Question

Addition of HBr to 1-phenylpropene yields only (1-bromopropyl)benzene. Propose a mechanism for the reaction, and explain using resonance structures why none of the other regioisomer is produced.

Short answer

1-bromopropylbenzene forms due to the stable benzylic carbocation via resonance stabilization.

Step-by-step solution

Identify initial reactants and reaction components

In this reaction, HBr is added to 1-phenylpropene. 1-phenylpropene is an alkene with a phenyl group attached to its double-bonded carbon atoms. HBr, a hydrogen halide, will add across the double bond in a typical electrophilic addition reaction.

Protonation of the Alkene

The first step in the mechanism is the protonation of the double bond. The \( ext{H}^{+}\) from HBr will attack the more substituted carbon of the alkene due to Markovnikov's rule, forming a more stable carbocation intermediate. This creates a carbocation at the benzylic position adjacent to the phenyl ring, which is stabilized by resonance.

Carbocation Stabilization Through Resonance

The benzylic carbocation is stabilized by resonance with the phenyl group. This allows the positive charge to be delocalized over several atoms, significantly increasing the stability of the carbocation. The resonance structures include the movement of positive charge between the carbon atoms adjacent to the phenyl group.

Bromide Ion Attack

The bromide ion (Br⁻), which was released when HBr donated a proton, now acts as a nucleophile. It attacks the positively charged carbon of the stable carbocation. Since the carbocation is at the benzylic position, the attack leads to the formation of 1-bromopropylbenzene.

Explanation of Regioselectivity

The formation of (1-bromopropyl)benzene over other possible regiomers is attributed to the stability of the benzylic carbocation through resonance. Since this carbocation is more stable compared to other potential intermediates, the reaction proceeds in this direction exclusively, hindering formation of other regioisomers. Alternative carbocation formations lead to less stability and are thus not favored.

Key concepts

Carbocation Stability

In electrophilic addition reactions, such as the addition of HBr to alkenes, carbocation intermediates are formed. These intermediates are crucial because their stability largely determines the major products of the reaction. Carbocation stability is influenced by several factors:

• Inductive effect: More alkyl groups donate electron density to the positively charged carbon, thereby stabilizing it.
• Hyperconjugation: The overlap of adjacent bonding orbitals with the empty p-orbital of the carbocation also provides stability.

Breaking down our example, once HBr adds to 1-phenylpropene, a carbocation forms at the benzylic position. This position is adjacent to the phenyl ring, making it especially stable. The lone pair electrons from the phenyl are able to delocalize the positive charge through resonance, increasing stability significantly.
As a result, lesser stable carbocations, which could form on other positions, are not favored. Only the more stable benzylic carbocation is central, driving the reaction toward the formation of (1-bromopropyl)benzene.

Markovnikov's Rule

Markovnikov's Rule is a guiding principle used to predict the outcome of electrophilic addition reactions. It states that the hydrogen atom from a hydrogen halide (e.g., HBr) will attach to the less substituted carbon of a double bond, while the halide becomes bonded to the more substituted carbon. This rule is based on the stability of carbocation intermediates, where more substituted carbocations are generally more stable due to electronic effects such as hyperconjugation and the inductive effect.
In the addition of HBr to 1-phenylpropene, according to Markovnikov's Rule, the hydrogen attaches to the terminal carbon of the double bond (the less substituted carbon). This leaves the more substituted carbon adjacent to the phenyl group as a carbocation.
This rule effectively predicts the major product because it aligns with the formation of the most stable intermediary, guiding the selectivity and speed of the reaction's progression, leading to the predominant formation of 1-bromopropylbenzene.

Resonance Structures

Resonance structures are a concept used to describe the delocalization of electrons in molecules that cannot be represented by a single Lewis structure. In the context of benzylic carbocations, resonance stabilization is vital. Typically, resonance involves shifting electrons in the structure to show different possible arrangements of electrons.
For the 1-phenylpropene addition reaction, once a carbocation forms at the benzylic position, the positive charge can be delocalized over the entire aromatic ring.

• Delocalization: The positive charge moves throughout the ring, dispersing the charge across multiple atoms. This reduces the energy of the molecule, thus stabilizing it.
• Resonance hybrid: The actual structure is a hybrid of its resonance forms, appearing more stable than any single configuration.

The resonance effect greatly enhances the stability of the intermediate carbocation. This stabilization is key to why alternative pathways, which lack such delocalization, are not taken. The resonantly stabilized benzylic carbocation is the cornerstone of why only the desired product, (1-bromopropyl)benzene, results from this reaction.