Electrophilic Addition Stereochemistry
Hardchemistry
Bromination of cis-2-butene yields which stereochemical relationship in product?
Select the correct option:
Solution
Incorrect! Answer:
Anti addition forming racemic mixture
- Mechanism: Addition of Br2 involves a cyclic bromonium ion intermediate.
- Stereospecificity: The attack of the second bromide ion always occurs from the side opposite to the bromonium bridge (anti-addition).
- Geometric Logic (CAR Mnemonic):
- Cis alkene + Anti addition → Racemic mixture.
- Trans alkene + Anti addition → Meso compound.
- Applying to 2-butene:
- Cis-2-butene reacts to form (2R,3R) and (2S,3S) dibromobutane in equal amounts.
- This is a racemic mixture.
- Contrast: Had we used trans-2-butene, we would get the meso-2,3-dibromobutane.
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About This Question
- Subject
- chemistry
- Chapter
- hydrocarbons
- Topic
- electrophilic addition stereochemistry
- Difficulty
- Hard
- Year
- 2025
Solution
Correct Answer:
Anti addition forming racemic mixture
- Mechanism: Addition of Br2 involves a cyclic bromonium ion intermediate.
- Stereospecificity: The attack of the second bromide ion always occurs from the side opposite to the bromonium bridge (anti-addition).
- Geometric Logic (CAR Mnemonic):
- Cis alkene + Anti addition → Racemic mixture.
- Trans alkene + Anti addition → Meso compound.
- Applying to 2-butene:
- Cis-2-butene reacts to form (2R,3R) and (2S,3S) dibromobutane in equal amounts.
- This is a racemic mixture.
- Contrast: Had we used trans-2-butene, we would get the meso-2,3-dibromobutane.
This hard difficulty chemistry question is from the chapter hydrocarbons, covering the topic of electrophilic addition stereochemistry. It appeared in the 2025 exam.
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