electrophilic addition stereochemistry

Question

Bromination of cis-2-butene yields which stereochemical relationship in product?

RankGuru · Accepted Answer

1. **Mechanism**: Addition of $Br_2$ involves a cyclic **bromonium ion** intermediate.
2. **Stereospecificity**: The attack of the second bromide ion always occurs from the side opposite to the bromonium bridge (**anti-addition**).
3. **Geometric Logic (CAR Mnemonic)**:
   - **C**is alkene + **A**nti addition $	o$ **R**acemic mixture.
   - **T**rans alkene + **A**nti addition $	o$ **M**eso compound.
4. **Applying to 2-butene**: 
   - Cis-2-butene reacts to form $(2R,3R)$ and $(2S,3S)$ dibromobutane in equal amounts.
   - This is a **racemic mixture**.
5. **Contrast**: Had we used trans-2-butene, we would get the meso-2,3-dibromobutane.

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electrophilic addition stereochemistry

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