Chemistry233 Chapter11Handout–SubstitutionReactions SN2 Reaction - Substitution Nucleophilic Bimolecular + H 3C X H 3C Nu Nu Only a single step in the reaction This single step must then = slow step + X Rate = k H 3C X Nu - Reaction Coordinate: TS G‡ H H SM E rel partial bond broken partial bond formed Br + HO HO Br HO HH H + Br H HH TS (Transition State) SM H P P Rxn G‡ = energy required to get to the transition state SN1 Reaction - Substitution Nucleophilic Unimolecular X + Nu Nu + X "slow step" More than one step involved in the reaction with discrete intermediate Carbocation formation is the rate determining step Only a single reactant (the alkyl) halide is involved in the rate determining step = unimolecular Rate = k Br Changing the concentration of R-X changes the reaction rate Chaing the concentration of any other reactants will not affect the rate X H 2O + Br TS-1 SM TS-2 Int 1 TS-1 G‡1 G ‡2 Int 1 H O H Int 2 H 2O TS-3 OH + H 3O P TS-2 G ‡3 E rel G‡1 > G‡2 and G‡3 TS-3 Int 2 Requires the most energy therefore = slow step of the reaction SM P Rxn AlkylHalideReactivity X X H 3C X methyl primary secondary allylic SN 2 Only Br I S HS RS H 2S O HO RO C N C R C H 2O RSH ROH GoodLeavingGroups R Cl R OTs (we will talk about this one later on) R OH2 CommonSolvents PolarProtic H H O H 3C H MeOH PolarAprotic O H N H H O F Cl O SN1 Only Weak X R Br tertiary benzilic SN1 or SN 2 CommonNucleophiles Strong R I X X X H dimethylsulfoxide (DMSO) H EtOH O O S OH AcOH hexane CCl4 carbontetrachloride O H 3C UsedinSN1Reactions N N,N-dimethylformamide (DMF) O acetone Non-Polar N acetonitrile tetrahydrofuran (THF) UsedinSN2Reactions DifferentiatingBetweenSN1andSN2 Factor FavoringSN2 Substrate Methylorprimary Nucleophile Strongnucleophile LeavingGroup Goodleavinggroup Solvent PolarAprotic NotUsedinSubstitution Reactions FavoringSN1 Tertiary WeakNucleophile Verygoodleavinggroup PolarProtic
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