Organic reaction mechanisms · Grade 12
A reaction mechanism shows, step by step, how bonds break and form. Curly arrows show pairs of electrons moving: from a lone pair or a bond, to where a new bond forms or to the atom that takes them. Haloalkanes are attacked by nucleophiles, alkenes by electrophiles, and alkanes by radicals.
Key facts
- A curly arrow shows a pair of electrons moving. It starts at a lone pair or a bond and points to where the new bond forms, or to the atom that keeps the electrons.
- A nucleophile is an electron-pair donor (OH⁻, CN⁻, NH₃). An electrophile is an electron-pair acceptor (H⁺, Br₂ once polarised, NO₂⁺). A radical has an unpaired electron.
- Heterolytic fission: one atom takes both bonding electrons, giving ions. Homolytic fission: each atom takes one electron, giving radicals.
- Nucleophilic substitution: the C–X bond in a haloalkane is polar (Cδ+–Xδ−). A nucleophile attacks the δ+ carbon and the halide ion leaves. Warm aqueous NaOH gives an alcohol; KCN in ethanol gives a nitrile; excess ammonia gives an amine.
- Elimination: with hot KOH in ethanol, OH⁻ acts as a base and removes H⁺ from the carbon next to the C–X carbon, forming an alkene.
- Electrophilic addition: the electron-rich C=C attacks an electrophile such as H–Br. A carbocation forms, then Br⁻ adds. With an unsymmetrical alkene the major product comes from the more stable carbocation: tertiary > secondary > primary.
- Free-radical substitution: UV light splits Cl₂ into radicals (initiation); chain reactions make the product (propagation); two radicals join (termination).