Question

An organic compound (A) (molecular formula C₆H₁₂O₂) was hydrolysed with dilute H₂SO₄ to give a carboxylic acid (B) and alcohol (C). ‘C’ gives white turbidity immediately when treated with anhydrous ZnCl₂ and conc. HCl. The organic compound (A) is:
(A)


(B)


(C)


(D)

Answer 1

Hint: In the given question anhydrous ZnCl2 and concentrated HCl are given. Both of these together form the Lucas reagent. Now, lucas reagent gives precipitates at different times when reacted with primary, secondary and tertiary alcohols. Only one of the three gives an immediate precipitate with Lucas reagent.

Complete Step by Step Solution:
Anhydrous zinc chloride is dissolved in strong hydrochloric acid to form "Lucas' reagent." The classification of low molecular weight alcohols is done using this method. The chloride substitutes for a hydroxyl group in the process, which is a substitution. A transition from clear and colourless to muddy, signifying the development of a chloroalkane, indicates a positive test.

Alcohols can be distinguished into primary, secondary, and tertiary alcohols using the Lucas test. The three classes of alcohols' varying reactivity with hydrogen halides via an SN₁ reaction provide the basis for this:
ROH + HCl → RCl + H₂O.

Before the reaction with Lucas Reagent the compound A asked in the question undergoes hydrolysis. On the hydrolysis reaction of the given compounds, one will always be a carboxylic acid whereas the other three will be either primary, secondary or tertiary alcohol.

From the given options, when the compound given in option (A) undergoes hydrolysis it gives secondary alcohol. Similarly, the compound given in option (C) and (D) gives primary alcohol. These primary and secondary alcohols do not give immediate but delayed precipitation with Lucas reagent. Only option (B) gives tertiary alcohol.

Thus, the alcohol obtained from the compound given in option (B) gives immediate precipitate with Lucas reagent.
Hence, the correct answer is option (B)

Note: The reagent is a combination of concentrated HCl and ZnCl₂ in an equimolar ratio. The nucleophile Cl, which is present in excess, rapidly attacks the protonated alcohol, causing the H₂O group to depart and form a carbocation, which then yields the chloroalkane. Due to the limited solubility of the organic chloride in the aqueous mixture, tertiary alcohols react with Lucas reagent instantly as shown by the turbidity. Secondary alcohols respond in about five minutes (depending on their solubility). At room temperature, primary alcohols do not significantly react with the Lucas reagent.