Problem 31 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

In the presence of peroxide, HCl and HI do not give Anti-Markovnikov's additon to alkenes because (a) All the steps are endothermic in both the cases (b) One is oxidising 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

The reason is (c): One of the steps is endothermic in both HCl and HI cases.

Step 1: Understanding Anti-Markovnikov's Addition

In anti-Markovnikov addition, the hydrogen atom and the halogen are added to the alkene in such a way that the hydrogen atom attaches to the less substituted carbon. This is often facilitated by the presence of a peroxide as it creates free radicals that help in this type of addition.

Step 2: Role of Peroxides

Peroxides can initiate radical reactions. In the presence of peroxides, bromine radicals are generated which allow HBr to add in an anti-Markovnikov manner. However, this is not observed with HCl or HI.

Step 3: Analyze HCl and HI Mechanism

For HCl and HI, the radical chain mechanism necessary for anti-Markovnikov addition does not proceed efficiently. This is because the bond dissociation energy of HCl and HI make the formation of chlorine and iodine radicals less favorable.

Step 4: Thermodynamics of the Reaction

The reaction thermodynamics and the energy requirement of each step must be considered. If any of the steps are endothermic, the overall reaction is less likely to proceed. For both HCl and HI, one of the crucial steps in the radical chain is endothermic, which prohibits anti-Markovnikov addition.

Key Concepts

Radical Chain Mechanism

A radical chain mechanism involves a series of repetitive and self-sustaining reactions that produce radicals to continue the reaction cycle. It usually consists of three main steps: initiation, propagation, and termination.

• **Initiation:** This is where radicals are initially generated. For anti-Markovnikov addition, peroxides can decompose to produce radicals. These radicals start the chain reaction.
• **Propagation:** In this step, radicals produced during initiation react with stable molecules to produce new radicals. This continues the chain reaction as new radicals react further in the system. In our context, bromine radicals attack the alkene in a sequence that leads to the addition of HBr across the double bond.
• **Termination:** Here, radicals may combine to form stable, non-radical molecules. This step effectively ends the chain reaction. However, in many radical additions, termination is a minor pathway, allowing the radicals to keep the chain going for multiple cycles.

In the specific case of HCl and HI, the radical initiation isn't efficient due to bond strengths, resulting in an unsuccessful chain mechanism.

Bond Dissociation Energy

Bond dissociation energy refers to the energy required to break a particular bond and generate radicals. It's a critical factor in determining whether a radical chain mechanism will proceed.
The effectiveness of anti-Markovnikov additions with different hydrogen halides (HCl, HBr, HI) correlates with their bond dissociation energies:

• **HCl:** Has a strong H-Cl bond. The energy required to break this bond is high, making the formation of chlorine radicals difficult. Therefore, it cannot effectively participate in the radical chain process needed for anti-Markovnikov addition.
• **HBr:** Has a weaker bond compared to HCl, making it easier to form bromine radicals, which actively partake in the chain reaction.
• **HI:** Iodine radicals should theoretically form as easily as bromine ones due to a weaker bond than HBr. However, iodine's reactivity in propagation steps may differ, affecting overall efficiency.

Hence, the varying bond dissociation energies of these halides directly influence their ability to participate in radical-based reactions.

Thermodynamics of Reactions

Thermodynamics describes the energy changes during a chemical reaction, influencing whether a reaction will spontaneously occur. In the context of radical reactions involving HCl and HI, thermodynamics plays a vital role.

• **Exothermic vs. Endothermic:** An exothermic reaction releases energy, often making it favorable and spontaneous. Conversely, an endothermic reaction absorbs energy, requiring an input of energy to proceed and making it less likely to happen spontaneously.
• **HCl and HI Reactions:** Both HCl and HI reactions involve key steps that are endothermic. This energy requirement inhibits the progression of the radical chain mechanism necessary for anti-Markovnikov additions.
• **Energy of Activation:** Even though a radical chain reaction may have exothermic elements, its requirement of an initial activation energy (particularly in initiating radicals) can also impede the process if the energy barrier is too high.

For a radical chain to proceed efficiently, critical steps should ideally be exothermic, or at the very least, not excessively endothermic, to favor the reaction's progress in the forward direction.