Question

The reaction of HBr with C=C(C)C in the presence of peroxide will give (a) CC(C)(C)C (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br}\) (c) CC(C)CBr (d) CCC(C)C

Short answer

The product is (c) \( \text{CC(C)CBr} \), following anti-Markovnikov addition.

Step-by-step solution

Understand the Reaction Type

This reaction involves the addition of HBr (hydrobromic acid) to an alkene in the presence of a peroxide. This is known as a radical addition reaction, specifically following the anti-Markovnikov rule due to the presence of peroxide.

Identify Peroxide Effect

In the presence of peroxides, the HBr addition to an alkene will proceed via a radical mechanism, which results in the bromine atom adding to the less substituted carbon atom of the double bond, following the anti-Markovnikov rule.

Analyze the Structure of Reactants

The reactant, as represented by the SMILES notation \( \text{C=C(C)C} \), is isobutylene, or 2-methylpropene, which has a double bond between the first carbon (C1) and the second carbon (C2).

Product Formation Analysis

According to the anti-Markovnikov rule, the bromine will attach to the less substituted carbon, on carbon 1 (C1), resulting in the formation of \( \text{CC(C)CBr} \), which corresponds to option (c).

Verify the Correct Product

Breaking down option (c) \( \text{CC(C)CBr} \): The main chain has 3 carbons with methyl (CH3) and bromine (Br) groups attached, forming 2-bromo-2-methylpropane. This matches the expected product from the reaction mechanism following the anti-Markovnikov pathway.

Key concepts

Radical Mechanism

In organic chemistry, a radical mechanism involves the formation and reaction of radicals. A radical is a species that has an unpaired electron, making it highly reactive. During the radical mechanism, bonds are broken and formed in a manner different from typical ionic or covalent interactions.

Radicals are often formed when a bond within a molecule breaks homolytically, meaning that each atom in the bond retains one electron. This is in contrast to heterolytic cleavage, where both electrons from a bond go to one atom, forming ions. In the context of the reaction between HBr and an alkene, radicals are responsible for the unique reactivity observed in the presence of peroxides.

In a common radical mechanism for addition reactions such as HBr addition in the presence of peroxides:

• Initiation: Peroxides decompose to form radicals that initiate the chain reaction.
• Propagation: Radicals react with the alkene to form new radicals, continuing the chain reaction.
• Termination: Radical species combine to form stable molecules, ending the reaction.

Understanding the radical mechanism is crucial for predicting the outcome of peroxide-mediated reactions, as it dictates the anti-Markovnikov addition of the halogen.

Peroxide Effect

The peroxide effect refers to the altered chemical behavior observed in reactions involving alkenes and hydrogen bromide (HBr) in the presence of peroxides. Normally, when HBr adds to an alkene, the product follows Markovnikov's rule, where the bromine atom adds to the more substituted carbon atom of the double bond. However, the presence of peroxides flips this usual rule.

This phenomenon occurs because peroxides decompose to create free radicals, which initiate a radical chain reaction. This initiation starts with the homolytic cleavage of peroxide, leading to the formation of free radicals that can affect the ener gy pathway of the reaction.

Key aspects of the peroxide effect include:

• Reversal of regiochemistry: The radical mechanism leads to anti-Markovnikov addition.
• Formation of less stable carbons: The bromine atom attaches to the less substituted carbon due to radical stability considerations.
• Reaction specificity: This effect is specific to the addition of HBr and does not typically occur with HCl or HI.

This effect is a classic example of how altering reaction conditions and initiating reagents can lead to different chemical pathways and outcomes.

HBr Addition

Hydrobromic acid (HBr) addition to alkenes is a fundamental reaction in organic chemistry that typically follows Markovnikov's rule. This rule states that in the addition of protic acids (HX) to alkenes, the hydrogen atom bonds to the carbon with the greater number of hydrogen substituents, while the halide attaches to the carbon with fewer hydrogens.

However, the addition of HBr becomes more interesting in the presence of peroxides. Then, the reaction proceeds through a radical mechanism rather than the typical ionic pathway, and the Markovnikov rule is not followed. Instead, the bromine adds to the less substituted carbon, leading to the anti-Markovnikov product.

Essential points about HBr addition include:

• Normal conditions: HBr typically adds in a Markovnikov manner without peroxides.
• With peroxides: The presence of peroxides switches the addition to an anti-Markovnikov pattern.
• Product formation: The reaction with peroxides yields a product like 2-bromo-2-methylpropane in the provided example, where bromine attaches to the less substituted carbon atom of the alkene.

Thus, understanding the conditions under which HBr additions occur can significantly influence the type of product formed, demonstrating the importance of reaction conditions.