Reactivity 3.4.11 – Electrophilic Addition Reactions of Alkenes

Alkenes are unsaturated hydrocarbons that contain at least one carbon-carbon double bond (\( \text{C} = \text{C} \)). This double bond is a region of high electron density due to the presence of a π bond in addition to the σ bond. As a result, it acts as a nucleophile and attracts electrophiles (electron-deficient species).

Why Alkenes Undergo Electrophilic Addition:

• The π electrons in the double bond are loosely held and are available to attack electrophiles.
• Electrophiles are attracted to these electrons, initiating the reaction.
• The product of electrophilic addition is a saturated compound, usually a haloalkane, alcohol, or other functionalized alkane depending on the reagent.

General Reaction Type:

Electrophilic addition involves two main steps:

1. Attack of the electrophile: The π electrons from the alkene attack the electrophile, forming a carbocation intermediate.
2. Nucleophilic attack: A nucleophile (e.g. halide ion, water) attacks the carbocation, completing the addition reaction.

General Equation:

\( \text{C}_n\text{H}_{2n} + \text{E} – \text{Nu} \rightarrow \text{C}_n\text{H}_{2n}\text{ENu} \)

Where E = electrophile and Nu = nucleophile

Common Electrophiles:

• \( \text{Br}_2 \), \( \text{Cl}_2 \) – react with alkenes to form dihaloalkanes
• \( \text{HBr}, \text{HCl}, \text{HI} \) – hydrogen halides give haloalkanes
• \( \text{H}_2\text{O} \) (in presence of acid catalyst like \( \text{H}_2\text{SO}_4 \)) – gives alcohols

Important Note:

For symmetrical alkenes like ethene or but-2-ene, the addition can occur identically on either side of the double bond, producing only one product. For unsymmetrical alkenes, regioselectivity (Markovnikov’s rule) becomes important – but this is covered later.

Physical Evidence of Electrophilic Addition:

• The decolourisation of orange bromine water (\( \text{Br}_2 \)) is a classic qualitative test for alkenes.

Mechanisms of Electrophilic Addition to Symmetrical Alkenes

This explains, step-by-step, the electrophilic addition mechanisms involving symmetrical alkenes (e.g. ethene, but-2-ene) with halogens (\( \text{Br}_2 \), \( \text{Cl}_2 \)), hydrogen halides (\( \text{HBr} \), \( \text{HCl} \)) and water (in acidic conditions). These mechanisms involve movement of electron pairs shown with curly arrows.

1. Reaction with Halogens (e.g. Bromine)

Example: Ethene + Bromine → 1,2-dibromoethane

Mechanism:

1. The bromine molecule (\( \text{Br}_2 \)) becomes temporarily polarised as it approaches the electron-rich double bond: \( \delta^+ \text{Br} – \delta^- \text{Br} \)
2. The π electrons from the double bond attack the slightly positive Br, breaking the Br-Br bond.
3. A bromonium ion intermediate forms (a three-membered ring with Br attached to both carbons).
4. The \( \text{Br}^- \) ion attacks the more positive carbon atom from the opposite side (anti-addition).

Overall Equation: \( \text{CH}_2 = \text{CH}_2 + \text{Br}_2 \rightarrow \text{CH}_2\text{BrCH}_2\text{Br} \)

Visual Summary: Curly arrows show π electrons attacking \( \text{Br}_2 \); then \( \text{Br}^- \) attacks the carbocation or bromonium ion.

2. Reaction with Hydrogen Halides (e.g. HBr)

Example: Ethene + HBr → Bromoethane

Mechanism:

1. The π electrons of the double bond attack the electrophilic H of HBr, forming a bond and breaking the H-Br bond.
2. This creates a carbocation intermediate on one of the carbon atoms.
3. The \( \text{Br}^- \) ion then attacks the carbocation to form the final product.

Overall Equation: \( \text{CH}_2 = \text{CH}_2 + \text{HBr} \rightarrow \text{CH}_3\text{CH}_2\text{Br} \)

Note: With symmetrical alkenes, only one product forms. With unsymmetrical ones, Markovnikov’s rule would apply (covered later).

3. Reaction with Water (Hydration) in Acidic Conditions

Example: Ethene + Water (acid-catalysed) → Ethanol

Mechanism:

1. Ethene reacts with an H⁺ from the acid catalyst to form a carbocation.
2. Water (\( \text{H}_2\text{O} \)) acts as a nucleophile and attacks the carbocation.
3. The resulting protonated alcohol loses a proton (H⁺) to form the neutral alcohol.

Overall Equation: \( \text{CH}_2 = \text{CH}_2 + \text{H}_2\text{O} \xrightarrow{\text{H}^+} \text{CH}_3\text{CH}_2\text{OH} \)

Note: This reaction is known as acid-catalysed hydration. The catalyst is usually \( \text{H}_2\text{SO}_4 \) or phosphoric acid.

General Mechanism Features:

• Step 1: Electrophile attacks π bond, forming a carbocation or cyclic intermediate.
• Step 2: Nucleophile attacks the carbocation or intermediate.
• All electron pair movements are shown using curly arrows.

Curly Arrows Summary:

• From π bond to electrophile
• From leaving group bond to its atom (e.g. Br-Br bond to Br)
• From nucleophile to carbocation