Organic Mechanisms (OCR)

Electrophilic addition is a fundamental reaction where electrophiles (electron-loving species) add across carbon-carbon double bonds. With Br₂ in non-polar solvents, the bromine molecule becomes polarised and attacks the electron-rich alkene. The π bond breaks to form a carbocation intermediate, followed by attachment of the bromide ion to form a dibromoalkane.

Similar mechanics occur with HBr, where the partially positive hydrogen acts as the electrophile. The hydrogen attaches to one carbon of the double bond, creating a carbocation, which is then attacked by the bromide ion. The result is a bromoalkane where HBr has been added across the C=C bond.

In benzene reactions, electrophilic substitution (rather than addition) occurs due to the stable aromatic ring. For halogenation, a catalyst like AlBr₃ or Fe polarises the bromine molecule, allowing it to accept electrons from benzene's delocalised ring. A hydrogen is replaced by bromine, preserving the aromatic structure. Nitration follows similar principles, with the nitronium ion (NO₂⁺) acting as the electrophile at 55°C in concentrated H₂SO₄.

💡 Remember that benzene undergoes substitution (not addition) reactions to preserve its stable aromatic structure - this is a key difference from alkene chemistry!

Nucleophilic addition to carbonyl compounds involves nucleophiles attacking the partially positive carbon. With KCN, the cyanide ion attacks the carbonyl carbon, forming a hydroxynitrile. When NaBH₄ is used, hydride ions reduce the carbonyl to an alcohol. The oxygen gains a negative charge which is then protonated to form a hydroxyl group.

Nucleophilic substitution in halogeanoalkanes occurs when nucleophiles like hydroxide ions (OH⁻) attack the partially positive carbon atom in the carbon-halogen bond. The halogen leaves as a halide ion, resulting in an alcohol.

Free radical substitution in alkanes involves three steps: initiation (forming radicals via UV light), propagation (chain reaction with the alkane), and termination (radicals combining to end the reaction). This mechanism converts alkanes to halogeanoalkanes.