Lecture 14 Outline of Last Lecture I Common Nucleophiles II Common Leaving Groups CHEM 333 1st Edition III Two different subsitution mechanisms a SN1 i Carbocations 1 Inductive effects 2 Hyperconjugation i Stereochemistry 1 Solvolysis a SN2 Outline of Current Lecture I SN2 examples II Energy Diagrams a Transition state b Activation energy G c G H T S d G RTlnKeq i Exergonic reaction ii Endergonic Reaction e Enthalpy H i Exothermic ii Endothermic f Rate determining step III SN1 vs SN2 Experimental Evidence d Kinetics e Solvent for nucleophilic reactions i Protic ii Aprotic iii Solvent Polarity 1 Dielectric constant 2 Non polar solvent nps 3 Polar solvent ps 4 Border line solvent Current Lecture I SN2 examples a 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 G difference in energy between reactants and transition state Smallest amount of energy needed to make reaction proceed determines the rate of reaction or how fast the reaction goes c G H T S equation realted to enthalpy and entropy d G RTlnKeq equation realted to equilibrium amount of product and starting material when reaction reaches equilibrium i Exergonic reaction G Means products are favored over starting material at equilibrium Energy is released ii Endergonic Reaction G Means starting material is favored over products at equilibrium Energy is taken from surroundings e Enthalpy H heat of reaction i Exothermic H Bonds formed are strong er than bonds broken Heat is given off ii Endothermic H Bonds formed are weaker than bonds broken Heat is absorbed f Rate determining step slowest step in reaction Cross high energy barrier i Example SN1 Unimolecular 1 molecule reacting in rds 2 steps overall exothermic ii Example SN2 Bimolecular 2 molecules reacting in rds alkyl halide nucleophile 1 step overall exothermic III SN1 vs SN2 Experimental Evidence d 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 concentration ii SN2 rate depends on alkyl halide and nucleophile concentration iii Rate only depends on things that happen before or during the rate determining step 1 Example SN1 x 2 Example SN2 iv If you double concentration of reactants involved in the rate the rate will increase by two 1 Example SN1 or SN2 increase by 3 and rate increases by 3 DON T KNOW increase OH by 3 and rate increases by 3 SN2 increase OH by 3 and rate remains the same SN2 b Solvent for nucleophilic reactions solvent is a medium for reactions to take place iii 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 H 1 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 well 1 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 15