Carbon and its Compounds
Catenation and tetravalency explain why carbon forms millions of compounds. The exam asks you to draw structures, name homologues and functional groups, distinguish substitution from addition, and explain the cleansing action of soap.
Bonding and series
Carbon forms four covalent bonds. Saturated compounds (alkanes, C-C single bonds) give substitution with chlorine in sunlight; unsaturated compounds (alkenes, alkynes) give addition and decolourise bromine water. A homologous series differs by CH2 and about 14 u per member: CH4, C2H6, C3H8.
Functional groups
Alcohol -OH (ethanol C2H5OH); carboxylic acid -COOH (ethanoic acid CH3COOH); aldehyde -CHO; ketone -CO-; halide -X. Ethanol with ethanoic acid and a little H2SO4 gives ethyl ethanoate, a sweet-smelling ester: esterification.
Soaps
Soap is a sodium salt of a long-chain fatty acid. Its hydrocarbon tail dissolves in grease and its ionic head in water, forming a micelle that lifts dirt away. In hard water Ca2+ and Mg2+ form insoluble scum, so detergents are preferred there.
Worked example
How many structural isomers does C4H10 have? A straight chain of four carbons gives butane, CH3-CH2-CH2-CH3. A chain of three carbons with a methyl branch gives 2-methylpropane. That is two isomers: same molecular formula, different structures and slightly different properties.
Traps
Drawing carbon with five bonds; writing C2H6O for ethanoic acid, which is ethanol; saying soap lathers well in hard water; forgetting ethanoic acid is weak yet still turns blue litmus red.