Question

The major product obtained when 1-phenyl-buta-1,3-diene is treated with 1 equivalent of \(\mathrm{HBr}\) is

Short answer

The major product obtained when 1-phenyl-buta-1,3-diene is treated with 1 equivalent of \(HBr\) is 1-phenyl-3-bromobut-1-ene. This product is formed due to the resonance stabilization of Intermediate B, which is the most stable intermediate in this electrophilic addition reaction.

Step-by-step solution

Draw the structure of 1-phenyl-buta-1,3-diene.

First, let's draw the structure of 1-phenyl-buta-1,3-diene: It has a phenyl group (C6H5) attached to one end of a conjugated diene (buta-1,3-diene).

Determine the possible intermediates.

When the molecule reacts with HBr, one of the carbon-carbon double bonds will be broken, and the molecule will reorganize by forming an intermediate carbocation. The carbocation can be formed in two different ways depending on which carbon atom in the double bond receives the hydrogen atom: Intermediate A (formed with the addition of H to C-2 of the diene): Intermediate B (formed with the addition of H to C-3 of the diene):

Determine the most stable intermediate.

The major product will be determined by which intermediate carbocation is most stable. The stability of the carbocation is due to hyperconjugation and resonance effects. In this case, Intermediate B is the most stable. Because the positive charge is on the carbon next to a phenyl ring, there is resonance stabilization due to the delocalization of the positive charge into the phenyl ring. This resonance stabilization is not present in Intermediate A.

Formation of the major product.

Once we have determined the most stable intermediate (Intermediate B), we can proceed with the final step, where the bromide anion will attack the carbocation to create the major product. In Intermediate B, the carbocation is attached to the carbon next to the phenyl ring, so the bromine will be added to that same position. The major product when 1-phenyl-buta-1,3-diene is treated with 1 equivalent of HBr is as follows:

Key concepts

Carbocation Stability

Carbocations are positively charged carbon atoms, typically formed during certain organic reactions, such as electrophilic addition. The stability of these carbocations is crucial in determining the outcome of a reaction. Think of carbocations as the backbone or the decision points in these chemical transformations. The more stable a carbocation, the more likely it will form and lead to the major reaction product.

Several factors contribute to carbocation stability:

• Inductive Effect: This involves the donation of electron density through sigma bonds. Alkyl groups attached to a positively charged carbon can stabilize the carbocation by pushing electron density toward it.
• Hyperconjugation: This occurs when adjacent hydrogen atoms' sigma bonds overlap with the empty p-orbital of a carbocation, allowing for the delocalization of charge and contributing to the carbocation's stability.
• Resonance: A carbocation is more stable when its positive charge can be delocalized over several atoms, often through pi bond systems or other adjacent electron-rich structures.

In the case of 1-phenyl-buta-1,3-diene, when treated with HBr, the stability of the possible carbocations (Intermediate A and B) dictates which leads to the major product. Intermediate B is more stable because the positive charge can be delocalized into the phenyl ring, showcasing an excellent example of resonance stabilization.

Resonance Effect

Resonance is a powerful stabilization mechanism in organic chemistry, as it allows the delocalization of electrons across a structure. By spreading out the charge or electron density, molecules can lower their energy state, leading to greater stabilization. This concept is central to understanding why certain products are favored in reactions involving organic compounds.

In the context of carbocations formed during electrophilic addition reactions, resonance can drastically influence which intermediate forms predominantly:

• Delocalization: If a carbocation's positive charge can be delocalized over an entire structure, such as through a conjugated pi system, it becomes significantly more stable.
• Phenyl Group Stabilization: Aromatic rings like phenyl groups are particularly effective in resonance stabilization. They can effectively accommodate and distribute the positive charge, which makes connected carbocations much more stable.

In our example, the carbocation formed as Intermediate B benefits from resonance as the positive charge can resonate into the adjacent phenyl ring. This resonance effect is absent in Intermediate A, making Intermediate B the more stable and favored intermediate, directly affecting the major product's formation.

Markovnikov's Rule

Markovnikov's Rule is an important guideline in organic chemistry, especially relevant to the addition reactions of electrophiles like hydrogen halides (HX) to alkenes. Simply put, this rule predicts that in the addition of HX to an asymmetric alkene, the hydrogen atom will attach to the carbon with more hydrogen atoms already attached to it. This creates a more stable carbocation, ultimately directing the product's formation.

The logic behind Markovnikov's Rule lies in the preference for the formation of the most stable carbocation intermediate:

• Carbocation Stability: By attaching the hydrogen atom to the carbon that can lead to the most stable carbocation, the reaction can proceed quickly and favorably.
• Electrophile Placement: In practical terms, during electrophilic addition reactions, like the treatment of 1-phenyl-buta-1,3-diene with HBr, Markovnikov's Rule suggests that the hydrogen ion would add to the carbon already bonded to more hydrogen atoms. As a result, the intermediate more likely to form is the more stabilized one adjacent to a phenyl group, as seen in Intermediate B.

By applying Markovnikov's Rule, chemists can predict the major product of a reaction, realizing that the most stable carbocation will mainly determine the pathway of the reaction. In our specific chemical example, despite seeming randomness, Markovnikov's Rule helps us understand why Intermediate B leads to the major product.