Question

On the addition of \(\mathrm{HBr}\) to propene in the absence of peroxides, the first step involves the addition of: (a) \(\mathrm{H}^{+}\) (b) \(\mathrm{Br}^{-}\) (c) \(\dot{\mathrm{H}}\) (d) \(\dot{\mathrm{B}} \mathrm{r}\)

Short answer

The first step is the addition of \( \text{H}^+ \) (option a).

Step-by-step solution

Understanding the Reaction Context

The addition of HBr to alkenes like propene in the absence of peroxides follows Markovnikov's rule, where the hydrogen atom (H) from HBr is added to the carbon atom in the double bond with the most hydrogen atoms already attached.

Identify the Electrophile and Nucleophile

In this scenario, HBr can dissociate into \( ext{H}^+\) and \( ext{Br}^-\) ions. The \( ext{H}^+\) ion is an electrophile, meaning it is an electron pair acceptor, whereas the \( ext{Br}^-\) ion is a nucleophile, meaning it is an electron pair donor.

Determine the Initial Step in the Reaction

The first step in the reaction between HBr and propene involves the electrophile, \( ext{H}^+\), attacking the double bond of propene. This step results in the formation of a carbocation intermediate and is crucial for the Markovnikov addition process.

Key concepts

Understanding Electrophiles and Nucleophiles

In organic chemistry, the terms "electrophile" and "nucleophile" are crucial for understanding how reactions occur. An **electrophile** is a species that is attracted to regions of high electron density, meaning it seeks out electrons because it is electron-deficient. Electrophiles are often positively charged or have a partial positive charge. A classic example of an electrophile is the proton,

• The positively charged \( \text{H}^+ \) ion is an electrophile, as it searches for electrons to stabilize itself.
• Nucleophiles, on the other hand, are rich in electrons and donate a pair of electrons to electrophiles. Nucleophiles can be negatively charged ions, such as \( \text{Br}^- \), or molecules with lone pairs.

In the reaction between \( \text{HBr} \) and propene without peroxides, \( \text{H}^+ \) acts as the electrophile, starting the reaction by attacking the double bond of the alkene. This process highlights the foundational role of electrophiles and nucleophiles in understanding and predicting chemical reactions.

The Process of Carbocation Formation

Carbocation formation is a pivotal step in many alkene reactions, including the addition of \( \text{HBr} \) to propene. When an electrophile like \( \text{H}^+ \) attacks the alkene's double bond, it breaks one of the bonds, leading to a rearrangement of electrons. This loss of the \( \pi \) bond within the alkene creates a positively charged carbon, known as a carbocation.

• The formation of carbocations is governed by stability.In the \( \text{HBr} \) addition to propene, the hydrogen is added to the carbon with more hydrogen atoms (Markovnikov's rule), forming the more stable secondary carbocation as opposed to a primary carbocation.
• Secondary and tertiary carbocations are more stable than primary carbocations due to hyperconjugation and the inductive effect, where surrounding alkyl groups donate electron density.

This understanding of carbocation stability helps chemists predict products and guide synthetic strategies in more complex reactions.

Mechanics of Alkene Reactions

Reactions involving alkenes are integral to organic synthesis, with the addition of \( \text{HBr} \) to an alkene being a fundamental example. Alkenes contain a \( \pi \) bond which is electron-rich, making them attractive targets for electrophiles. This is particularly apparent in Markovnikov reactions.
In an \( \text{HBr} \) addition without peroxides, the mechanism follows the typical pattern:

• Initially, the alkene's \( \pi \) bond attacks the \( \text{H}^+ \) ion, forming a \( \text{C–H} \) bond and generating a carbocation at the more substituted carbon.
• Then, the \( \text{Br}^- \) nucleophile attacks the carbocation, completing the addition reaction.

Incorporating concepts like Markovnikov's rule and carbocation stability allows chemists to control reactions by steering the outcome towards desired products, demonstrating the importance of these reactions in creating a wide variety of organic molecules.