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Halohydrin Formation – Alkene Reaction Mechanism

November 20, 2013 By Leah4sci 29 Comments

Reaction Overview: The Halohydrin formation reaction involves breaking a pi bond and creating a halohydrin in its place. Halo = halogen and Hydrin = OH. This reaction takes place in water and yields an anti-addition reaction which follows Markovnikov's rule.

halohydrin formation reaction

Reaction Summary

  • Nucleophilic pi bond attacks halogen molecule, pi bond breaks in the process
  • Halogen lone pair retaliates – attacks carbon forming a halogen bridge
  • Second halogen breaks away as negative halide – immediately surrounded by solvent molecules
  • While halide ‘caged' by water, another water molecule acts as nucleopile and breaks halide bridge
  • If asymmetric, water attacks at the more substituted carbon
  • Yet another water molecule removes the excess H-atom from the attacking H2O
  • Product is a halohydrin with halogen on one carbon and OH on its neighbor

Mechanism Overview And Explanation

halohydrin formation reaction mechanism

Asymmetric halohydrins have the OH on the more substituted carbon and the halogen on the less substituted carbon. This follows Markovnikov's rule due to water opening the halogen bridge at the more partially positive carbon atom.

Markovnikovs rule in halohydrin formation

(Watch my Halohydrin Formation Video to see the reaction details come to life)

Key Reaction Notes:

  • Halohydrin comes from Halo = Halogen and Hydrin = OH
  • Despite lack of carbocation intermediate, this reaction follows Markovnikov's rule
  • The more substituted bridged carbon holds more partial positive charge
  • Water attacks from the ‘anti' or opposite side of the halogen bridge
  • If carried out in an inert solvent halogenation occurs forming a vicinal dihalide

What Exactly Is Happening In This Reaction?

The halohydrin formation reaction is a viable starting point for many multi-step synthesis reactions. Placing a reactive group along with a leaving group on the same molecule allows for chain elongation, oxidation, epoxidation and more.

Understanding The Reactants:

bromonium bridge in halohydrin formation

We'll use Br2 for our explanation but realize that this can also take place with Cl2. At first glance, the product of this reaction is not very obvious. In fact, this reaction starts out just like the alkene halogenation mechanism. The nucleophilic pi electrons attack a neutral bromine molecule. The attacked bromine atom retaliates and attacks the carbon with one of its lone electron pairs. The second Br breaks away as a negative bromide in solution.

The two former sp2 (pi bound) carbon atoms are now each attached to bromine with a sigma bond forming a 3-atom ring. The bromonium bridge has an unstable positive charge. To compensate, the bromonium ion pulls on the bonds that connect it to carbon in the hopes of balancing its positive charge with the extra electron density. This leaves the bromine slightly ‘less' positive but also places a partial positive charge on the 2 bound carbon atoms.

The Caged Bromide Ion:

caged bromine in halohydrin formationThe negative bromide in solution would like to attack the partially positive carbon atom, but it can't. Given that this reaction does NOT take place in an inert solvent, we must account for the water's interference.

Water is a highly polar molecule with a partially negative oxygen atom and 2 partially positive hydrogen atoms. The partially positive hydrogen atoms surround and ‘cage' the bromide rendering it incapable of attack.

With the bromide ion out of the way, the next best nucleophile in solution swoops in to attack the partially positive carbon atoms. One of the water molecules will use its nucleophilic lone electron pairs to attack the partially positive carbon.

Formation of the Halohydrin

Water must approach from the opposite face of the molecule given that the bromonium bridge will stand in the way of attack from the same face of the molecule. This is the anti-addition.

anti attack of water in halohydrin formationWhen water attacks carbon, carbon lets go of the electrons binding it to bromine. (bromine wanted them anyway) thus breaking the bromonium bridge.

The attacking oxygen atom still has 2 bound hydrogen atoms for a total of 3 bonds and one lone pair. This results in an oxonium, or an oxygen atom with a positive formal charge.

Oxygen, an electronegative atom, is not thrilled with the positive charge and therefore pulls on the electrons binding it to hydrogen. This results in a slightly less positive oxygen, and a partially positive hydrogen atom.

Another water molecule in solution reaches out with one of its nucleophilic lone pairs to grab the partially positive hydrogen atom. The electrons break away and return to the oxygen giving it a second lone pair and neutral charge.

This resulting product is a halohydrin. If bromine you get a bromohydrin, if chlorine you'll get a chlorohydrin.

How Markovnikov's Rule Factors In:

Recognize that at no point in this explanation did I mention a carbocation. So why do we still follow Markovnikov's rule? The answer is in the partial charges. After all, Markovnikov's rule is all about creating the most stable carbocation intermediate. When the unstable bridged halogen pulls on its bonds to carbon, the pull will not be equal. If one of the bridged carbons is more substituted, it is more capable of holding a positive charge if a carbocation to form'. And so, even if there is no obvious carbocation forming, the fact that the more substituted carbon is capable of holding the charge, means that it will hold a greater amount of the partial positive charge.

Picture this scenario. You require an important task to be carried out and you have two willing participants. You know that one of your volunteers is more capable of completing the task. I assume you will assign this person the task in place of the other. Same goes for the carbocation.

Now when the nucleophilic water molecule comes to attack, it feels a stronger attraction towards the more substituted carbon. This of course is a direct result of its greater positive charge, and thus still follows Markovnikov's rule

See the Halohydrin Formation Reaction come to life along with practice examples in my tutorial video below:

Click for Halohydrin Formation - Organic Chemistry Tutorial Video
Halohydrin Formation – Organic Chemistry Tutorial Video

Ready to test your skills? Click HERE for my FREE Alkene Reactions Practice Quiz

Filed Under: Organic Chemistry Reaction Mechanisms

Comments

  1. Alex says

    February 12, 2019 at 11:31 am

    thx for easy breakdown of each reaction. really helps when a lot of orgo professors have no structure to their class or “how to study for orgo”. they just say to do a lot of problems.

    Reply
  2. Balqees Khader says

    February 8, 2019 at 2:23 pm

    You are the Goddess of Organic Chemistry. I No teacher explains it this simple.

    Reply
  3. Lim A says

    December 10, 2018 at 11:51 am

    Thank you Leah!

    Reply
  4. Gautham Raj P says

    November 15, 2018 at 6:59 pm

    The description is really helpful.No one can explain it in a short and sweet manner as you have did it.Can I know which carbon does the hydroxyl group attack after the formation of!three membered ring.Thanks a lot in advance.

    Reply
  5. ralphas says

    April 13, 2018 at 11:27 am

    why halohydrins is not a type of hydration reaction like oxymecuration and hydroboration..since its reacting in the presence of h2o

    Reply
  6. Elif says

    April 6, 2018 at 4:26 pm

    Couldnt understand teacher but you r the best! I love your phrase

    Reply
  7. Vijay says

    December 15, 2017 at 1:32 am

    could you plz explain the mechanism of Isoflurane synthesis from Tri-Fluro-Ethanol? Isoflurane is an inhalation anesthetic.

    Reply
  8. izzy_orgo says

    December 5, 2017 at 3:14 pm

    How do you know whether the OH or the Br ends up on the most substituted carbon?

    Reply
  9. Pranav says

    November 4, 2017 at 11:35 am

    Can you explain addition of HOX (X-halogen) on alkene

    Reply
  10. Dovlo Michael says

    October 24, 2017 at 4:01 pm

    You are awesome. Exceptional explanation. I found myself in the world of water and bromine will reading.

    Reply
  11. Meena says

    October 20, 2017 at 7:39 am

    Amazing explanation…your work is really worth appreciating.

    Reply
  12. Frankie says

    April 20, 2017 at 7:43 pm

    what exactly do you mean by a halide bridge?

    Reply
  13. Monica Limas says

    October 16, 2016 at 11:33 pm

    So there aren’t any halide or alkyl shifts in this reaction mechanism?

    Reply
  14. reb says

    October 7, 2016 at 8:47 pm

    I follow all of your lesson and its very helpful. Anw, some books says H from H3O reacts to Br-. And form HBr, it is possible?

    Reply
  15. Giselle says

    April 17, 2016 at 4:52 pm

    wow you are so awesome! no one actually takes the time to go into detail and explain it in a simple way. Really, Thanks!

    Reply
    • Leah4sci says

      April 20, 2016 at 5:51 pm

      Thank you Giselle, glad to help

      Reply
  16. Lakshita says

    April 17, 2016 at 4:33 am

    Thank you so much….I got an answer to my query… Your explanation is amazing Thanks once again

    Reply
  17. Lakshita says

    April 16, 2016 at 3:42 am

    Why and how the formation of 3 membered ring takes place?

    Reply
    • Leah4sci says

      April 16, 2016 at 11:25 am

      Lakshita: watch the halogenation video first as it’s explained there

      Reply
  18. biancakdash says

    August 11, 2015 at 4:54 pm

    Hi Leah,
    Really great lesson, loved the description of what was happening. But could you possibly explain why the CH3 goes from having the dashed away bond, to the coming out thick bond when the H20 joins?
    Thanks in advance and great work! 🙂

    Reply
    • Leah4sci says

      August 11, 2015 at 9:26 pm

      The backside attack pushes on of the groups to the opposite direction. Dashes = back, gets pushed forward on a wedge

      Reply
  19. Yannis Karamitas says

    October 31, 2014 at 12:26 am

    Amazing detail and answered every single question I had about this mechanism. I was especially curious as to how this followed Markovnikov when it looked more like Anti-Mark and you hit the nail on the head with your description. Thank you so so so much 😀

    Reply
    • Leah4sci says

      October 31, 2014 at 7:48 pm

      Thank you so much for the feedback and I’m glad I was able to help you understand the reaction. Are you currently in Orgo 1?

      Reply
  20. Samantha says

    October 15, 2014 at 10:09 pm

    thank you so much!! Us O-chem students really appreciate it 🙂

    Reply
    • Leah4sci says

      October 16, 2014 at 9:06 pm

      You are very welcome

      Reply
  21. Angelina says

    February 27, 2014 at 4:48 am

    Thank you again for all this work done on the cheat sheets! WOW! Organized! What a new way to do Ochem…… 🙂 (my desk is a horror scene)

    Reply
    • Leah4sci says

      March 2, 2014 at 12:22 pm

      Thank you Angelina,
      I’m glad you’re finding it helpful. Do you have an exam coming up? If so which topics?

      Reply
  22. raj muhammad says

    January 27, 2014 at 5:34 pm

    i realy enjoy the lesson i like you

    Reply
    • Leah4sci says

      January 28, 2014 at 12:03 am

      Thank you Raj

      Reply

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