Properties

Question 1

What are alkenes?

Answer 1

• Alkenes are unsaturated hydrocarbons. They have at least 1 carbon to carbon double bond.
• General formula CnH2n

Question 2

Double bonds

Answer 2

• Double bond is made up of a sigma bond and a pi bond.
• Sigma bond is formed by two orbitals overlapping in a straight line directly between the bonding atoms.
• Pi bond is formed by a sideways overlap of adjacent p-orbitals above and below the bonding C atoms.
• This restricts the rotation of the pi bond.

Question 3

Why are pi bonds weaker than sigma bonds?

Answer 3

• They are weaker because the electron density is spread out above and below the nuclei.
• This means the electrostatic attraction between nuclei and shared pair of elect. is weaker.
• So pi bonds have low bond enthalpy.

Question 4

Shape around double bond

Answer 4

• Shape around each of the carbon atoms in the double bond is trigonal planar.
• 3 regions of electron density around each of the carbon atoms.
• 3 regions repel each other as far apart as possible, so bond angle is 120.
• All of the atoms are in the same plane.