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why is anhydrous zncl2 is used in lucas reagent?

why is anhydrous zncl2 is used in lucas reagent?

Grade:12

5 Answers

Akshay Amol Revankar
35 Points
10 years ago

HCl is less reactive with alcohols since the bond between H and Cl is a bit stronger than HBr and HI. Hence its reaction with alcohols need a catalyst ZnCl2, which helps in breaking the bond between H and Cl. Sometimes, as with primary alcohols, heating is required to achieve this along with ZnCl2.

Ranjan
11 Points
8 years ago
Why is anhydrous ZnCl2 used in reaction of alcohols with HCl?
The reaction of alcohol with HCl involves a nucleophilic substitution reaction, which results in the formation of chloroalkans. The Cl- is not a good nucleophile and in alcohol —OH- acts a poor leaving group. Reaction with HC1 is thus, carried out in presence of ZnCl2 (Lewis acid). ZnCl2 forms a complex with oxygen of alcohol and converts the      -OH to a much better leaving group thereby giving way to a rapid formation of chioroalkane.
Shubham Gupta
11 Points
7 years ago
So it starts like this.We have to replace -OH group from alcohol by Cl-. That is a type of nucleophilic substitution reaction. Cl- has to replace -OH group.But -OH is a poor leaving group due to the high electronegativity of oxygen & Cl- is a moderate ( not so good ) nucleophile . So, it is slightly difficult.So, we use Lucas Reagent ( ZnCl2 + HCl) . ZnCl2 is a lewis base due to the empty d orbitals of Zn. Oxygen of -OH forms coordinate bond with Zn or in other words complexes with Zinc . Oxygen acquires a positive charge & Zn acquires a negative charge.So , basically what happens is that a poor leaving group -OH has now been converted into a good leaving group as oxygen now has a positive charge (increased electronegativity).
Ankur Saha
56 Points
7 years ago
Reaction with HCL is thus, carried out in presence of ZnCl2 (Lewis acid). ... Lucas reagent is a solution of anhydrous zinc chloride (Lewis acid) in concentrated hydrochloric acid. It is used as a reagent to test alcohols and classify them in accordance to their reactivity.
Kushagra Madhukar
askIITians Faculty 628 Points
3 years ago
Dear student,
Please find the answer to the question.
 
The reaction of alcohol with HCl involves a nucleophilic substitution reaction, which results in the formation of chloroalkanes.
The Cl- is not a good nucleophile and in alcohol –OH- acts a poor leaving group. Reaction with HCl is thus, carried out in presence of ZnCl2 (Lewis acid).
ZnCl2 forms a complex with oxygen of alcohol and converts the –OH- to a much better leaving group thereby giving a way to the rapid formation of chloroalkane.
 
Thanks and regards,
Kushagra

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