Question

In the given sequence of reactions, identify 'P, 'Q', 'R' and 'S' respectively.

• A. Br₂ , Alc.KOH, NaOH, Al₂O₃
• B. HBr, Alc.KOH, CaC₂, KMnO₄
• C. HBr, Alc.KOH, NaNH2, Red hot iron tube
• D. Br₂, Alc.KOH, NaNH₂, Red hot iron tube

Answer 1

The Correct Option is D

The given reaction sequence is:

1. CH₂=CH₂ → P: The alkene ethene (CH₂=CH₂) reacts with Br₂ in the first step, which typically leads to the formation of 1,2-dibromoethane (P) through an electrophilic addition mechanism.
2. P → Q: 1,2-dibromoethane (P) then undergoes dehydrohalogenation with alcoholic KOH, leading to the formation of vinyl bromide (Q). The elimination of HBr forms the alkene CH₂=CH-Br.
3. Q → R: In the third step, NaNH₂ (Sodium amide) is used to dehydrohalogenate the vinyl bromide, resulting in the formation of acetylene (R), which is ethyne (CH≡CH).
4. R → S: Finally, the acetylene (R) undergoes partial hydrogenation using a red-hot iron tube to form benzene (C₆H₆).

Thus, the reactions are:

• P: 1,2-dibromoethane.
• Q: Vinyl bromide.
• R: Acetylene (ethyne).
• S: Benzene.

This sequence of reactions corresponds to option (D).

Answer 2

• P: Br₂ reacts with the alkene to add across the double bond.
• Q: The compound undergoes alcoholic KOH elimination (dehydrohalogenation) to form a conjugated alkene.
• R: NaNH₂ in a base-induced elimination leads to the formation of an alkyne.
• S: The alkyne undergoes a reduction reaction with red hot iron tube to form benzene (C₆H₆).