Electrophilic addition to alkenes

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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. Examples: Br_{2}, HBr, H⁺.

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The **C=C double bond** has a **π bond** of high electron density that is easily attacked by **electrophiles**.

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It is **electron-rich** (two shared pairs / exposed π electrons), so it readily donates electrons and attacks electrophiles.

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Two molecules combine into **one**: a group adds to **each** carbon and the **double bond becomes single** (nothing is left over).

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Saturated = only **single** bonds (alkane); unsaturated = has a **C=C** (alkene) so more atoms can be **added**.

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The molecule contains a **C=C (or C≡C)** and can undergo **addition**; a saturated molecule has only single bonds.

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Arrow 1: the **C=C π electrons → a bromine** (new bond). Arrow 2: the **Br–Br bond → the other bromine**, which leaves as Br⁻.

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Arrow 1: the **C=C π bond → the electrophile** (new bond). Arrow 2: the **X–X / H–X bond → the leaving atom**, which breaks heterolytically (e.g. as Br⁻).

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**1,2-dibromoethane, CH_{2}BrCH_{2}Br** — a bromine atom adds to each carbon as the C=C opens.

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**1,2-dibromoethane, CH_{2}BrCH_{2}Br** — one bromine adds to each carbon.

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**Bromoethane, CH_{3}CH_{2}Br** — H and Br add across the C=C.

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A **halogenoalkane** — H adds to one carbon and the halogen to the other (e.g. ethene + HBr → bromoethane).

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**Ethanol, CH_{3}CH_{2}OH** — water adds across the C=C with an H_{3}PO_{4} catalyst at high T and P.

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An **alcohol** — using **steam with an H_{3}PO_{4} catalyst** (heat & pressure); –H and –OH add across the C=C.

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**Ethane, CH_{3}CH_{3}** (saturated) — hydrogen adds across the C=C with a Ni catalyst.

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Add **bromine water**: an alkene **decolourises** the orange bromine (it adds across the C=C).

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Add **bromine water**: an **alkene decolourises** it (orange → colourless); an **alkane** gives **no change**.

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Electrophile = electron-pair **acceptor** (electron-poor); nucleophile = electron-pair **donor** (electron-rich). Opposites.

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For an unsymmetrical alkene + HX, the **H adds to the carbon that already has more hydrogens**, giving the **major** product.

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Alkenes (unsaturated) react by **addition** (C=C opens, nothing left over); alkanes (saturated) by **substitution** (an atom is replaced).

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As Br_{2} meets the electron-rich C=C it becomes polarised (δ+/δ−); the far bromine leaves with **both** electrons as **Br⁻**.

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**2-bromopropane, CH_{3}CHBrCH_{3}** — H goes to the CH_{2} end, Br to the middle carbon (Markovnikov).