Free-radical substitution, cracking and alkene oxidation

Last time

Before we start: last time, in Electrophilic addition to alkenes, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

A reaction that needs sunlight

Mix methane and chlorine in the dark and nothing happens. Shine ultraviolet light on the mixture and it reacts explosively. The light does not heat the gas. It splits the chlorine molecules into highly reactive atoms with unpaired electrons: free radicals. A chain reaction follows.

Why alkanes are unreactive

The C–C and C–H bonds are strong. A large amount of energy is needed to break them. The bonds are non-polar, because carbon and hydrogen have similar electronegativities. So there is nothing for electrophiles or nucleophiles to attack. Alkanes react only with radicals, or by combustion at high temperature.

Free-radical substitution

Initiation: ultraviolet light breaks the Cl–Cl bond homolytically, each atom taking one electron: Cl two goes to 2 Cl radical. Propagation: Cl radical plus CH four goes to CH three radical plus HCl. Then CH three radical plus Cl two goes to CH three Cl plus a new Cl radical. A chain. Termination: two radicals combine, removing radicals: Cl radical plus Cl radical gives Cl two; CH three radical plus Cl radical gives CH three Cl; two CH three radicals give C two H six. Overall: CH four plus Cl two goes to CH three Cl plus HCl. A substitution reaction.

Terms

A free radical is a species with an unpaired electron. Homolytic fission is the breaking of a covalent bond with one electron going to each atom, forming two radicals. A substitution reaction is one where an atom or group is replaced by another atom or group. Use single-headed, fish-hook arrows for radical movement of one electron.