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Ch 10 Notes - A Word on Thermochemistry H H O H H H Br!H O...

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A Word on Thermochemistry Driving Force: Δ G° = - RT ln(K eq ) Δ G° = - 2.3 RT log(K eq ) = 2.3 * 0.0020 kcal/mol-K * T * pK eq or at room temp (298 K) Δ G° = 1.4 pK eq THERMODYNAMICS: A 1.4 kcal/mol difference between starting materials and reactants makes a 10X difference in relative amounts of material . KINETICS: A 1.4 kcal/mol difference in activation energy makes a 10X difference in reaction rate . IN THE REACTION ABOVE pK eq = pK a (fwd) - pK a (rev) = -9 - -2 = -7 K eq = 10 7 ; Δ G = 1.4 * -7 = -10 kcal/mol ! + ! - H Br + + O H H O H H H Br
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catalyst + H H H H H H H H H H H H Hydrogenation: How fast, how much How much: Δ G° = Δ H° - T Δ Δ H2 = Δ H f ° (products) - Δ H f ° (reactants) = -20.2 kcal/mol - ( + 12.5 kcal/M [ethene] + O [H 2 ] ) = -32.7 kcal/mol (this is the enthalpic contribution) ! H = -32.7 ethane ethene + H 2 ! H C-C = 83; C = C = 146; C - H = 99; H - H = 104; C = C π bond we will take as 66 Enthalpy can also be estimated from BDEs (kcal/mol): Δ H ~ energy required to break C = C π bond [66], break H-H [104] - energy gained in making 2 CH bonds [2 x 99] ~ - 30 kcal/mol which agrees reasonably well with our exact value Δ G° = Δ H° - T Δ = -31 kcal/mol - 300°K ( - 0.03 kcal/molK) = -22 kcal/mol [ entropy works against this reaction ]
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Chapter 10 Radical Reactions Heterolytic and Homolytic Bond Breaking So far, all the organic reaction mechanisms examined involve heterolytic bond breaking where ions are reactants, intermediates, or products. These reactions form a very large (and more common) category of mechanisms in organic chemistry. A second important type of organic reaction mechanism involves homolytic bond breaking where free radicals are key intermediates. A free radical has an unpaired electron in the valence shell. It is often uncharged, but radical cations and radical anions also exist.
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Free Radicals by Homolytic Bond Breaking Bond Homolysis A:B A + B . . Note: single-barbed arrows show movement of one electron. Some Examples of Free Radicals Cl : : : . chlorine atom or radical H-C-H H . methyl radical H-O : : . hydroxyl radical
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Homolytic Bond Dissociation Energies To break a covalent bond, energy must be supplied. Such a reaction is endothermic . The energy required to break a covalent bond homolytically is the homolytic bond dissociation energy, DH o . When chemical species (atoms or molecules) with unpaired electrons (radicals or atoms) combine to form a covalent bond, energy is released . The state with the covalent bond is more stable than the state with the unpaired (nonbonding) electrons. This reaction is exothermic. ...it evolves heat. . . + D H o = - 104 kcal/mol an exothermic reaction H H H-H energy . . + 104 kcal/mol H-H H H . . + ! H o =+ 104 kcal/mol an endothermic reaction H-H H H
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Homolytic Bond Dissociation and Heats of Reaction The standard enthalpy change, ! H o , for a reaction can be calculated from an evaluation of the bond dissociation energies ( DH o ) of the bonds being broken and the bonds being made.
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