Problem 199 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

(c) protonation of alcohol molecule.

Step 1: Understanding the problem

We are asked to identify the initiation step in the dehydration of alcohols to alkenes using concentrated sulfuric acid (H2SO4). The options given involve different chemical processes.

Step 2: Protonation of Alcohol

The dehydration of alcohols involves the acid-catalyzed removal of a water molecule to form an alkene. This reaction begins with the protonation of the alcohol hydroxyl group (-OH) by the acid, increasing the leaving group's ability and forming an oxonium ion.

Step 3: Evaluating Options

With the alcohol protonated, it is more reactive. The protonation occurs at the start of the reaction, making it the initiation step. We must check the options to see which one corresponds to this step.

Step 4: Choosing the Correct Answer

Option (c) states that the initiation step is the protonation of the alcohol molecule. This matches our explanation from the previous steps, as this is the first step that starts the reaction process.

Key Concepts

Protonation

In the context of dehydration of alcohols, protonation is an essential first step. Here, the alcohol molecule reacts with a strong acid, like concentrated sulfuric acid (\(\mathrm{H}_2\mathrm{SO}_4\)).

This reaction involves the hydrogen ion (\(\mathrm{H}^+\)) from the acid attaching itself to the oxygen atom in the alcohol group (\(-\mathrm{OH}\)). This transforms the hydroxyl group into water, which is a much better leaving group than the hydride itself.

The result is an oxonium ion, a highly reactive structure. Oxonium ions have a positive charge concentrated on the oxygen, making them eager to proceed to the next step of the reaction.

• Key point: Protonation enhances the reactivity of the alcohol.
• Resulting structure: Formation of the oxonium ion.

Understanding protonation is crucial because it sets up the conditions necessary for the alcohol to undergo the elimination reaction that eventually forms an alkene.

Carbocation Formation

Once the alcohol is protonated, the subsequent step involves the formation of a carbocation. This process is driven by the departure of water from the oxonium ion.

As water leaves, it takes with it the electrons from the oxygen bond, leaving behind a positively charged carbon. This is how the carbocation, a carbon-centered ion with a positive charge, comes into existence.

Carbocations are highly unstable and very reactive due to their positive charge. Their formation acts as a driving force for the reaction to proceed towards the elimination of a hydrogen and creation of a double bond, forming an alkene.

• Unstable intermediate: Carbocations have a positive charge due to the loss of electrons.
• Reaction driving force: The instability of the carbocation propels the reaction towards the final products.

Carbocation formation is a critical step because it sets the stage for the elimination reaction that results in alkene creation.

Elimination Reaction

The elimination reaction is where all the prior steps come together to complete the transformation. This is the stage where the alkene is finally formed.

After the carbocation is created, an adjacent hydrogen atom is generally removed, allowing a double bond to form between the two carbon atoms.
This results in the creation of an alkene, completing the dehydration process.

The elimination reaction is powerful because it leads to the unsaturation of the molecule, forming the desired alkene.

• Key reaction: Formation of the carbon-carbon double bond.
• End goal: Producing an alkene from the initial alcohol molecule.

This step highlights the elegance of the dehydration reaction by culminating in alkene formation, showcasing both the conceptual and practical importance of understanding each transformation phase.