Problem 142 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following compounds: 1\. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) 2\. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{OH}) \mathrm{CH}_{3}\) 3\. \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{COH}\) These compounds are dehydrated by treatment with sulphuric acid. The correct sequence of increasing order of the reactivity of these three compounds towards dehydration is (a) \(1,3,2\) (b) \(1,2,3\) (c) \(2,1,3\) (d) \(3,1,2\)

Step-by-Step Solution

Answer

(b) 1, 2, 3

Step 1: Identify the Type of Alcohols

The compounds given are alcohols, and we need to identify if they are primary, secondary, or tertiary. 1. **Compound 1**: \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) is a primary alcohol because the \(-OH\) group is attached to a carbon that is connected to only one other carbon atom.2. **Compound 2**: \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH} (\mathrm{OH}) \mathrm{CH}_{3}\) is a secondary alcohol because the \(-OH\) group is attached to a carbon that is connected to two other carbon atoms.3. **Compound 3**: \((\mathrm{CH}_{3})_{3} \mathrm{COH}\) is a tertiary alcohol because the \(-OH\) group is attached to a carbon that is connected to three other carbon atoms.

Step 2: Understand Dehydration Tendencies

Reactivity towards dehydration generally follows the order: tertiary > secondary > primary. This is because the formation of the more stable carbocation intermediate facilitates dehydration. - **Tertiary alcohols** form the most stable carbocations, so they dehydrate more easily. - **Secondary alcohols** form less stable carbocations compared to tertiary ones. - **Primary alcohols** form the least stable carbocations, making them less reactive towards dehydration.

Step 3: Arranging According to Reactivity

Based on the stability of the carbocations formed during dehydration, the reactivity order from least to greatest should be:1. **Compound 1** (Primary alcohol, least reactive)2. **Compound 2** (Secondary alcohol, moderately reactive)3. **Compound 3** (Tertiary alcohol, most reactive)Thus, the order of increasing reactivity is \(1, 2, 3\).

Key Concepts

Primary Alcohols

Primary alcohols are characterized by an -OH group attached to a carbon atom, which in turn is bonded to only one other carbon. This simpler structure means they form the least stable carbocations.
During dehydration, these alcohols are less reactive because they struggle to stabilize positive charges when losing a water molecule.

• Example: he compound CH₃CH₂CH₂CH₂OH (butanol) is a typical primary alcohol.
• Reactivity: Primary alcohols are the least reactive in dehydration reactions compared to secondary and tertiary alcohols.
• Carbocation Stability: Because their carbocations have no neighboring carbons to stabilize the charge, they often undergo rearrangements driving towards more stable forms.

Understanding this helps us know why primary alcohols require stronger conditions (high temperature) or catalysts to dehydrate effectively.

Secondary Alcohols

In secondary alcohols, the -OH group is attached to a carbon atom connected to two other carbon atoms. This layout allows for a medium level of reactivity during dehydration.
Because their carbocations are more stable than those in primary alcohols, they react faster under similar conditions.

• Example: CH₃CH₂CH(OH)CH₃ (butan-2-ol) is a secondary alcohol.
• Reactivity: They are moderately reactive, requiring less energy to undergo dehydration than primary alcohols.
• Carbocation Stability: The carbocation formed during dehydration benefits from hyperconjugation, making it more stable.

Secondary alcohols lie in the middle of the reactivity scale due to their ability to stabilize carbocations effectively, balancing between stability and reactivity.

Tertiary Alcohols

Tertiary alcohols feature an -OH group affixed to a carbon linked to three other carbon atoms. This dense arrangement enhances the formation of highly stable carbocations.
Their dehydration is hence swift and requires very mild conditions.

• Example: (CH₃)₃COH (tert-butanol) is a classic example of a tertiary alcohol.
• Reactivity: They are the most reactive among all alcohols in dehydration reactions due to their carbocation stability.
• Carbocation Stability: With three carbon groups, these carbocations have maximal hyperconjugation and inductive effects aiding stability.

Tertiary alcohols are favored in reactions requiring rapid dehydration, as their carbocation transitions smoothly towards forming alkenes.