2ndOrder Nucleophilic Substitution(SN2)nucleophile electrophile productleaving group2ndOrder because two molecules involved in rate expression:rate = k[OH-][CH3Br]++SN2 is a reaction mechanism that substitutes one functional group for another.Nucleophile-Electrophile CombinationsCommon feature of nucleophiles: electrons available to donate to a new bond.++2ndOrder Nucleophilic Substitution(SN2)Nucleophile approaches, adds from behind leaving group.As a result, stereochemistry at electrophile inverts.Halfway, reaction passes through its transition state.Backside Attack in SN2BrBr*(antibonding)(bonding)lone pairExplained by orbital interactions and mixing.Lone pair orbital stabilized by interaction with C-Br *.Backside Attack in SN2BrBr*(antibonding)(bonding)Explained by orbital interactions and mixing.lone pair• Interaction is only possible if nucleophile is oppositeleaving group.• This interaction will become the new C-O bonding orbital; only possible in this orientation.++2ndOrder Nucleophilic Substitution(SN2)Nucleophile approaches, adds from behind leaving group.As a result, stereochemistry at electrophile inverts.SN2 Proceeds with Inversion of Configuration(S)-2-bromobutane (R)-2-butanolWarning: Inversion of stereochemistry doesn’t necessarily mean inversion of naming. (S) doesn’t always become (R). ++The SN2 Transition State- -transition statepositive:Electron density distributed between nucleophile and leaving group.even more negative:5 repelling electron pairs at reaction center.++The SN2 Transition Statepotential energyreaction coordinateEaSN2 reactions are usually not equilibria.Success or failure of reaction depends on height of activation barrier (not on energy of products).activation energyk = Ae(-Ea/RT)-
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