Electrophilic Addition Mechanisms

In the previous lesson, you learned that alkenes undergo addition reactions. Now we explore how these reactions happen at the molecular level — the mechanism of electrophilic addition. Understanding this mechanism is one of the most important skills for Edexcel A-Level organic chemistry.

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What Is an Electrophile?

An electrophile is an electron-pair acceptor — a species that is attracted to regions of high electron density. The word comes from the Greek for "electron-loving".

Examples of electrophiles include:

• H⁺ (from HBr, H₂SO₄)
• Br₂ (the Br-Br bond becomes polarised as it approaches the electron-rich C=C)
• NO₂⁺ (nitronium ion)
• Any positively charged species or species with an electron-deficient atom

The C=C double bond in an alkene is a region of high electron density due to the pi bond. This makes alkenes susceptible to attack by electrophiles.

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Mechanism of Electrophilic Addition of HBr to Ethene

This is the simplest example and the one you must be able to draw.

Step 1: Electrophilic Attack

The H-Br bond is polar — hydrogen carries a partial positive charge (δ+) and bromine a partial negative charge (δ−). The δ+ hydrogen is the electrophile.