CHEM 2211 1st Edition Lecture 28 Outline of Last Lecture I Diels Alder Stereochemistry Outline of Current Lecture I II III IV V VI VII VIII Introduction to Sn2 reactions Mechanism for Sn2 reactions What affects Sn2 reactions Solvent effects on Nucleophilicity Aprotic Solvents Mechanism for Sn1 Steric effects Factors that effect Sn1 reactions Lecture Notes I II Introduction to Sn2 reactions A Substitution electronegative atom is replaced by a group B Elimination electronegative atom and hydrogen from adjacent carbon are removed C Leaving group is the group being eliminated or substituted Mechanism for Sn2 reactions A Aka Nucleophilic Substitution Reaction B Rate is dependent upon the concentration of BOTH the reactants i Doubling either reactant with double the product ii A large rate constant k means that the activation energy for the reaction is low and the reactants reach the transition state quicker C Sn2 reactions don t happen in extremely sterically hindered environments i REMEMBER that Sn2 reactions occur via backside attacks to an sp3 carbon in order to avoid contact with the leaving group ii Reactivity methyl group 1 2 3 III IV V VI What affects Sn2 reactions A Leaving group i Weak bases make excellent leaving groups because they are more stable 1 Weak bases are more able to accommodate the electric charge they have B Nucleophile strength i Basicity has an equilibrium constant and is considered weak or strong ii Nucleophilicity has a rate constant and is considered poor or good C Strong Bases Good Nucleophiles i APROTIC polar solvent 1 A polar solvent without a hydrogen bonded to either and oxygen or a nitrogen 2 Follows the rule that strong bases are good nucleophiles ii PROTIC polar solvent EXCEPTION to rule 1 Polar solvent with hydrogen bonded to nitrogen or oxygen 2 EXCEPTION Largest atom best nucleophile Solvent effects on Nucleophilicity A Protic solvents i Bases form ion dipole interactions with protons of solvent ii The more electronegative an atom is the less likely the nucleophile can break its hold on the solvent molecules 1 Aka SMALL molecules POOR nucleophiles Aprotic solvent A Solvate cations better than anions i Partial negative is located on outside of molecule while partial positive is located on an inaccessible inner atom B Fluorine is a good nucleophile in NONpolar solvents and in gas phase when no solvent is present C Ionic compounds do NOT dissociate in nonpolar solvents Steric effects A Large molecules may have the criteria to be a good nucleophile but their size prevents them from getting close enough to the target molecule to bond REMEMBER good leaving groups are weak bases VII Mechanism for an Sn1 Reaction A Rate is first order and dependent upon the concentration of the alkyl halide B Reacts only with tertiary alkyl halides p 417 in the book C Forms 2 products which are enantiomers of each other VIII D First step is slow because it is the rate determining step and only includes the alkyl halide E Forms a carbocation and follows Markovnikov s rule F Nucleophile can attack the carbocation from either side and form one enantiomer with regular stereochemistry and one enantiomer with inverted stereochemistry Factors that effect Sn1 reactions A Affected by i Ease of leaving group dissociation ii Stability of carbocation B Weaker bases are tightly bonded to carbon atom i Refer to Sn2 section for discussion about basicity and nucleophilicity C Solvent is generally the nucleophile D Solvolysis is a reaction with the solvent i Does not occur in methyl primary or secondary carbocations
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