IB Chemistry R3.4.11 Electrophilic Addition Notes

Why Are Alkenes So Reactive?

1. Alkenes are hydrocarbons containing at least one carbon-carbon double bond (C=C).
2. This double bond consists of a sigma (σ) bond and a pi (π) bond.
3. The $\pi$ bond is a region of high electron density, making it particularly attractive to electrophiles, species that seek electrons.
4. This feature makes alkenes highly reactive compared to alkanes.
5. During electrophilic addition reactions, the $π$ bond is broken, allowing the formation of two new covalent bonds.
6. 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:

1. Induced Dipole Formation:
   1. As the non-polar bromine molecule approaches the electron-rich $π$ bond, the electrons in bromine are repelled, creating a temporary dipole.
   2. One bromine atom becomes partially positive (δ⁺), while the other becomes partially negative (δ⁻).
2. Electrophilic Attack:
   1. The $π$ electrons of the C=C bond attack the δ⁺ bromine atom, breaking the Br-Br bond heterolytically.
   2. This forms a bromonium ion, a three-membered ring intermediate with a positive charge.
3. Nucleophilic Attack:
   1. The bromide ion (Br⁻), formed during the heterolytic fission of Br₂, attacks one of the carbon atoms in the bromonium ion, opening the ring.
   2. This opens the ring, resulting in the formation of 1,2-dibromoethane (C₂H₄Br₂). $$
      C_2H_4 + Br_2 \rightarrow C_2H_4Br_2
      $$