[lamhfhenry]

The reaction of 2-bromobut-2-ene and HBr will produce 2,2-dibromobutane as the major product under Mark's rule.  But actually Br is an electron-withdrawing group, which will draw electron from the carbocation formed and make the carbocation less stable. So why does the Mark's rule also valid in this situation?


CH3CBr=CHCH3 + HBr    CH3CBr2CH2CH3

why not

CH3CBr=CHCH3 + HBr    CH3CHBrCHBrCH3 which involves more stable carbocation?

[macman104]

Well...unless I'm missing something, I have to say I agree with you.  I can't seem to see anything wrong with your reasoning.  The only thing that I might think is that it would have something to do with neighboring group participation.  The bromine might form an intermediate bromonium ion, but that is a stretch I think.

Good question!

[JGK]

I thought Markovnikov's rule applied to the hydrogens.

In your example.  - CBR=CH-, the carbon in the carbon bromine bond may exhihbit slight positivity attracting the BR- ion, while the carbon in the carbon hydrogen bond may  exhihbit slight negativity (inductive effect for the CH3 group) attracting the H+. Consequently,  Markovnikov's rule is applicable

[azmanam]

Maybe it's best to think of them like halogens in electrophilic aromatic substitution (op, I know you probably haven't gotten there yet, thus this wont help you.  But keep it in the back of your mind until you get to the aromatics chapter next semester...)

Halogens are electron withdrawing substituents on the benzene ring, but still give ortho/para products.  The resonance from the halogen stabilizes buildup of positive charge when the alkene reacts.  The halide-substituted alkene is slower to react than unsubstituted alkenes (the electron withdrawing ability of the halogen DOES decrease the nucleophilicity of the alkene), but when it reacts, the positive charge on the carbon bearing the bromine atom is stabilized through resonance.

http://www.cem.msu.edu/~reusch/VirtTxtJml/addyne1.htm#add1bb
http://www.cem.msu.edu/~reusch/VirtTxtJml/benzrx1.htm#benz3

[macman104]

Ah, you are right azmanam, I was only thinking in terms of primary, secondary and tertiary, I didn't think about the halogen stabilizing the carbocation.