Problem 63 - A Complete Resource Book in Chemistry for JEE Main

Question

Peroxide effect can be checked by the addition of sufficient amount of (a) monophenyl amine (b) diphenyl amine (c) triphenyl amine (d) pentaphenyl amine

Step-by-Step Solution

Answer

(b) diphenylamine

Step 1: Understanding the Peroxide Effect

The peroxide effect, also known as the Kharasch effect, is the anti-Markovnikov addition of hydrogen bromide (HBr) to alkenes in the presence of organic peroxides. The presence of peroxides leads to the formation of free radicals that initiate the reaction along a pathway that opposes the usual Markovnikov rule.

Step 2: Role of Amine in Testing for Peroxide

Amines can be used to test for the presence of peroxides due to their ability to reduce them. Specifically, diphenylamine is known to be used in tests for detecting the peroxide effect, reacting with peroxides to produce a color change, thus confirming their presence.

Step 3: Identifying the Appropriate Amine

Among the given options, diphenylamine (option b) is the compound traditionally used to confirm the presence of peroxides due to its reliability in producing clearly identifiable results upon reaction with peroxides.

Key Concepts

Anti-Markovnikov Addition

Anti-Markovnikov addition is a fascinating aspect of chemical reactions where molecules add across a double bond in a specific way that differs from the typical Markovnikov rule. Typically, in the addition of hydrogen halides to alkenes, the hydrogen atom bonds with the carbon having more hydrogen atoms (the rich get richer). However, during anti-Markovnikov addition, the opposite occurs. The hydrogen atom attaches to the less-substituted carbon of the alkene.

This process is usually seen in the presence of peroxides, hence the term 'peroxide effect'. It occurs specifically with hydrogen bromide (HBr) adding to alkenes, facilitated by free radicals. This reaction path is significant in organic chemistry as it allows for the creation of molecular structures that otherwise require different reaction conditions or catalysis.

Free Radical Mechanism

The free radical mechanism is a chain reaction that occurs in several organic processes, such as polymerization and in the anti-Markovnikov addition of hydrogen bromide to alkenes. Free radicals are highly reactive atoms or molecules with unpaired electrons. They are formed when organic peroxides decompose under heat or light.

In the anti-Markovnikov addition of HBr, the free radical mechanism involves three main steps:

• Initiation: Peroxides (ROOR') break down into alkoxy radicals (RO●).
• Propagation: These radicals react with hydrogen bromide to form a new bromine radical (Br●) while releasing ROH.
• Termination: Bromine radicals react with an alkene, creating a product and another radical, perpetuating the cycle until the radicals combine to form stable molecules and end the reaction.

This mechanism explains why the addition does not follow the typical Markovnikov rule, as the bromine radical adds to the less substituted carbon atom.

Testing for Peroxide Presence

Testing for peroxide presence in a reaction is crucial since peroxides can significantly alter the reaction path. Special tests can identify their presence, ensuring the desired outcome of the reaction. Among the common test methods is the use of diphenylamine.

Diphenylamine is a popular choice due to its sensitivity and reliability. When it comes into contact with peroxides, diphenylamine undergoes an oxidation reaction which causes a visible color change. This change in color, usually to blue, is a qualitative indicator of the presence of peroxides.

This method helps chemists ensure that anti-Markovnikov reactions or any peroxide-dependent reactions are indeed proceeding under the right conditions. By confirming peroxide presence, chemists can take steps to control or utilize these compounds effectively, ensuring both safety and efficacy in their chemical processes.