Properties of selected organic compounds
The organic chemistry standard tests naming, drawing and interconverting selected organic compounds, and using reaction types and observations to identify an unknown. You need functional groups, IUPAC names, structural formulae, and the reagents and conditions for each reaction.
Naming and structure
- Stems: meth-, eth-, prop-, but-. Suffixes: -ane, -ene, -yne, -ol, -oic acid, -amine; number the chain to give the functional group the lowest locant.
- Structural isomers share a molecular formula. cis/trans isomers arise from restricted rotation about a C=C that carries two different groups on each carbon.
Reactions to know
- Alkanes: substitution with halogens under UV light; combustion.
- Alkenes: addition of H₂/Ni, HCl, Br₂ (rapid decolourisation), H₂O/H₂SO₄; addition polymerisation.
- Alcohols: primary alcohols oxidised by acidified dichromate to an aldehyde, then a carboxylic acid; secondary to a ketone; tertiary resistant. Esterification with a carboxylic acid and concentrated H₂SO₄ catalyst.
- Carboxylic acids: react with carbonates releasing CO₂, and with alcohols forming esters.
Tests
- Bromine water, orange to colourless - C=C present.
- Acidified permanganate, purple to colourless - alcohol or alkene.
- Tollens' or Fehling's/Benedict's - an aldehyde is oxidised, giving a silver mirror or a red precipitate.
Worked example
Compound C₄H₈ decolourises bromine water, so it has a C=C. It shows cis/trans isomerism, so each doubly bonded carbon must carry one H and one CH₃. The compound is but-2-ene; but-1-ene is a structural isomer that cannot be cis or trans.
Traps
- Drawing molecular formulae when the question asks for structural formulae.
- Omitting reagents and conditions, or writing 'heat' where 'reflux with acidified dichromate' is required.
- Confusing an aldehyde, which oxidises further, with a ketone, which resists mild oxidation.
- Forgetting water in the esterification equation, or forgetting that the sulfuric acid is a catalyst.