Question

What will be the product formed on reaction of styrene with hydrogen bromide?

Short answer

The product of styrene and hydrogen bromide reaction is 1-bromo-1-phenylethane.

Step-by-step solution

Identify the Reactants

Styrene is the reactant, which is an alkene structurally represented as C6H5-CH=CH2, with a phenyl group attached to the carbon-carbon double bond. The second reactant is hydrogen bromide (HBr), a typical reagent used in electrophilic addition reactions.

Understand the Reaction Type

The reaction between styrene and hydrogen bromide is an example of an electrophilic addition reaction. The double bond in styrene acts as a nucleophile, allowing HBr to add across the double bond.

Identify the Electrophile and Nucleophile

In this reaction, the double bond ( C=C ) in styrene acts as the nucleophile, while the hydrogen in HBr is the electrophile. The bromine will displace its position along the carbon chain after the electrophilic attack.

Determine the Reaction Mechanism

The reaction proceeds via Markovnikov addition. The hydrogen from HBr adds to the carbon atom of the double bond that already has the greater number of hydrogen atoms (the terminal carbon in styrene), forming a more stable carbocation intermediate.

Understand Carbocation Stability

Forming the carbocation at the benzylic position (adjacent to the phenyl group) is highly favorable because it is resonance stabilized by the phenyl group, resulting in an intermediate ion configuration that stabilizes the reaction.

Describe the Final Product Formation

After the carbocation intermediate forms, the bromide ion (Br⁻) acts as a nucleophile and attacks the carbocation. This results in the final product, where bromine is attached to the benzylic carbon, giving 1-bromo-1-phenylethane.

Key concepts

Markovnikov's Rule

In the context of organic chemistry, Markovnikov's Rule is a handy guideline to predict how certain reagents will add to alkenes. This principle states that in electrophilic addition reactions, the hydrogen atom from the reagent will attach to the carbon with more hydrogen atoms already present. This happens because it helps to form a more stable carbocation intermediate.
Using styrene and hydrogen bromide as an example, the hydrogen adds to the carbon in the double bond that has two hydrogen atoms. Consequently, the other carbon in the double bond, adjacent to the phenyl group, is left with a positive charge.
This prediction helps chemists determine the major product of a reaction. By applying Markovnikov's Rule, you can easily anticipate where the various parts of a reagent will attach during the reaction process.

• Hydrogen adds to the carbon with more hydrogen.
• Carbocation forms at the position allowing maximum stabilization.
• This rule simplifies predicting outcomes in alkene reactions.

Carbocation Stability

Carbocations are positively charged carbon species that play a crucial role in many organic reactions, including electrophilic addition. Stability of these intermediates greatly influences the pathway of the reaction. In the reaction of styrene with HBr, a benzylic carbocation is formed.
A benzylic position refers to the carbon atom adjacent to a benzene ring. This position is uniquely stable due to the phenomenon of resonance. The positive charge can be delocalized over the aromatic ring, stabilizing the carbocation.
The stability of a carbocation can be deduced from several principles:

• Tertiary carbocations are more stable than secondary, which are more stable than primary.
• Adjacent electron-donating groups and resonance structures further stabilize carbocations.

Understanding these principles can clarify why certain products form during chemical reactions, due to the preference for more stable, lower-energy intermediates.

Alkene Reactivity

Alkenes are hydrocarbons that contain a carbon-carbon double bond, making them highly reactive in a variety of reactions. The reactivity stems from the presence of the π-bond in the double carbon bond. This π-bond is a region of high electron density, which vulnerable electrophiles can attack.
In the case of styrene reacting with hydrogen bromide, the π-bond in the alkene reacts with the electrophile, bringing about the formation of a carbocation intermediate, as described previously.
Alkenes, due to their double bonds, react with:

• Hydrohalic acids (like HBr), leading to electrophilic addition reactions.
• Various other electrophiles that can break the double bond.

Understanding alkene reactivity helps in predicting the types of reactions they may undergo, and thus, determining the potential products of these reactions.

Nucleophile Electrophile Interaction

In chemical reactions, the interaction between nucleophiles and electrophiles is fundamental. A nucleophile is a chemical species that donates an electron pair to form a chemical bond. In contrast, an electrophile is a species that accepts an electron pair.
In the reaction between styrene and hydrogen bromide, the alkene acts as the nucleophile due to its electron-rich double bond. Meanwhile, the hydrogen atom in HBr is the electrophile. Initially, the nucleophile donates its electron pair to the electrophile, facilitating the addition of the Br atom across the double bond.
Key interactions include:

• The nucleophile (alkene) seeks electron-deficient sites.
• The electrophile (hydrogen in HBr) seeks electron-rich sites.

This fundamental interaction drives the reaction forward, leading to the formation of a more stable, saturated product.