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Organic chemistry

Chemistry · WAEC and JAMB · SS2 and SS3

Pattern recognition. Learn the homologous series, their general formulae and their characteristic reactions, and most questions answer themselves.

What you need to know

  • Alkanes CnH2n+2: saturated, generally unreactive, undergo substitution with halogens in sunlight.
  • Alkenes CnH2n: unsaturated with a double bond, undergo addition reactions, decolourise bromine water.
  • Alkynes CnH2n-2: contain a triple bond, ethyne being the common example.
  • Alkanols CnH2n+1OH: ethanol from fermentation of glucose by yeast.
  • Alkanoic acids: ethanoic acid is the familiar one, found in vinegar.
  • Esterification: alkanoic acid + alkanol -> ester + water, with concentrated H2SO4 as catalyst. Esters smell fruity.
  • Saponification: fat or oil + sodium hydroxide -> soap + glycerol.
  • Polymerisation joins many monomers; addition for alkene monomers, condensation where a small molecule is eliminated.

Key terms

Homologous series
A family of compounds with the same general formula and functional group, each differing by CH2.
Isomerism
Compounds with the same molecular formula but different structural arrangements.
Functional group
The atom or group that gives a compound its characteristic reactions.

Formulae

  • Fermentation: C6H12O6 --yeast--> 2C2H5OH + 2CO2
  • Esterification: CH3COOH + C2H5OH <-> CH3COOC2H5 + H2O

Worked example

How would you distinguish between ethane and ethene in the laboratory?

  1. Ethene is unsaturated and undergoes addition; ethane is saturated and does not.
  2. Add bromine water to each and shake.
  3. Ethene adds across the double bond and removes the colour; ethane leaves it unchanged.

The mistake to avoid

Alkenes undergo ADDITION, alkanes undergo SUBSTITUTION. Swapping the two is among the most common errors in the organic section.

In the exam

When a test is asked for, give the reagent, the observation and the conclusion. "Bromine water, decolourised, so unsaturated" earns all three marks.