1 a alkanes Alevel考试化学课件.pptx

Chapter16aalkanes

Themajorsourceofmethaneisnaturalgas.Crudeoilisthesourceofmanyotheralkanes.Itisfirstfractionallydistilled.Thefractionsareeitherusedasfuelsorarecrackedtogiveamixtureofalkenesandsmalleralkanes.

Alkanesaresaturatedhydrocarbons(thismeansthatthereareNOdoublebondsinthemolecule).TheycontainHydrogenandCarbonatomsonly.Thereareanimmensenumberofpossibilities.

MethaneCH4EthaneC2H6PropaneC3H8ButaneC4H10PentaneC5H12HexaneC6H14Youcanworkouttheformulaofanyofthemusing:

PhysicalProperties

PhysicalPropertiesBoilingpointSolubilityDensity

BoilingPoints

ExplanationsThereisntmuchelectronegativitydifferencebetweencarbonandhydrogen,sothereishardlyanybondpolarity.Themoleculesthemselvesalsohaveverylittlepolarity.Atotallysymmetricalmoleculelikemethaneiscompletelynon-polar.ThismeansthattheonlyattractionsbetweenonemoleculeanditsneighbourswillbeVanderWaalsdispersionforces.Thesewillbeverysmallforamoleculelikemethane,butwillincreaseasthemoleculesgetbigger.Thatswhytheboilingpointsofthealkanesincreasewithmolecularsize.

Whereyouhaveisomers,themorebranchedthechain,thelowertheboilingpointtendstobe.VanderWaalsdispersionforcesaresmallerforshortermolecules,andonlyoperateoververyshortdistancesbetweenonemoleculeanditsneighbours.Itismoredifficultforshortfatmolecules(withlotsofbranching)tolieasclosetogetheraslongthinones.

boilingpoint(K)

SolubilityAlkanesarevirtuallyinsolubleinwater,butdissolveinorganicsolvents.Theliquidalkanesaregoodsolventsformanyothercovalentcompounds.

densityLiquidalkanesarelessdensethanwater.Theirdensityincreaseasthemolarmassofthealkaneincrease.