The Formation of Halogenoalkanes Targets To define what a radical is To describe the reaction mechanism for free radical substitution of methane RSC mechanisms resource: http://www.rsc.org/learn-chemistry/resource/res00000638/curly-arrows-and-stereoselectivity-in-organic-reactions   RSC resource of misconceptions about mechanisms: http://www.rsc.org/learn-chemistry/resource/res00001107/reaction-mechanisms Mechanism animations http://science.jbpub.com/organic/movies/ Interactive mechanisms http://www.chem.ox.ac.uk/vrchemistry/iom/

What is a halogen? RSC mechanisms resource: http://www.rsc.org/learn-chemistry/resource/res00000638/curly-arrows-and-stereoselectivity-in-organic-reactions   RSC resource of misconceptions about mechanisms: http://www.rsc.org/learn-chemistry/resource/res00001107/reaction-mechanisms Mechanism animations http://science.jbpub.com/organic/movies/ Interactive mechanisms http://www.chem.ox.ac.uk/vrchemistry/iom/

ALKANES WITH HALOGENS, Cl2 or Br2 Initiated by energy from UV light  successive substitution (replacement) of H atoms in the alkane by Cl / Br atoms.  mixture of chloro- / bromoalkanes

HALOGEN-METHANE SUBSTITUTION : MECHANISM = a FREE RADICAL SUBSTITUTION mechanism Also a CHAIN REACTION  a product from one stage of process is used in the next stage. THREE stages : 1. INITIATION  (Start) (Products Grow ) (Finish) 2. PROPOGATION  3. TERMINATION CH4(g) + Cl2 (g)  CH3Cl(g) + HCl (g) C6H14(g) + Br2 (l)  C6H13Br(l) + HBr (g)

Cl  + Cl Cl UV energy 1. INITIATION Caused by energy input from UV light  covalent bond in a few Cl2 molecules broken  HOMOLYSIS - one e- from bond pair to each atom = FREE RADICALS The dot is used to show the unpaired electron. Free radicals are highly reactive. Cl  Cl + Cl UV energy

C H C H Cl H Cl Cl• + CH4  HCl + •CH3 2. PROPOGATION STEPS (a) Cl. free radical from initiation step collides with a molecule of CH4 and removes a H atom. C H C H Cl + H Cl +  first substitution product (HCl) + a methyl free radical (H3C.). Cl• + CH4  HCl + •CH3

C H C H Cl Cl Cl •CH3 + Cl2  CH3Cl + Cl• (b) CH3. free radical from step (a) collides with a NEW molecule of Cl2 and removes a Cl atom. C H C H Cl + Cl Cl +  second substitution product (CH3Cl) + a NEW chlorine free radical (Cl.). •CH3 + Cl2  CH3Cl + Cl•

Propagation Step Summary The two steps produce hydrogen chloride, chloromethane and a new Cl• free radical. This is ready to react with more methane and repeat the two steps. This is the chain part of the chain reaction. The steps may take place thousands of times before the radicals are destroyed.

Free Radicals in Mixture Possible termination products REMOVING THE FREE RADICALS! 3. TERMINATION STEPS  propagation steps stopped by consumption of free radicals  combine to form neutral molecules Free Radicals in Mixture Possible termination products CH3Cl Cl2 CH3CH3 CH3. Cl.

Termination Every case two free radicals react to form a stable compound with no unpaired electrons! Cl• + Cl•  Cl2 •CH3 + •CH3  C2H6 Cl• + •CH3  CH3Cl

Abbreviated mechanism : Initiation Propogation 1 Propogation 2 Termination 1 Termination 2 Termination 3 By energy from uv light : Cl2  2Cl. CH4 + Cl.  .CH3 + HCl .CH3 + Cl2  CH3Cl + Cl. .CH3 + Cl.  CH3Cl .CH3 + .CH3  CH3CH3 .Cl + Cl.  Cl2 Write a similar mechanism for CH3Br  CH2Br2 Initiation Propogation 1 Propogation 2 Termination 1 Termination 2 Termination 3 By energy from uv light : Br2  2Br. CH3Br + Br.  .CH2Br + HBr .CH2Br + Br2  CH2Br2 + Br. .CH2Br + Br.  CH2Br2 .CH2Br + .CH2Br  CH2BrCH2Br . Br + Br.  Br2

Other products of the chain reaction Further reactions can occur to produce di, tri- and tetra-haloalkanes E.g. dichloromethane may be made at the propagation stage, if chlorine radical reacts with some chloromethane that has already formed: CH3Cl + Cl•  •CH2Cl + HCl followed by •CH2Cl + Cl2  CH2Cl2 + Cl•

Other products of the chain reaction Longer-chain alkanes will produced many isomers because the Cl• can replace any of the hydrogen atoms. Chain reactions are not very useful because they produce such a mixture of products. Chain reactions will occur without light at high temperatures.

 mono-, di-, tri- and tetrachloromethanes ALKANES WITH HALOGENS, Cl2 or Br2   eg Methane + Cl2  mono-, di-, tri- and tetrachloromethanes Excess CH4  CH3Cl MORE likely Excess Cl2  CCl4 MORE likely

C H C H Cl U.V. + Cl2 + HCl + Cl2 C Cl C Cl H + HCl + HCl + Cl2 + Cl2 chloromethane + Cl2 C Cl C Cl H + HCl + HCl tetrachloromethane dichloromethane + Cl2 + Cl2 C Cl H + HCl Similarly for Br2 trichloromethane