8Chapter 04

Atoms one electron goes with one atom the second one

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Unformatted text preview: alent bond. required A chemical bond can be broken in two chemical different ways—heterolytically or homolytically. homolytically. Dr. Wolf's CHM 201 & 202 4- 118 Homolytic In a homolytic bond cleavage, the two In electrons in electrons the bond are divided equally between the two the atoms. atoms. One electron goes with one atom, the second One with the other atom. – + Heterolytic In a heterolytic cleavage, one atom retains both both electrons. Dr. Wolf's CHM 201 & 202 4- 119 Homolytic Homolytic The species formed by a homolytic bond The cleavage of a neutral molecule are free radicals. Therefore, measure energy cost of homolytic bond cleavage to gain information about stability of free radicals. radicals. The more stable the free-radical products, The the weaker the bond, and the lower the Dr. Wolf's CHM 201 & 202 4- 120 Measures of Free Radical Stability Measures of Free Radical Stability Bond-dissociation energy measurements tell Bond-dissociation us that isopropyl radical is 13 kJ/mol more stable than propyl. stable . CH3CH2CH2 . + CH3CHCH3 . H + 410 397 . H Dr. Wolf's CHM 201 & 202 CH3CH2CH3 4- 121 Measures of Free Radical Stability Measures of Free Radical Stability Bond-dissociation energy measurements tell Bond-dissociation us that tert-butyl radical is 30 kJ/mol more tert-butyl stable than isobutyl. stable . (CH3)2CHCH2 + (CH3)3C . . H + 410 380 H. Dr. Wolf's CHM 201 & 202 (CH3)3CH 4- 122 4.18 Mechanism of Chlorination of Mechanism Methane Methane Dr. Wolf's CHM 201 & 202 4- 123 Mechanism of Chlorination of Methane Mechanism of Chlorination of Methane Free-radical chain mechanism. Initiation step: .. .. .. .. : Cl: Cl: : Cl . + . Cl: .. .. .. .. 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. Dr. Wolf's CHM 201 & 202 4- 124 Mechanism of Chlorination of Methane Mechanism of Chlorination of Methane First propagation step: H3C : H + .. . Cl: .. Second propagation step: .. .. H3C . + : Cl: Cl: .. .. Dr. Wolf's CHM 201 & 202 H3C . .. + H : Cl: .. .. .. H3C : Cl: + . Cl: .. .. 4- 125 Mechanism of Chlorination of Methane Mechanism of Chlorination of Methane First propagation step: H3C : H + .. . Cl: .. Second propagation step: .. .. H3C . + : Cl: Cl: .. .. .. .. H3C : H + : Cl: Cl: .. .. Dr. Wolf's CHM 201 & 202 H3C . .. + H : Cl: .. .. .. H3C : Cl: + . Cl: .. .. .. .. H3C : Cl: + H : Cl: .. .. 4- 126 Almost all of the product is formed by repetitive cycles of the two propagation steps. First propagation step: H3C : H + .. . Cl: .. Second propagation step: .. .. H3C . + : Cl: Cl: .. .. .. .. H3C : H + : Cl: Cl: .. .. Dr. Wolf's CHM 201 & 202 H3C . .. + H : Cl: .. .. .. H3C : Cl: + . Cl: .. .. .. .. H3C : Cl: + H : Cl: .. .. 4- 127 Termination Steps Termination Steps stop chain reaction by consuming free radicals .. H3C . + . Cl: .. .. H3C : Cl: .. hardly any product is formed by termination step hardly because concentration of free radicals at any iinstant is extremely low nstant Dr. Wolf's CHM 201 & 202 4- 128 4.19 Halogenation of Higher Alkanes Dr. Wolf's CHM 201 & 202 4- 129 Chlorination of Alkanes Chlorination of Alkanes Chlorination can be used to prepare alkyl chlorides can from alkanes in which all of the hydrogens are equiv...
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