Problem 69 - A Complete Resource Book in Chemistry for JEE Main

Question

When pent-2-ene is treated with HI in presence of a peroxy acids the major product obtained is (a) 2-Iodo-pentane (b) 3-Iodo-pentane (c) both of these (d) none of these

Step-by-Step Solution

Answer

The major product obtained is (a) 2-Iodo-pentane.

Step 1: Understanding the Reaction Context

The problem involves an addition reaction where pent-2-ene is treated with hydroiodic acid (HI) in the presence of peroxy acids. This context suggests a free radical addition mechanism typical of anti-Markovnikov addition reactions because of the peroxy acid.

Step 2: Identifying the Reaction Type

In presence of peroxides, HI undergoes anti-Markovnikov addition to alkenes. This means that the iodine will be added to the least substituted carbon of the double bond in pent-2-ene.

Step 3: Analyzing the Structure of Pent-2-ene

Pent-2-ene is an alkene with a double bond between the second and third carbons. Its structure is CH3-CH=CH-CH2-CH3.

Step 4: Applying Anti-Markovnikov Rule

According to the anti-Markovnikov rule, iodine attaches to the less substituted carbon in the carbon-carbon double bond. Thus, iodine will attach to the second carbon, leading to the formation of 2-iodo-pentane.

Step 5: Conclusion

Based on the mechanism of the reaction, the major product formed will be 2-iodo-pentane.

Key Concepts

Anti-Markovnikov Addition

Anti-Markovnikov addition is an important concept in organic chemistry that helps dictate how certain reactions proceed. This principle states that in the presence of peroxides, such as peroxy acids, the electrophilic addition to alkenes occurs in a way that the hydrogen atom from the reagent attaches to the more substituted carbon atom. In this manner, the more electronegative part of the reagent, often a halogen like iodine, attaches to the less substituted carbon atom.

This is quite different from the usual Markovnikov addition, where the hydrogen attaches to the less substituted carbon atom.

• Anti-Markovnikov addition typically involves a radical mechanism, initiated by the presence of peroxides.
• It results in the product having the halogen bonded to the carbon with fewer hydrogen substituents (less substituted carbon).
• Understanding this mechanism is crucial for predicting the structure of the reaction products.

The anti-Markovnikov addition is crucial for understanding how certain compounds can be synthesized with desired substitution patterns.

Peroxides in Reactions

Peroxides play a pivotal role in facilitating anti-Markovnikov addition across carbon-carbon double bonds. They are often used as initiators in radical chain reactions.

How do peroxides work in these reactions? Here's a simplified view:

• Peroxides decompose to form radicals when subjected to heat or radiation.
• These radicals help initiate the formation of more radicals and propagate the reaction.
• In the context of the problem, peroxides convert the hydrogen iodide (HI) into radicals that favor the addition of the iodine radical to the less substituted carbon atom of pent-2-ene.

The presence of peroxides thus changes the typical addition pattern to favor anti-Markovnikov addition, a key point to be aware of in these mechanisms.

Pent-2-ene Reaction Mechanism

The reaction mechanism for pent-2-ene in the presence of HI and peroxy acids involves several key steps.

Let's walk through this mechanism to understand why 2-iodo-pentane is formed as the major product:

• The first step involves the generation of iodine radicals from the decomposition of peroxides, which interact with hydrogen iodide (HI).
• The iodine radical then adds to the less substituted carbon of pent-2-ene, which specifically is the second carbon when counting from the end closer to the double bond.
• This results in the formation of a radical intermediate on the more substituted carbon, which is then terminated by another HI molecule donating a hydrogen radical, thereby completing the reaction.
• The alkene structure of pent-2-ene, with its double bond placed between the second and third carbon, plays a critical role in determining the site of iodine attachment.

This mechanism elucidates why 2-iodo-pentane is the major product, aligning with the anti-Markovnikov rule due to the presence of peroxides that favor the addition to the least substituted carbon.