B3 - Alkenes

Question 1

What is the nature of the double bond in an alkene like?

Answer 1

• One electron on each carbon atom of the double bond is not involved in sigma bonds. This electron is in a p-orbital. A pi bond is formed by the sideways overlap of two p-orbitals, one from each carbon atom of the double bond. Each carbon atom contributes one electron to the electron pair in the pi bond. The pi-electron density is concentrated above and below the line joining the nuclei of the bonding atoms.

Question 2

What does the pi bond do to the carbon atoms involved with it?

Answer 2

It locks them in position, preventing them from rotating around the double bond.

Question 3

What is the shape around each of the carbon atoms involved the double bond?

Answer 3

Trigonal planar

Question 4

What is the shape around each of the carbon atoms involved the double bond, and why?

Answer 4

Trigonal planar because:
- There are three regions of electron density around each of the carbon atoms
- The three regions repel each other as far apart as possible, so the bond angle around each carbon atom is 120 degrees
- All of the atoms are in the same plane