Problem 184 - A Complete Resource Book in Chemistry for JEE Main

Question

During dehydration of alcohols to alkenes by heating with concentration \(\mathrm{H}_{2} \mathrm{SO}_{4}\) the initiation step is (a) protonation of alcohol molecule (b) formation of carbocation (c) elimination of water (d) formation of an ester

Step-by-Step Solution

Answer

(a) Protonation of alcohol molecule

Step 1: Understand Protonation of Alcohol

In the dehydration of alcohols to alkenes, the first step involves the interaction of the alcohol with the acid (concentrated \( \mathrm{H}_{2} \mathrm{SO}_{4} \)). The alcohol molecule gets protonated by the acid, resulting in the formation of a better leaving group, water. This is because the hydroxyl group (\( \text{OH}^- \)) becomes \( \text{H}_2\text{O} \) when protonated, which can leave more easily.

Step 2: Compare the Given Options

Evaluate each option against the typical sequence of reactions: - (a) Protonation of alcohol molecule: This is the first interaction between the alcohol and the acid. - (b) Formation of carbocation: This happens after the alcohol loses water. - (c) Elimination of water: Follows protonation, water leaves. - (d) Formation of an ester: Not part of the typical dehydration sequence to alkenes.

Step 3: Select the Correct Initiation Step

Based on the typical reaction mechanism for dehydration, the initiation step in the presence of concentrated \( \mathrm{H}_{2} \mathrm{SO}_{4} \) is the protonation of the alcohol molecule. This protonation facilitates the formation of the carbocation in later steps.

Key Concepts

Protonation

In the dehydration process of alcohols to form alkenes, protonation is the key initial step. This reaction generally occurs using concentrated sulfuric acid \((\mathrm{H}_2\mathrm{SO}_4)\). During protonation, the hydroxyl group \((\text{OH}^-)\) of the alcohol undergoes a transformation. The acid provides a proton \((\text{H}^+)\) which attaches to the alcohol, converting it into water \((\text{H}_2\text{O})\). This transformation is significant because water is a much better leaving group than hydroxyl ions, facilitating its departure from the molecule.

This conversion sets the stage for the next steps in dehydration, making protonation a crucial component of the reaction process. It's important to note that this protonation step is essential for weakening the bond between the hydroxyl group and the carbon it's attached to, making it easier for the molecule to proceed to the subsequent stages.

Carbocation Formation

After protonation and the formation of water as a leaving group, the alcohol molecule undergoes carbocation formation. This step is crucial and occurs once the water molecule has left.

Here's how it happens:

• The loss of water from the alcohol molecule leaves behind a positively charged ion known as a carbocation.
• Carbocations are unstable due to their positive charge, making them highly reactive. This reactivity is what drives the continuation of the reaction process in dehydration.
• This instability is compensated by the rearrangement of atoms, often leading to more stable carbocations if possible.
• The structure and potential rearrangement of the carbocation determine the type of alkene that will be formed eventually.

Understanding carbocation formation helps in predicting the outcome of the reaction and the type of alkene that will be synthesized.

Elimination of Water

The elimination of water is a pivotal step in the dehydration of alcohols. It occurs right after protonation and results in the formation of a carbocation. The actual elimination mechanism involves the following:

• Once the alcohol is protonated, the water molecule formed acts as a good leaving group due to its stability compared to hydroxide ions.
• The elimination of the water molecule effectively converts the alcohol into a carbocation, thereby setting the stage for the formation of double bonds in alkenes.
• This step is crucial as it drives the transformation process from alcohols to alkenes by facilitating the change in functional groups.

In essence, eliminating water is an integral part of converting alcohol into an alkene, as it helps form a double bond by removing components that hinder the stabilization of the newly formed carbocation.