why is pyridine used as a catalyst in the prepn of haloalkanes from alcohols by action of thionyl chloride?
because pyridine helps in creating nucleophile and the reaction of prepn of haloalkanes froms alcohols in nucleophilic substitution reaction.
The reaction of thionyl chloride with alcohol in presence / absence of pyridine is called as Darzen's reaction.
Here, the pyridine molecule acts as a catalyst which is used to neutralize the HCl -produced in the reaction.
CH3CH2OH + SOCl2 --(pyridine)-→ CH3CH2Cl + SO2 ↑+ HCl ↑
Here, pyridine acts as neucleophiles because, in the reaction a proton is added to the alcoholic oxygen due to the strong electronegative SOCl2 group,and this proton is removed by the pyridine and reforms the S=O and also the Cl- ion is removed from SOCl2 .
This Cl- ion reacts with electrophilic carbon of alcohol and displaces SO2 and releases another Cl- ion which makes the alkyl chloride.
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