Problem 76 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following is more reactive than the rest towards a mixture of anhydrous. \(\mathrm{ZnCl}_{2}\) and concentrated \(\mathrm{HCl}\) ? (a) 2 -methylpropan-2-ol (b) methanol (c) butan-2-ol (d) butan-1-ol

Step-by-Step Solution

Answer

2-methylpropan-2-ol (a) is the most reactive towards Lucas reagent.

Step 1: Understanding Lucas Reagent

The mixture of anhydrous \(\text{ZnCl}_2\) and concentrated \(\text{HCl}\) is known as Lucas reagent. It is used to classify alcohols of low molecular weight into primary, secondary, and tertiary alcohols based on their reactivity toward the reagent.

Step 2: Reactivity Trend

The reactivity of alcohols toward Lucas reagent follows the order: tertiary alcohols are the most reactive, followed by secondary alcohols, and then primary alcohols are the least reactive. This is due to the stability of the carbocation intermediate formed during the reaction.

Step 3: Analyzing Options

Let's identify the type of alcohol for each option: (a) 2-methylpropan-2-ol is a tertiary alcohol. (b) Methanol is a primary alcohol. (c) Butan-2-ol is a secondary alcohol. (d) Butan-1-ol is a primary alcohol.

Step 4: Determine the Most Reactive

Since tertiary alcohols react fastest with Lucas reagent, 2-methylpropan-2-ol (option a), being a tertiary alcohol, is the most reactive toward the mixture of anhydrous \(\text{ZnCl}_2\) and concentrated \(\text{HCl}\).

Key Concepts

Reactivity of Alcohols

Alcohols are compounds that contain one or more hydroxyl (-OH) groups attached to a carbon atom. When we talk about the reactivity of alcohols, we often refer to how they behave in chemical reactions, especially those that involve the loss of the hydroxyl group.
One common reaction used to measure this is their response to Lucas reagent, a mixture of anhydrous zinc chloride (\(\text{ZnCl}_2\)) and concentrated hydrochloric acid (\(\text{HCl}\)).
Lucas reagent helps differentiate between types of alcohols based on how quickly they form a cloudy solution, indicating the formation of an alkyl chloride.

• Tertiary alcohols: These react immediately or within a few minutes, forming a cloudy mixture almost instantly.
• Secondary alcohols: Their reaction is slower, typically taking several minutes to turn cloudy.
• Primary alcohols: These take the longest and often require heating to react and show cloudiness.

The different reaction rates are important in determining the type or classification of an unknown alcohol sample. With our example compounds, 2-methylpropan-2-ol, being a tertiary alcohol, reacts the fastest, followed by butan-2-ol and then the primary alcohols like methanol and butan-1-ol.

Carbocation Stability

Carbocations are positively charged ions that form when the alcohol's hydroxyl group leaves, typically in reactions involving Lucas reagent.
The stability of these carbocations greatly influences the reactivity of alcohols.
Stability depends on the number of alkyl groups attached to the positively charged carbon atom.

• Tertiary carbocations: These are the most stable because the carbon atom is bonded to three other carbon groups. The groups help stabilize the positive charge through a phenomenon called hyperconjugation and the inductive effect.
• Secondary carbocations: These are less stable than tertiary ones, as they have only two carbon groups attached, providing less stabilization.
• Primary carbocations: These are the least stable due to having only one carbon group attached. Consequently, they form and react quite slowly.

The more stable the carbocation, the faster it forms, and thus, the faster the corresponding alcohol will react with the Lucas reagent. In our exercise, 2-methylpropan-2-ol forms the most stable tertiary carbocation, hence, it reacts the fastest.

Alcohol Classification

One of the key roles of Lucas reagent is in the classification of alcohols. Alcohols can be classified into primary, secondary, and tertiary based on the number of carbon-containing groups attached to the carbon attached to the hydroxyl group.

• Primary alcohols: These have the \(\text{OH}\) group attached to a carbon with only one carbon-containing group. Examples include methanol and butan-1-ol.
• Secondary alcohols: The hydroxyl group is attached to a carbon with two carbon-based groups. Butan-2-ol is a prime example.
• Tertiary alcohols: These have the \(\text{OH}\) group on a carbon atom bonded to three other carbon groups. Such as 2-methylpropan-2-ol in our example.

This classification is essential because it determines how quickly alcohols will react with certain reagents like those in acid-base and substitution reactions. This understanding is crucial not just for academic purposes but also in practical applications, like organic synthesis, where knowing the type of alcohol helps predict reaction pathways and products.