Question

The alcohol, \(\mathrm{C}_{4} \mathrm{H}_{9} \mathrm{OH}\), when shaken with a mixture of anhydrous \(\mathrm{ZnCl}_{2}\) and concentrated HCl gave an immediate oil layer product. The alcohol is (a) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCH}_{2}-\mathrm{OH}\) (b) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{OH}\) (c) \(\mathrm{H}_{3} \mathrm{C}-\mathrm{CH}(\mathrm{OH}) \mathrm{CH}_{2} \mathrm{CH}_{3}\) (d) \(\mathrm{H}_{3} \mathrm{C}-\left(\mathrm{CH}_{2}\right)_{3}-\mathrm{OH}\)

Short answer

(b) \((\mathrm{CH}_3)_3 \mathrm{C}-\mathrm{OH}\) is the alcohol that reacts immediately.

Step-by-step solution

Understanding the Lucas Test

The given reaction is essentially a Lucas test, which is used to classify the alcohols into primary, secondary, and tertiary based on the formation rate of the alkyl halides. When an alcohol reacts with Lucas reagent (a mixture of anhydrous \(ZnCl_2\) and concentrated HCl), tertiary alcohols form turbidity (or an oil layer) almost immediately, secondary alcohols take a few minutes, and primary alcohols take much longer or do not react at all.

Identify Alcohol Type

Evaluate the list of given alcohols:1. \((\mathrm{CH}_3)_2 \mathrm{CHCH}_2-\mathrm{OH}\) is a secondary alcohol.2. \((\mathrm{CH}_3)_3 \mathrm{C}-\mathrm{OH}\) is a tertiary alcohol.3. \(\mathrm{H}_3 \mathrm{C}-\mathrm{CH}(\mathrm{OH}) \mathrm{CH}_2 \mathrm{CH}_3\) is a secondary alcohol.4. \(\mathrm{H}_3 \mathrm{C}-(\mathrm{CH}_2)_3-\mathrm{OH}\) is a primary alcohol.

Determine the Response

Since tertiary alcohols produce an oil layer almost immediately when mixed with Lucas reagent, we expect only the tertiary alcohol to respond this way. From the options provided, \((\mathrm{CH}_3)_3 \mathrm{C}-\mathrm{OH}\) fits this description as it is a tertiary alcohol.

Key concepts

Tertiary Alcohols

Tertiary alcohols are a class of alcohols wherein the hydroxyl group (\(-\mathrm{OH}\)) is bonded to a carbon atom that is itself bonded to three other carbons. This structure distinguishes tertiary alcohols from primary and secondary alcohols, which are bonded to one and two carbon atoms, respectively.

One of the most important chemical properties of tertiary alcohols is their rapid reaction with the Lucas reagent, an anhydrous solution of zinc chloride (\(\mathrm{ZnCl}_2\)) in concentrated hydrochloric acid (HCl). Upon reaction with this reagent, tertiary alcohols quickly form a turbid solution, displaying an oily layer.

The structure of tertiary alcohols leads to this quick reactivity. Due to steric hindrance, or the crowding of atoms around the carbon atom, the substitution reaction to form alkyl halides happens almost instantly, unlike in primary and secondary alcohols.

Alkyl Halides Formation

Alkyl halides are compounds in which an alkyl group (a carbon chain) is bonded to a halogen (such as Cl, Br, or I). The formation of alkyl halides from alcohols, especially tertiary ones, involves the cleavage of the carbon-oxygen bond (C-O) in the alcohol.

When tertiary alcohols react with the Lucas reagent, one of the key steps is the protonation of the \(-\mathrm{OH}\) group. This step facilitates its conversion into a better leaving group (water), allowing the carbon atom to bind with the chloride ion from the hydrochloric acid. As a consequence, an alkyl chloride is formed.

This process is indicative of the nucleophilic substitution type of reaction, which in chemistry, is characterized by the exchange of a leaving group for a nucleophile. Here, the chloride ion acts as the nucleophile, replacing the \(-\mathrm{OH}\) group.

ZnCl2 and HCl Reagent

The ZnCl2 and HCl mixture, known as the Lucas reagent, is fundamental in differentiating between primary, secondary, and tertiary alcohols.

ZnCl2 acts as a Lewis acid, accepting electron pairs, while HCl offers a source of protons (\(\mathrm{H}^+\)) and chloride ions (\(\mathrm{Cl}^-\)). When combined, they create a powerful reagent capable of protonating the hydroxyl group of an alcohol.

This protonation is the first step, allowing the \(-\mathrm{OH}\) group to leave as water. The resulting carbocation is intermediate and is highly reactive, allowing the chloride ion to attach.

The overall reaction rates differ among the types of alcohols due to their structure. Tertiary alcohols react almost instantly, forming an oily layer due to the efficient formation of alkyl halides. This instant turbidity forms the basis of the Lucas Test, making it a practical tool in organic chemistry for identifying the type of alcohol at hand.