Alkenes and their nomenclature
Alkenes are open-chain unsaturated hydrocarbons with one C=C double bond; the general formula is CₙH₂ₙ and the first member is ethylene (ethene) C₂H₄. The double bond consists of one σ- and one π-bond; the π-bond is weak, so alkenes are reactive. The CH₂=CH– radical is called vinyl. In systematic names “-ane” of the alkane becomes “-ene” and the position of the double bond is given by a number: propene CH₂=CH–CH₃, butene-1, butene-2. The main chain must contain the double bond and numbering starts from the end nearer to it; substituents are listed alphabetically: 2-methylbutene-2. In rational nomenclature an alkene is a derivative of ethylene: propene is methylethylene, butene-2 is symmetrical dimethylethylene, 2-methylpropene is unsymmetrical dimethylethylene. In halogen derivatives the halogen is also an alphabetical substituent: 3-bromopropene-1.
Compare molecules with and without a double bond: build propane and propene, show the double bond with two parallel sticks and test whether the ends can “rotate”. Be careful with the sticks.