columbia大学有机化学课件08_07 html

1、8.7Nucleophiles and NucleophilicityThe nucleophiles described in Sections 8.1-8.6have been anions.C:.:.:.:etc.: NHS.HO.CH3O.NucleophilesThe nucleophiles described in Sections 8.1-8.6have been anions.C:.:.:.:etc.: NHS.HO.CH3O.Not all nucleophiles are anions.Many are neutral.HO. H.CH3O. H: NH3for exam

2、pleNucleophilesThe nucleophiles described in Sections 8.1-8.6have been anions.C:.:.:.:etc.: NHS.HO.CH3O.Not all nucleophiles are anions.Many are neutral.HO. H.CH3O. H: NH3for exampleAll nucleophiles, however, are Lewis bases.NucleophilesMany of the solvents in which nucleophilic substitutions are ca

3、rried out are themselvesnucleophiles.HO. H.CH3O. Hfor exampleNucleophilesThe term solvolysis refers to a nucleophilicsubstitution in which the nucleophile is the solvent.Solvolysissubstitution by an anionic nucleophileRX + :NuRNu+:Xsolvolysis+ RNuH+ :XRX + :NuHstep in which nucleophilicsubstitution

4、occursSolvolysissubstitution by an anionic nucleophileRX + :NuRNu+:Xsolvolysis+ RNuH+ :XRX + :NuHproducts of overall reactionRNu+HXSolvolysisMethanolysis is a nucleophilic substitution in which methanol acts as both the solvent andthe nucleophile.CH3CH3: O:CH3+H+R:RO:RXO.HHThe product is a methyleth

5、er.Example:Methanolysissolventproduct from RXwater (HOH) methanol (CH3OH) ethanol (CH3CH2OH)OROH ROCH3 ROCH2CH3OROCformic acid (HCOH)OHOROCCH3acetic acid (CH3COH)Typical solvents in solvolysisTable 8.4 compares the relative rates of nucleophilic substitution of a variety of nucleophiles toward methy

6、l iodide as thesubstrate.The standard of comparison ismethanol, which is assigned a relative rate of 1.0.Nucleophilicity is a measure of the reactivity of a nucleophileRankNucleophileRelative rate105104stronggoodI-, HS-, RS-Br-, HO-,RO-, CN-, N3-NH3, Cl-, F-, RCO2- H2O, ROHRCO2Hfair weakvery weak103

7、110-2Table 8.4 Nucleophilicitybasicity solvationsmall negative ions are highly solvated in protic solventslarge negative ions are less solvated polarizabilityMajor factors that control nucleophilicityRankNucleophileRelative rate1041031goodfair weakHO, RO RCO2H2O, ROHWhen the attacking atom is the sa

8、me (oxygen in this case), nucleophilicity increases withincreasing basicity.Table 8.4 Nucleophilicitybasicity solvationsmall negative ions are highly solvated in protic solventslarge negative ions are less solvated polarizabilityMajor factors that control nucleophilicitySolvation of a chloride ion b

9、y ion-dipole attractiveforces with water.The negatively charged chlorideion interacts with the positively polarized hydrogensof water.Figure 8.4RankNucleophileRelative rate105104103strong goodfairI-Br-Cl-, F-A tight solvent shell around an ion makes itless reactive.Larger ions are less solvated thansmaller ones and are more nucleophilic.Table 8.4 Nucleophilicitybasicity solvationsmall negative ions are highly solvated in protic solventslarge negative ions are less solvated polarizabilityMajor factors that control nucleophilicityRankNucleophileRelative reactivity105104103strong goodfairI