Problem 31 - A Complete Resource Book in Chemistry for JEE Main

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

Step-by-Step Solution

Answer

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

Step 1: Understanding Anti-Markownikoff's Addition

Anti-Markownikoff's addition involves the addition of a compound such as extbf{HX} (where X is a halide) to an alkene in such a way that the halogen adds to the less substituted carbon. This usually occurs in the presence of peroxides, which facilitate a radical mechanism for the addition.

Step 2: Analyzing Peroxide Effect in Radical Reaction

Peroxides typically decompose to form radicals, which can initiate radical chain reactions. For anti-Markownikoff's addition to occur, the reaction should proceed through a radical mechanism that favors the addition of halogens to the least substituted carbon due to stability factors.

Step 3: Evaluating Options for Reaction Characteristics

(a) If both are highly ionic, they wouldn't follow a radical mechanism necessary for anti-Markownikoff's pattern but rather an ionic one. (b) One being oxidizing and the other reducing would imply incompatible interactions either causing different reactions or neutralization, disrupting a consistent radical pathway. (c) An endothermic step in the reaction would hinder progression as additional energy input is required, making radical formation less favorable. (d) All exothermic steps suggest the reaction proceeds spontaneously with energy release but could imply a non-radical path.

Step 4: Identifying the Correct Option Based on Analysis

The statement about one of the steps being endothermic (option c) is pivotal. If there is an endothermic step in both reactions, it implies additional energy is required for radical formation, making the peroxide-mediated radical mechanism unfavorable. Thus, anti-Markownikoff's addition does not occur.

Key Concepts

Radical Mechanism

In organic chemistry, a radical mechanism refers to a reaction pathway involving radicals, which are atoms or molecules with an unpaired electron. Radicals are highly reactive due to their tendency to form paired electrons, driving them to participate readily in chemical reactions. In the context of anti-Markownikoff's addition, a radical mechanism is crucial.

When peroxides are involved, they decompose to generate radicals. These radicals initiate a chain reaction with the alkene and halogen (e.g., HX). The radicals first interact with the alkene, generating new radicals that further propagate the chain, contributing to the unique positioning of the halogen atom on the less substituted carbon. This distinct placement can be explained by the stabilizing effect of the radical intermediate. Radicals typically favor more stable configurations, making the less substituted carbon a favorable site for halogen attachment. This mechanism deviates from the classical ionic addition seen in Markownikoff's rule, where the more substituted carbon typically bears the halogen.

The chain reaction in radical mechanisms continues until termination occurs, where radicals combine to form non-radical products, completing the reaction.

Peroxide Effect

The peroxide effect is an interesting phenomenon that specifically refers to the influence of peroxides in promoting radical reactions. This effect is primarily observed in the addition of hydrogen halides to alkenes, leading to anti-Markownikoff's addition.

Peroxides contain an -O-O- single bond that is relatively weak and susceptible to breaking, forming radical species. During this initiation step, peroxides typically decompose thermally or via ultraviolet light to release radicals. These radicals play a vital role in altering the pathway of the reaction toward a radical mechanism.

The peroxide effect hinges on the capability of these radicals to interact with hydrogen halides like HBr, generating halogen radicals that can attack the alkene. This process is regulated by the experimental conditions such as temperature and the presence of light, both of which can influence the efficiency of peroxide decomposition. Without peroxides, the reaction would favor a different path, typically resulting in a Markownikoff addition where the halogen attaches to the more substituted carbon.

Alkenes and Halogen Addition

The addition of halogens to alkenes is a fundamental reaction in organic chemistry, involving the transformation of a carbon-carbon double bond into two new single bonds. This addition can follow different pathways based on the reagents and conditions used.

Under typical circumstances, halogen addition to alkenes follows Markownikoff’s rule. This rule states that in the addition of HX to an unsaturated double bond, the hydrogen atom attaches to the less substituted carbon, and the halogen to the more substituted carbon, facilitating a stable carbocation intermediate.

However, in the presence of peroxides, we observe a shift in this pattern to favor an anti-Markownikoff addition. This switch arises because of the radical mechanism induced by the peroxide effect. The radicals generated prefer adding the halogen to the less substituted carbon, bypassing the ionic pathway typically observed.

This unique behavior contrasts standard halogen additions to alkenes, making it a topic of interest, especially in synthetic organic chemistry where such tailored reactions provide routes to otherwise challenging target molecules.