Question

Addition of HBr to propene yields 2-bromopropane, while in the presence of benzoyl peroxide, the same reaction yields 1-bromopropane. Explain and give mechanism.

Short answer

HBr adds to propene via ionic addition for 2-bromopropane (without benzoyl peroxide). With benzoyl peroxide, free radical addition yields 1-bromopropane.

Step-by-step solution

Understand the Reaction

In the absence of benzoyl peroxide, HBr adds to propene (an alkene) following Markovnikov's rule. This rule states that the hydrogen atom from HBr will attach to the carbon atom with the greater number of hydrogen atoms, and the bromine will attach to the carbon with fewer hydrogen atoms. This results in 2-bromopropane as the main product.

Identify the Role of Benzoyl Peroxide

When benzoyl peroxide is present, the reaction proceeds via a radical mechanism instead due to the initiation of radical formation. Benzoyl peroxide decomposes upon heating to form free radicals, which initiate the reaction chain.

Initiation Step

In this step, benzoyl peroxide decomposes to generate free benzoyl radicals. The benzoyl radicals react with HBr to form bromine radicals and benzaldehyde molecules.

Propagation Step 1

The bromine radical formed in the initiation step adds to the terminal carbon of the propene molecule, creating the more stable secondary radical (Markovnikov radical addition is not favored in this case). This is because radical stability is influenced by both hyperconjugation and the electron-donating nature of substituents.

Propagation Step 2

The new radical formed on the propene reacts with another molecule of HBr. The radical substitutes the hydrogen atom which binds to the radical center, and a new bromine radical is formed. This results in the formation of 1-bromopropane.

Termination Step

The chain reaction is concluded when two radical species combine to form a stable molecule, effectively removing radicals from the solution. This could be the combination of two bromine radicals or other possible radicals formed during the process.

Key concepts

Markovnikov's Rule

Markovnikov's Rule is an essential concept in organic chemistry, especially in electrophilic addition reactions. It helps predict the outcome of certain reactions involving alkenes like the addition of hydrogen halides to alkenes. According to this rule, when hydrogen halides such as HBr are added to an alkene, the hydrogen atom will attach itself to the carbon atom that already has more hydrogen atoms attached. This can be summed up as "the rich get richer," meaning the more substituted carbon will receive the additional hydrogen, while the halide will bind to the less substituted carbon.
For example, when HBr is added to propene without any radical initiator, the hydrogen attaches to the terminal carbon (CH extsubscript{2}), which already has more hydrogen atoms, resulting in the formation of 2-bromopropane as the product.

Benzoyl Peroxide

Benzoyl peroxide is a common radical initiator used in various chemical reactions, making it a crucial element for initiating radical mechanisms. This compound is known for its ability to decompose into free radicals upon heating or exposure to light. The structure of benzoyl peroxide contains an O-O bond, known as a peroxide bond, which easily breaks under heat to form two benzoyl radicals.
The presence of these radicals changes the course of the reaction from a traditional Markovnikov addition to a free radical mechanism. Benzoyl peroxide is especially useful in industrial applications such as polymerization reactions and in organic synthesis for the introduction of specific functional groups.

Radical Initiation

Radical initiation is the initial step in a radical reaction mechanism where radicals are generated from non-radical species. This crucial step sets off a chain reaction that leads to the final product.
In the case of benzoyl peroxide, radical initiation occurs when the O-O bond in benzoyl peroxide undergoes homolytic cleavage upon heating. This cleavage results in the formation of benzoyl radicals, which are highly reactive species. These benzoyl radicals then engage with HBr molecules, leading to the creation of bromine radicals.
This is the turning point, playing a pivotal role in setting the stage for the subsequent propagation steps of the radical chain reaction.

Addition Reactions

Addition reactions are fundamental to organic chemistry and involve the addition of two or more atoms or groups to a molecule with multiple bonds, such as double or triple bonds. In the context of alkenes like propene, addition reactions often involve the opening of the double bond to allow new groups or atoms to bind.
The outcome of addition reactions can vary significantly depending on the presence of radical initiators. For example, with benzoyl peroxide present, the typical Markovnikov-directed addition of HBr to propene is altered, resulting in a radical addition mechanism. This shift leads to the formation of 1-bromopropane instead of the 2-bromopropane that would form under normal electrophilic conditions.

Propagation Steps

Propagation steps are a crucial phase in a radical chain mechanism and involve a series of repetitive steps that build up the main product through radical intermediates. These steps ensure that the radical chain continues and multiplies until a sizeable amount of product is created.
In the HBr addition to propene with benzoyl peroxide, the propagation steps involve a bromine radical adding to the terminal carbon of propene to form a secondary radical. This radical is more stable due to factors like hyperconjugation. The newly formed radical then reacts with another HBr molecule, where the radical abstracts a hydrogen, forming 1-bromopropane and regenerating the bromine radical to continue the reaction cycle.
These steps repeat continuously and are responsible for magnifying the effects of radical initiation in forming significant amounts of the desired product.