Question

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

Short answer

The major product is (b) 3-Iodo-pentane.

Step-by-step solution

Understanding the Reaction

This problem describes the reaction of pent-2-ene with HI in the presence of peroxy acids. The presence of peroxy acids suggests that a radical addition process will occur. This is similar to the anti-Markovnikov addition often seen in reactions involving peroxy acids.

Identifying the Reaction Mechanism

In the presence of peroxy acids, HQ (hypervalent iodine) forms, which initiates a radical chain reaction. This radical reaction usually leads to the anti-Markovnikov addition of HI across the double bond in alkenes.

Determining the Anti-Markovnikov Product

For the anti-Markovnikov addition of HI to pent-2-ene, iodine will add to the less substituted carbon (end carbon of the double bond). Hence, the iodine will attach to the carbon 3 of the chain, resulting in 3-iodo-pentane.

Checking the Major Product

Since the reaction proceeds through a radical mechanism facilitated by peroxy acids, the main product formed will be 3-iodo-pentane. The iodine attaches at the less substituted carbon due to the radical process favoring this pathway.

Key concepts

Radical Reaction Mechanism

The radical reaction mechanism is a fascinating process that differs from more common ionic mechanisms. It often involves the initial formation of free radicals, which are highly reactive species with unpaired electrons. Within this mechanism, radicals play a central role in initiating and propagating a chain reaction, leading to product formation. Unlike ionic mechanisms that may favor more substituted and stable carbocations, radical reactions often proceed through anti-Markovnikov pathways.
The reaction mechanism typically unfolds through several stages:

• Initiation: This step involves the creation of radicals, often catalyzed by agents like peroxy acids, which produce radicals capable of starting the reaction.
• Propagation: Radicals generated in the initiation step react with substrate molecules to form new radicals, continuing the chain reaction.
• Termination: The chain reaction ends when radicals combine to form a stable product, reducing the total number of radicals available for reaction.

Radical reactions are key to many industrial applications and synthetic processes, including polymerization and halogenation.

Peroxy Acids

Peroxy acids, also known as peracids, are organic acids containing a peroxide functional group (–O–O–). They play a vital role in organic synthesis due to their strong oxidizing properties. When it comes to radical reactions, peroxy acids serve as crucial initiators. They can generate free radicals necessary for specific reactions, such as the anti-Markovnikov addition in alkenes.
These acids are highly versatile and are often used in:

• Epoxidation: Conversion of alkenes into epoxides through oxygen transfer.
• Baeyer-Villiger Oxidation: Transformation of ketones into esters.
• Initiation of Radical Reactions: Their ability to produce radicals makes them useful for initiating polymerization and addition reactions.

Due to their strength, handling peroxy acids requires caution, as they can be explosive under certain conditions. Thus, understanding their reactivity and applications is essential for anyone working with organic reactions.

Iodine Addition

Iodine addition in organic chemistry typically involves the reaction of iodine or iodine compounds with alkenes or alkynes, leading to the formation of iodinated organic products. The addition of iodine often follows the anti-Markovnikov rule, especially in the presence of radical initiators like peroxy acids.
In the specific case of adding hydriodic acid (HI) to alkenes, such as pent-2-ene, the reaction favors the formation of less substituted alkyl iodides. This outcome is directed by the radical reaction mechanism initiated by peroxy acids, where the iodine is added to achieve a more stable radical intermediate.
The steps for iodine addition are similar to those seen in other radical reactions:

• Formation of a radical initiator, often facilitated by peroxy acids, which starts the reaction.
• Addition of iodine to the less substituted carbon, consistent with the anti-Markovnikov pattern.
• Stabilization of the resulting iodinated product, frequently seen as more stable due to radical stability considerations.

Iodine additions are highly practical in organic chemistry for introducing iodine atoms into complex molecules, useful in both industrial and laboratory syntheses.