ALKANES with HALOGENS Reactions – what happens Mechanism – how it happens AS Chemistry

ALKANES WITH HALOGENS, Cl 2 or Br 2 Initiated by energy from UV light  successive substitution (replacement) of H atoms in the alkane by Cl / Br atoms.  mixture of chloro- / bromoalkanes eg Methane + Cl 2 Excess Cl 2 Excess CH 4  CH 3 Cl MORE likely  CCl 4 MORE likely  mono-, di-, tri- and tetrachloromethanes

C H H H H + Cl 2 U.V. C H Cl H H + HCl C Cl H H + HCl C Cl H + HCl C Cl + HCl + Cl 2 Similarly for Br 2 chloromethane dichloromethane trichloromethane tetrachloromethane

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 2. PROPOGATION 3. TERMINATION (Start) (Products Grow ) (Finish)

1. INITIATION Caused by energy input from UV light  neutral halogen atoms= FREE RADICALS Cl  + UV energy  HOMOLYSIS = homolytic fission  covalent bond in a few Cl 2 molecules broken - one e - from bond pair to each atom

2. PROPOGATION STEPS (a)Cl. free radical from initiation step collides with a molecule of CH 4 and removes a H atom.  first substitution product (HCl) + a methyl free radical (H 3 C. ). Cl C H H H H C H H H H + +

(b)CH 3. free radical from step (a) collides with a NEW molecule of Cl 2 and removes a Cl atom.  second substitution product (CH 3 Cl) + a NEW chlorine free radical (Cl. ). C H Cl H H C H H H +

Propagation steps (a) and (b) ALTERNATE  HCl + CH 3 Cl each repetition of (a) and (b) 3. TERMINATION STEPS  propagation steps stopped by consumption of free radicals  combine to form neutral molecules Free Radicals in Mixture Possible termination products CH 3. Cl. CH 3 Cl Cl 2 CH 3

Abbreviated mechanism : Initiation Propogation 1 Propogation 2 Termination 1 Termination 2 Termination 3 Initiation Propogation 1 Propogation 2 Termination 1 Termination 2 Termination 3 Write a similar mechanism for CH 3 Br  CH 2 Br 2 By energy from uv light : Cl 2  2Cl. CH 4 + Cl. . CH 3 + HCl. CH 3 + Cl 2  CH 3 Cl + Cl.. CH 3 + Cl.  CH 3 Cl. CH 3 +. CH 3  CH 3 CH 3. Cl + Cl.  Cl 2 By energy from uv light : Br 2  2Br. CH 3 Br + Br. . CH 2 Br + HBr. CH 2 Br + Br 2  CH 2 Br 2 + Br.. CH 2 Br + Br.  CH 2 Br 2. CH 2 Br +. CH 2 Br  CH 2 BrCH 2 Br. Br + Br.  Br 2

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