Organic chemistry
Organic chemistry tests hydrocarbons, functional groups, reactions and polymers.
Hydrocarbons
Alkanes have the general formula CnH2n+2 and contain only single bonds; they are saturated. They undergo substitution with halogens in UV light. Alkenes have CnH2n and a C=C double bond; they are unsaturated and undergo addition reactions. Bromine water is orange-brown and is decolourised by alkenes but not by alkanes, which is a test for unsaturation. Cracking breaks larger alkanes into smaller alkanes and alkenes.
Functional groups
Alcohols contain −OH. Ethanol is made by fermentation of glucose with yeast at about 30–40 °C in the absence of air, or by hydration of ethene with steam and a catalyst. Carboxylic acids contain −COOH and react with alcohols to form esters, which have characteristic smells. Addition polymerisation joins alkene monomers; condensation polymerisation joins monomers with loss of a small molecule such as water.
Worked example
Ethene reacts with bromine: C2H4 + Br2 → C2H4Br2. The product is 1,2-dibromoethane. The C=C double bond opens and one bromine atom adds to each carbon. The orange-brown bromine water becomes colourless, showing that an addition reaction has occurred.
Traps
Alkanes do not decolourise bromine water in the dark. Isomers have the same molecular formula but different structures, so they have different properties. In a polymer repeat unit, the C=C double bond is opened; do not draw the double bond inside the repeat unit. Fermentation needs no air; if air is present, ethanol is oxidised to ethanoic acid.