Electrophilic addition and substitution (HL)

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It is **electron-rich** — the exposed **π electrons** make it an electron-pair **donor** that attacks an electrophile.

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An **electron-pair acceptor** — an electron-poor (often positive) species attracted to an electron-rich centre such as a C=C (e.g. δ+ H of HBr, NO_{2}⁺).

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A carbon atom bearing a **positive charge** (only 3 bonds / 6 electrons) — the reactive **intermediate** in electrophilic addition.

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The **C=C π electrons → the δ+ H** of HBr (arrow 1); the **H–Br bond → Br** (arrow 2), which leaves as **Br⁻**. A **carbocation** forms.

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The **bromide ion (Br⁻) attacks the positive carbon** of the carbocation, forming the final C–Br bond — the reagent has now **added across** the C=C.

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The major product forms via the **more stable carbocation**; equivalently, the **H of HX adds to the C with more H's**.

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**tertiary (3°) > secondary (2°) > primary (1°)** — more alkyl groups donate electron density and spread the positive charge.

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**2-bromopropane, CH_{3}CHBrCH_{3}** — via the more stable **secondary** carbocation (Markovnikov).

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**2-bromo-2-methylpropane, (CH_{3})_{3}CBr** — via the most stable **tertiary** carbocation.

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Its **delocalised** ring is very **stable**; addition would destroy the delocalisation, while **substitution restores** the aromatic ring.

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Electrophile = **nitronium ion, NO_{2}⁺**; catalyst = conc. **H_{2}SO_{4}**; product = **nitrobenzene, C_{6}H_{5}NO_{2}** (plus H⁺ lost).

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**Addition**: reagent adds across C=C, nothing leaves (alkenes). **Substitution**: one group replaces a ring H, ring restored (benzene).