chapter7

chapter7 - 7-7-1Organic Organic ChemistryChemistryWilliam...

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Unformatted text preview: 7-7-1Organic Organic ChemistryChemistryWilliam H. BrownWilliam H. BrownChristopher S. FooteChristopher S. FooteBrent L. IversonBrent L. Iverson7-7-2AlkynesChapter 7Chapter 77-7-3NomenclatureNomenclatureIUPAC: use the infix -ynyn-to show the presence of a carbon-carbon triple bondCommon names: prefix the substituents on the triple bond to the word acetyleneCommon name:IUPAC name:D imethylacetyleneV inylacetylene2-Butyne1-Buten-3-yne11,6-Heptadiyne3-Methyl-1-butyne6,6-Dimethyl-3-heptyne234123456123456777-7-4CycloalkynesCycloalkynesCyclononyne is the smallest cycloalkyne isolatedit is quite unstable and polymerizes at room tempthe C-C-C bond angle about the triple bond is approximately 155, indicating high angle strainCyclononyne7-7-5Physical PropertiesPhysical PropertiesSimilar to alkanes and alkenes of comparable molecular weight and carbon skeleton0.766174-360.746125-790.71671-1320.69040-900.69127-32(a gas)8-126(a gas)-23-102(a gas)-84-81Densityat 20�C(g/mL)Boiling Point(� C)MeltingPoint(� C)FormulaName1-Decyne1-Octyne1-Hexyne1-Pentyne2-Butyne1-ButynePropyneEthyneHCCHCH3CCHCH3CCCH3CH3(CH2)2CCHCH3CH2CCHCH3(CH2)3CCHCH3(CH2)5CCHCH3(CH2)7CCH7-7-6AcidityAcidityThe pKaof acetylene and terminal alkynes is approximately 25, which makes them stronger acids than ammonia but weaker acids than alcohols (Section 4.1)terminal alkynes react with sodium amide to form alkyne anions NH2H-CC-HH-CC:-NH3+pKa 38(Weaker acid)pKa 25(Stronger acid)+7-7-7AcidityAcidityterminal alkynes can also be converted to alkyne anions by reaction with sodium hydride or lithium diisopropylamide (LDA)because water is a stronger acid than terminal alkynes, hydroxide ion is not a strong enough base to convert a terminal alkyne to an alkyne anionNa+H[( CH3)2CH]2N Li+Sodium hydrideLithium diisopropylamide (LDA)Keq = 10-9.3pKa 15.7pKa 25--++(Stronger acid)W(eaker acid)HCCHHCCOHH2O7-7-8Alkylation of Alkyne Alkylation of Alkyne AnionsAnionsAlkyne anions are both strong bases and good nucleophilesThey participate in nucleophilic substitutionnucleophilic substitutionreactions with alkyl halides to form new C-C bonds to alkyl groups; they undergo alkylationalkylationbecause alkyne anions are also strong bases, alkylation is practical only with methyl and 1 halideswith 2 and 3 halides, elimination is the major reactionHCCHHCC- Na+HBrSodium acetylideBromo-cyclohexaneAcetylene CyclohexeneNa+Br -+++elimin-ation(Ch 9)7-7-9Alkylation of Alkyne AnionsAlkylation of Alkyne Anionsalkylation of alkyne anions is the most convenient method for the synthesis of terminal alkynesalkylation can be repeated and a terminal alkyne can be converted to an internal alkyneHC C - Na+BrNa+Br-Sodiumacetylide++1-Bromobutane1-HexyneCH3CH2CC- Na+CH3CH2CCCH2CH3CH3CH2-BrNa+Br-+Bromoethane3-Hexyne+Sodium butynide7-7-1010Preparation from AlkenesPreparation from AlkenesTreatment of a vicinal dibromoalkane with two...
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chapter7 - 7-7-1Organic Organic ChemistryChemistryWilliam...

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