chapter9

chapter9 - 9-9-1Organic Organic H BrownWilliam H...

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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 ββ-Elimination-EliminationChapter 9Chapter 99-9-3Nucleophilic SubstitutionNucleophilic Substitution◆Nucleophilic substitution:Nucleophilic substitution:any reaction in which one nucleophile substitutes for another at a tetravalent carbon◆Nucleophile: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 Substitution◆Some 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:++:::::::9-9-5SolventsSolvents◆Protic solvent:Protic solvent:a solvent that is a hydrogen bond donor •the most common protic solvents contain -OH groups◆Aprotic solvent:Aprotic solvent:a solvent that cannot serve as a hydrogen bond donor•nowhere in the molecule is there a hydrogen bonded to an atom of high electronegativity9-9-6Dielectric ConstantDielectric Constant◆Solvents are classified as polar and nonpolar•the most common measure of solvent polarity is dielectric constant◆Dielectric constant:Dielectric constant:a measure of a solvent’s ability to insulate opposite charges from one another•the greater the value of the dielectric constant of a solvent, the smaller the interaction between ions of opposite charge dissolved in that solvent•polar solvent: dielectric constant > 15•nonpolar solvent: dielectric constant < 159-9-7Protic SolventsProtic SolventsH2OHCOOHCH3OHCH3CH2OHCH3COOHSolventStructureDielectricConstant(25&#2; 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)48.937.536.720.7PolarNonpolarHexane1.99-9-9MechanismsMechanisms◆Chemists propose two limiting mechanisms for nucleophilic substitution•a fundamental difference between them is the timing of bond-breaking and bond-forming steps◆At one extreme, the two processes take place simultaneously; designated SN2•S = substitution•N = nucleophilic•2 = bimolecular (two species are involved in the rate-determining step)9-9-1010Mechanism - SMechanism - SN22•both reactants are involved in the transition state of the rate-determining stepCBrHHHHO:-+CHHHHOBrδ-δ-Τραν σιτιον στατε ϖιτη σιμ υλταν εουσβονδ βρεακινγ αν δ βονδ φορμ ινγΧΗΗΗΗΟ+Βρ9-9-1111Mechanism - SMechanism - SN229-...
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This note was uploaded on 03/21/2008 for the course CHEM 261 taught by Professor Austell during the Spring '08 term at UNC.

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chapter9 - 9-9-1Organic Organic H BrownWilliam H...

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