Problem 31 - A Complete Resource Book in Chemistry for JEE Main

Question

In the presence of peroxide, hydrogen chloride and hydrogen iodide do not undergo anti-Markownikoff's addition to alkenes because (a) both are highly ionic (b) one is oxidizing and the other is reducing (c) one of the steps is endothermic in both the cases (d) all the steps are exothermic in both the cases

Step-by-Step Solution

Answer

The correct option is (c): one of the steps is endothermic in both cases.

Step 1: Understand Anti-Markownikoff's Addition

In anti-Markownikoff's addition, the addition reaction proceeds against the typical Markownikoff’s rule, where the more electronegative atom attaches to the less substituted carbon atom. This generally happens when an alkene reacts with HBr in the presence of peroxides (initiation of a free radical chain reaction).

Step 2: Analyze the Role of Peroxide

Peroxides are responsible for initiating free radical chain reactions. These reactions alter the typical course of addition (anti-Markownikoff rather than Markownikoff). However, it specifically works well with HBr due to the comparable bond dissociation energies and radical stabilities.

Step 3: Consider Reaction Environments for HCl and HI

Hydrogen chloride (HCl) and hydrogen iodide (HI) are unable to produce anti-Markownikoff products in the presence of peroxide. This is because the bond energies and radical reactions for these compounds are not favorable for the free radical mechanism required for anti-Markownikoff addition.

Step 4: Evaluate Option (a)

Consider if both HCl and HI being highly ionic affects the addition process. Ionic nature allows for typical electrophilic addition but doesn’t impact anti-Markownikoff addition directly.

Step 5: Evaluate Option (b)

Consider if one being oxidizing and the other reducing affects their reactivity with alkenes. While HI is a reducing agent, it doesn't affect the anti-Markownikoff addition for HCl which is not generally recognized as oxidizing.

Step 6: Evaluate Option (c)

Consider if one of the steps being endothermic for both HCl and HI could suppress their radical reactions. Endothermic steps do not favor chain propagation in radical reactions.

Step 7: Evaluate Option (d)

Consider if all steps being exotheric affects their radical addition pathway. Exothermic steps usually favor chain reactions but do not explain why HCl and HI differ from HBr.

Step 8: Select the Correct Answer

After evaluating all options, option (c) emerges as logical, where one of the steps (forming free radicals) is endothermic in both HCl and HI, thus hindering the progression of a free radical chain reaction.

Key Concepts

Free Radical Chain Reaction

Free radical chain reactions are a fascinating type of chemical reaction where free radicals, which are highly reactive atoms or molecules with unpaired electrons, form and react to generate new radicals, thereby sustaining the reaction.
The process generally includes three main stages: initiation, propagation, and termination.

• Initiation: The reaction is kicked off by breaking a chemical bond, usually with heat or light. This generates two radicals. For example, in the presence of peroxide, the O-O bond splits to form two peroxide radicals.
• Propagation: The radicals produced in the initiation step react with stable molecules, like alkenes, to form new radicals. These new radicals can continue reacting with other molecules to sustain the reaction.
• Termination: Eventually, the radicals may combine with each other or different radicals to form stable molecules, effectively halting the chain reaction.

Free radical chain reactions can occur under various conditions but commonly require a spark—such as light or heat—to start the process, making the role of initiators like peroxides crucial.

Hydrogen Halides Reactivity

Hydrogen halides such as HCl, HBr, and HI show varied reactivity in chemical reactions, including their interaction with alkenes. The key to understanding this reactivity lies in the stability of the bonds and the nature of the halide.

• Bond Strength: The bond dissociation energy changes among the hydrogen halides, fortifying their reactivity levels. For instance, in HBr, bond strengths are aligned favorably for breaking in the presence of peroxides.
• Radical Formation: The ability to stabilize free radicals is essential. HBr can facilitate radical mechanisms due to favorable bond dissociation energies, unlike HCl and HI.
• Electronegativity and Size: Affects the nature of interactions. HCl is too stable, while HI is too reactive, resulting in difficulties in forming the stable radical required for anti-Markownikoff addition.

The interplay of these factors dictates how each hydrogen halide responds in radical reactions, impacting their ability to undergo typical vs. anti-Markownikoff additions when in the presence of peroxides.

Peroxides Role in Reactions

Peroxides are pivotal participants in free radical reactions, acting as initiators that can drive the reaction away from the more common Markownikoff path to the less expected anti-Markownikoff route.

• Initiators of Radical Formation: The weak O-O bond in peroxides is easily cleaved, allowing the formation of radicals that kickstart the reaction. This step is crucial in promoting radical chain mechanisms.
• Influence on Reaction Paths: By generating radicals, peroxides can shift an addition reaction from the electrophilic route to a free radical pathway. This radical path is what facilitates anti-Markownikoff addition as seen with HBr.
• Compatibility with Hydrogen Halides: Peroxides specifically alter the reaction course of HBr when reacting with alkenes, allowing the inversion seen in anti-Markownikoff's addition. This does not occur effectively with HCl and HI due to unfavorable stepwise energies.

Understanding the precise role of peroxides in these reactions helps clarify why only certain hydrogen halides, like HBr, proceed with anti-Markownikoff additions.