Markovnikov Rxn

[2PacClone23]

I seem to be a little confused regarding this.


When it's a normal Markov reaction, what adds first, the H or the Br? (say it was HBr), and does it add to the most substituted carbon or LEAST?


What about if it's anti-markov (HBr in ROOR, Peroxide, or something). Which adds first and to which carbon? I keep getting mixed answers.


Also, if it's a radical (say Br2 in UV light), it's Anti right?


If someone could clear this up, I'd appreciate it. THANKS!

---

Members don't see this ad.

 

[slm266]

i think Markovnikov is when H is added to the least substituted (and the carbocation forms at the most substituted which is the most stable).

and i think for radical rxns, first initiation forms the radical which pulls off H from HBr. alkene + Br radical --> anti-markovnikov addition of Br to the alkene (at the least substituted carbon) with a radical now on the most substituted carbon.

see my ghetto paint drawing of arrow pushing, let me know if it's wrong.

 

[zellyen]

If you are talking about Alkenes

HBR- markovnikov, intermediate is carbocation. possible rearrangement with the Br adding to the MOST substituted (I THINK H adds first and creates the carbocation, then the Br- ion adds to it)

(this is not the case with radicals)

in anti Markovnikov, with peroxides and what not, the most stable radical is formed meaning that Br adds to the least substituted carbon first, then H adds to the most substituted carbon

and Br2 or Cl2 doesnt have a markovnikov orientation since 2 halogens add, but yes, they do add in an anti (in respect to the plane) fasion through a cyclic bromonium ion

i may be wrong but ive going with these definitions

 

[americanpierg]

I seem to be a little confused regarding this.


When it's a normal Markov reaction, what adds first, the H or the Br? (say it was HBr), and does it add to the most substituted carbon or LEAST?


What about if it's anti-markov (HBr in ROOR, Peroxide, or something). Which adds first and to which carbon? I keep getting mixed answers.


Also, if it's a radical (say Br2 in UV light), it's Anti right?


If someone could clear this up, I'd appreciate it. THANKS!

both markov and anti-markov adds to make the most stable radical/carbocation. the peroxide thats present breaks hbr into a Br radical. the br attaches to the ethene first, leaving the other, more stable, spot for the H. only works for HBr and not HX where X=Cl I F. NBS also works like this (when theres a benzyllic hydrogen, in a normal double bond br will take the place of a allylic hydrogen if theres no rearrangement), as well as BH3 or antyhing else in H2O2.

Br2 is in the same way, but this time light initates the propagation step and not peroxide. 2 Br's are added (i think) so u dont have to worry about regiochemistry