Lecture and Chapter 11 Notes

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University of Toronto St. George
Thavarajah/ Woolley/ Nitz

CHM138H1Jasmyn Lee Chapter 11 Nucleophilic Substitutions and Eliminations 111 The Discovery of Nucleophilic Substitution Reactions Nucleophilic Substitution Reactions Nucleophilic Substitution Reactioneach step involves the substitution of one nucleophile by anotherTwo limiting mechanisms of nucleophilic substitution o S2 Substitution Nucleophilic Bimolecular No S1 Substitution Nucleophilic Unimolecular N A fundamental difference between them is the timing of bondbreaking and bondforming steps112 S2 Substitution Nucleophilic Bimolecular NS2substitution Nucleophilic bimolecular two Nmolecules nucleophile and alkyl halide take part in the step whose kinetics are measured Takes place in a single step without intermediates whenthe nucleophile reacts with the substrate from adirection opposite the group that is displaced theleaving group Mechanism Stereochemistry changesKineticsthe relationship between the rate at which the reaction occurs and the concentration of the reactants CHM138H1Jasmyn Lee Evidence a Reaction Kinetics Reaction RateRate of disappearance of reactantRatek Nu substratesecond orderRate depends on both concentration of electrophile and nucleophileTherefore both must participate in rate limiting stepConsistent with proposed mechanism b Stereochemistry S2 reaction occurs when an electron Npair on the nucleophile Nu forces out the group X which takes with it the electron pair from the former CXTransition statenew NuC bondS2 reactions at a stereocenter occur with inversion of Npartially forming and old CX bond is configuration partially breaking negative charge is shared by both incoming Nu and outgoing halide ion oPlanar arrangement 113 Characteristics of the S2 Reaction NFactors a Structure of the SubstrateSteric Strainrepulsive interaction in a molecule due to interpenetrating electron clouds when atoms are forced close togetherBulky substratessterically hindered prevent easy approach of nucleophile making bond formation difficult o Transition state is higher in energyforms slower than less sterically hindered substrate o Carbon is shielded from nucleophileSteric effects slow down S2 reactions Nb The Nature of the NucleophileNucleophilemust have an unshared pair of electrons Lewis Base o Negative nucleophileneutral product o Neutral nucleophilepositive productNucleophilicitya kinetic property measured by the rate at which a Nu attacks a reference compound
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