ChemistryWorkWord文档格式.docx

ChemistryWorkWord文档格式.docx

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Wehearandreadalotthesedaysaboutthedangersof"

chemicals"

-aboutpesticideresidues,toxicwastes,unsafemedicines,andsoforth.What'

sapersontobelieve?

Lifeisnotrisk-free;

wealltakemanyriskseachday.Wedecidetorideabikeratherthandrive,eventhoughthereisatentimesgreaterlikelihoodpermileofdyinginabicyclingaccidentthaninacar.Wedecidetowalkdownstairsratherthantakeanelevator,eventhough7000peoplediefromfallseachyearintheUnitedStates.Wedecidetosmokecigarettes,eventhoughitincreasesourchanceofgettingcancerby50%.Makingjudgmentsthataffectourhealthissomethingwedoroutinelywithouteventhinkingaboutit.

Whataboutrisksfromchemicals?

Riskevaluationofchemicalsiscarriedoutbyexposingtestanimals（usuallyrats）tothechemicalandthenmonitoringforsignsofharm.Tolimittheexpenseandtimeneeded,theamountsadministeredarehundredsorthousandsoftimesgreaterthanthoseapersonmightnormallyencounter.DataarethenreducedtoasinglenumbercalledanLD50value,theamountofasubstanceperkilogrambodyweightthatislethalto50%ofthetestanimals.TheLD50valuesofvarioussubstancesareshowninTable1.4.Thelowerthevalue,themoretoxicthesubstance.

Evenwithanimaldataavailable,riskisstillhardtoassess.Ifasubstanceisharmfultoanimals,isitnecessarilyharmfultohumans?

Howcanalargedoseforasmallanimalbetranslatedintoasmalldoseforalargehuman?

Allsubstancesaretoxictosomeorganismstosomeextent,andthedifferencebetweenhelpandharmisoftenamatterofdegree.VitaminA,forexample,isnecessaryforvision,yetitcancausecancerathighdosages.Arsenictrioxideisthemostclassicofpoisons,yetrecentworkhasshownittobeeffectiveatinducingremissionsinsometypesofleukemia.Furthermore,howweevaluateriskisstronglyinfluencedbyfamiliarity.Manyfoodscontainnaturalingredientsfarmoretoxicthansyntheticadditivesorpesticideresidues,buttheingredientsareignoredbecausethefoodsarefamiliar.Alldecisionsinvolvetradeoffs.Doesthebenefitofincreasedfoodproductionoutweighpossiblehealthrisksofapesticide?

Dothebeneficialeffectsofanewdrugoutweighapotentiallydangeroussideeffecttoatinyfractionofusers?

Theanswersarerarelyobvious,butweshouldatleasttrytobaseourresponsesonfactsratherthanemotions.

2.

Alkaloids:

NaturallyOccurringBases

Justasammonia,NH3,isaweakbase,therearealargenumberofnitrogencontainingorganiccompoundscalledaminesthatarealsoweakbases.Intheearlydaysoforganicchemistry,basicaminesderivedfromnaturalsourceswereknownasvegetablealkali,buttheyarenowcalledalkaloids.Thestudyofalkaloidsprovidedmuchoftheimpetusforthegrowthoforganicchemistryinthe19thcentury,anditremainstodayafascinatingareaofresearch.

Alkaloidsvarywidelyinstructure,fromthesimpletotheenormouslycomplex.Theodorofrottingfish,forexample,iscausedbymethylamine,asimplerelativeofammoniainwhichoneoftheNH3hydrogenshasbeenreplacedbyanorganicCH3group.（Infact,theuseofacidiclemonjuicetomaskfishodorsissimplyanacid-basereaction.）

Manyalkaloidshavepronouncedbiologicalproperties,andmanyofthepharmaceuticalagentsusedtodayarederivedfromnaturallyoccurringamines.Asafewexamples,atropine,anantispasmodicagentusedforthetreatmentofcolitis,isobtainedfromthefloweringplantAtropabelladonna,commonlycalledthedeadlynightshade.Cocaine,bothananestheticandacentralnervoussystemstimulant,isobtainedfromthecocabushErythroxyloncoca,endemictouplandrainforestareasofColombia,Ecuador,Peru,Bolivia,andwesternBrazil.Reserpine,atranquilizerandantihypertensive,comesfrompowderedrootsofthesemitropicalplantRauwolfiaserpentina.Ephedrine,abronchodilatoranddecongestant,isobtainedfromtheChineseplantEphedrasinica.

3.

Gasoline---Chemistry@Work

Manyalkanesoccurnaturallyintheplantandanimalworld.Thewaxycoatingoncabbageleaves,forexample,containsnonacosane（C29H60）,andthewoodoiloftheJeffreypinecommontotheSierraNevadamountainsofCaliforniacontainsheptane（C7H16）.Byfarthemajorsourcesofalkanes,however,aretheworld'

snaturalgasandpetroleumdeposits.Laiddowneonsago,thesedepositsarethoughttobederivedfromthedecompositionofplantandanimalmatter,primarilyofmarineorigin.Naturalgasconsistschieflyofmethanebutalsocontainsethane,propane,andbutane.Petroleumisacomplexmixtureofhydrocarbonsthatmustbeseparatedintofractionsandthenfurtherrefinedbeforeitcanbeused.

ThepetroleumerabeganinAugust1859,whentheworld'

sfirstoilwellwasdrillednearTitusville,Pennsylvania.Thepetroleumwasdistilledintofractionsaccordingtoboilingpoint,butitwashigh-boilingkerosene,orlampoil,ratherthangasolinethatwasprimarilysought.Literacywasbecomingwidespread,andpeoplewantedbetterlightforreadingthanwasavailablefromcandles.Gasolinewastoovolatileforuseinlampsandwasinitiallyconsideredawasteby-product.Theworldhaschangedgreatlysincethoseearlydays,however,anditisnowgasolineratherthanlampoilthatisprized.

Modernpetroleumrefiningbeginsbyfractionaldistillationofcrudeoilintothreeprincipalcuts,accordingtotheirboilingpoints（bp）:

straight-rungasoline（bp30-200°

C）,kerosene（bp175-300°

C）,andheatingoilordieselfuel（bp275-400°

C）.Finally,distillationunderreducedpressureyieldslubricatingoilsandwaxesandleavesanundistillabletarryresidueofasphalt.

Thedistillationofcrudeoilisonlythefirststepingasolineproduction.Straight-rungasoline,whichcontainsahighproportionofstraight-chainalkanes,turnsouttobeapoorfuelbecauseofengineknock.Inthetypicalfour-strokeautomobileengine,apistondrawsamixtureoffuelandairintoacylinderonitsdownwardstrokeandcompressesthemixtureonitsupwardstroke.Justbeforetheendofthecompression,asparkplugignitesthemixtureandcombustionoccurs,drivingthepistondownwardandturningthecrankshaft.

Notallfuelsburnequallywell,though.Whenpoorfuelsareused,uncontrolledcombustioncanbeinitiatedbyahotsurfaceinthecylinderbeforethesparkplugfires.Thispreignition,detectedasanengineknock,candestroytheenginebyputtingirregularforcesonthecrankshaftandraisingenginetemperature.

Theoctanenumberofafuelisthemeasurebywhichitsantiknockpropertiesarejudged.Itwasrecognizedlongagothatstraight-chainhydrocarbonsarefarmorepronetoinduceengineknockthanarehighlybranchedcompounds.Heptane,aparticularlybadfuel,isassignedabasevalueof0octanenumber;

2,2,4-trimethylpentane（commonlyknownasisooctane）hasaratingof100.

4.

Terpenes:

NaturallyOccurringAlkenes

Ithasbeenknownforcenturiesthatdistillationofmanyplantmaterialswithsteamproducesafragrantmixtureofliquidscalledessentialoils.Forhundredsofyears,suchplantextractshavebeenusedasmedicines,spices,andperfumes.Theinvestigationofessentialoilsalsoplayedamajorroleintheemergenceoforganicchemistryasascienceduringthe19thcentury.

Chemically,plantessentialoilsconsistlargelyofmixturesofcompoundscalledterpenes—smallorganicmoleculeswithanimmensediversityofstructure.Thousandsofdifferentterpenesareknown,andmanyhavecarbon-carbondoublebonds.Somearehydrocarbons,andotherscontainoxygen;

someareopen-chainmolecules,andotherscontainrings.Forexample:

Allterpenesarerelated,regardlessoftheirapparentstructuraldifferences.Accordingtoaformalismcalledtheisoprenerule,terpenescanbethoughtofasarisingfromhead-to-tailjoiningoffive-carbonisoprene（2-methyl-1,3-butadiene）units.Carbon1istheheadoftheisopreneunit,andcarbon4isthetail.Forexample,myrcenecontainstwoisopreneunitsjoinedheadtotail,formingan8-carbonchainwithtwo1-carbonbranches.a-Pinenesimilarlycontainstwoisopreneunitsassembledintoamorecomplexcyclicstructure.（Seeifyoucanidentifytheisopreneunitsina-pinene.）

Terpenesareclassifiedaccordingtothenumberofisopreneunitstheycontain.Thus,monoterpenesare10-carbonsubstancesbiosynthesizedfromtwoisopreneunits,sesquiterpenesare15-carbonmoleculesfromthreeisopreneunits,diterpenesare20-carbonsubstancesfromfourisopreneunits,andsoon.Monoterpenesandsesquiterpenesarefoundprimarilyinplants,butthehigherterpenesoccurinbothplantsandanimals,andmanyhaveimportantbiologicalroles.Thetriterpenelanosterol,forexample,istheprecursorfromwhichallsteroidhormonesaremade.

Researchhasshownthatisopreneitselfisnotthetruebiologicalprecursorofterpenes.Natureinsteadusestwo"

isopreneequivalents"

—isopentenyldiphosphateanddimethylallyldiphosphate—five-carbonmoleculesthatarethemselvesmadefromaceticacid.Everystepinthebiologicalconversionfromaceticacidthroughlanosteroltohumansteroidshasbeenworkedout—animmenseachievementforwhichseveralNobelPrizeshavebeenawarded.

5.

Explosives

NobelMedicine

Mostchemicalreactionstakeplaceinoneormorediscretesteps,eachofwhichhasarate,anequilibriumconstant,andawell-definedmechanism.Thestepscanoftenbeidentified,theratesandequilibriumconstantscanbemeasured,andthemechanismscanbestudieduntilthereactioniswellunderstood.Explosions,how