Isomerism and preparation of alkenes
Alkenes show three kinds of isomerism. Chain isomerism: butene-1 and 2-methylpropene. Position isomerism: the place of the double bond differs, e.g. butene-1 and butene-2, pentene-1 and pentene-2. Geometric isomerism: there is no rotation about the double bond, so if each double-bonded carbon carries two different substituents they lie on the same side (cis-) or opposite sides (trans-); cis- and trans-butene-2 are separate substances with different properties. For more complex substituents the Z/E system is used: the larger groups on the same side is Z, on opposite sides is E. Methods of preparation: (1) heating an alcohol with concentrated H₂SO₄ above 140 °C: C₂H₅OH → C₂H₄ + H₂O; (2) dehydrogenating an alkane over a catalyst: C₂H₆ → C₂H₄ + H₂; (3) zinc acting on a vicinal dibromoalkane: CH₂Br–CH₂Br + Zn → C₂H₄ + ZnBr₂; (4) alcoholic KOH acting on a haloalkane: C₃H₇Cl + KOH → C₃H₆ + KCl + H₂O.
Build cis- and trans-butene-2 with two colours of plasticine and two parallel sticks, then try to “rotate” one carbon — the sticks block it. Use this to explain the lack of rotation.