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16th_lecture_ReviewCh4_10_5_2011(1)

16th_lecture_ReviewCh4_10_5_2011(1) - Halogenation of...

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carried out at high temperature (400 °C) CH 4 + Cl 2 CH 3 Cl + HCl CH 3 Cl + Cl 2 CH 2 Cl 2 + HCl CH 2 Cl 2 + Cl 2 CHCl 3 + HCl CHCl 3 + Cl 2 CCl 4 + HCl This is an example of a radical reaction Chlorination of Methane 1 Halogenation of alkanes: RH + X 2 RX + HX
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C H H H . Methyl radical less stable than C H 3 C H H . Ethyl radical (primary) C H 3 C CH 3 H . Isopropyl radical (secondary) less stable than less stable than C H 3 C CH 3 CH 3 . tert -Butyl radical (tertiary) Alkyl Radicals 2
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The initiation step "gets the reaction going" by producing free radicals—chlorine atoms from chlorine molecules in this case. Initiation step is followed by propagation steps. Each propagation step consumes one free radical but generates another one. : : .. .. Cl Cl .. .. : : Cl Cl .. .. : : .. .. .. .. Cl Cl .. .. : : . . : : Cl Cl .. .. .. .. . . + + Free-radical chain mechanism. Free-radical chain mechanism. Initiation step: Initiation step: Mechanism of Chlorination of Methane 3
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Cl Cl : : .. .. .. .. . . H H 3 C C : : H H H H : : Cl Cl : : .. .. .. .. H H 3 C C . . + + + + First propagation step: First propagation step: Second propagation step: Second propagation step: + + + + Cl Cl : : .. .. .. .. . . H H 3 C C . . : : .. .. Cl Cl .. .. : : Cl Cl .. .. : : .. .. H H 3 C C .. .. .. .. : : Cl Cl : : Mechanism of the Chlorination of Methane 4 + CH 4
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Cl Cl : : .. .. .. .. . . H H 3 C C : : H H H H : : Cl Cl : : .. .. .. .. H H 3 C C . . + + + + Second propagation step: Second propagation step: + + + + + + + + First propagation step: First propagation step: Cl Cl : : .. .. .. .. . . H H 3 C C . . : : .. .. Cl Cl .. .. : : Cl Cl .. .. : : .. .. H H 3 C C : : H H H H : : Cl Cl : : .. .. .. .. : : .. .. Cl Cl .. .. : : Cl Cl .. .. : : .. .. H H 3 C C .. .. .. .. : : Cl Cl : : H H 3 C C .. .. .. .. : : Cl Cl : : Mechanism of the Chlorination of Methane 5 Overall: Overall:
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stop chain reaction by consuming free radicals hardly any product is formed by termination step because concentration of free radicals at any instant is extremely low + + H H 3 C C .
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