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Alkenes are not easily added directly to iodine (I₂). The main reasons are as follows:
**Thermodynamic Limitations**:
- The iodination reaction (forming 1,2-dibromoalkanes) is typically exothermic but reversible, and the product readily undergoes **reversible decomposition** back into alkenes and iodine at room temperature (*J. Org. Chem.* **1983**, *48*, 1627).
2. **Kinetic barrier**:
- The low polarization degree of iodine molecules makes it difficult to form a strong electrophilic I⁺ intermediate, resulting in a **slow electrophilic attack rate on the π bonds of alkenes** (*Chem. Rev.* **2006**, *106*, 4628).
3. **Solvent effect**:
- In non-polar solvents (such as CCl₄), the iodine addition efficiency is extremely low. Strong polar solvents (such as acetic acid) or Lewis acids (such as FeCl₃) are required to facilitate the dissociation of I₂ to form I⁺ (*Tetrahedron* **2005**, *61*, 12377).
**Exception**: Electron-rich alkenes (such as styrene) or reactions can be facilitated under high pressure conditions.
