14.5 Alkenes
Cracking: is a thermal decomposition reaction, in which an alkene (and sometimes hydrogen) are produced from an alkane. Cracking always produces a short chain compound with a C=C bond. Cracking of ethane will give ethene and hydrogen.
In the lab it looks like this:
Saturated hydrocarbons:
-have NO double bonds
-do not react with aqueous bromine, so the mixture stays orange.
Unsaturated hydrocarbons:
-have double bonds
-react with aqueous bromine, turning the mixture from orange to colourless.
Poly(ethene) / Polythene: is a polymer produced from ethene by addition polymerisation. A polymer is a compound with very long carbon chains made up of monomer units.
Alkenes’ Addition Reactions:
-with bromine: (the test for saturation)
e.g. ethene (g) + bromine (aq) → 1,2-dibromomethane (l)
-with steam: forms alcohols with heat, pressure and a catalyst
e.g. ethene (g) + steam (g) ⇌ ethanol (l)
-with hydrogen: double bond breaks down to for an alkane with heat, pressure and a catalyst
e.g. ethene (g) + hydrogen (g) → ethane (g)