13.1 Properties of Alkenes
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What are alkenes?
homologous group of unsaturated hydrocarbons with the general formula CnH2n
Describe how a double bond forms between 2 adjacent carbon atoms in an alkene:
• each carbon in double bond uses 3 of 4 outer electrons in bonding • leaves 1 p-orbital electron • above & below overlap of p-orbitals forms pi-bond
Which of sigma & pi bonds restricts rotation?
pi
What is a pi-bond?
sideways overlap of 2 p-orbitals consisting of 2 electrons. electron density concentrated above & below the line joining the nuclei of the bonded atoms.
In what way does a double bond restrict movement of parts of alkene molecules?
restricts carbon atoms from rotating around double bond
Describe the shape of atoms around a double bond in an alkene:
• bond angle of 120°,trigonal planar shape around each double-bonded carbon • each carbon has 3 bonding groups • repel equally
Explain the difference between a σ-bond & a π-bond
• pi bond is sideways overlap of p-orbitals • sigma bond is head-on overlap of orbitals
Explain why the bond angle & shape of the carbons in the double bond of an alkene is different to that of a carbon in an alkane:
• alkane, each carbon has 4 bonding groups • repel equally, giving bond angle 109.5, tetrahedral shape • alkene, each carbon in double bond has 3 bonding groups due to double bond • repel equally, giving bond angle of 120, trigonal planar shape