Key ideas
Organic synthesis is a map of conversions between functional groups. Knowing the reagent for each arrow, and the type of reaction it represents, lets you plan a route from one compound to another.
- The main reaction types are addition (to C=C and C=O), substitution (at saturated carbon or on a ring), elimination and oxidation/reduction.
- Alkene to alkane: H₂ with Ni or Pt. Alkene to alcohol: water with an acid catalyst. Alcohol to alkene: concentrated H₂SO₄ and heat.
- Haloalkane to alcohol: aqueous NaOH (substitution). Haloalkane to alkene: alcoholic KOH (elimination). Nitrile or amide to amine: LiAlH₄.
- Primary alcohol to aldehyde: PCC. Primary alcohol to acid, or secondary to ketone: acidified K₂Cr₂O₇ or KMnO₄.
Key equations
alkene + H₂ → alkane (Ni)alkene + H₂O → alcohol (H⁺)alcohol → alkene (conc. H₂SO₄, heat)RX + OH⁻(aq) → ROH + X⁻RX + KOH(alc.) → alkene
Worked example
To go from ethene to ethanoic acid: add water to ethene (H₃PO₄, steam) to give ethanol, oxidize with acidified dichromate under reflux to give ethanoic acid. With PCC instead you would stop at ethanal. Aqueous NaOH turns bromoethane into ethanol, but alcoholic KOH turns it into ethene.
Common mistakes
- Mixing up aqueous and alcoholic KOH. Water gives substitution, ethanol as solvent gives elimination.
- Using a strong oxidant when the aldehyde is the target. It runs on to the acid.
- Forgetting that reduction of an aldehyde gives a primary alcohol and of a ketone a secondary alcohol.
Practice
Test yourself with practice questions on this topic: Reactions and mechanisms