I was taught that $\ce{PCl5}$ reacts with alcohols by $S_Ni$ mechanism as shown below: alcohol reaction with PCl5 Here it can be seen that $\ce{PCl5}$ exists as $\ce{[PCl4]+[PCl6]-}$ and so lone pair of the alcohol is donated to the electron deficient Phosphorus. Then nucleophilic Phosphorus deprotonates the good leaving +vely charged Oxygen. Now a Chlorine of the Oxygen-bonded Phosphorus attacks the alkyl group in a 4-centre cyclic transition state mechanism just like that in a reaction of an alcohol with $\ce{SOCl2}$. This yields an alkyl halide or two (if it was an ether) and $\ce{POCl3}$.

Once, I searched about this on internet and I couldn't find this mechanism but found written that $\ce{PCl3}$ and $\ce{PCl5}$ do $S_N2$ while $\ce{SOCl2}$ do $S_Ni$ (if there is no pyridine). However, the products were the same, leaving the stereochemistry.

I know the mechanism of $\ce{SOCl2}$ with alcohols and if I looked upon the above mechanism then I found no reason for why it should be wrong.

Also, the fact that $\ce{PCl5}$ is unstable and dissociates into ions to distribute its charge is true and can be validated by a simple Google search.

I am looking for some reasonable arguments to contradict or validate the above mechanism, if possible.

  • $\begingroup$ Have you seen chemistry.stackexchange.com/questions/33675/… $\endgroup$
    – Buck Thorn
    Sep 13, 2023 at 8:10
  • 1
    $\begingroup$ @BuckThorn Yeah... Completely from top to bottom but, it discusses totally on reaction with PCl3 and it is disjoint to the mechanism above. $\endgroup$
    – D13G
    Sep 13, 2023 at 9:12


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