Question

Identify the correct set of reagents or reaction conditions ‘X’ and ‘Y’ in the following set of transformation.

• A. \( X = \text{conc. alc. NaOH, 80}^\circ \text{C}, \, Y = \text{Br}_2/\text{CHCl}_3 \)
• B. \( X = \text{dil. aq. NaOH, 20}^\circ \text{C}, \, Y = \text{HBr/acetic acid} \)
• C. \( X = \text{conc. alc. NaOH, 80}^\circ \text{C}, \, Y = \text{HBr/acetic acid} \)
• D. \( X = \text{dil. aq. NaOH, 20}^\circ \text{C}, \, Y = \text{Br}_2/\text{CHCl}_3 \)

Answer 1

The Correct Option is C

Determine the Reaction with Reagent X:
The starting compound, CH₃ − CH₂ − CH₂ − CH₂ − Br, is a 1-bromobutane.
When treated with concentrated alcoholic NaOH at 80°C, an elimination reaction (dehydrohalogenation) occurs, leading to the formation of an alkene.
The product after elimination is CH₃ − CH = CH − CH₃ (1-butene).

Reaction with Reagent Y:
After the formation of 1-butene, adding HBr in the presence of acetic acid will convert it back into an alkyl halide by electrophilic addition.
The final product is 1-bromo-2-butene.

Conclusion:
The correct set of reagents for X and Y is:
X = conc. alc. NaOH, 80°C
Y = HBr/acetic acid
This corresponds to Option (3).

Answer 2

Step 1: Understanding the given reaction sequence.
The given reaction starts from 1-bromopropane (CH₃–CH₂–CH₂–Br) and proceeds through two steps — reagent ‘X’ and reagent ‘Y’ — to finally form 2-bromopropane (CH₃–CH(Br)–CH₃).

Step 2: Step 1 — Reaction with reagent ‘X’.
When 1-bromopropane is treated with concentrated alcoholic NaOH at 80°C, it undergoes β-elimination (dehydrohalogenation) to form propene:
\[ \text{CH₃–CH₂–CH₂–Br} \xrightarrow[\text{80°C}]{\text{alc. NaOH}} \text{CH₃–CH=CH₂} + \text{HBr} \] This step produces an alkene (propene) as the intermediate product.

Step 3: Step 2 — Reaction with reagent ‘Y’.
Propene reacts with HBr in acetic acid, following the Markovnikov rule, where the bromine atom attaches to the more substituted carbon atom. This gives 2-bromopropane:
\[ \text{CH₃–CH=CH₂} + \text{HBr} \rightarrow \text{CH₃–CH(Br)–CH₃} \]
Step 4: Final identification.
Therefore, the reagents are:
\[ X = \text{conc. alc. NaOH, 80°C} \quad \text{and} \quad Y = \text{HBr/acetic acid.} \]
Step 5: Final Answer.
\[ \boxed{X = \text{conc. alc. NaOH, 80°C}, \, Y = \text{HBr/acetic acid}} \]