CHEM 2211 1st Edition Lecture 28Outline of Last LectureI. Diels-Alder StereochemistryOutline of Current LectureI. Introduction to Sn2 reactionsII. Mechanism for Sn2 reactionsIII. What affects Sn2 reactionsIV. Solvent effects on NucleophilicityV. Aprotic SolventsVI. Mechanism for Sn1VII. Steric effectsVIII. Factors that effect Sn1 reactionsLecture NotesI. Introduction to Sn2 reactionsA. Substitution = electronegative atom is replaced by a groupB. Elimination = electronegative atom and hydrogen from adjacent carbon are removedC. Leaving group is the group being eliminated or substitutedII. Mechanism for Sn2 reactionsA. Aka Nucleophilic Substitution ReactionB. Rate is dependent upon the concentration of BOTH the reactantsi. Doubling either reactant with double the productii. A large rate constant (k) means that the activation energy for the reactionis low and the reactants reach the transition state quickerC. Sn2 reactions don’t happen in extremely sterically hindered environmentsi. REMEMBER that Sn2 reactions occur via backside attacks to an sp3 carbonin order to avoid contact with the leaving groupii. Reactivity : methyl group > 1° > 2° > 3°III. What affects Sn2 reactionsA. Leaving groupi. Weak bases make excellent leaving groups because they are more stable1. Weak bases are more able to accommodate the electric charge they haveB. Nucleophile strengthi. Basicity has an equilibrium constant and is considered weak or strongii. Nucleophilicity has a rate constant and is considered poor or goodC. Strong Bases = Good Nucleophiles i. APROTIC polar solvent1. A polar solvent without a hydrogen bonded to either and oxygen or a nitrogen2. Follows the rule that strong bases are good nucleophilesii. PROTIC polar solvent (EXCEPTION to rule)1. Polar solvent with hydrogen bonded to nitrogen or oxygen2. EXCEPTION: Largest atom = best nucleophileIV. Solvent effects on NucleophilicityA. Protic solventsi. Bases form ion dipole interactions with protons of solventii. The more electronegative an atom is, the less likely the nucleophile can break its hold on the solvent molecules1. Aka SMALL molecules = POOR nucleophilesV. Aprotic solventA. Solvate cations better than anionsi. Partial negative is located on outside of molecule while partial positive is located on an inaccessible inner atomB. Fluorine is a good nucleophile in NONpolar solvents and in gas phase when no solvent is presentC. Ionic compounds do NOT dissociate in nonpolar solventsVI. Steric effectsA. 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 bondREMEMBER: good leaving groups are weak basesVII. Mechanism for an Sn1 ReactionA. Rate is first order and dependent upon the concentration of the alkyl halideB. Reacts only with tertiary alkyl halides (p. 417 in the book)C. Forms 2 products which are enantiomers of each otherD. First step is slow because it is the rate determining step and only includes the alkyl halideE. Forms a carbocation and follows Markovnikov’s ruleF. Nucleophile can attack the carbocation from either side and form one enantiomer with regular stereochemistry and one enantiomer with inverted stereochemistryVIII. Factors that effect Sn1 reactionsA. Affected byi. Ease of leaving group dissociationii. Stability of carbocationB. Weaker bases are tightly bonded to carbon atomi. Refer to Sn2 section for discussion about basicity and nucleophilicityC. Solvent is generally the nucleophileD. Solvolysis is a reaction with the solventi. Does not occur in methyl, primary, or secondary,
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