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Question: The problem involves a two-step reaction sequence starting from cyclopentene. **Step 1: Reaction of...

The problem involves a two-step reaction sequence starting from cyclopentene.

Step 1: Reaction of Cyclopentene with Br₂

Cyclopentene is an alkene. Alkenes undergo electrophilic addition with halogens like bromine (Br₂). This reaction is typically an anti-addition, meaning the two bromine atoms add to opposite faces of the double bond.

The product (A) formed is 1,2-dibromocyclopentane.

  • Structure of Cyclopentene: SMILES: C1C=CCC1

       CH₂-CH₂
  /       \
 CH=CH     CH₂

  • Structure of A (1,2-dibromocyclopentane): SMILES: BrC1C(Br)CCC1

       CH₂-CH₂
  /       \
 CHBr-CHBr CH₂

Step 2: Reaction of A (1,2-dibromocyclopentane) with Alcoholic KOH

Alcoholic KOH is a strong base and is used for dehydrohalogenation (elimination reaction). When a vicinal dihalide (like 1,2-dibromocyclopentane) is treated with alcoholic KOH, it undergoes double elimination of HBr to form an alkyne.

The reaction proceeds in two steps:

  1. First elimination: One molecule of HBr is removed from 1,2-dibromocyclopentane to form a bromoalkene (specifically, 1-bromocyclopentene).

    1,2-dibromocyclopentane + alc. KOH → 1-bromocyclopentene + KBr + H₂O

  2. Second elimination: Another molecule of HBr is removed from 1-bromocyclopentene to form a cycloalkyne.

    1-bromocyclopentene + alc. KOH → Cyclopentyne (B) + KBr + H₂O
  • Structure of B (Cyclopentyne): SMILES: C1#CCCC1 
       CH₂-CH₂
  /       \
 C≡C       CH₂

Stability of Cyclopentyne:

Cyclopentyne is a highly strained molecule. The ideal bond angle for a carbon-carbon triple bond is 180°, which is impossible to achieve in a five-membered ring without significant angle strain and ring strain. Therefore, cyclopentyne is extremely unstable and cannot be isolated under normal conditions; it is typically observed only as a transient intermediate or polymerizes rapidly. However, in the context of predicting reaction products in organic chemistry, it is the theoretical product of this double dehydrohalogenation.

Summary of Products:

  • A: 1,2-dibromocyclopentane
  • B: Cyclopentyne
Answer

A is 1,2-dibromocyclopentane. Smiles: BrC1C(Br)CCC1

B is cyclopentyne. Smiles: C1#CCCC1

Explanation

Solution

  1. Cyclopentene undergoes electrophilic addition with Br₂ to form 1,2-dibromocyclopentane (A).

  2. 1,2-dibromocyclopentane (A) undergoes double dehydrohalogenation with alcoholic KOH. The first elimination forms 1-bromocyclopentene, and the second elimination forms cyclopentyne (B).