Problem 100 - A Complete Resource Book in Chemistry for JEE Main

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) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}(\mathrm{Br}) \mathrm{CH}=\mathrm{CH}_{2}\)

Step-by-Step Solution

Answer

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

Step 1: Recognize the Reactant and Product

The reactant, 3-phenylpropene, is an alkene that will undergo an addition reaction with HBr. We're looking for the major product of this reaction.

Step 2: Apply Markovnikov's Rule

According to Markovnikov's rule, in the addition of HX (like HBr) to alkenes, the hydrogen (H) from HBr attaches to the carbon with more hydrogen atoms already bonded to it, and the bromine (Br) attaches to the carbon with fewer hydrogen atoms.

Step 3: Identify the Alkene's Structure

In 3-phenylpropene, the double bond is between the second and third carbon atoms. The phenyl group is bonded to the first carbon, giving it the structure: \[ \text{C}_6\text{H}_5 - \text{CH}=\text{CH} - \text{CH}_3 \] Here, the terminal carbon (C connected to CH₃) is more substituted than the one bonded to the phenyl group.

Step 4: Determine the Major Product

Following Markovnikov's rule, the Br will attach to the more substituted carbon (second carbon), forming 2-bromo-3-phenylpropane. This corresponds to option (a): \[ \text{C}_6\text{H}_5\text{CH}_2\text{CH}(\text{Br})\text{CH}_3 \]

Step 5: Check for Any Rearrangements

Typically, we should check if carbocation rearrangements could occur for greater stability, but in this simple system, the given option (a) forms as expected due to Markovnikov's addition without any rearrangement.

Key Concepts

Addition Reactions

Addition reactions are fundamental in organic chemistry, especially when dealing with alkenes like 3-phenylpropene. These reactions involve the breaking of the double bond between carbon atoms and the addition of atoms or groups from the reactants to these carbons. For alkenes reacting with HBr, the process often follows Markovnikov's rule which guides where each part of the reagent (H and Br in this case) will bond.

This type of reaction typically occurs in a few stages. Initially, the alkene interacts with the acid (HBr), leading to the formation of a carbocation intermediate at one of the carbon atoms. This is where the pi bond of the alkene opens up to bond with the proton (H⁺) from HBr.

The bromine ion (Br⁻) then attaches to the electron-deficient carbon (carbocation), completing the addition reaction. Understanding these steps is crucial since the position and stability of the carbocation significantly affect the final structure of the product.

3-phenylpropene

3-phenylpropene is a specialized alkene, featuring a three-carbon chain with a phenyl group connected to the first carbon. Its structure can be depicted as \ \( \text{C}_6\text{H}_5 - \text{CH}=\text{CH} - \text{CH}_3 \ \). This compound is key in studying addition reactions, especially with HBr, due to its potential to form stable intermediates during reaction.

The double bond in 3-phenylpropene specifically exists between the second and third carbon atoms. This highlights an important aspect when considering reactions like Markovnikov additions: the differing substitution levels at each end of the alkene.

• The first carbon, attached to the phenyl group, offers stability due to resonance, whereas
• the third carbon is merely bonded to a methyl group.

This difference in substitution helps guide the regioselectivity of the addition reaction, aligning with Markovnikov's principle for where the bromine will attach.

Carbocation Rearrangement

Carbocation rearrangement is a possibility during addition reactions with alkenes. It involves shifting hydrogen or alkyl groups around to form a more stable carbocation. However, in the case of 3-phenylpropene reacting with HBr, such rearrangements are not needed nor do they occur.

This is because, upon initial application of Markovnikov's rule, the intermediate carbocation is already located at the second carbon (adjacent to the more stable phenyl group). This proximity to a phenyl group provides resonance stabilization, limiting any further shifts or rearrangement needs.

In more complicated reactions, carbocation rearrangements would be essential to ensure the most stable intermediate is formed. Factors influencing these reactions include carbocation stability order (3° > 2° > 1°) and the presence of other groups or atoms capable of participating in stabilization through conjugation or hyperconjugation effects.