Problem 92 - A Complete Resource Book in Chemistry for JEE Main

Question

In the presence of peroxide, and hydrogen chloride, hydrogen iodide does not undergo antiMarkovnikov's addition to alkenes because (a) both are highly ionic (b) one is oxidizing and 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

(c) one of the steps is endothermic in both the cases.

Step 1: Analyze Conditions

Identify the conditions given: the reaction involves the presence of peroxide and hydrogen chloride with hydrogen iodide. Commonly, peroxides aid in anti-Markovnikov addition for reactions such as free-radical additions.

Step 2: Understand Hydrogen Iodide's Behavior

Hydrogen iodide (HI) is known to not participate in anti-Markovnikov addition with alkenes in the presence of peroxides. This exception is due to the energetics of the radical formation process involving HI.

Step 3: Examine Oxidation and Reduction Potential

Evaluate if the mismatch in oxidizing and reducing potentials plays a role. Typically, hydrogen iodide forms radicals more hesitantly, making one pathway not favorable.

Step 4: Consider Reaction Energetics

HI radicals are relatively high energy and the propagation step involving HI is endothermic, making the radical chain process inefficient as the formed radicals are less stabilized.

Step 5: Evaluate Step Energetics

Determine which specific steps in the process are endothermic, confirming that one of the propagation steps involves energy input rather than release, making option (c) "one of the steps is endothermic in both the cases" correct for preventing the reaction.

Key Concepts

Hydrogen Iodide

Hydrogen iodide, or HI, is a binary compound consisting of hydrogen and iodine. It is a strong acid, dissolving in water to form hydroiodic acid. Hydrogen iodide is less commonly seen in organic reactions involving anti-Markovnikov additions when compared to its counterpart, hydrogen bromide (HBr).

In the context of reactions with alkenes, hydrogen iodide behaves differently due to its reaction energetics. It does not engage in anti-Markovnikov addition in the presence of peroxides. This is primarily due to the formation of an endothermic radical in the chain reaction step, which is energetically unfavorable. Unlike HBr, which forms relatively stable radicals that facilitate the anti-Markovnikov pathway, HI radicals are less stable and demand more energy to form and sustain in a reaction.

This higher energy requirement prevents the typical anti-Markovnikov mechanism from proceeding efficiently with hydrogen iodide.

Alkene Reactions

Alkenes are hydrocarbons characterized by the presence of at least one carbon-carbon double bond. These double bonds are unsaturated, meaning they can undergo additions, a versatile type of reactions where atoms are added to the carbon atoms of the double bond.

In an anti-Markovnikov addition, the addition occurs contrary to the more traditional Markovnikov's rule, which states that the hydrogen atom from an asymmetric reagent will attach to the less substituted carbon atom. Peroxides can instigate this different mechanism through a radical process, where the more stable carbon radical forms first, leading to different regiochemistry.

When it comes to hydrogen iodide, the addition to alkenes doesn’t follow this antiflow. This is because HI doesn't effectively form or sustain the necessary radical intermediates due to unfavorable energetic conditions, preventing the chain reaction essential for anti-Markovnikov addition.

Reaction Energetics

Reaction energetics refers to the energy changes that occur during a chemical reaction. Energetics are critical in determining whether a reaction proceeds. If a reaction's steps require too much energy, the reaction may become non-spontaneous.

For hydrogen iodide's role in potential anti-Markovnikov additions, the formation of radical intermediates is influenced by energetics. The process of creating these HI radicals is endothermic — it absorbs energy rather than releasing it, unlike other reactions which are exothermic and release energy, helping drive the reaction forward.

The unfavorable energetics of an endothermic propagation step mean that maintaining the radical chain reaction with HI is less efficient. Consequently, the entire reaction pathway becomes energetically disfavored. Therefore, the presence of an endothermic step is pivotal in understanding why hydrogen iodide fails to undergo anti-Markovnikov addition with alkenes, even with oxidizing agents like peroxides present.