Problem 24 - A Complete Resource Book in Chemistry for JEE Main

Question

The compound which reacts fastest with Lucas reagent at room temperature is (a) butan - 1 -ol (b) butan-2-ol (c) 2 -methylpropan-2-ol (d) 2-methylpropan-1-ol

Step-by-Step Solution

Answer

2-methylpropan-2-ol reacts the fastest with Lucas reagent.

Step 1: Understanding Lucas Reagent Test

Lucas reagent is a mixture of concentrated hydrochloric acid and zinc chloride. It is used to classify alcohols of low molecular weight. The test is based on the reactivity of the alcohol with the reagent, producing an alkyl chloride. Tertiary alcohols react the fastest, turning cloudy instantly, secondary alcohols take a few minutes, and primary alcohols do not react or take a long time.

Step 2: Analyzing the Given Alcohols

The options given include four alcohols: butan-1-ol (primary), butan-2-ol (secondary), 2-methylpropan-2-ol (tertiary), and 2-methylpropan-1-ol (primary). According to the Lucas test, a tertiary alcohol will react the fastest.

Step 3: Identifying the Fastest Reacting Compound

Among the given options, 2-methylpropan-2-ol is a tertiary alcohol as the -OH group is attached to a carbon that is bonded to three other carbon atoms. Therefore, based on the Lucas test, it will react the fastest with Lucas reagent.

Key Concepts

Tertiary Alcohols Reactivity

Tertiary alcohols are the most reactive in the Lucas Reagent Test. In this test, tertiary alcohols react quickly, often producing an immediate cloudy solution. This reaction is indicative of alkyl chloride formation, where the alcohol group (-OH) is replaced by a chloride ion. Tertiary alcohols have their -OH group connected to a carbon atom that is bonded to three other carbon atoms. This configuration leads to a stable carbocation formation during the test, which enhances the alcohol's reactivity. The tertiary carbocation is more stable than secondary or primary carbocations, which explains why tertiary alcohols are the fastest to react. Understanding this reactivity trend helps in identifying the type of alcohol simply by observing the reaction time and extent.

Alcohol Classification

Alcohol classification is critical in determining reactivity in chemical reactions like the Lucas Reagent Test. Alcohols are categorized into primary, secondary, and tertiary, based on the number of alkyl groups attached to the carbon atom bearing the hydroxyl group (-OH).

• Primary Alcohols: Only one alkyl group directly attaches to the carbon with the -OH group. Examples include ethanol and butan-1-ol.
• Secondary Alcohols: Two alkyl groups are connected to the carbon with the -OH group, such as butan-2-ol.
• Tertiary Alcohols: These have three alkyl groups attached to the carbon with the -OH group, a prime example being 2-methylpropan-2-ol.

The classification affects the physical and chemical properties, including boiling points, solubility, and reactivity in various tests like Lucas' test.

Alkyl Chloride Formation

Alkyl chloride formation is a key result seen when alcohols react with the Lucas reagent. The primary goal of using the Lucas reagent, which consists of concentrated hydrochloric acid and zinc chloride, is to convert alcohols into alkyl chlorides. This is done through a substitution reaction where the hydroxyl group of the alcohol is replaced by a chlorine ion, forming an alkyl chloride. For example, in the reaction of tertiary alcohols, such as 2-methylpropan-2-ol, the process is swift. The chloride ion from the Lucas reagent quickly replaces the hydroxyl group, leading to the immediate formation of a cloudy suspension, which indicates the production of an alkyl chloride. Observing this transformation not only aids in understanding the reactivity of different alcohols but also showcases the impact of molecular structure on chemical behavior.