Electron sharing reactions

A species with an **unpaired electron**, written with a dot (e.g. Cl•, •CH_{3}); very reactive.

A bond breaks **evenly** — **one electron goes to each** atom, forming two **radicals**.

A bond breaks **unevenly** — **both electrons go to one** atom, forming **ions** (a cation and an anion).

**Homolytic** fission makes radicals; **heterolytic** fission makes ions.

An alkane reacts with a halogen in **UV light**, replacing an H atom with a halogen atom, via a radical chain.

**UV light** breaks the halogen molecule by **homolytic** fission, e.g. Cl_{2} → 2 Cl•.

A radical reacts to give a product **and a new radical**, so the chain continues (radical count unchanged).

Cl• + CH_{4} → •CH_{3} + HCl, then •CH_{3} + Cl_{2} → CH_{3}Cl + Cl•.

**Two radicals combine** into one molecule, removing radicals and **stopping** the chain (e.g. •CH_{3} + Cl• → CH_{3}Cl).

It supplies the energy to break the halogen bond **homolytically** and create the first radicals.

Each propagation step **regenerates** a radical, so one initiation triggers many cycles (and a mixture of products).

With a **dot (•)** next to it, showing the single unpaired electron (e.g. Cl•).

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

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

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

**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**.

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

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

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

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

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

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

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

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