Reactivity 3.4.5 — Electrophilic Addition in Alkenes

Alkenes contain a carbon–carbon double bond (\( \text{C} = \text{C} \)) which consists of a sigma (σ) bond and a pi (π) bond. The π bond is formed by the sideways overlap of p-orbitals and contains electrons in a region of high electron density above and below the plane of the molecule. This makes the π bond accessible to electrophiles—species that are attracted to electron-rich regions.

Thus, alkenes undergo electrophilic addition reactions because the π electrons in the double bond can be donated to an electrophile, resulting in the formation of new sigma bonds.

Mechanism of Electrophilic Addition:

The general mechanism proceeds in two steps:

1. Step 1: The electrophile is attracted to the π electrons of the double bond. The π bond breaks and forms a new bond with the electrophile. A carbocation intermediate is often formed.
2. Step 2: A nucleophile attacks the carbocation, forming the final addition product.

Curly arrows are used to indicate the movement of electron pairs during this process.

Electrophilic Addition Reactions of Alkenes:

1. Addition of Hydrogen Halides (\( \text{HX} \))

Example with ethene:

\( \text{CH}_2=CH_2 + \text{HBr} \rightarrow \text{CH}_3\text{CH}_2\text{Br} \)

Mechanism:

1. π electrons attack the electrophilic hydrogen (\( \text{H}^+ \)) of HBr, forming a carbocation intermediate
2. \( \text{Br}^- \) attacks the carbocation

Regioselectivity: For unsymmetrical alkenes, Markovnikov’s rule applies — the hydrogen attaches to the carbon with more hydrogens already attached, and the halide goes to the more substituted carbon.

2. Addition of Halogens (\( \text{X}_2 \))

Example with bromine:

\( \text{CH}_2=CH_2 + \text{Br}_2 \rightarrow \text{CH}_2\text{Br}-\text{CH}_2\text{Br} \)

Mechanism:

1. Electron-rich double bond induces polarity in the \( \text{Br}_2 \) molecule
2. One Br atom forms a cyclic bromonium ion intermediate with both carbon atoms
3. The other \( \text{Br}^- \) attacks the more substituted carbon to open the ring

3. Addition of Water (in the presence of acid catalyst)

Hydration of ethene:

\( \text{CH}_2=CH_2 + \text{H}_2\text{O} \xrightarrow{\text{H}_2\text{SO}_4} \text{CH}_3\text{CH}_2\text{OH} \)

Mechanism:

1. Electrophilic \( \text{H}^+ \) from acid adds to the double bond, forming a carbocation
2. Water (nucleophile) attacks the carbocation
3. Loss of \( \text{H}^+ \) regenerates the acid catalyst

Note on Carbocation Stability:

• More substituted carbocations (e.g. tertiary > secondary > primary) are more stable due to inductive effects and hyperconjugation
• This stability influences the major product in electrophilic additions (explains Markovnikov’s rule)