专业英语.docx

专业英语.docx

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专业英语

ChapterIStructure&Bonding

Thestudyoforganicchemistrymustatsomepointextendtothemolecularlevel,forthephysicalandchemicalpropertiesofasubstanceareultimatelyexplainedintermsofthestructureandbondingofmolecules.Thismodule模块introducessomebasicfactsandprinciplesthatareneededforadiscussionoforganicmolecules.

1-1ElectronicConfigurations

ElectronConfigurationsinthePeriodicTable

Theperiodictableshownhereisseverelytruncated.Thereare,ofcourse,overeightyotherelements.

Thehalogensareoneelectronshortofavalenceshelloctet,andareamongthemostreactiveoftheelements.Intheirchemicalreactionshalogenatomsachieveavalenceshelloctetbycapturingorborrowingtheeighthelectronfromanotheratomormolecule.

Thealkalimetalsarealsoexceptionallyreactive,butfortheoppositereason.Theseatomshaveonlyoneelectroninthevalenceshell,andonlosingthiselectronarriveatthelowershellvalenceoctet.Asaconsequenceofthiselectronloss,theseelementsarecommonlyencounteredascations（positivelychargedatoms）.

1-2ChemicalBondingandValence

1-2-1IonicBonding

Whensodiumisburnedinachlorineatmosphere,itproducessodiumchloride.Thishasahighmeltingpoint（800ºC）anddissolvesinwatertogiveaconductingsolution.Sodiumchlorideisanioniccompound,inwhichanelectronofsodiumatomwastransferredtoachlorineatomandgeneratesasodiumcationandachlorideanion.

Electrostaticattractionresultsintheseoppositelychargedionspackingtogetherinalattice.Theattractiveforcesholdingtheionsinplacecanbereferredtoasionicbonds.

1-2-2CovalentBonding

Waterisaliquidatroomtemperature;carbondioxideandcarbontetrafluoridearegases.Noneofthesecompoundsiscomposedofions.Adifferentattractiveinteractionbetweenatoms,calledcovalentbonding,isinvolvedhere.

Covalentbondingoccursbysharingofvalenceelectrons,ratherthananoutrightelectrontransfer.Similaritiesinphysicalproperties（theyareallgases）suggestthatthediatomicelementsH2,N2,O2,F2&Cl2alsohavecovalentbonds.

Carbondioxideisnotablebecauseitisacaseinwhichtwopairsofelectronsaresharedbythesametwoatoms.Thisisanexampleofadoublecovalentbond.

1-2-3Valence

Thenumberofelectronsanatomgainorlosetoachieveavalenceoctetiscalledvalence.

Thevalenceshererepresentthemostcommonform普通形式inorganiccompounds.Manyelements,suchaschlorine,bromineandiodine,areknowntoexistinseveralvalencestatesindifferentinorganiccompounds.

1-3ChargeDistribution

Iftheelectronpairsincovalentbondsweresharedabsolutelyevenlytherewouldbenofixedlocalchargeswithinamolecule.AlthoughthisistruefordiatomicelementssuchasH2,N2andO2,mostcovalentcompoundsshowsomedegreeoflocalchargeseparation,resultinginbondand/ormoleculardipoles.

1-3-1electronegativity

Differentatomshavedifferentaffinitiesfornearbyelectrons.Theabilityofanelementtoattractorholdontoelectronsiscalledelectronegativity.

Fluorinehasthegreatestelectronegativityofalltheelements,andtheheavieralkalimetalssuchaspotassium,rubidiumandcesiumhavethelowestelectronegativities.Itshouldbenotedthatcarbonisaboutinthemiddleoftheelectronegativityrange,andisslightlymoreelectronegativethanhydrogen.

1-3-2PolarCovalentBonds

Whentwodifferentatomsarebondedcovalently,thesharedelectronsareattractedtothemoreelectronegativeatomofthebond,resultinginashiftofelectrondensitytowardthemoreelectronegativeatom.Suchacovalentbondispolar,andwillhaveadipole.

Thedegreeofpolarityandthemagnitudeofthebonddipolewillbeproportionaltothedifferenceinelectronegativityofthebondedatoms.ThusaO–HbondismorepolarthanaC–Hbond,withthehydrogenatomoftheformerbeingmorepositivethanthehydrogenbondedtocarbon.

Likewise,C–ClandC–Libondsarebothpolar,butthecarbonendispositiveintheformerandnegativeinthelatter.Thedipolarnatureofthesebondsisoftenindicatedbyapartialchargenotation（δ+/–）orbyanarrowpointingtothenegativeendofthebond.

Theshiftofelectrondensityinacovalentbondtowardthemoreelectronegativeatomorgroupcanbeobservedinseveralways.Forbondstohydrogen,acidityisonecriterion.Ifthebondingelectronpairmovesawayfromthehydrogennucleustheprotonwillbemoreeasilytransferedtoabase（itwillbemoreacidic）.

Methaneisalmostnon-acidic,sincetheC–Hbondisnearlynon-polar.TheO–Hbondofwaterispolar,anditisatleast25powersoftenmoreacidicthanmethane.H–Fisover12powersoftenmoreacidicthanwaterasaconsequenceofthegreaterelectronegativitydifferenceinitsatoms.

Electronegativitydifferencesmaybetransmittedthroughconnectingcovalentbondsbyaninductiveeffect.Thisinductivetransferofpolaritytapersoffasthenumberoftransmittingbondsincreases,andthepresenceofmorethanonehighlyelectronegativeatomhasacumulativeeffect.Forexample,trifluoroethanol,CF3CH2–O–Hisabouttenthousandtimesmoreacidicthanethanol,CH3CH2–O–H.

1-3-2FunctionalGroups

Functionalgroupsareatomsorsmallgroupsofatoms（twotofour）thatexhibitacharacteristicreactivitywhentreatedwithcertainreagents.Aparticularfunctionalgroupwillalmostalwaysdisplayitscharacteristicchemicalbehaviorwhenitispresentinacompound.

Becauseoftheirimportanceinunderstandingorganicchemistry,functionalgroupshavecharacteristicnamesthatoftencarryoverinthenamingofindividualcompoundsincorporatingspecificgroups.

1-4TheShapeofMolecules

Thethreedimensionalshapeorconfigurationofamoleculeisanimportantcharacteristic.Threedimensionalconfigurationsarebestviewedwiththeaidofmodels.Inordertorepresentsuchconfigurationsonatwo-dimensionalsurface（paper,blackboardorscreen）,weoftenuseperspectivedrawingsinwhichthedirectionofabondisspecifiedbythelineconnectingthebondedatoms.

Asimplestraightlinerepresentsabondlyingapproximatelyinthesurfaceplane.ThetwobondstosubstituentsAinthestructureontheleftareofthiskind.

Awedgeshapedbondisdirectedinfrontofthisplane（thickendtowardtheviewer）,asshownbythebondtosubstituentB;andahatchedbondisdirectedinbackoftheplane（awayfromtheviewer）,asshownbythebondtosubstituentD.

Thefollowingexamplesmakeuseofthisnotation,andalsoillustratetheimportanceofincludingnon-bondingvalenceshellelectronpairs（coloredblue）whenviewingsuchconfigurations.

Bondingconfigurationsarereadilypredictedbyvalence-shellelectron-pairrepulsiontheory,commonlyreferredtoasVSEPRinmostintroductorychemistrytexts.

Thissimplemodelisbasedonthefactthatelectronsrepeleachother,andthatitisreasonabletoexpectthatthebondsandnon-bondingvalenceelectronpairswillprefertobeasfarapartaspossible.Thebondingconfigurationsofcarbonareeasytoremember,sincethereareonlythreecategories.

Inthethreeexamplesshownabove,thecentralatom（carbon）doesnothaveanynon-bondingvalenceelectrons;consequentlytheconfigurationmaybeestimatedfromthenumberofbondingpartnersalone.

Formoleculesofwaterandammonia,however,thenon-bondingelectronsmustbeincludedinthecalculation.Ineachcasetherearefourregionsofelectrondensityassociatedwiththevalenceshellsothatatetrahedralbondangleisexpected.

Themeasuredbondanglesofthesecompounds（H2O104.5º&NH3107.3º）showthattheyareclosertobeingtetrahedralthantrigonalorlinear.Ofcourse,itistheconfigurationofatoms（notelectrons）thatdefinesthetheshapeofamolecule,andinthissenseammoniaissaidtobepyramidal（nottetrahedral）.Thecompoundborontrifluoride,BF3,doesnothavenon-bondingvalenceelectronsandtheconfigurationofitsatomsistrigonal.

1-5Isomers

1-5-1StructuralFormulas

Itisnecessarytodrawstructuralformulasfororganiccompoundsbecauseinmostcasesamolecularformuladoesnotuniquelyrepresentasinglecompound.Differentcompoundshavingthesamemolecularformulaarecalledisomers.

Whenthegroupofatomsthatmakeupthemoleculesofdifferentisomersarebondedtogetherinfundamentallydifferentways,werefertosuchcompoundsasconstitutionalisomers.

TherearesevenconstitutionalisomersofC4H10O,andstructuralformulasforthesearedrawninthefollowingtable.TheseformulasrepresentallknownandpossibleC4H10Ocompounds,anddisplayacommonstructuralfeature.Therearenodoubleortriplebondsandnoringsinanyofthesestructures.

Simplificationofstructuralformulasmaybeachievedwithoutanylossoftheinformationtheyconvey.Incondensedstructuralformulasthebondstoeachcarbonareomitted,buteachdistinctstructuralunit（group）iswrittenwithsubscriptnumbersdesignatingmultiplesubstituents,includingthehydrogens.

Shorthand（line）formulasomitthesymbolsforcarbonandhydrogenentirely.Eachstraightlinesegmentrepresentsabond,theendsandintersectionsofthelinesarecarbonatoms,andthecorrectnumberofhydrogensiscalculatedfromthetetravalencyofcarbon.Non-bondingvalenceshellelectronsareomittedintheseformulas.

1-5-2DistinguishingCarbonAtoms

Whendiscussingstructuralformulas,itisoftenusefultodistinguishdifferentgroupsofcarbonatomsbytheirstructuralcharacteristics.Aprimarycarbon（1º）isonethatisbondedtonomorethanoneothercarbonatom.Asecondarycarbon（2º）isbondedtotwo