Problem 207 - A Complete Resource Book in Chemistry for JEE Main

Question

The main product of the following reaction is \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{OH}) \mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2} \stackrel{\text { conc. } \mathrm{H}_{2} \mathrm{SO}_{4}}{\longrightarrow}\) ?

Step-by-Step Solution

Answer

The main product is \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{C}(\mathrm{=CHCH}_{3}) \left(\mathrm{CH}_{3}\right)\).

Step 1: Recognize Reactant Structure

Identify the molecular structure of \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{OH}) \mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2}\). This compound is an alcohol with a phenyl group (C6H5-, which is a benzene ring) and a tertiary alcohol group (\(-\mathrm{CH}(\mathrm{OH})\mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2}\)).

Step 2: Analyze Conditions

Consider the reactions promoted by concentrated sulfuric acid (\(\mathrm{H}_{2} \mathrm{SO}_{4}\)). Concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\) is a strong dehydrating agent and can facilitate dehydration reactions, where water is removed from alcohols to form alkenes.

Step 3: Determine Possible Dehydration Paths

Understand that dehydration of an alcohol typically forms the more substituted alkene (Saytzeff's rule). Remove the \(\mathrm{OH}\) group and a hydrogen atom from the neighboring carbon to form a double bond. Identify potential carbons for double bond formation: in this structure, between the carbon bonded to \(\mathrm{OH}\) and either adjacent carbon.

Step 4: Predict Major Alkene Product

By removing the \(\mathrm{OH}\) group from the alcohol carbon and a hydrogen from the most substituted adjacent carbon (second carbon from phenyl ring), the major product will be \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{C} \left(=\mathrm{CHCH}_{3}\right) \left(\mathrm{CH}_{3}\right)\). This is the more stable (and therefore major) alkene product as it adheres to Saytzeff's rule.

Key Concepts

Alcohol Dehydration

Alcohol dehydration is a chemical reaction where an alcohol undergoes a transformation to lose a molecule of water, resulting in the formation of an alkene. This reaction is commonly facilitated by using strong acids, such as concentrated sulfuric acid (\( \text{H}_2\text{SO}_4 \)). During this process, the hydroxyl group (\(-\text{OH}\)) from the alcohol and a hydrogen atom from an adjacent carbon are removed to form water, and a new carbon-carbon double bond is established in its place.

This reaction is vital in organic chemistry because it is a method to convert alcohols into alkenes, which are key building blocks in industrial chemistry and organic synthesis. **Key points to remember about alcohol dehydration include:**

• It typically requires an acid catalyst.
• The reaction involves the removal of water from the alcohol compound.
• A carbon-carbon double bond is formed, resulting in an alkene.

Understanding this process is crucial for predicting the outcomes of chemical reactions involving alcohols and planning efficient synthesis routes for alkenes.

Saytzeff's Rule

Saytzeff's Rule, also spelled Zaitsev's Rule, is a fundamental principle in organic chemistry. It helps predict which alkene will be the major product when an alcohol undergoes dehydration. According to this rule, the more substituted alkene product is favored. In simpler terms, the double bond tends to form between carbons that have more alkyl groups attached because this provides greater stability to the resulting alkene.

**Saytzeff's Rule is based on the following observations:**

• Alkenes are more stable when they are highly substituted because of hyperconjugation and the stabilizing effect of alkyl groups.
• When there is a choice of forming multiple alkenes, the most stable (usually the more substituted) alkene predominates.

**It's important because:**

• It helps predict the outcome of elimination reactions, like alcohol dehydration.
• It guides chemists in controlling reaction conditions to achieve the desired product.

In the dehydration reaction of a tertiary alcohol, such as in the original exercise, Saytzeff's Rule would suggest that the alkene formed at the more substituted position is the major product.

Alkene Formation

Alkene formation occurs when an alcohol undergoes dehydration, resulting in the creation of a carbon-carbon double bond. Alkenes are hydrocarbons with at least one double bond in their structure, making them unsaturated compounds. They have the general formula \( \text{C}_n\text{H}_{2n} \), and their presence and formation are central to many synthesis and industrial processes in chemistry.

**The significance of alkene formation includes:**

• Providing feedstock for a wide range of chemical products, such as polymers and solvents.
• Offering pathways for more complex synthetic reactions, including cycloadditions and polymerization.

In the dehydration process, once water is eliminated, the remaining parts of the molecule rearrange to form the double bond. This process is driven by the stability of the formed alkene, which can be predicted using Saytzeff's Rule.

Thus, understanding alkene formation is critical for chemists looking to synthesize a variety of organic compounds and to manipulate molecular structures for desired reactions. Ensuring the control over which alkene is formed can greatly affect the efficiency and success of chemical synthesis projects.