Problem 51 - A Complete Resource Book in Chemistry for JEE Main

Question

The alcohol which does not give a stable compound on dehydration is (a) methyl alcohol (b) ethyl alcohol (c) \(\mathrm{n}\)-butyl alcohol (d) n-propyl alcohol

Step-by-Step Solution

Answer

(a) Methyl alcohol does not give a stable compound on dehydration.

Step 1: Understanding Dehydration Reaction

Dehydration of alcohols typically involves the removal of a water molecule, leading to the formation of an alkene. This process is influenced by the stability of the resulting carbocation intermediate. Tertiary carbocations are more stable than secondary, which are more stable than primary carbocations.

Step 2: Analyzing Methyl Alcohol

Methyl alcohol ( ext{CH}_3 ext{OH}) undergoes dehydration but does not lead to a stable carbocation, as it would require the formation of a primary carbocation, which is unstable.

Step 3: Analyzing Ethyl Alcohol

Ethyl alcohol ( ext{C}_2 ext{H}_5 ext{OH}) can be dehydrated to form ethene by forming a primary carbocation, which is not very stable but commonly occurs in reactions.

Step 4: Analyzing n-Butyl Alcohol

n-Butyl alcohol ( ext{C}_4 ext{H}_9 ext{OH}) can undergo dehydration to form 1-butene via a primary carbocation, which is relatively unstable but can still occur.

Step 5: Analyzing n-Propyl Alcohol

n-Propyl alcohol ( ext{C}_3 ext{H}_7 ext{OH}) dehydrates to form propene through a primary carbocation. While not as stable as secondary or tertiary carbocations, it can nonetheless occur.

Step 6: Identifying the Least Stable Option

Among the given options, methyl alcohol results in the least stable intermediate upon dehydration, as its primary carbocation is the most unstable, leading to no stable compound.

Key Concepts

Carbocation Stability

Carbocations are positively charged ions, typically involving a carbon atom that has only six electrons, making it quite reactive. The stability of these ions is crucial in many organic reactions, including the dehydration of alcohols. An easy way to remember the stability of carbocations is by considering their degree of substitution:

• Tertiary carbocation: Three alkyl groups attached. Most stable due to the inductive effect and hyperconjugation.
• Secondary carbocation: Two alkyl groups attached. Moderately stable.
• Primary carbocation: One alkyl group attached. Least stable and less commonly observed.

The reasons behind this ranking involve both inductive effects, where electron-donating alkyl groups stabilize the positive charge, and hyperconjugation, which allows delocalization of charge through \(\sigma\) bonds. Because of this, reactions that form more stable carbocations are typically faster and more favored.

Primary Carbocation

A primary carbocation is a specific type of carbocation where the carbon atom bearing the positive charge is bonded to only one other carbon. This limited substitution makes primary carbocations the least stable among the different types.When discussing reactions like alcohol dehydration, primary carbocations are often intermediates. However, because they are not very stable, these reactions can sometimes need extra energy or a catalyst to proceed. In synthetic chemistry, chemists often try to avoid reactions that will form primary carbocations or instead find ways to stabilize them with neighboring groups or rearrangement.Knowing that primary carbocations are unstable illuminates why reactions like the dehydration of methyl alcohol (\( ext{CH}_3 ext{OH} \)) are problematic. The resulting primary carbocation is so unstable that the reaction does not proceed effectively under normal conditions.

Alcohol Dehydration Mechanism

The mechanism of alcohol dehydration involves multiple steps where the hydroxyl group (\( - ext{OH} \)) is removed to eventually form a double bond between carbon atoms (an alkene). This process generally requires acidic conditions, often using sulfuric acid (\( ext{H}_2 ext{SO}_4 \)) as a catalyst. The steps involved in the dehydration mechanism typically include:

• Protonation: The alcohol oxygen attracts a proton, making the water molecule a better leaving group.
• Carbocation Formation: The protonated alcohol loses water to form a carbocation. This is the step where the stability of the carbocation plays a critical role.
• Alkene Formation: Finally, a base removes a proton adjacent to the carbocation, leading to the formation of an alkene.

The entire process is an example of an E1 mechanism, dependent on the formation and stability of the carbocation. Because it requires carbocation stability, the dehydration of primary alcohols (like n-butyl and n-propyl alcohol) can be less efficient than that of secondary or tertiary alcohols.