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 option is (c).

Step 1: Understanding the Reaction

We are dealing with anti-Markownikoff's addition to alkenes in the presence of peroxides. Anti-Markownikoff's rule states that in the presence of peroxides, hydrogen halides add to alkenes with the hydrogen atom attaching to the more substituted carbon.

Step 2: Analyzing Reaction Mechanism

Typically, this reaction mechanism is free radical. Peroxides initiate the formation of radicals that facilitate the reaction. However, for anti-Markownikoff's addition to proceed, steps within the reaction mechanism need to be exothermic.

Step 3: Reviewing Hydrogen Halides Behavior

Hydrogen chloride (HCl) and hydrogen iodide (HI) do not undergo anti-Markownikoff's addition because their reaction pathways in the presence of peroxides are not favorable.

Step 4: Identifying Thermodynamic Constraints

A critical point in the reaction involving ext{HCl} and ext{HI} is that one of the necessary steps is endothermic, making the reaction not proceed spontaneously in these cases.

Step 5: Concluding the Correct Option

Among the given options, (c) 'one of the steps is endothermic in both cases' correctly explains why ext{HCl} and ext{HI} do not undergo the reaction in the presence of peroxides.

Key Concepts

Free Radical Mechanism

In chemistry, understanding how reactions proceed can be quite exciting! One important reaction type is the free radical mechanism. This type of reaction plays a crucial role in the anti-Markownikoff's addition to alkenes. When peroxides are present, they kickstart the creation of radicals. Here's how it works:

• Radicals are highly reactive species that have unpaired electrons.
• Peroxides, due to their structure, easily form radicals under the right conditions.
• Free radicals then attack double bonds in alkenes, allowing for the addition of hydrogen halides like HBr.

The presence of radicals can change how atoms add to alkenes. Normally, under regular conditions, atoms add in a pattern known as Markownikoff's rule. However, free radicals alter this, resulting in an anti-Markownikoff addition where the hydrogen atom attaches to the less substituted carbon. By understanding free radical mechanisms, chemists can choose the right conditions to steer reactions towards desired products!

Peroxide Effect

The peroxide effect is a fascinating phenomenon that occurs when peroxides alter the normal path of chemical reactions. Under typical conditions, hydrogen halides add to alkenes in a process guided by Markownikoff's rule. However, the presence of peroxides flips the script, resulting in anti-Markownikoff's addition.

• The peroxide effect demonstrates how certain catalysts can change the course of chemical reactions.
• Peroxides can initiate the formation of free radicals, which in turn direct the unique pathway of addition reactions.
• In the context of anti-Markownikoff's addition, peroxides enable the addition of hydrogen atoms to the more substituted carbon due to radical intermediates.

It is important to note that not all hydrogen halides follow this pathway. For example, hydrogen chloride and hydrogen iodide do not undergo anti-Markownikoff's addition in the presence of peroxides due to thermodynamic constraints. This highlights the selective nature of the peroxide effect in guiding specific reaction pathways.

Exothermic and Endothermic Reactions

Chemical reactions can either release energy or require additional energy input, and understanding this energy flow is crucial in predicting reaction behavior.

• Exothermic reactions release energy, usually in the form of heat, making them favorable or spontaneous under certain conditions.
• Endothermic reactions absorb energy, thus requiring an input of energy to proceed.

In the context of anti-Markownikoff's addition, the reaction steps need to be exothermic for the process to occur smoothly. When a hydrogen halide like HBr undergoes this addition, its reaction pathway is generally exothermic when facilitated by peroxides and radicals.

However, for hydrogen chloride and hydrogen iodide, one of the steps is endothermic in the presence of peroxides. This increased energy requirement means that these reactions do not proceed readily, explaining their exception to the typically observed anti-Markownikoff's addition pattern. By understanding which steps require additional energy, chemists can better predict and manipulate how and whether reactions will occur.