Characteristic organic reactions

When a covalent bond breaks, the shared pair of electrons can split two ways:

• Homolytic fission — the bond breaks evenly, each atom taking one electron, forming two free radicals (species with an unpaired electron, shown with a dot, e.g. $\text{Cl}\bullet$). Typical of non-polar bonds under UV light.
• Heterolytic fission — the bond breaks unevenly, one atom takes both electrons, forming a negative ion (got the pair) and a positive ion (lost the pair). Typical of polar bonds.

Curly-arrow notation matches this: a half-headed (fish-hook) arrow moves one electron (homolytic); a full (double-headed) arrow moves an electron pair (heterolytic).

• A nucleophile is an electron-pair donor — a species with a lone pair (often negative or δ−) that attacks electron-poor (δ+) carbon atoms. Examples: $\text{OH}^-$, $\text{CN}^-$, $\text{NH}_3$, $\text{H}_2\text{O}$.
• An electrophile is an electron-pair acceptor — an electron-deficient species (often positive or δ+) that attacks electron-rich centres such as C=C double bonds. Examples: $\text{H}^+$, $\text{NO}_2^+$, the δ+ end of a polar molecule, $\text{Br}^{\delta+}$.

Memory hook: Nucleophile loves the nucleus (positive); electrophile loves electrons.

Reaction type	What happens
Addition	Two molecules combine into one (e.g. across a C=C); no atoms lost.
Substitution	One atom/group is replaced by another.
Elimination	A small molecule (e.g. HX or H₂O) is removed, forming a C=C.
Hydrolysis	A bond is broken by reaction with water (or aqueous acid/alkali).
Condensation	Two molecules join with the loss of a small molecule (often H₂O).
Oxidation / reduction	Addition of O / removal of H (oxidation) or addition of H / removal of O (reduction).

In organic redox equations, [O] represents one oxygen atom from an oxidising agent, and [H] one hydrogen atom from a reducing agent — a useful shorthand for balancing.

You must recognise and draw four mechanism types (developed in later topics):

• Free-radical substitution — e.g. alkanes + halogens in UV light (initiation → propagation → termination), Topic 14.1.
• Electrophilic addition — e.g. alkenes + Br₂/HBr, Topic 14.2.
• Nucleophilic substitution — e.g. halogenoalkanes + OH⁻, Topic 15.1.
• Nucleophilic addition — e.g. carbonyls + HCN, Topic 17.1.

Curly arrow rules (essential for mechanism marks):

• A full curly arrow shows the movement of an electron pair.
• It must start at a bond or a lone pair (the source of the electrons).
• It must point to where the pair goes (a new bond, or an atom becoming negative).

Getting the start and end points of each arrow right is exactly what mark schemes credit.