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

Question

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

Step-by-Step Solution

Answer

The preferred product is (d) 2-methyl but-2-ene.

Step 1: Understand the Dehydration Process

Dehydration is a reaction where water is removed from the alcohol. The hydroxyl group (OH) from the alcohol is eliminated, along with a hydrogen atom from an adjacent carbon, forming an alkene.

Step 2: Identify the Alcohol Structure

2-methyl butan-1-ol has the structure: CH3-CH(CH3)-CH2-CH2-OH. It is a primary alcohol because the hydroxyl group is attached to the first carbon atom.

Step 3: Determine the Carbocation Intermediate

In dehydration reactions for alcohols, a carbocation intermediate is usually formed. After the removal of the hydroxyl group as water, the molecule's structure allows the formation of a carbocation at the more substituted carbon, increasing stability. Here, a shift leads to a tertiary carbocation at the 2nd carbon (CH3-C^+(CH3)-CH2-CH3).

Step 4: Identify the Most Stable Alkene

From the tertiary carbocation, the most stable alkene forms through the elimination of a proton from the adjacent carbon. The most substituted and stable product (Zaitsev's rule) is formed by eliminating a hydrogen from the tertiary butyl group, leading to 2-methyl but-2-ene.

Step 5: Choose the Correct Product

According to Zaitsev's rule, the more substituted alkene (the one with more alkyl groups attached) is preferred. Thus, 2-methyl but-2-ene is the major product in this dehydration reaction.

Key Concepts

Alkene Formation

When alcohols undergo a dehydration reaction, they are transformed into alkenes. This process involves removing a water molecule from the alcohol. An alkene is an unsaturated hydrocarbon, meaning it has at least one carbon-carbon double bond. In the case of 2-methyl butan-1-ol, the dehydration helps create this double bond by removing the hydroxyl group (OH) from the alcohol and a hydrogen from an adjacent carbon.

This removal forms a double bond between two carbon atoms, creating the alkene product. The overall reaction can be summarized as:

• Loss of -OH group from the alcohol
• Loss of a hydrogen (H) atom from the neighboring carbon
• Formation of a new double bond to create the alkene

Alkene formation is crucial in organic chemistry because alkenes serve as building blocks for many more complex molecules.

Carbocation Intermediate

In the process of dehydration of alcohols, a carbocation intermediate often plays a key role. It occurs after the initial step where the -OH group is removed as water, leaving behind a positively charged carbon atom, known as a carbocation.

The stability of this carbocation is critical, as more substituted carbocations are generally more stable. Substitution refers to the number of alkyl groups attached to the positively charged carbon.

For 2-methyl butan-1-ol, the formation of a carbocation leads to a rearrangement, where initially a secondary carbocation is formed, but it quickly rearranges to a more stable tertiary carbocation. The tertiary state is more stable because it can better distribute the positive charge.

• The carbocation stabilizes through rearrangement
• A tertiary carbocation (three alkyl groups attached) is more stable
• Stability is essential for successful alkene formation

Carbocation intermediates are crucial in determining the stability and outcome of the dehydration reaction.

Zaitsev's Rule

Zaitsev's rule guides us in predicting the major alkene product formed during elimination reactions such as dehydration. The rule states that the more substituted alkene, or the alkene with more alkyl groups attached to the double-bonded carbons, will generally be the favored product. This is because these alkenes are more stable due to hyperconjugation and the electron-donating effects of the alkyl groups.

Applying Zaitsev's rule to the dehydration of 2-methyl butan-1-ol results in the formation of 2-methyl but-2-ene as the major product. This alkene is more substituted compared to alternatives and therefore takes precedence.

• Zaitsev's rule emphasizes stability and substitution in product formation
• More substituted alkenes are more thermodynamically stable
• Understanding this rule helps predict major products in dehydration reactions

Zaitsev's rule acts as a helpful predictor in determining the outcome of many organic reactions, allowing chemists to predict which products will be formed.