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SC CHEM 333 - Energy Diagrams & SN1 vs. SN2

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Lecture 14Outline of Last Lecture I. Common NucleophilesII. Common Leaving GroupsCHEM 333 1st EditionIII. Two different subsitution mechanismsa. SN1i. Carbocations1. Inductive effects2. Hyperconjugationi. Stereochemistry1. Solvolysisa. SN2Outline of Current LectureI. SN2 examplesII. Energy Diagramsa. Transition stateb. Activation energy (ΔG+)c. ΔG = ΔH - TΔSd. ΔG = -RTlnKeqi. Exergonic reactionii. Endergonic Reactione. Enthalpy (ΔH)i. Exothermicii. Endothermicf. Rate determining stepIII. SN1 vs. SN2 - Experimental Evidenced. Kineticse. Solvent for nucleophilic reactionsi. Proticii. Aproticiii. Solvent Polarity1. Dielectric constant2. Non-polar solvent (nps)3. Polar solvent (ps)4. Border line solventCurrent LectureI. SN2 examplesa. Example:b. Example:c. Example:II. Energy Diagrams: show change in energy during a reaction. When bonds break, there is an increase in energy. When bonds form, there is a decrease in energy.a. Transition state: cannot be isolated, just a path to get somewhere. Bonds are startign to break and/or form.b. Activation energy (ΔG+): difference in energy between reactants and transitionstate. Smallest amount of energy needed to make reaction proceed determines the rate of reaction, or how fast the reaction goes.c. ΔG = ΔH - TΔS: equation realted to enthalpy and entropyd. ΔG = -RTlnKeq: equation realted to equilibrium (amount of product and starting material when reaction reaches equilibrium)i. Exergonic reaction: -ΔG. Means products are favored over starting material at equilibrium. Energy is released.ii. Endergonic Reaction: ΔG. Means starting material is favored over products at equilibrium. Energy is taken from surroundings.e. Enthalpy (ΔH): heat of reactioni. Exothermic: -ΔH. Bonds formed are strong er than bonds broken. Heatis given off.ii. Endothermic: ΔH. Bonds formed are weaker than bonds broken. Heat is absorbed.f. Rate determining step: slowest step in reaction. Cross high energy barrier.i. Example: SN1Unimolecular, 1 molecule reacting in rds, 2 steps overall, exothermicii. Example: SN2Bimolecular, 2 molecules reacting in rds (alkyl halide/nucleophile), 1 step overall, exothermicIII. SN1 vs. SN2 - Experimental Evidenced. Kinetics: study the rate of reactions. Look at how rate is affected when changing concentrations of reactants.i. SN1 rate only depends on alkyl halide concentrationii. SN2 rate depends on alkyl halide and nucleophile concentrationiii. Rate only depends on things that happen before or during the rate determining step1. Example: SN1x2. Example: SN2iv. If you double concentration of reactants involved in the rate, the rate will increase by two1. Example: SN1 or SN2?increase [ ] by 3 and rate increases by 3? DON’T KNOWincrease [OH] by 3 and rate increases by 3? SN2increase [OH] by 3 and rate remains the same? SN2b. Solvent for nucleophilic reactions: solvent is a medium for reactions to take placeiii. Protic: solvents with acidic H’s. Polar X-H bonds R-OH.1. Common protic solvents: H2O, low molecular weight alcohols (MeOH, EtOH, etc.), low molecular weight carboxylic acids (formic acid, acetic acid, etc.)iv. Aprotic: solvents that lack an acidic H1. Common aprotic solvents: DMSO (dimethylsulfoxide), CH3CN (acetonitrile), DMF (dimethylformamide), acetone, CH2Cl2 (dichloromethane), THF (tetrahydrofuran), Et2O (diethyl ether), toluene, hexane v. Solvent Polarity: needs to be taken into consideration, as well1. Dielectric constant: measure of solvent polarity. Ability of solvent to isolate opposite charges from each other.2. Non-polar solvent (nps): cannont isolate opposite charges. Charges stick together. Dielectric constant < 5.3. Polar solvent (ps): separate opposite charges. Charges stabalized by solvent. Dielectric constant > 15.4. Border line solvent: dielectric constant between 5 and


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