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 species (often δ+ or positive) attracted to electron-rich regions. Examples: Br_{2}, HBr, H⁺.

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

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

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

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⁻).

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

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

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

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

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

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

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

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

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}⁺).

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

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.

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.

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

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

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

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

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

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

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