Bromine water is decolourised upon reaction with (E)-3-hexene.
Q.25 The structure of the product obtained is

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Key Concept: Reaction of Alkenes with Bromine Water

• Bromine water reacts with alkenes via electrophilic addition.

• The reaction proceeds through a bromonium ion intermediate.

• Addition of Br₂ across a C=C bond is always anti-addition.

• The stereochemistry of the alkene (E or Z) determines the stereochemistry of the product.

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Structure of (E)-3-Hexene

• Double bond between C-3 and C-4

• (E)-configuration → alkyl groups are on opposite sides

• Structure:

  CH3–CH2–CH=CH–CH2–CH3
(E)


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Reaction Mechanism

1. The π-bond attacks Br₂ forming a cyclic bromonium ion

2. Br⁻ attacks from the opposite side

3. Results in anti-addition of bromine

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Nature of the Product

• Anti-addition across an E-alkene produces a meso or racemic vicinal dibromide

• In this case:

  • Product = 3,4-dibromohexane

  • Configuration = anti (one Br wedge, one Br dash)

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Correct Answer

Option (A)

This structure correctly represents anti-addition of Br₂ across the double bond of (E)-3-hexene.

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Explanation of All Options

Option (A) – Correct

✔ Shows vicinal dibromide
✔ Bromine atoms are added anti
✔ Matches reaction via bromonium ion
✔ Correct stereochemistry for (E)-alkene

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Option (B) – Incorrect

 Contains a double bond
 Bromine addition should remove unsaturation
 Not a final addition product

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Option (C) – Incorrect

 Shows syn-addition
 Bromine addition to alkenes is always anti, not syn

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Option (D) – Incorrect

 Both bromine atoms are on the same side
 Violates bromonium ion mechanism
 Not stereochemically feasible

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Final Answer

Option (A)

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Key Takeaways

• Bromine water decolourisation confirms presence of C=C bond

• Br₂ adds via anti-addition

• (E)-alkenes give anti-vicinal dibromides

• Bromonium ion controls stereochemistry