Problem 109 - A Complete Resource Book in Chemistry for JEE Main

Question

When 2 -methyl butan-1-ol is dehydrated to give an alkene, the preferred product is (a) but-1-ene (b) but-2-ene (c) 2 -methyl but-1-ene

Step-by-Step Solution

Answer

(c) 2-methylbut-2-ene is the preferred product due to greater stability from increased substitution.

Step 1: Identify the Given Compound

The given compound is 2-methylbutan-1-ol, which is an alcohol. The structure of the compound includes a hydroxyl group (-OH) attached to the first carbon of a four-carbon backbone with a methyl group attached to the second carbon.

Step 2: Understand Dehydration Reaction

Dehydration of an alcohol involves the removal of a water molecule (H2O) from the alcohol, resulting in the formation of a double bond, thus converting the alcohol into an alkene.

Step 3: Determine the Major Product Using Zaitsev's Rule

According to Zaitsev's Rule, during an elimination reaction, the more substituted alkene (the alkene with more alkyl groups attached to the double-bonded carbons) is generally the more stable and favored product.

Step 4: Apply Zaitsev's Rule to Identify Preferred Alkene

In this reaction, the removal of water (specifically the -OH and a hydrogen from the second carbon) will lead to the formation of 2-methylbut-2-ene as the major product, since it has a more substituted and thus more stable double bond.

Key Concepts

Zaitsev's Rule

When dealing with reactions that involve the removal of atoms or groups, Zaitsev's Rule becomes incredibly important. This rule predicts the predominant arrangement of the atoms in the alkene product formed by the dehydration of alcohols. In simple terms, during an elimination reaction, the more stable alkene is favored. Stability in this context means that the alkene should have as many alkyl groups attached to the double-bonded carbons as possible.

• The more alkyl substituents around the double bond, the more stable the alkene.
• This is because alkyl groups are electron-releasing, which stabilizes the positive charge in the transition state.

This rule is particularly useful because it allows chemists to predict not just any product, but the major product of a reaction without having to perform an experimental analysis first. Hence, understanding Zaitsev's Rule is a cornerstone in studying organic chemistry reactions.

2-methylbutan-1-ol

2-methylbutan-1-ol is an alcohol, which is an organic compound that contains a hydroxyl group (-OH) attached to a saturated carbon atom. In 2-methylbutan-1-ol, the structure is particularly special because of its branching.

Here's the breakdown:

• A four-carbon chain forms the backbone of this molecule.
• A methyl group ( -CH₃) is attached to the second carbon atom, giving it a branched structure.
• The alcohol functional group (-OH) is on the first carbon, which classifies it as a primary alcohol.

This structure is crucial because during dehydration reactions, the location of the hydroxyl group in the chain strongly influences which hydrogen atom is removed and thus determines the possible alkenes that can be formed. Therefore, knowing the structure formats the foundation of predicting reaction outcomes.

Alkene Formation

In organic chemistry, alkene formation is a central reaction, particularly when it comes from alcohol dehydration. Dehydration is a chemical reaction where water is removed from the alcohol molecule, resulting in an alkene.

The process involves two steps:

• Removing the hydroxyl group (-OH) from the alcohol.
• Removing a hydrogen atom from a neighboring carbon atom.

This process creates a double bond, formatting the alkene structure. The key here is to identify which hydrogen atom is removed, leading to different stereochemistry and structure of the resulting alkene.
Zaitsev's Rule helps in predicting this step by favoring more substituted alkenes, which are more stable. In the case of 2-methylbutan-1-ol, the dehydration most likely leads to 2-methylbut-2-ene, a more stable, substituted alkene, thus demonstrating the efficiency of Zaitsev's Rule in experimental reactions.