Reactions of Alkenes: Electrophilic Addition and Mechanism
- Alkenes are hydrocarbons that contain at least one carbon-carbon double bond (C=C).
- This double bond consists of two parts: a strong sigma (σ) bond and a weaker pi (π) bond.
- The pi bond lies above and below the plane of the molecule, making it a region of high electron density.
- This electron-rich area is highly attractive to electrophiles—species that are electron-deficient and seek out electrons to form bonds.
- Electrophiles are molecules or ions that accept a pair of electrons to form a covalent bond.
- When alkenes encounter electrophiles, they undergo electrophilic addition reactions, where the double bond is broken, and new covalent bonds are formed.
Examples include halogen molecules (e.g., $Br_2$, hydrogen halides (e.g., HBr), and protons $H^+$ in acidic solutions.
TipThe pi bond is weaker than the sigma bond, making it the reactive part of the double bond in alkenes.
Electrophilic Addition Reactions of Alkenes
Reaction with Halogens
When alkenes react with halogens like bromine $Br_2$ or chlorine $Cl_2$, the halogen atoms add across the double bond to form a dihalogenoalkane.
Example$$\text{C}_2\text{H}_4 + \text{Br}_2 \rightarrow \text{C}_2\text{H}_4\text{Br}_2$$
- This reaction is often used as a test for unsaturation because bromine water, which is orange, becomes colorless in the presence of an alkene.
- The disappearance of color indicates that the double bond has reacted with bromine.
