Problem 77 - A Complete Resource Book in Chemistry for JEE Main

Question

Which is dehydrated with the greatest ease using concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4} ?\) (a) CC1(O)CCCC(=O)C1 (b) O=C1CCC(O)CC1 (c) O=C1CCCCC1O (d) CC1C(=O)CCCC1O

Step-by-Step Solution

Answer

Compound (a) dehydrates with the greatest ease because it is a tertiary alcohol.

Step 1: Understanding Dehydration

Dehydration is a chemical reaction that involves the removal of a water molecule from a compound. Alcohols can undergo dehydration to form alkenes when treated with strong acids such as concentrated \( \mathrm{H}_2\mathrm{SO}_4 \). The ease of dehydration generally follows the order: tertiary > secondary > primary alcohols, due to the stability of the resulting carbocation.

Step 2: Analyzing Each Compound

We need to examine the structure of each compound to identify the alcohol group and classify it as primary, secondary, or tertiary. Tertiary alcohols dehydrate the most easily, followed by secondary and then primary.

Step 3: Analyzing Compound (a)

Compound (a) has the structure CC1(O)CCCC(=O)C1. This is a tertiary alcohol because the carbon attached to the -OH group is connected to three other carbons. Tertiary alcohols have the greatest ease of dehydration.

Step 4: Analyzing Compound (b)

Compound (b), O=C1CCC(O)CC1, has a secondary alcohol structure. The carbon with the -OH group is attached to two other carbons.

Step 5: Analyzing Compound (c)

Compound (c), O=C1CCCCC1O, contains a primary alcohol. The carbon bearing the -OH group is attached to only one other carbon.

Step 6: Analyzing Compound (d)

Compound (d), CC1C(=O)CCCC1O, is a secondary alcohol due to the carbon with the -OH group being attached to two other carbons.

Step 7: Conclusion

From the analysis, compound (a) is a tertiary alcohol, whereas (b), (c), and (d) are secondary or primary. Tertiary alcohols dehydrate with the greatest ease. Therefore, compound (a) will dehydrate more easily when treated with concentrated \( \mathrm{H}_2\mathrm{SO}_4 \).

Key Concepts

Tertiary Alcohol

Tertiary alcohols are key players in organic chemistry, recognized for their high reactivity in dehydration reactions. In a tertiary alcohol, the hydroxyl group (-OH) is connected to a carbon atom that is itself bonded to three other carbon atoms. This unique structure provides strong stability to the carbocation formed during the dehydration process.
The process of dehydration involves removing a water molecule, typically producing an alkene. Tertiary alcohols dehydrate more easily than their secondary and primary counterparts.
The reason? The resultant carbocation is highly stable due to the inductive effect and hyperconjugation provided by the surrounding carbon groups.

• More stable carbocation
• Enhanced by surrounding carbons

The stability ensures the carbocation intermediate does not readily revert to the alcohol form, facilitating the elimination reaction.

Secondary Alcohol

Secondary alcohols feature a -OH group attached to a carbon atom connected to two other carbon atoms. While not as reactive as tertiary alcohols in dehydration reactions, they still play a significant role, forming somewhat stable carbocations upon dehydration.
During the dehydration process with concentrated sulfuric acid, secondary alcohols form a carbocation intermediate, which is less stable than that formed from a tertiary alcohol but more stable than a primary one.
This intermediate stability means secondary alcohols can partake in dehydration fairly readily, albeit not as efficiently as tertiary alcohols. Considerations include:

• Moderate stability of carbocation
• Intermediary reactivity in dehydration

Understanding the balance of these properties is crucial when predicting alcohol dehydration outcomes.

Primary Alcohol

Primary alcohols are typically the least reactive form when it comes to dehydration reactions. Here, the -OH group is connected to a carbon atom that is bonded with only one other carbon.
When dehydrating primary alcohols, the resultant carbocation is the least stable. This instability arises due to the lack of adjacent carbon atoms to stabilize the positive charge effectively.
Because these molecules form the least stable carbocations, primary alcohols are least suited for dehydration reactions in the presence of strong acids like concentrated sulfuric acid. Key points include:

• Formation of least stable carbocation
• Least reactive in dehydration

Understanding this can help predict when a primary alcohol will struggle in dehydration processes compared to secondary and tertiary alcohols.