R3.4.11 Electrophilic addition (Higher Level only)

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 $\pi$ 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 $\pi$ 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 $\pi$ 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 $\pi$ 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).

$$C_2{H}_4+B{r}_2\to C_2{H}_{4}B{r}_2$$

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 $\pi$ electrons of the C=C bond.
  • This breaks the H-Br bond heterolytically, forming a bromide ion (Br⁻) and a carbocation intermediate.
• Carbocation Formation:
  • One of the carbon atoms in the double bond becomes positively charged, forming a carbocation intermediate.
  • In symmetrical alkenes like ethene, the carbocation is identical regardless of which carbon becomes positively charged.
• Nucleophilic Attack:
  • The bromide ion (Br⁻) attacks the carbocation, forming bromoethane (C₂H₅Br).

$$C_2{H}_4+HBr\to C_2{H}_{5}Br$$

Electrophilic Addition of Water to Symmetrical Alkenes (Hydration)

The addition of water to alkenes occurs in the presence of an acid catalyst, such as sulfuric acid.

This process is known as hydration and is widely used in industry to produce alcohols.

Mechanism Steps:

• Protonation:
  • A proton (H⁺) from the acid catalyst acts as the electrophile.
  • It attacks the $\pi$ bond, breaking it and forming a carbocation intermediate.
• Nucleophilic Attack by Water:
  • A water molecule acts as a nucleophile, attacking the carbocation to form an oxonium ion (R-OH₂⁺).
• Deprotonation:
  • The oxonium ion loses a proton, yielding the alcohol.

For example, the hydration of ethene produces ethanol:
$$C_2{H}_4+H_{2}O\stackrel{H^{+}}{\to }{C}_2{H}_{5}OH$$

Key Features of Electrophilic Addition Mechanisms

When drawing electrophilic addition mechanisms, keep these points in mind:

1. Curly Arrows: Always show the movement of electron pairs with curly arrows.
2. Intermediate Formation: Highlight the formation of carbocations or other intermediates like bromonium ions.
3. Electrophile and Nucleophile Roles: Clearly identify which species acts as the electrophile and which as the nucleophile.

Reflection