Why is thionyl chloride ($\ce{SOCl2}$) preferred for preparing alkyl chlorides from alcohols, as opposed to say $\ce{PCl5}$ or $\ce{PCl3}$ ?


Thionyl chloride is preferred for preparing alkyl chlorides from alcohols because the by-products formed in the reaction are $\ce{SO2}$ and $\ce{HCl}$ which are in gaseous form and escape into the atmosphere leaving behind pure alkyl chlorides.

$$\ce{CH3CH2-OH + SOCl2 → CH3CH2-Cl + SO2 ↑ + HCl ↑}$$

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    $\begingroup$ A comment about the possible side-products when using phosphorus pentachloride would be useful too. $\endgroup$ – vapid Oct 12 '16 at 7:39
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    $\begingroup$ add pyridine to your reaction vessel and see the jump in yield! $\endgroup$ – Amritansh Singhal Dec 26 '16 at 19:30

Preparation of alkyl chlorides from alcohols by thionyl chloride is preferred over other methods mentioned because of the following reasons :

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

$$\ce{3R-OH + PCl3 → 3R-Cl + H3PO3}$$

$$\ce{R-OH + SOCl2 → R-Cl + SO2 ↑ + HCl ↑}$$

  • Both the byproducts formed in the last reaction i.e., $\ce{SO2}$ and $\ce{HCl}$ are gases and can easily escape out into the atmosphere, so the equilibrium shifts more to the right side and hence, more products are obtained.

  • Moreover as both the gases easily escape out leaving behind pure alkyl halide in the last reaction, the purification process is minimized and thus the alkyl halide obtained can be easily purified.


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