[co_life]

Hi all,
So I'm trying to do the mechanism for this reaction:
CH3-CH2-CH=CH2 + Br2 <----(hv, high pressure)----> 

Watch out, this is not a simple Markovnikov mechanism (notice high pressure and hv...)

Anyone has an idea?

[nj_bartel]

hv would homolyze the Br₂ into two Br radicals.

 http://www.cem.msu.edu/~reusch/VirtualText/addene2.htm

I'm pretty sure the high pressure only serves to make the reaction more favorable.

[macman104]

Allylic bromination.

[co_life]

The teacher said something about resonance structures...
What if we consider that the 3rd carbon is (the first being the one with the double bond) is:
- a secondary carbon
- stabilized by resonance

Also, the Br2 is selective, which offers him even more stability...

Could the Br (1 of them or both) attach to the 3rd carbon?

[co_life]

Also, this is not allylic bromination as there is no NBS...
The only reagents are those mentioned above.

[macman104]

What you just described is exactly allylic bromination.  This is a radical mechanism, and you form the radical carbon, with the secondary radical being more stable than a primary radical.

[AWK]

This is much complex bromination, but some data are missed - ie concentration, phase, temperature.

Always a mixture of products are observed - 1,2 bromination and allylic bromination (even vinyl bromination at some extent)
http://iopscience.iop.org/0036-021X/42/9/R01/pdf?ejredirect=.iopscience.

Concerming NBS bromination - this is nearly ideal reagent for this purpose, but it does mean other method of allylic bromination can exist.

[co_life]

You were wrong.
It was indeed Allylic bromination, sorry about that.
Now my question is:
Do i make an allylic bromination EVERYTIME I see an alkene (propene and longer) and a Br2 or Cl2? Or must there be a certain catalyst... Does hv suffice to make an allylic bromination instead of the Marakovnikov mechanism... do we need heat, or pressure or more?

Thanks

[nj_bartel]

Ah, I'd ask your professor what he wants.  For my class, we just wrote NBS/hv in DCM I think it was.  I believe a lot of the allylic bromination reactions are run with reagents that are gaseous at STP, which might be why pressure was added.  Heat might be there to decrease the the energy of the light needed for homolyzation.

[intrepid_nerd]

Br₂  hv 
is the same (basically) as
NBS  hv 

NBS is used because it doesn't kill people every time it is transported, plus it's cheaper.
another positive of NBS for radical halogenation is that as the cycle propagates, NBS can sometimes recycle itself, thus only a little catalytic amount is needed.

obviously you'll get many many different results as you're working with radicals from the hv, but you'll get mostly allylic bromination.
i'm pretty sure the high pressure is arbitrary, but it could potentially increase the amount of dibromos you get because of (1/2)mv². but that could be looking too much into it.

hv is a sufficient catalyst in an equation.  your prof may require you write the solvent but probably not.

markovnikov has nothing to do with this.  even if this was X₂ H₂O you would be dealing with stereochemistry and not regiochemistry.  markovnikov only comes into play when your OVERALL reaction shows an H replacing an alkene (note overall because mercuroxidation isn't really markovnikov but overall shows markovnikov characteristics).