Free Radical Substitution TYPICAL PROPERTIES • reactive species (atoms or groups) which possess an unpaired electron • their reactivity is due to them wanting to pair up the single electron https://www.youtube.com/watch?v=E5Wowh3aRts

CHLORINATION OF METHANE All Combined Initiation Cl2 ——> 2Cl• radicals created Propagation Cl• + CH4 ——> CH3• + HCl radicals used and Cl2 + CH3• ——> CH3Cl + Cl• then re-generated Termination Cl• + Cl• ——> Cl2 radicals removed Cl• + CH3• ——> CH3Cl CH3• + CH3• ——> C2H6

CHLORINATION OF METHANE The single dots represent UNPAIRED ELECTRONS Initiation Cl2 ——> 2Cl• RADICALS CREATED The single dots represent UNPAIRED ELECTRONS During initiation, the WEAKEST BOND IS BROKEN as it requires less energy. There are three possible bonds in a mixture of alkanes and chlorine. 412 348 242 Average bond enthalpy kJ mol-1 The Cl-Cl bond is broken in preference to the others as it is the weakest and requires less energy to separate the atoms.

Free Radical Substitution Moving a single electron, use a half-headed curved arrow, sometimes called a fishhook. A full headed curved arrow shows the movement of an electron pair. A reactive intermediate with a single unpaired electron is called a radical.

The Cl-Cl bond breaks. One electron goes to each atom Chain initiation The chain is initiated (started) by UV light breaking a chlorine molecule into free radicals. unpaired electron Cl2                 2Cl   chlorine molecule chlorine radicals The Cl-Cl bond breaks. One electron goes to each atom The curly half-arrows are used to show the movement of one electron. Cl Cl Cl Cl This process is known as homolytic fission.

CHLORINATION OF METHANE Propagation Cl• + CH4 ——> CH3• + HCl RADICALS USED and Cl2 + CH3• ——> CH3Cl + Cl• then RE-GENERATED A chlorine radical is produced and the whole process can start over again

CH4 + Cl CH3 + HCl CH3 + Cl2 CH3Cl + Cl CH3Cl + Cl CH2Cl + HCl Chain propagation reactions These are the reactions which keep the chain going. CH4  +  Cl                  CH3    +  HCl chlorine radical methyl radical CH3    +  Cl2                CH3Cl  +  Cl   chlorine radical The new chlorine radical can then react with methane or chloromethane molecules. CH3Cl    +  Cl                CH2Cl  +  HCl   CH2Cl    +  Cl2                CH2Cl2  +  Cl   And so on until CCl4 is formed

H3C H + Cl CH3 + H-Cl CH3 + Cl Cl CH3 Cl + Cl Homolytic fission Chain propagation reactions - using curly arrows H3C H   +   Cl                  CH3    +  H-Cl Homolytic fission New HCl bond formed from electron from chlorine radical and one of the electrons in the C-H bond Draw the curly arrows for the next step which is the methyl radical reacting with a chlorine molecule CH3    +   Cl Cl                CH3 Cl  +  Cl  

CHLORINATION OF METHANE Termination Cl• + Cl• ——> Cl2 RADICALS REMOVED Cl• + CH3• ——> CH3Cl CH3• + CH3• ——> C2H6 Removing the reactive free radicals brings an end to the reaction. This is not very likely at the start of the reaction because of their low concentration.

2Cl Cl2 CH3 + Cl CH3Cl CH3 + CH3 CH3CH3 Chain termination reactions These are reactions which remove free radicals from the system without replacing them by new ones. 2Cl                  Cl2 CH3    +  Cl                  CH3Cl CH3    +  CH3                  CH3CH3

CHLORINATION OF METHANE OVERVIEW Initiation Cl2 ——> 2Cl• radicals created Propagation Cl• + CH4 ——> CH3• + HCl radicals used and Cl2 + CH3• ——> CH3Cl + Cl• then re-generated Termination Cl• + Cl• ——> Cl2 radicals removed Cl• + CH3• ——> CH3Cl CH3• + CH3• ——> C2H6 Summary Due to lack of reactivity, alkanes need a very reactive species to persuade them to react Free radicals need to be formed by homolytic fission of covalent bonds You only need one chlorine radical to start things off With excess chlorine you get further substitution and a mixture of chlorinated products

CHLORINATION OF METHANE Further propagation If excess chlorine is present, further substitution takes place The equations show the propagation steps for the formation of... dichloromethane Cl• + CH3Cl ——> CH2Cl• + HCl Cl2 + CH2Cl• ——> CH2Cl2 + Cl• trichloromethane Cl• + CH2Cl2 ——> CHCl2• + HCl Cl2 + CHCl2• ——> CHCl3 + Cl• tetrachloromethane Cl• + CHCl3 ——> CCl3• + HCl Cl2 + CCl3• ——> CCl4 + Cl• Mixtures Because of the many possible reactions there will be a mixture of products. Individual haloalkanes can be separated by fractional distillation.