Problem 192 - A Complete Resource Book in Chemistry for JEE Main

Question

Acid catalysed hydration of alkenes except ethene leads to the formation of (a) primary alcohol (b) secondary or tertiary alcohol (c) mixture of primary and secondary alcohols (d) mixture of secondary and tertiary alcohols

Step-by-Step Solution

Answer

(b) Acid-catalyzed hydration of alkenes leads to secondary or tertiary alcohols.

Step 1: Understanding the Reaction

The reaction involves the acid-catalyzed hydration of alkenes, a process where a water molecule is added across the double bond of an alkene in the presence of an acid catalyst such as sulfuric acid.

Step 2: Mechanism of Reaction

The reaction proceeds via the formation of a carbocation intermediate. The proton from the acid catalyst adds to one of the carbons in the double bond, forming a more stable carbocation.

Step 3: Carbocation Stability

In general, more substituted carbocations are more stable. Therefore, the secondary or tertiary carbocations are preferred over primary ones during this reaction process.

Step 4: Formation of Alcohol

Once the carbocation is formed, water acts as a nucleophile and attacks the carbocation, leading to the formation of an alcohol. The position of the alcohol depends on the stability of the carbocation.

Step 5: Determining the Product Type

Given that secondary or tertiary carbocations are favored due to their stability, the resultant alcohol is typically secondary or tertiary if the starting alkene is more than ethene.

Step 6: Conclusion

With the exception of ethene, acid-catalyzed hydration of other alkenes predominantly forms secondary or tertiary alcohols due to the stability of the intermediates.

Key Concepts

Carbocation Stability

In acid-catalyzed hydration of alkenes, carbocation stability is a key player. When an alkene undergoes this reaction, the protonation of the double bond occurs, leading to the formation of a carbocation. The stability of this carbocation greatly influences the outcome of the reaction. Why is carbocation stability important? Well, more stable carbocations are formed more rapidly and are less likely to react further into less ideal products. The stability of a carbocation is determined by its level of substitution.

• Primary carbocations are the least stable, as they have only one alkyl group donating electrons.
• Secondary carbocations have two alkyl groups providing electron donation, making them more stable.
• Tertiary carbocations with three alkyl groups are the most stable due to maximum electron donation.

This is why in the acid-catalyzed hydration process, you will often see tertiary and secondary carbocations being formed, as they result in lower energy and more stable intermediates.

Formation of Alcohol

After the formation of a stable carbocation, the reaction proceeds towards the formation of alcohol. This step is crucial as it defines the overall product of the reaction. Once the carbocation is in place, a water molecule - acting as a nucleophile - attacks it. This subsequently leads to the formation of an oxonium ion. Next, a deprotonation event occurs, where the oxonium ion loses a proton to stabilize into an alcohol. The location of the alcohol group is determined by the initial site of nucleophilic attack on the most stable carbocation. Hence, the end product is usually a secondary or tertiary alcohol, unless the starting material is simpler like ethene. The stability of the alcohol often mirrors the stability of the carbocation from which it was derived.

Nucleophilic Attack

Nucleophilic attack is a fundamental step during the acid-catalyzed hydration of alkenes. After the formation of a carbocation, a water molecule steps in as a nucleophile. This molecule is drawn to the electron-deficient center of the carbocation. Here's how nucleophilic attack works:

• The water molecule, which possesses lone pairs of electrons, donates these electrons to the carbocation.
• This donation completes the octet of the positively charged carbon, converting it into an oxonium ion.

The oxonium ion then undergoes deprotonation to form a stable alcohol. This step is central to the transformation of alkenes into alcohols and highlights the role of nucleophiles in organic reactions.

Reaction Mechanism

Understanding the reaction mechanism of acid-catalyzed hydration of alkenes gives insights into how simple reactants transform into valuable products like alcohols. The overall mechanism can be broken down into several key steps: - **Protonation**: - The reaction kicks off with protonation of the alkene double bond by the acid catalyst. This results in the formation of a more stable carbocation. - **Carbocation Formation**: - Following protonation, a carbocation is formed. Its stability depends on the structure of the starting alkene, where more substituted carbocations are preferred. - **Nucleophilic Attack by Water**: - As a next step, the newly formed carbocation gets attacked by a water molecule, creating an oxonium ion intermediate. - **Deprotonation and Alcohol Formation**: - Finally, the oxonium ion is deprotonated to yield the final alcohol product. This systematic process shows how clever manipulation of chemical interactions and intermediate formations can create significant chemical transformations.