Problem 13 - A Complete Resource Book in Chemistry for JEE Main

Question

Substitution of chlorine takes place at higher temperature in (a) \(\mathrm{CH}_{2}=\mathrm{CH}_{2}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2}\) (c) \(\mathrm{CH} \equiv \mathrm{CH}\) (d) none of the above

Step-by-Step Solution

Answer

(b) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2}\) (propene) undergoes substitution with chlorine at higher temperatures.

Step 1: Understand the Reaction Type

Chlorination can occur via substitution or addition reactions. At higher temperatures, substitution reactions are favored. Therefore, we're looking for a molecule where chlorination will proceed through substitution rather than addition.

Step 2: Analyze Each Option

Option (a) is ethene \(\mathrm{CH}_{2}=\mathrm{CH}_{2}\); chlorine will add across the double bond at high temperatures. Option (b) is propene \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2}\); chlorine can substitute at the allylic position rather than add to the double bond at high temperatures. Option (c) is acetylene \(\mathrm{CH} \equiv \mathrm{CH}\); chlorine adds, not substitutes, across the triple bond at high temperatures. Option (d) states none of the above.

Step 3: Identify the Correct Option

Since option (b), propene, is the only option that involves substitution at high temperatures, it is the correct answer.

Key Concepts

Allylic Substitution

In organic chemistry, allylic substitution is a significant reaction where an atom or group near a double bond is replaced by another atom or group. It typically occurs in molecules possessing the allyl group, characterized by the general formula

• Allyl Group: \( ext{-CH}_2- ext{CH}=CH_2} \)

In the allylic substitution, the reaction focuses on removing a hydrogen atom adjacent to a double bond and substituting it with a different atom, commonly a halogen like chlorine. This reaction is unique due to:

• Resonance stabilization: The intermediate carbocation, formed when a hydrogen leaves, is stabilized by resonance. This stability makes the allylic position more reactive compared to other positions in the molecule.
• High temperature preference: Allylic substitution is favored at elevated temperatures over other types of reactions, such as addition across double bonds.

Understanding the allylic substitution pathway helps in predicting the behavior of certain organic molecules under conditions like high temperatures, making it a critical concept in organic synthesis.

Halogenation

Halogenation refers to the reaction where a halogen atom, such as chlorine or bromine, is added to an organic compound. This process can proceed via:

• Addition reactions: Halogen atoms add across double or triple carbon-carbon bonds, converting alkene or alkyne into a dihalide.
• Substitution reactions: A hydrogen atom is replaced by a halogen atom in a molecule, often observed in alkanes in the form of free-radical substitution.

During halogenation, the reaction mechanism often determines whether the process leads to substitution or addition. At increased temperatures, substitution tends to predominate, especially in allylic halogenation, because the allylic position allows radical and ionic stabilization. Understanding this helps chemists tailor reactions to synthesize desired products effectively, tailoring conditions such as temperature to achieve specific outcomes.

Reaction Mechanisms

Reaction mechanisms in organic chemistry are step-by-step descriptions of the process by which a chemical change occurs. Understanding these mechanisms allows chemists to predict how molecules will react under specific conditions. Some core principles of reaction mechanisms include:

• Electron movement: Curved arrows are used to depict the flow of electrons during a reaction, showing bond formation or breaking.
• Intermediates and Transition States: Intermediates are highly reactive, short-lived species formed during the reaction. Transition states represent high-energy configurations molecules must pass through.
• Energy Profile Diagrams: These diagrams illustrate the energy changes through different stages of a reaction, highlighting activation energy and reaction exothermicity or endothermicity.

Grasping these concepts is crucial for predicting and planning chemical reactions. In allylic substitution, for instance, understanding the resonance stability of intermediates lends insight into why certain reactions are preferred under specific conditions such as high temperatures. This knowledge enables chemists to manipulate reactions to guide synthesis in the desired direction, optimizing yields and efficiency.