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For the syntheses of organohalides from alcohols, what would plausible reaction mechanisms be for the three following reactions?

$$ \ce{R-OH + NaBr + H2SO4 -> R-Br + NaHSO4 + H2O} $$

$$ \ce{R-OH + PCl5 -> R-Cl + POCl3 + HCl} $$

$$ \ce{R-OH ->[red P][Br2 or I2] R-X} $$

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  • $\begingroup$ Explain what exactly? Also, could you give a reference of where you found them? They look wierd to me. $\endgroup$ – M.A.R. Feb 1 '15 at 18:50
  • $\begingroup$ @MARamezani I found those reactions in organic chemistry textbook $\endgroup$ – jNerd Feb 1 '15 at 19:02
  • $\begingroup$ Oh, you edited your question and removed the typo. That was what was giving me the creeps. Still, this remains unanswered: What should be explained? $\endgroup$ – M.A.R. Feb 1 '15 at 19:04
  • $\begingroup$ @MARamezani Explain the mechanism involved in those reactions $\endgroup$ – jNerd Feb 1 '15 at 19:06
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    $\begingroup$ I think this counts as homework, not is literal homework. I'm letting it to play out. Hopefully it won't get closed. sigh $\endgroup$ – M.A.R. Feb 1 '15 at 19:25
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Nucleophilic substitutions, regardless whether proceeding via $S_N1$ or $S_N2$ require a decent leaving group. $\ce{HO-}$, however, is a lousy leaving group!

To overcome this, several methods exist:

  1. Perform the reaction under acidic conditions, where the alkanol is protonated. Your leaving group now is $\ce{H2O}$. In addition, the corresponding anion of sulfuric acid as a bad nucleophile.

  2. React the alkanol with an acid chloride to form an ester and replace the ester group with a halide. Consider $\ce{PCl5}$ to be an acid chloride of phosphoric acid.

  3. Form the acid halide in situ by reaction of phosphorous and iodine.

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