Electrophilic Addition Reactions in Alkenes: Mechanisms and Applications
Why Are Alkenes So Reactive?
- Alkenes are hydrocarbons containing at least one carbon-carbon double bond (C=C).
- This double bond consists of a sigma (σ) bond and a pi (π) bond.
- The $π$ bond is a region of high electron density, making it particularly attractive to electrophiles—species that seek electrons.
- This feature makes alkenes highly reactive compared to alkanes.
- During electrophilic addition reactions, the $π$ bond is broken, allowing the formation of two new covalent bonds.
- These reactions proceed via a stepwise mechanism that often involves the formation of a carbocation intermediate or another reactive intermediate, such as a bromonium ion.
Electrophilic Addition of Halogens to Symmetrical Alkenes
Let’s begin with the reaction between ethene (C₂H₄) and bromine (Br₂), a classic example of electrophilic addition.
Mechanism Steps:
- Induced Dipole Formation:
- As the non-polar bromine molecule approaches the electron-rich $π$ bond, the electrons in bromine are repelled, creating a temporary dipole.
- One bromine atom becomes partially positive (δ⁺), while the other becomes partially negative (δ⁻).
- Electrophilic Attack:
- The $π$ electrons of the C=C bond attack the δ⁺ bromine atom, breaking the Br-Br bond heterolytically.
- This forms a bromonium ion, a three-membered ring intermediate with a positive charge.
- Nucleophilic Attack:
- The bromide ion (Br⁻), formed during the heterolytic fission of Br₂, attacks the positively charged carbon in the bromonium ion.
- This opens the ring, resulting in the formation of 1,2-dibromoethane (C₂H₄Br₂). $$
C_2H_4 + Br_2 \rightarrow C_2H_4Br_2
$$
- In this reaction, the bromine water test can be used to detect the presence of alkenes.
- The orange color of bromine water disappears as bromine reacts with the alkene to form a colorless product.
- Many students forget that the bromine molecule must first become polarized before the $π$ bond can attack it.
- Always show the induced dipole in your mechanism.
Electrophilic Addition of Hydrogen Halides to Symmetrical Alkenes
- Now consider the reaction between ethene and hydrogen bromide (HBr).
- Unlike bromine, HBr is already polar due to the electronegativity difference between hydrogen and bromine.
Mechanism Steps:
- Electrophilic Attack:
- The partially positive hydrogen atom (δ⁺) in HBr is attacked by the $π$ electrons of the C=C bond.
