chapter9

chapter9 - 9-9-1Organic Organic ChemistryChemistryWilliam...

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Unformatted text preview: 9-9-1Organic Organic ChemistryChemistryWilliam H. BrownWilliam H. BrownChristopher S. FooteChristopher S. FooteBrent L. IversonBrent L. Iverson9-9-2Nucleophilic Nucleophilic Substitution and Substitution and -Elimination-EliminationChapter 9Chapter 99-9-3Nucleophilic SubstitutionNucleophilic SubstitutionNucleophilic substitution:Nucleophilic substitution:any reaction in which one nucleophile substitutes for another at a tetravalent carbonNucleophile:Nucleophile:a molecule or ion that donates a pair of electrons to another molecule or ion to form a new covalent bond; a Lewis basenucleophilicsubstitutionNucleophile++CNuCLvNu:-LvLeavinggroup9-9-4Nucleophilic SubstitutionNucleophilic SubstitutionSome nucleophilic substitution reactions I -HS -RO -HO -- CNHCC -Nu NH3HOHCH3ICH3SHCH3OHCH3ORCH3O-HHCH3CNCH3BrCH3NH3+CH3CCHCH3NuBr+an alcohol (after proton transfer) an alkylammonium ionan alkyl iodidea nitrilean alkyne::::::::a thiol (a mercaptan)an etheran alcoholReaction:++:::::::9-9-5SolventsSolventsProtic solvent:Protic solvent:a solvent that is a hydrogen bond donor the most common protic solvents contain -OH groupsAprotic solvent:Aprotic solvent:a solvent that cannot serve as a hydrogen bond donornowhere in the molecule is there a hydrogen bonded to an atom of high electronegativity9-9-6Dielectric ConstantDielectric ConstantSolvents are classified as polar and nonpolarthe most common measure of solvent polarity is dielectric constantDielectric constant:Dielectric constant:a measure of a solvents ability to insulate opposite charges from one anotherthe greater the value of the dielectric constant of a solvent, the smaller the interaction between ions of opposite charge dissolved in that solventpolar solvent: dielectric constant > 15nonpolar solvent: dielectric constant < 159-9-7Protic SolventsProtic SolventsH2OHCOOHCH3OHCH3CH2OHCH3COOHSolventStructureDielectricConstant(25 C)WaterFormic acidMethanolEthanol79593324Acetic acid69-9-8Aprotic SolventsAprotic SolventsCH2Cl2CH3CH2OCH2CH3CH3CNCH3( CH2)4CH3( CH3)2S=O( CH3)2NCHO( CH3)2C=OC6H5CH32.3Toluene4.3Diethyl ether9.1DichloromethaneSolventDielectricConstantStructureDimethyl sulfoxide (DMSO)AcetonitrileAcetoneN,N-Dimethylformamide (DMF)48.937.536.720.7PolarNonpolarHexane1.99-9-9MechanismsMechanismsChemists propose two limiting mechanisms for nucleophilic substitutiona fundamental difference between them is the timing of bond-breaking and bond-forming stepsAt one extreme, the two processes take place simultaneously; designated SN2S = substitutionN = nucleophilic2 = bimolecular (two species are involved in the rate-determining step)9-9-1010Mechanism - SMechanism - SN22both reactants are involved in the transition state of the rate-determining stepCBrHHHHO:-+CHHHHOBr-- +9-9-1111Mechanism - SMechanism - SN229-...
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chapter9 - 9-9-1Organic Organic ChemistryChemistryWilliam...

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