Question

Choose the correct set of reagents for the following conversion:

• A. \( \text{Cl}_2/\text{Fe}; \text{Br}_2/\text{anhy.} \text{AlCl}_3; \text{aq. KOH} \)
• B. \( \text{Br}_2/\text{Fe}; \text{Cl}_2, \Delta; \text{alc. KOH} \)
• C. \( \text{Cl}_2/\text{anhy.} \text{AlCl}_3; \text{Br}_2/\text{Fe}; \text{alc. KOH} \)
• D. \( \text{Br}_2/\text{anhy.} \text{AlCl}_3; \text{Cl}_2, \Delta; \text{aq. KOH} \)

Hint:

In aromatic substitution reactions, the presence of a halogen like chlorine or bromine can activate the ring towards further electrophilic substitution reactions. Use specific reagents and conditions (e.g., heat and alcoholic KOH) to control the outcome.

Answer 1

The Correct Option is B

To determine the correct set of reagents for the given conversion from ethyl benzene to a brominated styrene, we need to follow specific steps and utilize appropriate reactions:

1. The first step involves the bromination of ethyl benzene. This can be achieved using \(\text{Br}_2/\text{Fe}\). Iron acts as a catalyst to facilitate the bromination at the benzylic position.
2. Next, we need to form a double bond by elimination. This can be done by using \(\text{Cl}_2, \Delta\) (heat), which helps in dehydrohalogenation, converting the side chain to a vinyl group (removing the halogen and hydrogen to form a double bond).
3. The final step is to ensure the position of the double bond and get rid of any remaining groups to form the desired vinyl substituted product. This is done using \(\text{alc. KOH}\), which removes the hydrogen adjacent to the halogen, finalizing the elimination step.

Therefore, the correct sequence of reactions involves the use of bromination, followed by elimination to form the desired styrene structure. Hence, the correct set of reagents is:

\(\text{Br}_2/\text{Fe}; \text{Cl}_2, \Delta; \text{alc. KOH}\)

Answer 2

The given transformation involves the conversion of ethyl benzene (C\(_6\)H\(_5\)C\(_2\)H\(_5\)) to the desired product where a Br atom is added to the benzenoid ring. 
- First, bromination of ethylbenzene occurs using \( \text{Br}_2 \) in the presence of iron (Fe), which acts as a catalyst.
The electrophilic aromatic substitution reaction results in the formation of a brominated product. 
- After bromination, the next step involves chlorination using \( \text{Cl}_2 \) under heat (denoted by \( \Delta \)), leading to the introduction of the chlorine atom at the desired position. 
- Finally, the product undergoes dehydrohalogenation in the presence of alcoholic potassium hydroxide (alc.
KOH), leading to the formation of the double bond as required. 

Thus, the correct set of reagents to form the desired product is option (2).