Problem 7 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Rate of dehydration of alcohols follows the order: (a) \(2^{\circ}>1^{\circ}>\mathrm{CH}_{3} \mathrm{OH}>3^{\circ}\) (b) \(3^{\circ}>2^{\circ}>1^{\circ}>\mathrm{CH}_{3} \mathrm{OH}\) (c) \(2^{\circ}>3^{\circ}>1^{\circ}>\mathrm{CH}_{3} \mathrm{OH}\) (d) \(\mathrm{CH}_{3} \mathrm{OH}>1^{\circ}>2^{\circ}>3^{\circ}\)

Step-by-Step Solution

Answer

Option (b) is correct: 3° > 2° > 1° > \(\mathrm{CH}_3\mathrm{OH}\).

Step 1: Understanding Alcohol Classifications

Alcohols are classified based on the number of carbon atoms attached to the carbon bearing the hydroxyl group (-OH). Primary (1°) alcohols have one alkyl group attached, secondary (2°) alcohols have two, and tertiary (3°) alcohols have three. Methanol (\( \mathrm{CH}_3\mathrm{OH} \)) does not have a carbon directly attached to the hydroxyl; it is in a class by itself.

Step 2: Dehydration Reaction Basics

Dehydration of alcohols involves the removal of water to form alkenes, facilitated by acid catalysts and heat. The ease of dehydration typically increases with the degree of substitution: 3° > 2° > 1° alcohols. Methanol, being a simple alcohol, does not fit neatly into this trend and is generally less reactive to dehydration.

Step 3: Comparing the Given Options

Review the given options and compare them to the known trend for alcohol dehydration rates, which is 3° > 2° > 1° > \(\mathrm{CH}_3\mathrm{OH}\). This arrangement corresponds to the decreasing stability of carbocations that form during the reaction.

Step 4: Identify the Correct Order

From the trend, tertiary (3°) alcohols dehydrate fastest, followed by secondary (2°), primary (1°), and methanol (\(\mathrm{CH}_3\mathrm{OH}\)). Thus, option (b) \(3^{\circ}>2^{\circ}>1^{\circ}>\mathrm{CH}_{3} \mathrm{OH}\) is correct.

Key Concepts

Alcohol Classification

Alcohols are an important class of organic compounds characterized by the presence of a hydroxyl group (-OH). The classification of alcohols is based on the number of alkyl groups attached to the carbon atom bearing the hydroxyl group. This classification is crucial for understanding the reactivity and chemical behavior of different alcohols.

- **Primary (1°) Alcohols**: These have one alkyl group attached to the carbon with the hydroxyl group. An example is ethanol, where the carbon is bonded to one other carbon atom.- **Secondary (2°) Alcohols**: In these, the hydroxyl-bearing carbon is connected to two alkyl groups. Isopropanol is a common example, with two carbon atoms attached to the central carbon.- **Tertiary (3°) Alcohols**: Tertiary alcohols have three alkyl groups attached. Tert-butyl alcohol is an example where three carbon atoms surround the central one.
Methanol (\(\mathrm{CH}_3\mathrm{OH}\)) is treated differently since the carbon with the hydroxyl group is only bonded to hydrogen, making its reactivity unique compared to other alcohols. Understanding these classifications helps predict how different alcohols will behave in chemical reactions such as dehydration.

Carbocation Stability

Carbocations are positively charged ions formed during reactions like the dehydration of alcohols. The stability of these carbocations greatly influences the rate and outcome of the reaction. When alcohols undergo dehydration, they lose a water molecule, and a carbocation is temporarily formed.

The stability of a carbocation is determined by several factors:

• **Degree of Substitution:** Carbocations are more stable if they are more substituted. Tertiary (3°) carbocations are the most stable because the three alkyl groups around the positively charged carbon help stabilize the positive charge through hyperconjugation and inductive effects.
• **Hyperconjugation:** Involves the overlap of \(\sigma\)-bonds (C-H or C-C) from adjacent atoms with the empty \( p\)-orbital of the carbocation, enhancing stability.
• **Inductive Effects:** These arise due to the electron-donating effects of alkyl groups, which help disperse the positive charge throughout the molecule.

As a result, tertiary carbocations are more stable than secondary, and secondary are more stable than primary. Methanol, when dehydrated, does not form a typical carbocation due to its structure, which contributes to its low reactivity in dehydration reactions.

Alkene Formation

Alkene formation through the dehydration of alcohols is a common reaction where alcohol molecules lose water to form double-bonded hydrocarbons known as alkenes. This process is typically facilitated by acid catalysts and requires heat to occur efficiently.

- **Mechanism**: The dehydration mechanism involves three main steps: 1. **Protonation:** The hydroxyl group in alcohol is protonated to form a better leaving group like water. 2. **Formation of a Carbocation:** The water molecule leaves, resulting in a carbocation intermediate. The stability of this carbocation is crucial and influences the rate of the reaction. 3. **Alkene Formation:** Finally, a hydrogen atom is removed from a neighboring carbon atom, facilitating the creation of a double bond, leading to alkene formation.
The ease of alkene formation in dehydration reactions mirrors the stability of carbocations generated. Thus, tertiary alcohols yield alkenes more readily than secondary or primary alcohols, aligning with the sequence of carbocation stability. Recognizing this process is essential for predicting the products and conditions needed for efficient alkene synthesis from alcohols.