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 [2003] (a) formation of an ester (b) elimination of water (c) protonation of alcohol molecule (d) formation of a carbocation

Step-by-Step Solution

Answer

The initiation step is the protonation of the alcohol molecule.

Step 1: Understanding the Question

The problem involves the dehydration of alcohols to form alkenes using concentrated sulfuric acid (\(\mathrm{H}_2\mathrm{SO}_4\)). The task is to identify the initiation step in this process, which involves recognizing the role each option plays in the mechanism.

Step 1: Protonation of Alcohol

In the dehydration of alcohols, the first step is the protonation of the alcohol molecule. The hydroxyl group of the alcohol acts as a Lewis base and accepts a proton (\(\mathrm{H}^+\)) from the acid, leading to the formation of an alkyloxonium ion.

Step 2: Formation of Carbocation (Elimination of Water)

Following protonation, the alkyloxonium ion loses a molecule of water, resulting in the formation of a carbocation. This step is crucial but it is not the initiation. It follows after the initial protonation.

Step 4: Conclusion: Determining the Initiation Step

From the understanding of the reaction mechanism, the initiation step is protonation of the alcohol, where the alcohol molecule gains a proton before forming a carbocation and eventually leading to alkene formation.

Key Concepts

Protonation of Alcohols

The dehydration of alcohols begins with a crucial step called protonation. This is where the alcohol molecule receives a proton (\(\text{H}^+\)) from the acid, typically sulfuric acid (\(\text{H}_2\text{SO}_4\)). Understanding this step is key to understanding the entire mechanism of this reaction. Here's what happens:

• The hydroxyl (\(\text{-OH}\)) group in the alcohol acts as a Lewis base. This means it has a pair of non-bonding electrons that attract the \(\text{H}^+\) ion from the acid.
• When the alcohol's \(\text{-OH}\) group gains a proton, it transforms into an alkyloxonium ion, \(\text{-OH}_2^+\).

This step is crucial because it prepares the alcohol for the subsequent steps of the reaction. Without protonation, the dehydration reaction won't proceed further. This preparation then leads to the critical creation of carbocations.

Carbocation Formation

Once the alcohol is protonated, the next stage is the formation of a carbocation. This is an important part of the dehydration reaction as it sets the stage for the final formation of the alkene.

• After the alcohol is protonated, it becomes an alkyloxonium ion. In this state, the molecule is unstable, and losing a water molecule - a process referred to as dehydration - makes it more stable.
• When water (\(\text{H}_2\text{O}\)) departs, it leaves behind a positively charged carbon atom, known as a carbocation.

Carbocations are vital intermediates in many reactions, but they can also rearrange to form more stable structures, which can affect the outcome of your reaction. The care and observation of how carbocations form and rearrange can greatly influence the efficiency of producing the desired alkene.

Alkene Formation

Finally, after carbocation formation, the reaction proceeds to produce an alkene. This stage essentially completes the dehydration of alcohol.

• With the newly formed carbocation, an adjacent hydrogen atom can be removed by the counter ion (often the bisulfate ion from sulfuric acid).
• The removal of a hydrogen ion (\(\text{H}^+\)) leads to the formation of a double bond between carbon atoms, resulting in an alkene.

Producing alkenes is one of the significant motives for dehydrating alcohols, as alkenes are useful building blocks in organic synthesis. The elegance of this reaction lies in its ability to convert simple alcohols into more complex structures quickly and efficiently, which are frequently used in synthesizing polymers and pharmaceuticals.