Problem 9 - A Complete Resource Book in Chemistry for JEE Main

Question

The compound which gives the most stable carbonium ion on dehydration is (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) (b) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{COH}\) (c) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCH}_{2} \mathrm{OH}\) (d) \(\mathrm{CH}_{3} \mathrm{CHOHCH}_{2} \mathrm{CH}_{3}\)

Step-by-Step Solution

Answer

Compound (b) \((\mathrm{CH}_3)_3\mathrm{COH}\) forms the most stable carbonium ion.

Step 1: Understand Carbonium Ion Formation

Carbonium ions (also known as carbocations) are positively charged ions with three bonds around the positively charged carbon. During dehydration, alcohols lose a water molecule, leading to the formation of a carbonium ion.

Step 2: Consider Carbocation Stability

Carbocation stability is influenced by hyperconjugation and resonance. Tertiary carbocations are more stable than secondary carbocations, which are more stable than primary carbocations due to greater alkyl group donations and possible resonance stabilization.

Step 3: Analyze Each Compound

Examine each option:(a) \(\mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_2\mathrm{CH}_2\mathrm{OH}\), dehydration produces a primary carbocation.(b) \(\left(\mathrm{CH}_3\right)_3\mathrm{COH}\), dehydration gives a tertiary carbocation.(c) \(\left(\mathrm{CH}_3\right)_2\mathrm{CHCH}_2\mathrm{OH}\), dehydration produces a secondary carbocation.(d) \(\mathrm{CH}_3\mathrm{CHOHCH}_2\mathrm{CH}_3\), dehydration yields a secondary carbocation.

Step 4: Identify the Most Stable Carbocation

Since tertiary carbocations are generally the most stable due to hyperconjugation and inductive effects from three surrounding alkyl groups, the tertiary carbocation from compound (b) \((\mathrm{CH}_3)_3\mathrm{COH}\) is the most stable after dehydration.

Step 5: Conclude the Answer

From the analysis, the compound \((\mathrm{CH}_3)_3\mathrm{COH}\) in option (b) forms the most stable carbonium ion upon dehydration because it is the only compound that forms a tertiary carbocation.

Key Concepts

Hyperconjugation

Have you ever wondered why some carbocations are more stable than others? A key player in this game is a phenomenon called hyperconjugation. To keep it simple, hyperconjugation is the dispersal of positive charge over multiple atoms, rather than having it concentrated entirely on one. This happens when the electrons in the sigma bonds (single bonds, like C-H bonds) near the positively charged carbon help stabilize the carbocation.
The more alkyl groups (like methyl groups) attached to the charged carbon, the more opportunities for hyperconjugation. Think of it like having more hands to help carry a heavy load. More hands make the load much lighter!

• Increased hyperconjugation leads to increased stability.
• Tertiary carbocations (those with three carbon groups attached) have the most hyperconjugation.

Summary: The more alkyl groups around a carbocation, the better hyperconjugation can help stabilize it.

Inductive Effects

Carbocation stability doesn't stop at hyperconjugation; inductive effects also play a crucial role. Inductive effects refer to the ability of a group or atom to withdraw or donate electrons through sigma bonds. This effect helps in stabilizing or destabilizing carbocations.
Alkyl groups are known for their electron-donating ability, pushing electrons toward the positively charged carbon center, making it less positive and hence more stable.

• Electron-donating groups increase carbocation stability.
• More alkyl groups mean increased electron donation through inductive effects.
• Tertiary carbocations experience the greatest stabilizing inductive effects.

In simple terms, when more alkyl groups surround a carbocation, they act like cushions, easing the stress on the positively charged carbon by donating some electron density.

Dehydration of Alcohols

Understanding carbocation stability is especially important in reactions like the dehydration of alcohols. Dehydration of alcohols involves removing a water molecule ( H ₂ O ) from an alcohol, forming a carbocation in the process. The type of carbocation formed—primary, secondary, or tertiary—depends on the structure of the alcohol.
Here's how it generally works:

• Alcohols undergo protonation, making the OH group a better leaving group as a molecule of water.
• The alcohols then lose this water, forming a carbocation.
• The resulting carbocation may rearrange to a more stable form, like from a primary to a tertiary one.

Tertiary alcohols often form the most stable carbocations directly due to both hyperconjugation and inductive effects. Thus, in exams or practice problems, when you see options like exttt{( C H ₃ ) ₃ C O H } , expect them to form stable carbocations upon dehydration. In sum, always look out for the type of carbocation an alcohol forms during dehydration to determine the stability.