to yield the dibromo-alkane.

[devenpatel2O]

That makes more sense now...it's a two step reaction. Maybe an electrophilic addition reaction? Is there any way to know whether a double bond will act as a nucleophile or an electrophile other than just past experience? Maybe by looking at the lewis structures?

[devenpatel2O]

Hey Christine, do you think that a Br(sub)2 molecule could be polarized to begin with? Dipole moments will cancel out in both directions...Br has the same electronegativity as Br, right?

[Chem_Mod]

Yes, this is an electrophilic addition since one of the bromine atoms in Br2 acts as an electrophile. Even though Br2 by itself would not be polarized on average, its large size is always causing fluctuations in the electron density. So you can imagine that at one brief moment or another, it COULD be polarized. And then, in this moment, the alkene comes in, and dictates a favorable reaction, therefore polarizing the Br2 even more until it splits apart into positive and negative.

The only electrophilic additions you would have to know for this class are Addition of hydrogen halide (HX) and Addition of halogen (X2).

[Christine Pham 1K]

So in general polarizable neutral molecules like Br2 can also act as electrophiles...

[Justin Le 2I]

Double bonds are usually nucleophiles because their pi bonds are electron rich.