Characteristic organic reactions — common mistakes

For mechanism questions, always include all relevant dipoles (e.g., C$^{\delta+}$-Cl$^{\delta-}$), charges, and curly arrows. A curly arrow must start from a lone pair or a bond and point to the atom it is forming a bond with. Examiners are very strict about the origin and destination of curly arrows.

Alkenes have localised $\pi$ electrons in the C=C double bond, which are readily available to attack electrophiles, leading to addition. Benzene has delocalised $\pi$ electrons in a stable ring system. An addition reaction would destroy this delocalisation and the associated stability. Therefore, benzene undergoes electrophilic substitution, where a hydrogen atom is replaced, preserving the stable aromatic ring.

The solvent plays a crucial role. For SN1 reactions, polar protic solvents (like water or ethanol) are preferred because they can stabilise both the carbocation intermediate and the leaving group through solvation. For SN2 reactions, polar aprotic solvents (like propanone or DMSO) are better as they solvate the cation but not the nucleophile, leaving the nucleophile 'naked' and more reactive.

Instead of rote memorisation, try to understand the role of each reagent. For example, concentrated H₂SO₄ is often a dehydrating agent or a catalyst. NaOH(aq) provides OH⁻ as a nucleophile, while NaOH in ethanol favours elimination. Creating a mind map or a summary table linking functional groups to their characteristic reactions, reagents, and conditions is a very effective revision strategy.

Yes, reduction is the gain of electrons, loss of oxygen, or gain of hydrogen. You need to know specific reducing agents for different functional groups. For example, NaBH₄ (sodium borohydride) reduces aldehydes and ketones to alcohols, but not carboxylic acids. LiAlH₄ (lithium aluminium hydride) is a stronger reducing agent and will reduce all three. Hydrogen gas (H₂) with a metal catalyst (Ni, Pt, Pd) is used to reduce alkenes to alkanes.