Problem 201 - A Complete Resource Book in Chemistry for JEE Main

Question

Among the following compounds which one can be dehydrated very easily? (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) (b) CCCC(C)O (c) CCC(C)O (d) CCC(C)CCO

Step-by-Step Solution

Answer

Compound (c) CCC(C)O is the easiest to dehydrate.

Step 1: Identify Dehydration Potential

To determine which alcohol can be dehydrated most easily, we need to consider the stability of the resulting carbocation after dehydration. Alcohol dehydration typically follows an E1 mechanism where the leaving group (OH) is converted into water and leaves, resulting in a carbocation intermediate. The more stable the intermediate carbocation, the easier the dehydration process will be.

Step 2: Analyze Each Compound's Structure

Let's evaluate each compound:(a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH} \\) is 1-pentanol, forming a primary carbocation.(b) \(\mathrm{CCCC(C)O} \\) is 2-pentanol, which forms a secondary carbocation upon dehydration.(c) \(\mathrm{CCC(C)O} \\) is 2-butanol, resulting in a secondary carbocation.(d) \(\mathrm{CCC(C)CCO} \\) is 2-hexanol, also leading to a secondary carbocation.

Step 3: Determine Carbocation Stability

The stability of carbocations follows the order: tertiary > secondary > primary. - Compound (a) forms a primary carbocation, which is least stable. - Compounds (b), (c), and (d) form secondary carbocations, which are more stable but less stable than tertiary carbocations.

Step 4: Identify the Most Easily Dehydrated Compound

All things being equal, secondary carbocations are considerably more stable than primary ones. Among compounds (b), (c), and (d) which form secondary carbocations, the carbon chain length doesn't affect the dehydration ease. However, 2-butanol (c) is the smallest molecule requiring less energy for dehydration due to less steric hindrance; therefore option (c) will dehydrate the easiest.

Key Concepts

Carbocation Stability

Understanding carbocation stability is crucial in predicting the ease of alcohol dehydration. A carbocation is an intermediate with a positively charged carbon atom. This positive charge makes it very reactive. The stability of a carbocation can be assessed by looking at the type of carbon atom holding the positive charge.

• Primary carbocations: These have the positive charge on a carbon attached to only one other carbon. They are the least stable.
• Secondary carbocations: The positive charge is on a carbon attached to two other carbons. They are more stable than primary carbocations.
• Tertiary carbocations: The positive charge is on a carbon connected to three other carbons. These are the most stable.

Stability increases with more alkyl groups attached to the charged carbon, as they can donate electron density to stabilize the positive charge. This is why tertiary carbocations are the most stable, and primary ones are the least.

E1 Mechanism

The E1 mechanism is a common pathway for alcohol dehydration. In this mechanism, the reaction proceeds in two main steps. First, the leaving group, often a hydroxyl group (OH), is protonated and leaves the molecule as water, forming a carbocation intermediate. This step is slow and determines the reaction's rate. Once the carbocation is formed, the molecule can lose a proton in the second step, forming a double bond and completing the elimination process. The result is an alkene. Since the first step involves the formation of a carbocation, the stability of this carbocation is a key factor in determining how quickly the reaction will proceed. The E1 mechanism is generally favored in conditions where the carbocation can be stabilized, such as in secondary or tertiary alcohols. Primary alcohols are less likely to proceed through an E1 mechanism due to their unstable primary carbocations.

Primary and Secondary Alcohols

Primary and secondary alcohols differ in their reaction behaviors because of the carbocations they form. A primary alcohol, when dehydrated, will form a primary carbocation, and as such, it will undergo reactions less readily compared to secondary alcohols. For primary alcohols:

• The hydroxyl group is connected to a carbon only bonded to one other alkyl group.
• This leads to the formation of an unstable, high-energy primary carbocation.
• As a result, dehydration is often difficult for primary alcohols.

In secondary alcohols:

• The hydroxyl group is attached to a carbon bonded to two other carbons, resulting in a more stable secondary carbocation after water leaves.
• This increased stability makes secondary alcohols more likely to dehydrate, often via an E1 mechanism.

Hence, in dehydration reactions at moderate temperatures, secondary alcohols typically proceed with greater ease than their primary counterparts.

Steric Hindrance

Steric hindrance refers to the prevention of chemical reactions due to the spatial arrangement of atoms within a molecule. In the context of alcohol dehydration, steric hindrance becomes significant when bulky groups are present near the reactive site. The process of dehydration involves the removal of water, forming a carbocation intermediate. If too many bulky groups surround the hydroxyl-bearing carbon, the steric hindrance can make it difficult for the necessary rearrangements or departures to occur. Factors affecting steric hindrance:

• Size of substituents: Larger groups around the carbocation can slow down or hinder its formation.
• Chain length: While longer chain molecules might increase hindrance, it is often the bulky branching in smaller molecules that critically impacts the reaction.

In practice, molecules with less steric hindrance can more easily form and stabilize the carbocation intermediate, allowing the dehydration process to occur more smoothly. Therefore, smaller alcohols like 2-butanol, which offers less steric hindrance, tend to dehydrate more readily.