Question

3-phenylpropene on reaction with HBr gives (as a major product) (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{Br}) \mathrm{CH}_{3}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}(\mathrm{Br}) \mathrm{CH}_{2} \mathrm{CH}_{3}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}(\mathrm{Br}) \mathrm{CH}=\mathrm{CH}_{2}\)

Short answer

The major product is (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH} (\mathrm{Br}) \mathrm{CH}_{2} \mathrm{CH}_{3}\).

Step-by-step solution

Understanding the Reaction Context

3-phenylpropene is an alkene containing a phenyl group attached to a propene chain. The reaction with HBr is a hydrohalogenation, in which the alkene undergoes an electrophilic addition of HBr. Here, the double bond in the alkene will react with HBr, with the possibility of forming a more stable carbocation.

Analyzing the Reaction Mechanism

First, the pi bond of the 3-phenylpropene attacks the hydrogen atom of HBr. This forms a carbocation intermediate. Due to carbocation stability trends, the more substituted carbocation is preferred. In this case, a secondary or tertiary carbocation will be more stable than a primary carbocation.

Identifying the Most Stable Carbocation

The formation of a secondary benzylic carbocation is favored because it is more stable than other possible carbocations. The phenyl group provides resonance stabilization to the intermediate, enhancing the reaction's likelihood of proceeding via this path.

Forming the Major Product

The bromide ion (Br⁻) will attack the most stable carbocation. The most stable intermediate here is the secondary benzylic carbocation, leading to the formation of the product \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH} (\mathrm{Br}) \mathrm{CH}_{2} \mathrm{CH}_{3} \). This is the most likely product due to its formation via the most stable carbocation.

Key concepts

Electrophilic Addition

Electrophilic addition is a fundamental reaction in organic chemistry, especially when working with alkenes. Alkenes, which contain a carbon-carbon double bond, are reactive due to the presence of pi bonds. These pi bonds are electron-rich, making them attractive to electrophiles, which are electron-seeking species. In an electrophilic addition reaction, an electrophile attacks the alkene, initiating the reaction.
During hydrohalogenation of an alkene, the electrophile is often a hydrogen atom from a hydrogen halide like HBr. The double bond attacks the hydrogen atom, breaking the H-Br bond and generating a carbocation as an intermediate. This step is crucial as it sets the stage for the remainder of the reaction.
The specificity of the reaction is often dictated by factors like carbocation stability, which influences which product will be favored as the reaction progresses.

Carbocation Stability

Carbocation stability is critical in determining the pathway and outcome of many alkene reactions. A carbocation is a positively charged carbon atom that emerges when an alkene reacts with an electrophile. In the context of alkene hydrohalogenation, after the electron-rich pi bond attacks the electrophile, a carbocation forms as a key intermediate.
The stability of this carbocation depends on several factors:

• Substitution: Tertiary carbocations (attached to three carbon groups) are more stable than secondary and primary carbocations. More substitution means better electron donation capable of stabilizing the positive charge.
• Resonance: Carbocations in resonance with electron-donating groups, such as a phenyl group, gain additional stability. This resonance effect is why a benzylic carbocation is particularly stable.

In our exercise, the secondary benzylic carbocation is the preferred intermediate due to its resonance stabilization, which significantly surpasses other possible structures.

Hydrohalogenation

Hydrohalogenation is a process where hydrogen halides (like HBr) add across the double bond of alkenes to form alkyl halides. This type of reaction is a specific example of electrophilic addition and is common in organic chemistry.
The reaction typically proceeds in two main steps: first, the alkene's pi bond attacks the hydrogen of the HBr, creating a carbocation. Then, the bromide ion (Br⁻), which is formed as a result of the HBr bond breaking, attacks the carbocation. This marks the completion of the reaction, yielding a single non-substituted halide.

• Regioselectivity: Often, this reaction is regioselective, meaning it forms preferentially one constitutional isomer over another. The nature of the alkene and the formation of the most stable carbocation dominate this selectivity.
• Markovnikov's Rule: This rule states that, in the addition of HX to alkenes, the hydrogen atom bonds to the less substituted carbon atom, allowing the halide to attach to the more substituted carbocation.

Understanding these principles is vital for predicting and explaining the outcomes observed in such addition reactions.

Alkene Reactions

Alkene reactions are a central topic of study in organic chemistry due to the diversity and abundance of alkenes in chemical synthesis and biological systems. These reactions characteristically exploit the reactivity of the carbon-carbon double bond, primarily through addition reactions like hydrohalogenation.
Here are some essential types of alkene reactions:

• Hydration: Adds water (H₂O) across the double bond, typically in the presence of an acid catalyst, forming alcohols.
• Halogenation: Involves the addition of halogen molecules (such as Br₂ or Cl₂) across the double bond, resulting in dihalides.
• Hydrogenation: The addition of hydrogen (H₂), usually in the presence of a metal catalyst like palladium, reducing the alkene to an alkane.

Each of these reactions involves the transformation of the double bond in different ways, yet they all rely on the initial vulnerability of the pi electrons to electrophilic attack, which is a cornerstone of how alkenes behave in various chemical environments.