13.1 Properties of Alkenes

Question 1

What are alkenes?

Answer 1

homologous group

of unsaturated hydrocarbons

with the general formula C_nH_2n

Question 2

Describe how a double bond forms between 2 adjacent carbon atoms in an alkene:

Answer 2

• 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

Question 3

Which of sigma & pi bonds restricts rotation?

Answer 3

pi

Question 4

What is a pi-bond?

Answer 4

sideways overlap

of 2 p-orbitals

consisting of 2 electrons.

electron density concentrated above & below the line joining the nuclei of the bonded atoms.

Question 5

In what way does a double bond restrict movement of parts of alkene molecules?

Answer 5

restricts carbon atoms from rotating around double bond

Question 6

Describe the shape of atoms around a double bond in an alkene:

Answer 6

• bond angle of 120°,trigonal planar shape around each double-bonded carbon
  
  • each carbon has 3 bonding groups
  • repel equally

Question 7

Explain the difference between a σ-bond & a π-bond

Answer 7

• pi bond is sideways overlap of p-orbitals
• sigma bond is head-on overlap of orbitals

Question 8

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:

Answer 8

• 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