organic chemistry4

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1、4.StereochemistryofAlkanesandCycloalkanesBasedonMcMurry’sOrganicChemistry,6thedition,Chapter4TheShapesofMoleculesThethree-dimensionalshapesofmoleculesresultfrommanyforcesAmoleculemayassumedifferentshapes,calledconformations,thatareinequilibriumatroomtemperature(theco

2、nformationalisomersarecalledconformers,emphasisonthefirstsyllable)ThesystematicstudyoftheshapesmoleculesandpropertiesfromtheseshapesisstereochemistryThefieldofstereochemistryisoneofthecentralpartsoforganicchemistryandincludesmanyimportanttopics4.1ConformationsofEthane

3、ConformersinterconvertrapidlyandastructureisanaverageofconformersMolecularmodelsarethreedimensionalobjectsthatenableustovisualizeconformersRepresentingthreedimensionalconformersintwodimensionsisdonewithstandardtypesofdrawingsRepresentingConformationsSawhorserepresenta

4、tionsshowmoleculesatanangle,showingamolecularmodelC-CbondsareatanangletotheedgeofthepageandallC-HbondsareshownNewmanprojectionsshowhowtheC-Cbondwouldprojectend-onontothepaperBondstofrontcarbonarelinesgoingtothecenterBondstorearcarbonarelinesgoingtotheedgeofthecircleEt

5、hane’sConformationsTherebarriertorotationbetweenconformationsissmall(12kJ/mol;2.9kcal/mol)ThemoststableconformationofethanehasallsixC–Hbondsawayfromeachother(staggered)TheleaststableconformationhasallsixC–Hbondsascloseaspossible(eclipsed)4.2ConformationsofPropanePropa

6、ne(C3H8)torsionalbarrieraroundthecarbon–carbonbonds14kJ/molEclipsedconformerofpropanehastwoethane-typeH–HinteractionsandaninteractionbetweenC–HandC–Cbond4.3ConformationsofButaneanticonformationhastwomethylgroups180°awayfromeachotherRotationaroundtheC2–C3giveseclipsedc

7、onformationStaggeredconformationwithmethylgroups60°apartisgaucheconformation4.4StabilityofCycloalkanes:TheBaeyerStrainTheoryBaeyer(1885):sincecarbonpreferstohavebondanglesofapproximately109°,ringsizesotherthanfiveandsixmaybetoostrainedtoexistRingsfrom3to30C’sdoexistb

8、utarestrainedduetobondbendingdistortionsandstericinteractions4.5TheNatureofRingStrainRingslargerthan3atomsarenotflatCyclicmo