Bromine water is decolourised upon reaction with (E)-3-hexene.
Q.24 It is due to
(A) electrophilic addition of bromine to C=C (B) nucleophilic addition of bromine to C=C
(C) electrophilic allylic bromination (D) nucleophilic allylic bromination
Bromine Water Decolourised by (E)-3-Hexene
Bromine water loses its color when reacting with (E)-3-hexene due to a key organic chemistry reaction. The correct answer is (A) electrophilic addition of bromine to C=C, a standard test for unsaturation in alkenes.
Correct Answer
The reaction decolourises bromine water because of electrophilic addition of bromine to the C=C double bond (Option A).
In this process, the electron-rich π bond of the alkene in (E)-3-hexene attacks the electrophilic Brδ⁺ from Br₂, forming a bromonium ion intermediate. The bromide ion (Br⁻) then attacks from the opposite side, adding across the double bond to yield a colorless 3,4-dibromohexane product. This consumes the brown Br₂, causing decolourisation.
Option Breakdown
(A) Electrophilic Addition of Bromine to C=C
Correct. Alkenes like (E)-3-hexene undergo electrophilic addition with Br₂ in water. The C=C acts as a nucleophile, breaking the Br-Br bond and forming a three-membered bromonium ion. Nucleophilic attack by Br⁻ completes the anti-addition, saturating the double bond and removing the red-brown color instantly.
(B) Nucleophilic Addition of Bromine to C=C
Incorrect. Bromine (Br₂) is not a nucleophile; it behaves as an electrophile due to its polarizable nature. Nucleophilic addition applies to electron-poor substrates like carbonyls, not electron-rich alkenes.
(C) Electrophilic Allylic Bromination
Incorrect. Allylic bromination (e.g., with NBS or Br₂ at high temperature/light) substitutes a hydrogen at the allylic position (carbon next to C=C), not adding to the double bond. It typically doesn’t decolourise bromine water quickly and requires radical conditions, unlike the direct room-temperature addition here.
(D) Nucleophilic Allylic Bromination
Incorrect. No such common mechanism exists. Allylic bromination is radical/electrophilic, not nucleophilic, and wouldn’t target the C=C for decolourisation in this context.
Reaction Scheme
For (E)-3-hexene (CH₃CH₂CH=CHCH₂CH₃):
Br₂ adds across C3=C4, giving meso-3,4-dibromohexane (colorless). This confirms unsaturation via the classic bromine test.
This mechanism is symmetric for (E)-3-hexene, yielding a single achiral product despite stereochemistry.
