Problem 88 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following will most readily be dehydrated in acidic condition? (a) (b) CC(=O)CC(C)O CCCC(C)O (c) (d) CCC(O)C(C)=O CC(=O)C(C)CC(C)O

Step-by-Step Solution

Answer

Compound (d) is most readily dehydrated in acidic conditions.

Step 1: Understand the Context

Dehydration in acidic conditions typically involves the removal of water (H2O) to form an alkene. Generally, alcohols undergo dehydration more readily than ketones or carboxylic acids. The hydroxyl group (OH) in alcohols can be protonated in acidic conditions, leading to the formation of a good leaving group.

Step 2: Analyze the Functional Groups

Look for compounds with hydroxyl groups, as these can be protonated in acidic conditions. Upon examination of the given SMILES strings: - (b) CCCC(C)O contains a hydroxyl (OH) group on a secondary carbon. - (d) CC(=O)C(C)CC(C)O contains a hydroxyl group on a tertiary carbon. The tertiary alcohol is typically dehydrated more readily because carbocations are more stable when tertiary.

Step 3: Verify Carbocation Stability

In dehydration reactions, carbocation stability is crucial. Assess the stability: - (b) forms a secondary carbocation. - (d) forms a tertiary carbocation upon dehydration. Tertiary carbocations are more stable compared to secondary, making dehydration easier in (d).

Step 4: Conclusion

Considering the functional groups and the stability of the resulting carbocations, compound (d) is more readily dehydrated under acidic conditions.

Key Concepts

Carbocation Stability

When discussing dehydration reactions, understanding carbocation stability is essential. Carbocations are positively charged carbon ions, and their stability can significantly impact the reaction. Here are some key points:

• Types of Carbocations: They are classified as primary, secondary, or tertiary, depending on the number of alkyl groups attached to the positively charged carbon.
• Tertiary Carbocations: These are stabilized by three alkyl groups, which provide electron-donating effects. This makes them more stable compared to primary or secondary carbocations.
• Importance in Dehydration: During dehydration, the formation of a stable carbocation intermediary is crucial for the reaction to proceed smoothly.

Overall, the more substituted the carbocation (i.e., more alkyl groups attached), the greater its stability, which enhances the likelihood of a successful and efficient dehydration process.

Alcohol Dehydration

Alcohol dehydration is a chemical reaction most commonly used to form alkenes from alcohols. In simple terms, it's the process of removing a water molecule from an alcohol. Here's how it works:

• Role of Alcohols: Alcohols contain a hydroxyl group (-OH), which can be easily removed as part of the water molecule in dehydration reactions.
• Reaction Steps: First, the alcohol's hydroxyl group is protonated, making the -OH a better leaving group. Following this, water is effectively "removed," forming an alkene.
• Ease of Reaction: Tertiary alcohols tend to dehydrate more easily than secondary and primary alcohols, due to the stability of the resulting carbocation.

Understanding these steps is vital for predicting how readily an alcohol will undergo dehydration.

Acid-Catalyzed Reactions

Acid-catalyzed reactions are a key type of chemical reaction used to accelerate dehydration processes. Specifically, they involve acids playing a crucial role as catalysts:

• Catalyst Function: An acid, like sulfuric acid or phosphoric acid, donates protons (H⁺) during the reaction process. This protonation increases the electrophilicity of the hydroxyl group, facilitating its departure as water.
• Reaction Environment: This type of reaction typically occurs in acidic conditions, where the abundance of protons can enhance reaction rates.
• Resulting Alkene Formation: After leaving water (H₂O), a double bond is formed, resulting in the production of an alkene.

In summary, acid catalysts are indispensable in dehydration reactions, as they effectively lower the activation energy required and speed up the process while enabling the transformation of alcohols into alkenes.