ChemistryWork.docx

ChemistryWork.docx

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ChemistryWork

1.

Chemicals,Toxicity,andRisk

    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