Question

Lucas test of alcohols involves the following reaction:
$R-OH+conc.HCl\xrightarrow{anhydrous\,ZnC{{l}_{2}}}RCl+{{H}_{2}}O$
Select the correct statements.
A. ROH behaves as a base.
B. Lesser is the acidic character of alcohol, greater is its reactivity towards Lucas reagent.
C. Reactivity of $1{}^\circ ,2{}^\circ ,3{}^\circ$alcohol lies in the following sequence (for Lucas reaction)- $3{}^\circ >2{}^\circ >1{}^\circ$
D.$C{{H}_{3}}OH$ gives Lucas test most quickly

Answer 1

Lucas reagent is a reagent that contains HCl and anhydrous$ZnC{{l}_{2}}$ (zinc chloride) in the ratio 1:1. This reagent is used to distinguish between, primary, secondary and tertiary alcohols. Alcohols are soluble in Lucas reagent.

Complete answer:
We have been given an example of an alcohol reacting with Lucas reagent. As we know Lucas reagent is 1: 1 ratio of HCl : $ZnC{{l}_{2}}$, and dissolves alcohols. So the correct statements will be.
A. an alcohol, ROH behaves as a base- this is because a base has the tendency to react with an acid, and so does ROH reacts with HCl present in Lucas reagent. So, this statement is correct.
B. Lesser is the acidic character of alcohol, greater is its reactivity towards Lucas reagent- this is true, as bases react with Lucas reagent, but acidic compounds do not react with this reagent. For example phenol does not react with Lucas reagent due to its high acidity.
C. Reactivity of $1{}^\circ ,2{}^\circ ,3{}^\circ$alcohol lies in the following sequence (for Lucas reaction)- $3{}^\circ >2{}^\circ >1{}^\circ$- this statement is also correct as $3{}^\circ$alcohols when added with Lucas reagent gives turbidity easily, while $2{}^\circ$alcohols give turbidity after 5 minutes, and $1{}^\circ$alcohols after heating. This is due to the fact that $3{}^\circ$carbocation is more stable due to resonance than $1{}^\circ$or $2{}^\circ$

Hence, options A, B and C are correct statements.

Note:
The option D that states $C{{H}_{3}}OH$ gives Lucas test most quickly is incorrect as it
is a primary $1{}^\circ$alcohol which will give turbidity with Lucas reagent only after heating. Alcohols are soluble in Lucas reagent but their halides are immiscible in Lucas reagent.