Chapter 13.1 - Properties of alkenes

Question 1

Describe the structure of alkenes

Answer 1

• Unsaturated hydrocarbons
• Contain at least one C=C
• General formula CₙH₂ₙ (aliphatic alkenes with only one double bond)

Question 2

What is a pi bond?

Answer 2

The sideways overlap of two electrons in p orbitals

Question 3

Describe how the pi bond affects the movement of molecules?

Answer 3

• The pi bond electron density is concentrated above and below the planar of bonded atoms
• This locks the two carbon atoms in place and prevents them from rotating around the double bond
• So the geometry is different from alkanes where rotation is possible around every atom

Question 4

Draw the bonding between a molecule of ethene (sigma and pi bond)

Answer 4

:)

Question 5

What is the shape around each carbon atom in the double bond of alkenes and give the bond angle?

Answer 5

Trigonal planar
Bond angle = 120°

Question 6

Explain why alkenes have this geometry and bond angle

Answer 6

• The three regions of electron density around each of the carbon atoms repel each other as far as possible
• This leads to the geometry of a trigonal planar(all atoms are in the same plane) and the bond angle of 120°
• This gives the minimum repulsion of electrons