The correct sequence of reagents used in the preparation of 4-bromo-2-nitroethylbenzene from benzene is:
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In multi-step electrophilic aromatic substitution, the order of reactions is crucial. Pay close attention to the directing effects (ortho, para vs. meta) and activating/deactivating nature of the substituents introduced at each step. It's often strategic to perform reductions like Clemmensen or Wolff-Kishner to change a meta-director (-COR) into an ortho,para-director (-R).
Step 1: Understanding the target molecule
The final compound is 4-bromo-2-nitroethylbenzene.
Thus, the benzene ring contains:
an ethyl group ($-C_2H_5$),
a bromo group at the para position to ethyl,
a nitro group at the ortho position to ethyl.
Hence, the ethyl group must be introduced before nitration so that it can control orientation.
Step 2: Introduction of ethyl group (indirect method)
Direct Friedel–Crafts alkylation can lead to rearrangement, so the ethyl group is introduced via an acylation–reduction sequence.
\[
\text{Benzene}
\xrightarrow{\text{CH}_3\text{COCl/AlCl}_3}
\text{Acetophenone}
\]
This is Friedel–Crafts acylation. The acyl group is meta-directing but will be removed later.
Step 3: Reduction of acyl group
\[
\text{Acetophenone}
\xrightarrow{\text{Zn–Hg/HCl}}
\text{Ethylbenzene}
\]
Clemmensen reduction converts the $-COCH_3$ group into an ethyl group ($-CH_2CH_3$).
Ethyl group is:
activating,
ortho/para-directing.
Step 4: Bromination
\[
\text{Ethylbenzene}
\xrightarrow{\text{Br}_2/\text{AlBr}_3}
\text{4-bromoethylbenzene (major)}
\]
Bromination occurs mainly at the para position due to:
ortho/para-directing effect of ethyl group,
less steric hindrance at para position.
Step 5: Nitration
\[
\text{4-bromoethylbenzene}
\xrightarrow{\text{HNO}_3/\text{H}_2\text{SO}_4}
\text{4-bromo-2-nitroethylbenzene}
\]
Now two directing groups are present:
Ethyl group: strong activator, ortho/para-directing
Bromine: weak deactivator, ortho/para-directing
The ethyl group dominates orientation.
The para position is already occupied by Br, so nitration occurs at the ortho position to ethyl (position 2).
Step 6: Why other options are incorrect Options (A) and (C): Bromination or nitration occurs on acetophenone, which is meta-directing → wrong substitution pattern.
Option (D): Nitration first strongly deactivates benzene, preventing Friedel–Crafts reactions.
Conclusion
The correct reagent sequence is:
\[
\boxed{
\text{CH}_3\text{COCl/AlCl}_3 \;\rightarrow\;
\text{Zn–Hg/HCl} \;\rightarrow\;
\text{Br}_2/\text{AlBr}_3 \;\rightarrow\;
\text{HNO}_3/\text{H}_2\text{SO}_4
}
\]
Hence, the correct answer is Option (B).
