Question 1

Why is the π bond weaker than the σ bond?

Accepted Answer

The π bond is formed by the sideways overlap of p-orbitals, which is a less effective and weaker form of overlap compared to the direct, head-on overlap of orbitals that forms a σ bond. This weaker overlap means the π bond has a lower bond enthalpy, requiring less energy to break.

Question 2

What happens if you add bromine water to an alkane?

Accepted Answer

Nothing. Alkanes are saturated and do not have an electron-rich π bond. They do not undergo electrophilic addition. Therefore, the orange-brown colour of the bromine water will persist. This difference in reactivity is the basis for the chemical test to distinguish between alkanes and alkenes.

Question 3

What is the difference between heterolytic and homolytic fission?

Accepted Answer

In electrophilic addition, the bond in the electrophile (e.g., Br-Br or H-Br) breaks via **heterolytic fission**. This is where one atom takes both electrons from the covalent bond, forming a positive and a negative ion. **Homolytic fission**, which is not seen in this mechanism, is when a bond breaks and each atom takes one electron, forming two neutral radicals. This is typical of free-radical substitution of alkanes.

Question 4

Can alkenes undergo polymerisation?

Accepted Answer

Yes, alkenes are the monomers for addition polymerisation. Under high pressure and temperature, and with a catalyst, the π bond in many alkene molecules can break and the molecules link together to form a long saturated chain called a polymer. For example, poly(ethene) is made from ethene monomers.

Alkenes

What you need to know

Alkenes: The Reactive Double Bond