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

Short answer

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

Step-by-step solution

Define Anti-Markownikoff's Addition

Anti-Markownikoff's addition refers to a reaction where the addition of a hydrogen halide to an alkene occurs with the halogen atom attaching to the less substituted carbon atom. This is typically facilitated by the presence of peroxides, which initiate a radical chain mechanism.

Analyze the Role of Peroxide in Reactions

Peroxide initiates a radical chain reaction that usually allows for anti-Markownikoff's addition in alkenes with hydrogen bromide (HBr), but not necessarily with other hydrogen halides like HCl or HI. This is because the radical mechanism requires suitable stability and energy changes which might not be present in HCl or HI reactions.

Examine Ionic and Reducing/Oxidizing Behaviour

Hydrogen chloride (HCl) and hydrogen iodide (HI) have different properties: HCl is not a strong oxidizing agent, and HI is a reducing agent. The oxidation-reduction nature could influence their reactivity and compatibility with the radical mechanism initiated by peroxides.

Analyze Energy Requirements in the Reaction Steps

Anti-Markownikoff's addition includes multiple reaction steps. If one of the steps is endothermic, it requires an input of energy which may not favor the reaction's progress with HCl or HI in the presence of peroxides, unlike HBr where all reaction steps are exothermic.

Evaluate Given Options

Given the properties of HCl and HI and their reactions, option (c) suggests that one step being endothermic could hinder the progress of the reaction. This aligns with why these halides do not undergo anti-Markownikoff's addition with peroxides.

Key concepts

Radical Chain Mechanism

The radical chain mechanism is a fascinating process that involves a sequence of steps wherein radicals are created and subsequently used to propagate a reaction. This mechanism is pivotal in the context of anti-Markownikoff's addition, particularly when peroxides are involved.
Radical reactions traditionally start with the homolytic cleavage of a peroxide bond, producing radicals.
These radicals are highly reactive species, seeking to pair their unpaired electrons with another.
In the addition of hydrogen bromide to alkenes, peroxides generate radicals that facilitate the addition of the bromine atom to the less substituted carbon of the alkene, exemplifying anti-Markownikoff's addition.
However, the stability of these radicals varies between different hydrogen halides.

• In the initiation step, radicals are produced from peroxides.
• In the propagation step, radicals react with alkenes to form new radicals and products.
• The reaction cycle continues until the termination step, where radicals combine to form stable molecules.

This chain sequence is critical because it determines the thermodynamics and kinetics of the overall reaction. Successful anti-Markownikoff addition relies on energy-efficient pathways, which are not always achievable with hydrogen chloride or hydrogen iodide due to their specific energy profiles.

Hydrogen Halides

Hydrogen halides consist of hydrogen bonded to halogen elements, such as iodine, bromine, or chlorine, creating compounds like hydrogen iodide (HI), hydrogen bromide (HBr), and hydrogen chloride (HCl). These compounds exhibit different reactivity in chemical processes because of their distinct chemical properties.
The typical trend among hydrogen halides is that acidity increases down the group, but this also corresponds to variations in bond strength and radical stability.

• Hydrogen bromide (HBr) is known for effectiveness in anti-Markownikoff's addition because the formed radical intermediates are sufficiently stable.
• On the other hand, hydrogen iodide (HI) and hydrogen chloride (HCl) do not undergo anti-Markownikoff addition with peroxides easily due to their respective radical strengths and reaction energies.
• The energy balance and radical chain reaction efficiency significantly determine their reactivity in specific conditions.

The choice of hydrogen halide directly affects whether an anti-Markownikoff reaction pathway can proceed, with HBr generally being more favorable under these conditions than HCl or HI.

Peroxides in Reactions

Peroxides are compounds containing an O-O single bond that can act as radical initiators due to their ability to homolytically cleave and form radicals.
In the context of anti-Markownikoff's addition, peroxides play a crucial role by initiating the radical chain mechanism that favors the attachment of a halogen to a less substituted carbon atom in alkenes.

• The presence of peroxides is vital for kick-starting the radical formation from hydrogen bromide in reactions conducted under these conditions.
• They have a dual role of providing both initiation energy and ensuring a suitable pathway for radicals to propagate through the reaction.
• The efficiency of peroxide-mediated processes depends on the ability to maintain a stable radical flow and manage the energy requirements involved.

While hydrogen bromide can efficiently carry out anti-Markownikoff's addition with peroxides, hydrogen chloride and hydrogen iodide struggle due to unfavorable energy profiles of radical steps, making peroxides less impactful in achieving the desired reaction pathway with them.