It is necessary to add zncl 2 to promote the reaction of hcl with some alcohols as this reaction is reversible to make it irreversible ZnCl2 is used.
ZnCl2 is used as Lewis acid catalyst. Since Cl- is weaker nucleophile than Br. The lewis acid ZnCl2 coordinate with oxygen of alcohol and proceed the reaction in forward direction.
HCl is less reactive with alcohols whereas the bond between H and Cl is stronger than HBr and HI.
Hence its reaction with alcohols need a catalyst ZnCl2, which helps in the breaking of bond between H and Cl.
Therefore, Luca's reagent is used in the reaction of HCl and alcohol.
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