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11.1 Free Radicals 11.1 Free Radicals • Free radicals form when bonds break HOMOLYTICALLY. • Recall the orbital hybridization in carbocations and carbanions. • Note the single‐barbed or fishhook arrow used to show the electron movement. Copyright 2012 John Wiley & Sons, Inc. 11 -1 Klein, Organic Chemistry 1e Copyright 2012 John Wiley & Sons, Inc. 11 -2 Klein, Organic Chemistry 1e 11.1 Free Radicals 11.1 Free Radicals – Stability • Free radicals can be thought of as sp2 hybridized or • Free radicals do not have a formal charge but are quickly interconverting sp3 hybridized. unstable because of an incomplete octet. • Groups that can push (donate) electrons toward the free radical will help to stabilize it. WHY? HOW? Consider hyperconjugation. Copyright 2012 John Wiley & Sons, Inc. 11 -3 Klein, Organic Chemistry 1e Copyright 2012 John Wiley & Sons, Inc. 11 -4 Klein, Organic Chemistry 1e 11.1 Free Radicals – Stability 11.1 Free Radicals –Resonance • Use arguments that involve hyperconjugation and an • Drawing resonance for free radicals using fishhook energy diagram to explain the differences in bond arrows to show electron movement. dissociation energy (BDE) below. • Remember, for resonance, the arrows don’t ACTUALLY show electron movement. WHY? • Draw the resonance hybrid for an allyl radical. • HOW and WHY does resonance affect the stability of the free radical? • Practice with CONCEPTUAL CHECKPOINT 11.1. Copyright 2012 John Wiley & Sons, Inc. 11 -5 Klein, Organic Chemistry 1e Copyright 2012 John Wiley & Sons, Inc. 11 -6 Klein, Organic Chemistry 1e 11.1 Free Radicals – Resonance 11.1 Free Radicals –Resonance Stabilization • The benzylic radical is a hybrid that consists of four • How does resonance affect the stability of a radical? contributors. WHY? • Draw the remaining contributors. • Draw the hybrid. • What is a bigger factor, hyperconjugation or resonance? • Practice with SKILLBUILDER 11.1. Copyright 2012 John Wiley & Sons, Inc. 11 -7 Klein, Organic Chemistry 1e Copyright 2012 John Wiley & Sons, Inc. 11 -8 Klein, Organic Chemistry 1e 11.1 Free Radicals – Resonance 11.2 Radical Electron Movement Stabilization • Vinylic free radicals are especially unstable. • Free radical electron movement is quite different from • No resonance stabilization. electron movement in ionic reactions. – For example, free radicals don’t undergo rearrangement. • What type of orbital is the vinylic free radical located in, • There are SIX key arrow‐pushing patterns that we will and how does that affect stability? discuss. • Practice with SKILLBUILDER 11.2. Copyright 2012 John Wiley & Sons, Inc. 11 -9 Klein, Organic Chemistry 1e Copyright 2012 John Wiley & Sons, Inc. 11 -10 Klein, Organic Chemistry 1e 11.2 Radical Electron Movement 11.2 Radical Electron Movement 1. Homolytic cleavage, initiated by light or heat: 5. Elimination: the radical from the α carbon is pushed toward the β carbon to eliminate a group on the β carbon (reverse of addition to a pi bond): 2. Addition to a pi bond: – The –X group is NOT a leaving group. WHY? 3. Hydrogen abstraction (NOT the same as proton transfer): 6. Coupling, the reverse of homolytic cleavage: 4. Halogen abstraction: • Note that radical electron movement generally involves two or three fishhook arrows. Copyright 2012 John Wiley & Sons, Inc. 11 -11 Klein, Organic Chemistry 1e Copyright 2012 John Wiley & Sons, Inc. 11 -12 Klein, Organic Chemistry 1e 11.2 Radical Electron Movement 11.2 Radical Electron Movement • Note the reversibility of radical processes: • Radical electron movement is generally classified as either initiation, termination, or propagation. INITIATION TERMINATION occurs when occurs when radicals are radicals are created. destroyed. • Practice with SKILLBUILDER 11.3. Copyright 2012 John Wiley & Sons, Inc. 11 -13 Klein, Organic Chemistry 1e Copyright 2012 John Wiley & Sons, Inc. 11 -14 Klein, Organic Chemistry 1e 11.3 Radical Reactions – 11.2 Radical Electron Movement Chlorination of Methane • PROPAGATION occurs when radicals are moved from • Let’s apply our electron‐pushing skills to a reaction. one location to another. • We must consider each pattern for any free radical that forms during the reaction. • Is homolytic cleavage also likely for CH4? Copyright 2012 John Wiley & Sons, Inc. 11 -15 Klein, Organic Chemistry 1e Copyright 2012 John Wiley & Sons, Inc. 11 -16 Klein, Organic Chemistry 1e 11.3 Radical Reactions – 11.3 Radical Reactions – Chlorination of Methane Chlorination of Methane • Why are termination reactions less common than propagation reactions? Copyright 2012 John Wiley & Sons, Inc. 11 -17 Klein, Organic Chemistry 1e Copyright 2012 John Wiley & Sons, Inc. 11 -18 Klein, Organic Chemistry 1e 11.3 Radical Reactions – 11.3 Radical Reactions – Chlorination of Methane Chlorination of Methane • The propagation steps give the net reaction: • Reactions that have self‐sustaining propagation steps are called CHAIN REACTIONS. • In a chain reaction, the products from one step are reactants for a different step in the mechanism. • Polychlorination is difficult to prevent, especially when an excess of Cl is present. WHY? 1. Initiation produces a small amount Cl• radical. 2 2. H abstraction consumes the Cl• radical. 3. Cl abstraction generates a Cl• radical, which can go on to start another H abstraction. • Propagation steps are self‐sustaining. • Practice with SKILLBUILDER 11.4. Copyright 2012 John Wiley & Sons, Inc. 11 -19 Klein, Organic Chemistry 1e Copyright 2012 John Wiley & Sons, Inc. 11 -20 Klein, Organic Chemistry 1e 11.3 Radical Reactions – 11.3 Radical Reactions – Radical Chlorination of Methane Initiators • Draw a reasonable mechanism that shows how each of • An initiator starts a free radical chain reaction the products might form in the following reaction. ∆Hº = +159 kJ/mol Cl ∆Hº = +243 kJ/mol Cl2 heat Cl Cl ∆Hº = +121 kJ/mol Cl • Which of the products above are major, and which are • Which initiator above initiates reactions most readily? minor? WHY? • The acyl peroxide will be effective at 80 °C. Copyright 2012 John Wiley & Sons, Inc. 11 -21 Klein, Organic Chemistry 1e Copyright 2012 John Wiley & Sons, Inc. 11 -22 Klein, Organic Chemistry 1e 11.3 Radical Reactions – Radical 11.3 Radical Reactions – Radical Inhibitors Inhibitors • Inhibitors act in a reaction to scavenge free radicals to • Hydroquinone is also often used as a radical inhibitor. stop chain reaction processes. • Oxygen molecules can exist in the form of a diradical, which reacts readily with other radicals. Use arrows to show the process. • How can reaction conditions be modified to stop • oxygen from inhibiting a desired chain reaction? Draw in necessary arrows in the reactions above. Copyright 2012 John Wiley & Sons, Inc. 11 -23 Klein, Organic Chemistry 1e Copyright 2012 John Wiley & Sons, Inc. 11 -24 Klein, Organic Chemistry 1e 11.4 Halogenation Thermodynamics 11.4 Halogenation Thermodynamics • If we want to determine whether a process is product • Because the ‐TΔS term should be close to zero, we can favored, we must determine the sign (+/‐) for ΔG. simplify the free energy equation • Considering the bonds that break and form in the reaction, estimate ΔHhalogenation for each of the halogens in the table below. • In a halogenation reaction, will ΔH or ‐TΔS have a bigger effect on the sign of ΔG? WHY? Copyright 2012 John Wiley & Sons, Inc. 11 -25 Klein, Organic Chemistry 1e Copyright 2012 John Wiley & Sons, Inc. 11 -26 Klein, Organic Chemistry 1e 11.4 Halogenation Thermodynamics 11.4 Halogenation Thermodynamics • Fluorination is so exothermic that it is impractical. WHY does that make it impractical? • Iodination is nonspontaneous, so does not favor products. The reaction is not PRODUCT‐FAVORED. Copyright 2012 John Wiley & Sons, Inc. 11 -27 Klein, Organic Chemistry 1e Copyright 2012 John Wiley & Sons, Inc. 11 -28 Klein, Organic Chemistry 1e 11.4 Halogenation Thermodynamics 11.4 Halogenation Thermodynamics • Consider chlorination and bromination in more detail. • Which step in the mechanism is the slow step? Which reaction has a faster rate? Which is more product favored? –Both steps are – First step is endothermic exothermic – Second step is exothermic • Chlorination is more product‐favored than bromination. Copyright 2012 John Wiley & Sons, Inc. 11 -29 Klein, Organic Chemistry 1e Copyright 2012 John Wiley & Sons, Inc. 11 -30 Klein, Organic Chemistry 1e 11.5 Halogenation Regioselectivity 11.5 Halogenation Regioselectivity • With substrates more complex than ethane, multiple • For the CHLORINATION process, the actual product MONOHALOGENATION products are possible. distribution favors 2‐chloropropane over 1‐ chloropropane. • If the halogen were indiscriminant, predict the product • Which step in the mechanism determines the ratio? regioselectivity? • Show the arrow pushing for that key mechanistic step. Copyright 2012 John Wiley & Sons, Inc. 11 -31 Klein, Organic Chemistry 1e Copyright 2012 John Wiley & Sons, Inc. 11 -32 Klein, Organic Chemistry 1e 11.5 Halogenation Regioselectivity 11.5 Halogenation Regioselectivity • In one reaction, a 1° free radical forms, and in the other, a 2° radical forms. • For the BROMINATION process, the product distribution • Is the chlorination process thermodynamically or vastly favors 2‐bromopropane over 1‐bromopropane. kinetically controlled? • WHY? • Which step in the mechanism determines regioselectivity? • Show the arrow pushing for that key mechanistic step. Copyright 2012 John Wiley & Sons, Inc. 11 -33 Klein, Organic Chemistry 1e Copyright 2012 John Wiley & Sons, Inc. 11
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