I2

Definition

I2, or diatomic iodine, is a chemical compound consisting of two iodine atoms bonded together. It is a key element in the context of the addition of halogens to alkenes, as well as the formation of halohydrins from alkenes.

5 Must Know Facts For Your Next Test

1. I2 is a solid at room temperature and has a distinctive violet color.
2. The addition of I2 to an alkene, known as halogenation, occurs via an electrophilic addition mechanism.
3. The addition of HO-I (hypoiodous acid) to an alkene, known as halohydrin formation, also occurs through an electrophilic addition mechanism.
4. The presence of I2 can lead to the formation of cyclic iodonium ion intermediates during halogenation reactions.
5. Halohydrins formed from the addition of HO-I to alkenes can be useful synthetic intermediates for further reactions.

Review Questions

• Describe the mechanism of the halogenation of alkenes using I2.
  • The halogenation of alkenes using I2 proceeds through an electrophilic addition mechanism. First, the I2 molecule polarizes, creating a partial positive charge on one of the iodine atoms. This electrophilic iodinium ion then adds to the carbon-carbon double bond of the alkene, forming a cyclic iodonium ion intermediate. Finally, the iodonium ion is opened by the nucleophilic attack of the iodide ion, resulting in the formation of the dihalogenated alkane product.
• Explain the role of I2 in the formation of halohydrins from alkenes.
  • The addition of HO-I (hypoiodous acid) to an alkene, known as halohydrin formation, also occurs through an electrophilic addition mechanism. In this reaction, the HO-I molecule acts as the electrophile, with the iodine atom bearing a partial positive charge. The electrophilic iodinium ion then adds to the carbon-carbon double bond of the alkene, forming a cyclic iodonium ion intermediate. This intermediate is then opened by the nucleophilic attack of the hydroxide ion, resulting in the formation of the halohydrin product.
• Analyze the importance of I2 in the context of halogenation and halohydrin formation reactions with alkenes.
  • I2 plays a crucial role in both the halogenation and halohydrin formation reactions with alkenes. In the halogenation reaction, the polarization of the I2 molecule creates an electrophilic iodinium ion that can add to the carbon-carbon double bond, leading to the formation of a cyclic iodonium ion intermediate. This intermediate is then opened by the nucleophilic attack of the iodide ion, resulting in the dihalogenated alkane product. Similarly, in the halohydrin formation reaction, the HO-I molecule acts as the electrophile, with the iodinium ion adding to the alkene and forming a cyclic iodonium ion intermediate. This intermediate is then opened by the nucleophilic attack of the hydroxide ion, leading to the formation of the halohydrin product. The presence and reactivity of I2 are therefore critical for the successful completion of these important organic reactions.

Related terms

Halogenation: The process of adding a halogen (such as chlorine, bromine, or iodine) to a compound, often an alkene, to form a new product.

Halohydrin: A compound formed by the addition of a halogen (such as chlorine, bromine, or iodine) and a hydroxyl group (-OH) to an alkene.

Electrophilic Addition: A type of reaction where an electrophile (a species attracted to electrons) adds to the carbon-carbon double bond of an alkene, forming a new product.