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I have read that hypohalous acids react with alkene to give halohydrin. For example the below reaction of hypochlorous acid with 2-methyl-2-butene which I found in Wikipedia.

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Now my question is: Since hypochlorous acid is an acid it should give $\ce{H+}$ and $\ce{OCl-}$ ions which it does according to Wikipedia (although it is a weak acid), so shouldn't it add as $\ce{H+}$ and $\ce{OCl-}$ like other hydrogen halides add to alkene? Why would it add as $\ce{HO-}$ and $\ce{Cl+}$?

chemhead
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1 Answers1

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Let us deconstruct our reagent:

Hypochlorous acids, more commonly, 'bleach', is an oxidant. Additionally, it is not a great acid, pKa 7.40.

And let us deconstruct our product:

Additionally the chloride is on the least substituted carbon (anti-Markovnikoff) without any indication of added radical source.

In this context it is unwise to view this as an analogous to HCl addition.

Your product could be readily explained by an epoxidation-type reaction followed by a nucleophilic substitution with residual chloride ion. The regioselectivity of Cl on the less substituted carbon would seem to support that, as well as the absence of a O-Cl bond in the product.

Lighthart
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  • Cyclic chlorinium ion would have OH on the secondary carbon, not the teritiarry – Lighthart Mar 25 '16 at 21:17
  • What I am saying is correct as I read it in the "Solomons and Fryhle Organic chemistry". Please follow this link to the snapshot of the concept from the book I mentioned. – chemhead Mar 27 '16 at 20:55
  • I agree that since HOCl is a weak acid it is not correct to compare it with addition of HCl but I am not sure whether it will follow epoxide-type reaction but this link which I found in the duplicate of my question seems to answer my question. Look half way down this link to see the explanation. The reaction goes through a cyclic chlronium ion and then attack by OH- ion, similar to the addition of Cl2 in H2O. – chemhead Mar 27 '16 at 20:58
  • or you can also say that Cl+ ion adds as an electrophile and then OH- ion attacks the more substituted carbon which bears more stable carbocation(Markovnikov's rule), both ways you get the same result. – chemhead Mar 27 '16 at 21:04
  • The link you provided is tangentailly related. 2) Markovnikoff's rule says nothing about carbocations, only substitution. We infer it is caused by cation stability. 3) You comment about ~H attacking the more substituted is appropriate, but in the absence of any indication of alkene formation as a likely byproduct, we can't be sure those mechanisms are appropriate. 4) You seem to have some good skills at getting more info, nice.
    • – Lighthart Mar 27 '16 at 23:54