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This question already has an answer here:

General equation -

Alcohol - $\ce{ROH + HX -> RX + H2O} $

Ethers - $\ce{ROR + HX -> RX + ROH} $

I've been told that X = $\ce{Cl-}$ , $\ce{Br-}$ , $\ce{I-}$

Then I wonder ...

  • How Cl , Br and I nucleophile is going to Attack OH ?

All of the 3 are weaker nucleophile than OH as well as a better leaving group. So ...

  • How does it replace and substitute OH ?

Or

  • OR (with ethers)?
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marked as duplicate by Mithoron, Pritt Balagopal, Buttonwood, Jon Custer, jerepierre Jun 26 '17 at 15:45

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ Is it $\ce{OH-}$ that leaves? $\endgroup$ – Pritt Balagopal Jun 25 '17 at 4:38
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This reaction generally takes place in an environment where there are free $\ce{H3O+}$ ions. The oxygen atom in the alcohol or ether takes up $\ce{H+}$ from these ions. Now the leaving group is $\ce{H\bond{-}O+\bond{-}H}$ or $\ce{H\bond{-}O+\bond{-}R}$ which are much better leaving groups than $\ce{O\bond{-}H}$ or $\ce{O\bond{-}R}$. This makes the substitution of the halide ions easier.

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