Problem 22 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The addition of HI in the presence of peroxide catalyst does not follow anti- Markovnikov's rule because : [Main Online April 9, 2013] (a) \(\mathrm{HI}\) is a strong reducing agent. (b) H-I bond is too strong to be broken homolytically. (c) I atom combines with \(\mathrm{H}\) atom to give back HI. (d) Iodine atom is not reactive enough to add across a double bond.

Step-by-Step Solution

Answer

(b) H-I bond is too strong to be broken homolytically.

Step 1: Understand Anti-Markovnikov's Rule

Anti-Markovnikov's rule involves the addition of a molecule to a double bond such that the more electronegative component attaches to the less substituted carbon. It typically applies to reactions involving peroxides which can initiate free radical mechanisms.

Step 2: Examine the Role of Hydrogen Iodide

In the presence of peroxides, some molecules like HBr follow the anti-Markovnikov rule due to free radical addition. This involves the formation of free radicals initiated by the peroxides.

Step 3: Evaluate the H-I Bond

The H-I bond is very strong, making it difficult to break homolytically (i.e., forming free radicals). Without the generation of iodine radicals, the anti-Markovnikov addition mechanism cannot proceed.

Step 4: Consider Radical Formation

The formation of iodine radicals from HI is not favorable because the reactivity required to form such radicals exceeds the bond strength of H-I. Thus, HI lacks the radical reactivity needed.

Step 5: Conclusion Based on Options

Given the options, (b) H-I bond is too strong to be broken homolytically is correct. The lack of radical formation prevents anti-Markovnikov addition, and therefore, HI does not follow the rule in presence of peroxides.

Key Concepts

HI Bond Strength

The hydrogen iodide (HI) bond is characterized by its significant strength, which plays a crucial role in various chemical reactions. A strong bond like H-I resists breaking under normal conditions, a behavior central to why HI doesn't usually participate in free radical reactions.
Breaking a bond homolytically means splitting it so that each atom retains one of the shared electrons, forming radicals.

• The H-I bond, due to its strength, does not easily undergo homolytic cleavage.
• This difficulty in breaking means that free iodine radicals, which are necessary for certain reaction mechanisms, are not readily formed.

This resistance to homolytic cleavage is a barrier to many radical-based reactions, such as the anti-Markovnikov reactions. In contrast, other compounds like HBr have weaker bonds, allowing easier radical formation and participation in anti-Markovnikov processes.

Radical Mechanism

A radical mechanism is a type of reaction mechanism involving intermediates with unpaired electrons, known as free radicals. These radicals often originate from the homolytic cleavage of bonds catalyzed by peroxides.
Free radicals, because they are electron-deficient, react rapidly, making them integral to mechanisms like the anti-Markovnikov addition. In reactions where HI is involved, however, the formation of iodine radicals from homolytic cleavage is unfavorable due to the strength of the H-I bond.

• Without iodine radicals, the chain reaction necessary for anti-Markovnikov addition doesn't initiate.
• Consequently, HI behaves differently from other hydrogen halides under similar conditions.

This explains why, even in the presence of peroxides, HI does not follow the typical radical mechanism pathway as HBr does.

Peroxide-Catalyzed Reactions

Peroxide-catalyzed reactions are initiated by compounds known as peroxides, which contribute to radical formation by breaking down to produce radicals themselves. These reactions are notable for their ability to initiate anti-Markovnikov addition in certain molecules like HBr.
Peroxides are chosen because they can readily decompose to form radicals, which in turn can propagate a radical chain reaction.

• The efficiency of the peroxide catalyst relies heavily on the ease of radical formation from the reactants.
• However, if the compound in question, such as HI, has a strong bond that resists radical formation, the usual peroxide-induced pathways cannot proceed.

Thus, while peroxides are powerful agents for initiating certain reactions, their effectiveness is limited by the intrinsic characteristics of the reactants involved, such as bond strength and radical stability.