Problem 51 - A Complete Resource Book in Chemistry for JEE Main

Question

In the presence of peroxide, hydrogen chloride and hydrogen iodide do not undergo antiMarkownikoff'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 answer is (c) one of the steps is endothermic in both cases.

Step 1: Understanding Anti-Markovnikov Addition

Anti-Markovnikov addition refers to the addition reaction where, contrary to Markovnikov's rule, the negative part of the adding molecule (e.g., halide ion) attaches to the less substituted carbon atom of the alkene. This reaction usually involves peroxide-initiated mechanism with the formation of radicals.

Step 2: Analyzing Peroxide Effect

Peroxides can initiate radical reactions, such as the anti-Markovnikov addition. For alkenes to undergo this type of addition, the presence of a radical chain reaction mechanism is key, generally facilitated by peroxides and certain halogen acids, like HBr.

Step 3: Applying to Hydrogen Chloride and Iodide

The anti-Markovnikov addition does not happen with HCl and HI even in the presence of peroxides. This could be explored by examining the energy changes in the steps of the radical mechanism.

Step 4: Evaluating Energy Changes in Reaction Steps

For a radical-mediated reaction, some steps must fulfill specific thermodynamic criteria to proceed. An important aspect is that some of the radical intermediate formation steps are endothermic for both HCl and HI, meaning they absorb energy and are less favorable energetically, inhibiting the radical chain propagation.

Key Concepts

Radical Chain Reaction

A radical chain reaction is a process where radical intermediates are continuously reused to sustain the reaction. These reactions are commonly seen in organic chemistry, especially in mechanisms like anti-Markovnikov addition that involve radicals.

Here's how it works:

• Initiation: Radicals are generated, often using heat or light. For instance, in the presence of peroxides, alkanes can form radicals that start the chain reaction.
• Propagation: In this step, the radicals react with other molecules, turning these into additional radicals. This step is crucial because it continues the chain of reactions.
• Termination: The reaction concludes when radicals collide with each other, forming stable non-radical products.

This chain reaction is crucial for reactions like the anti-Markovnikov addition because it allows for the continual availability of radicals necessary for the reaction to proceed successfully.

Thermodynamics of Reactions

Thermodynamics is the study of heat and energy changes in a chemical reaction. For the radical chain reaction, specifically in anti-Markovnikov additions, understanding thermodynamics is essential as it impacts whether the reaction will proceed.

Consider the importance of energy changes in reactions:

• Endothermic Reactions: These reactions absorb energy. They require an input of energy to proceed. For HCl and HI reacting in a radical chain, certain steps are endothermic, making the reaction less favorable.
• Exothermic Reactions: These release energy, often making them more likely to occur because they lead to a stable energy state. In radical chain reactions, steps that are exothermic help drive the propagation phase.

In the case of HCl and HI, their endothermic intermediate steps hinder successful anti-Markovnikov addition, as energy absorption without sufficient release makes the continuation of the chain reaction energetically unfavorable.

Peroxide Effect

The peroxide effect, also known as the Kharasch effect, is critical in facilitating radical reactions, including the anti-Markovnikov addition of alkenes.

Here's how peroxides come into play:

• Peroxides decompose to form free radicals (typically at elevated temperatures) that initiate radical chain reactions.
• They break down to generate radicals capable of attacking alkenes to produce more radical species, a necessary condition for anti-Markovnikov additions.

The importance of peroxides lies in their ability to consistently generate radicals, sustaining the chain reaction. However, not all radical reactions proceed smoothly. With HCl and HI, the peroxide-initiated mechanism is incomplete due to unfavorable intermediate steps, thus inhibiting successful anti-Markovnikov addition.

Markovnikov's Rule

Markovnikov's Rule is a principle that guides the addition of compounds to alkenes. According to this rule, in a normal addition reaction, the hydrogen atom adds to the less substituted carbon atom, while the other component (e.g., halide ion) attaches to the more substituted carbon.

In contrast, anti-Markovnikov addition does not follow this rule. Instead, the incoming component adds to the less substituted carbon. The difference arises due to the mechanism:

• Markovnikov: Typically involves ionic mechanisms where stability is paramount. The carbocation intermediate prefers more stable arrangements in accordance with Markovnikov's rule.
• Anti-Markovnikov: Initiated by radicals, relies on the presence of peroxides to form the less stable configurations not favored by ionic pathways.

Thus, while anti-Markovnikov additions deviate from the rule, they depend on specific radical conditions and do not apply universally, as seen with HCl or HI, even when peroxides are present.