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what is lucas test?

what is lucas test?

Grade:8

5 Answers

Digant Dubey
31 Points
9 years ago

Lucas reagent is a solution of anhydrous ZnCl2 & conc.HCl. This solution is used to classify alcohols of low molecular weight. This reaction is substitution in which a chloride replaces hydroxyl group. It is to differentiate between Primary, Secondary & tertiary alcohols.

                        ROH + HCl ------> RCl  + H2O

On reaction,

Primary-  no effect

Secondary- solution turns cloudy after 3-5 minutes

Tertiary- solution turns cloudy immediately

Vinayak Gupta
56 Points
9 years ago

HCl + Anhydrous ZnCl2 is known as Lucas Reagent. It is used to determine the DEGREE of an alcohol.

 

The alcohol is reacted with Lucas reagent, and give alkyl halide along with water. The reaction follows SN1 mechanism, and a carbocation intermediate is formed, the stability of which determines the rate of the reaction. The alkyl halide formed is insoluble, and its formation causes the solution to become turbid, ie. cloudy.

 

Now, Tertiary alcohols (which give 3 degree carbocations) and alcohols which give very stable carbocations on losing water after protonation (eg. resonance stabilised carbocations) react fastest and give immediate turbidity. Eg: t-butyl alcohol

 

Secondary alcohols react a bit slower as their carbocations (2 degree) are not as stable as above mentioned ones. They give turbidity after about 5-10 minutes. Eg.: isopropyl alcohol

 

Primary alcohols react very slowly (1 degree carbocations are very unstable), and their turbidity comes after more than 45 minutes, and this is why they are said to produce no turbidity in the reaction mixture. Eg: Ethanol

 

This way, degree of an alcohol can be found in most of the cases.

 

Those alcohols which form very unstable carbocations do not give positive lucas test. Eg: phenol, cyclopropanol, etc.


kajal goel
18 Points
9 years ago

lucas test involves lucas reagent which is ZnCl2+HX .it is particularly used in preparation of chloroalkanes.It can also be used as a test to distinguish between 1,2 and 3 degree alcohols.3 degree alcohols attains turbity i.e white ppt formation imediately when reated with lucas reagent.Whereas 2 degree alcohol attains turbidity after 5-10 minutes.1 degree alcohol attains turbidity on boiling.

tejaswini tiwale
18 Points
7 years ago

but why is anhydrous zncl2 is used in lucas reagent?

kartik
11 Points
5 years ago
why lucas reagent follows sn1 mechanism?....what if,l lucas reagent test follows sn2 mechanism???.....why it can’t follow sn2 mechanism????
 

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