Problem 151 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following species: 1\. \(\mathrm{RCHCH}_{3}\) 2\. \(\mathrm{RCH}_{2} \mathrm{CH}_{2}^{+} \quad 3 . \mathrm{RCH}_{2} \mathrm{CH}_{2} \mathrm{OH}_{2}\) In the dehydration of primary alcohols, the correct sequence of formation of the species involved is (a) \(1,2,3\) (b) \(1,3,2\) (c) \(3,2,1\) (d) \(2,1,3\)

Step-by-Step Solution

Answer

The correct sequence of formation is (c) 3,2,1.

Step 1: Identify the starting species

The problem involves the dehydration of the primary alcohol \(\mathrm{RCH}_{2}\mathrm{CH}_{2}\mathrm{OH}_{2}\). This means the reaction will start with this species.

Step 2: First step of dehydration: Protonation

During dehydration, the alcohol first undergoes protonation to become a good leaving group. The hydroxyl group \(\mathrm{OH}\) of the alcohol receives a proton \(\mathrm{H}^+\) and becomes water, represented as \(\mathrm{RCH}_{2}\mathrm{CH}_{2}\mathrm{OH}_{2}\). Thus, the first species in the sequence is 3.

Step 3: Formation of carbocation

After protonation, the water group leaves, forming a carbocation at the carbon where the hydroxyl group was attached. This forms the secondary carbocation \(\mathrm{RCH}_{2}\mathrm{CH}_{2}^{+}\). The second species in the sequence is 2.

Step 4: Formation of the alkene

In the final step, the carbocation rearranges or directly loses a proton to form the more stable alkene. The final product is an alkene \(\mathrm{RCHCH}_{3}\), due to a double bond formation between the carbons, which corresponds to species 1.

Key Concepts

Dehydration of Alcohols

Dehydration is a common chemical reaction in organic chemistry, specifically where alcohols lose a water molecule. It usually involves heating the alcohol with a strong acid catalyst, like sulfuric acid or phosphoric acid. This process starts with the protonation of the alcohol.

The hydroxyl group (OH) in alcohols is typically a poor leaving group. To facilitate its removal, the acid donates a proton (H\(^+\)) to the alcohol, converting the OH into water, which is a much better leaving group.

Once protonated, let's say starting with a primary alcohol like \( \mathrm{RCH}_2\mathrm{CH}_2\mathrm{OH} \), it changes into \( \mathrm{RCH}_2\mathrm{CH}_2\mathrm{OH}_2 \).

In summary, the dehydration of alcohols involves:

• Using a strong acid as a catalyst
• Protonation of the alcohol
• Formation of water as a better leaving group

These steps prepare the alcohol for subsequent reactions, enabling further transformations such as carbocation formation.

Carbocation Formation

Carbocations are positively charged ions with a carbon atom carrying the charge. These are key intermediates in many organic reactions, especially in dehydration of alcohols. After the hydroxyl group leaves as water, a carbocation is formed at the carbon that formerly bore the hydroxyl group.

If starting from a compound like \( \mathrm{RCH}_2\mathrm{CH}_2\mathrm{OH}_2 \), the departure of water results in a secondary carbocation: \( \mathrm{RCH}_2\mathrm{CH}_2^+ \). The stability of carbocations depends on the surrounding carbon atoms. More alkyl groups stabilize the carbocation via hyperconjugation and induction.

Key points about carbocation formation include:

• Carbocation acts as an intermediate in reactions
• Stability increases with more substituted carbocations
• Potential for rearrangement to form a more stable structure

Understanding carbocation stability is crucial, as it influences the next steps in reaction pathways, leading to different products.

Alkene Formation

Alkene formation typically follows carbocation formation in dehydration reactions of alcohols. This step involves the loss of a proton from a carbocation to form a double bond between carbon atoms. The resulting compound is called an alkene, characterized by its C=C double bond.

From our example, the secondary carbocation \( \mathrm{RCH}_2\mathrm{CH}_2^+ \) will lose a proton to form an alkene such as \( \mathrm{RCH}=\mathrm{CH}_2 \). This is part of the last step in the sequence, where the structure rearranges to achieve a more stable form.

Important aspects of alkene formation include:

• Preference for more substituted, stable alkenes
• Loss of a proton leading to double bond creation
• Potential rearrangements to form more stable structures

Overall, understanding these steps helps in predicting the products formed in the dehydration of alcohols and other similar reactions.