Alkenes — common mistakes

When drawing mechanisms, curly arrows must always start from a source of electrons (a bond or a lone pair) and point to the atom that accepts the electrons. Remember to show the induced dipole on non-polar electrophiles like $Br_2$ and the full dipole on polar ones like H-Br.

Reaction conditions are crucial and frequently earn marks. For hydrogenation, remember 'Nickel catalyst, 150°C'. For hydration of ethene, remember 'Steam, 300°C, 60 atm, $H_3PO_4$ catalyst'. Be precise!

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.

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.

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.

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.