1 Physical and chemical properties

Alkenes are unsaturated hydrocarbons General formula is CnH2n Alkenes contain a carbon- carbon double bond somewhere in their structure C C H H H H Ethene C C H H H C H H H Propene Numbers need to be added to the name when positional isomers can occur C H H C C C H H H H H H But-1-ene But-2-ene C=C double covalent bond consists of one sigma (σ) bond and one pi (π) bond. π bonds are exposed and have high electron density. They are therefore vulnerable to attack by species which ‘like’ electrons: these species are called electrophiles. C C C C H H H H H H H H N Goalby chemrevise.org 1 The π bond is formed by sideways overlap of two p orbitals on each carbon atom forming a π-bond above and below the plane of molecule. The π bond is weaker than the σ bond. p orbitals C-C sigma bond C-C pi bond C-C sigma bond C-C pi bond The arrangement of bonds around the >C=C< is planar and has the bond angle 120o Formation of π bond C C H H H H