Nucleophilic substitution: SN1 and SN2 (HL)

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A **nucleophile** replaces a **halide leaving group** on a halogenoalkane at the **δ+ carbon**: R–X + Nu⁻ → R–Nu + X⁻.

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An **electron-pair donor** that attacks an electron-poor (δ+) atom; it has a **lone pair** (e.g. OH⁻, CN⁻, NH_{3}).

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The halogen is more **electronegative** than carbon, so the polar C–halogen bond leaves the carbon partially positive (**δ+**).

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**One** concerted step: the nucleophile attacks the carbon from the **side opposite** the leaving group, via a **transition state** with partial bonds; configuration is **inverted**.

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**Two** steps: (1) slow **heterolysis** of C–halogen forms a **carbocation**; (2) fast attack of the nucleophile on the carbocation.

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rate = k[halogenoalkane][Nu⁻] — **second** order (first order in each reactant).

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rate = k[halogenoalkane] — **first** order; the nucleophile is **absent** (it joins in the fast step).

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**Primary** (1°): the carbon is **uncrowded**, so the nucleophile can reach it for back-side attack.

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**Tertiary** (3°): it forms a **stable tertiary carbocation** (alkyl groups spread the positive charge).

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The three attached alkyl groups push electron density onto the positive carbon (**positive inductive effect**), spreading out the charge.

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**C–I > C–Br > C–Cl > C–F** — the weaker (longer) the bond, the better the leaving group, the faster the reaction.

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The **C–I bond is the weakest**, so it breaks most easily and **iodide is the best leaving group**.