Electron-pair sharing reactions

An **electron-pair donor** — it has a lone pair and is attracted to a δ+ (electron-poor) carbon. Examples: OH⁻, CN⁻, NH_{3}.

The C–halogen bond is **polar**: the more electronegative halogen pulls the bonding electrons, leaving carbon slightly positive (**δ+**).

The movement of a **pair of electrons** — the tail is at the electrons that move, the head is where the pair ends up.

Arrow 1: the nucleophile's **lone pair → δ+ carbon** (new bond). Arrow 2: the **C–X bond → halogen**, which leaves as X⁻.

The atom/ion that departs **with the bonding pair** — here the **halide ion, X⁻** (e.g. Br⁻, Cl⁻).

An **alcohol** (the –halogen is replaced by –OH), plus a halide ion.

**Warm** (gentle heat) and **aqueous** sodium or potassium hydroxide.

A reaction in which **one group replaces another** on the carbon skeleton, which is otherwise unchanged.

Nucleophile = electron-pair **donor** (attacks δ+); electrophile = electron-pair **acceptor** (attacks δ−). Opposites.

**C–I** — it is the **weakest** bond, so it breaks most easily. C–F is strongest, so the fluoroalkane is slowest.

A **nitrile** (–CN) — and the chain gains one carbon atom.

An **amine** (–NH_{2}), using excess ammonia.

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⁺.

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

Two molecules join to form **one** product; the C=C opens to a single bond and the reactant adds across it. **Nothing leaves**.

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

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

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

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

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

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

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

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

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