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1、ChemistryWorkChemistryWork Organic Chemistry Organic Chemistry 6e 6e McMurryMcMurry11.Chemicals,Toxicity,and RiskChemicals,Toxicity,and Risk We hear and read a lot these days about the dangers of chemicals-about pesticideresidues,toxic wastes,unsafe medicines,and so forth.Whats a person to believe?L

2、ife isnot risk-free;we all take many risks each day.We decide to ride a bike rather than drive,even though there is a ten times greater likelihood per mile of dying in a bicycling accidentthan in a car.We decide to walk down stairs rather than take an elevator,even though7000 people die from falls e

3、ach year in the United States.We decide to smoke cigarettes,even though it increases our chance of getting cancer by 50%.Making judgments thataffect our health is something we do routinely without even thinking about it.What about risks from chemicals?Risk evaluation of chemicals is carried out byex

4、posing test animals(usually rats)to the chemical and then monitoring for signs of harm.To limit the expense and time needed,the amounts administered are hundreds orthousands of times greater than those a person might normally encounter.Data are thenreduced to a single number called anLD50 value,the

5、amount of a substance per kilogrambody weight that is lethal to 50%of the test animals.The LD50 values of varioussubstances are shown in Table 1.4.The lower the value,the more toxic the substance.1ChemistryWorkChemistryWork Organic Chemistry Organic Chemistry 6e 6e McMurryMcMurry2Even with animal da

6、ta available,risk is still hard to assess.If a substance is harmful toanimals,is it necessarily harmful to humans?How can a large dose for a small animal betranslated into a small dose for a large human?All substances are toxic to some organismsto some extent,and the difference between help and harm

7、 is often a matter of degree.Vitamin A,for example,is necessary for vision,yet it can cause cancer at high dosages.Arsenic trioxide is the most classic of poisons,yet recent work has shown it to be effectiveat inducing remissions in some types of leukemia.Furthermore,how we evaluate risk isstrongly

8、influenced by familiarity.Many foods contain natural ingredients far more toxicthan synthetic additives or pesticide residues,but the ingredients are ignored because thefoods are familiar.All decisions involve tradeoffs.Does the benefit of increased foodproduction outweigh possible health risks of a

9、 pesticide?Do the beneficial effects of a newdrug outweigh a potentially dangerous side effect to a tiny fraction of users?The answersare rarely obvious,but we should at least try to base our responses on facts rather thanemotions.2.Alkaloids:Naturally Occurring BasesAlkaloids:Naturally Occurring Ba

10、sesJust as ammonia,NH3,is a weak base,there are alarge number of nitrogen containing organic compoundscalledamines that are also weak bases.In the early daysof organic chemistry,basic amines derived from naturalsources were known asvegetable alkali,but they arenow calledalkaloids.The study of alkalo

11、ids providedmuch of the impetus for the growth of organic chemistryin the 19th century,and it remains today a fascinatingarea of research.Alkaloids vary widely in structure,from the simple to theenormously complex.The odor of rotting fish,forexample,is caused by methylamine,a simple relative ofammon

12、ia in which one of the NH3 hydrogens has been replaced by an organic CH3 group.(In fact,the use of acidic lemon juice to mask fish odors is simply an acid-base reaction.)2ChemistryWorkChemistryWork Organic Chemistry Organic Chemistry 6e 6e McMurryMcMurry3Many alkaloids have pronounced biological pro

13、perties,and many of the pharmaceuticalagents used today are derived from naturally occurring amines.As a few examples,atropine,an antispasmodic agent used for the treatment of colitis,is obtained from theflowering plantAtropa belladonna,commonly called thedeadly nightshade.Cocaine,bothan anesthetic

14、and a central nervous system stimulant,is obtained from the coca bushErythroxylon coca,endemic to upland rain forest areas of Colombia,Ecuador,Peru,Bolivia,and western Brazil.Reserpine,a tranquilizer and antihypertensive,comes from powderedroots of the semitropical plantRauwolfia serpentina.Ephedrin

15、e,a bronchodilator anddecongestant,is obtained from the Chinese plantEphedra sinica.3.Gasoline-ChemistryWorkGasoline-ChemistryWork3ChemistryWorkChemistryWork Organic Chemistry Organic Chemistry 6e 6e McMurryMcMurry4Many alkanes occur naturally in the plant and animal world.The waxy coating oncabbage

16、 leaves,for example,contains nonacosane(C29H60),and the wood oil of theJeffrey pine common to the Sierra Nevada mountains of California contains heptane(C7H16).By far the major sources of alkanes,however,are the worlds natural gas andpetroleum deposits.Laid down eons ago,these deposits are thought t

17、o be derived fromthe decomposition of plant and animal matter,primarily of marine origin.Natural gasconsists chiefly of methane but also contains ethane,propane,and butane.Petroleum is acomplex mixture of hydrocarbons that must be separated into fractions and then furtherrefined before it can be use

18、d.The petroleum era began in August 1859,when the worlds first oil well was drilled nearTitusville,Pennsylvania.The petroleum was distilled into fractions according to boilingpoint,but it was high-boiling kerosene,or lamp oil,rather than gasoline that was primarilysought.Literacy was becoming widesp

19、read,and people wanted better light for readingthan was available from candles.Gasoline was too volatile for use in lamps and was initiallyconsidered a waste by-product.The world has changed greatly since those early days,however,and it is now gasoline rather than lamp oil that is prized.Modern petr

20、oleum refining begins by fractional distillation of crude oil into three principalcuts,according to their boiling points(bp):straight-run gasoline(bp 30-200C),kerosene(bp 175-300C),and heating oil or diesel fuel(bp 275-400C).Finally,distillation underreduced pressure yields lubricating oils and waxe

21、s and leaves an undistillable tarry residueof asphalt.4ChemistryWorkChemistryWork Organic Chemistry Organic Chemistry 6e 6e McMurryMcMurry5The distillation of crude oil is only the first step in gasoline production.Straight-rungasoline,which contains a high proportion of straight-chain alkanes,turns

22、 out to be a poorfuel because ofengine knock.In the typical four-stroke automobile engine,a piston drawsa mixture of fuel and air into a cylinder on its downward stroke and compresses themixture on its upward stroke.Just before the end of the compression,a spark plug ignitesthe mixture and combustio

23、n occurs,driving the piston downward and turning thecrankshaft.Not all fuels burn equally well,though.When poor fuels are used,uncontrolled combustioncan be initiated by a hot surface in the cylinder before the spark plug fires.This preignition,detected as an engine knock,can destroy the engine by p

24、utting irregular forces on thecrankshaft and raising engine temperature.Theoctane number of a fuel is the measure by which its antiknock properties are judged.It was recognized long ago that straight-chain hydrocarbons are far more prone to induceengine knock than are highly branched compounds.Hepta

25、ne,a particularly bad fuel,isassigned a base value of 0 octane number;2,2,4-trimethylpentane(commonly known asisooctane)has a rating of 100.5ChemistryWorkChemistryWork Organic Chemistry Organic Chemistry 6e 6e McMurryMcMurry64.Terpenes:Naturally OccurringTerpenes:Naturally OccurringAlkeneAlkenesIt h

26、as been known for centuries that distillation of many plant materials with steamproduces a fragrant mixture of liquids calledessential oils.For hundreds of years,suchplant extracts have been used as medicines,spices,and perfumes.The investigation ofessential oils also played a major role in the emer

27、gence of organic chemistry as a scienceduring the 19th century.Chemically,plant essential oils consist largely of mixtures of compounds calledterpenes small organic molecules with an immense diversity of structure.Thousands of differentterpenes are known,and many have carbon-carbon double bonds.Some

28、 are hydrocarbons,and others contain oxygen;some are open-chain molecules,and others contain rings.Forexample:All terpenes are related,regardless of their apparent structural differences.Accordingto a formalism called theisoprene rule,terpenes can be thought of as arising fromhead-to-tail joining of

29、 five-carbon isoprene(2-methyl-1,3-butadiene)units.Carbon 1 is thehead of the isoprene unit,and carbon 4 is the tail.For example,myrcene contains twoisoprene units joined head to tail,forming an 8-carbon chain with two 1-carbon branches.6ChemistryWorkChemistryWork Organic Chemistry Organic Chemistry

30、 6e 6e McMurryMcMurry7a-Pinene similarly contains two isoprene units assembled into a more complex cyclicstructure.(See if you can identify the isoprene units ina-pinene.)Terpenes are classified according to the number of isoprene units they contain.Thus,monoterpenes are 10-carbon substances biosynt

31、hesized from two isoprene units,sesquiterpenes are 15-carbon molecules from three isoprene units,diterpenes are20-carbon substances from four isoprene units,and so on.Monoterpenes andsesquiterpenes are found primarily in plants,but the higher terpenes occur in both plantsand animals,and many have im

32、portant biological roles.The triterpene lanosterol,forexample,is the precursor from which all steroid hormones are made.Research has shown that isoprene itself is not the true biological precursor of terpenes.Nature instead uses two isoprene equivalents isopentenyl diphosphate anddimethylallyl dipho

33、sphate five-carbon molecules that are themselves made from aceticacid.Every step in the biological conversion from acetic acid through lanosterol to humansteroids has been worked out an immense achievement for which several Nobel Prizeshave been awarded.7ChemistryWorkChemistryWork Organic Chemistry

34、Organic Chemistry 6e 6e McMurryMcMurry85.ExplosivesExplosivesNobel MedicineNobel MedicineMost chemical reactions take place in one ormore discrete steps,each of which has a rate,an equilibrium constant,and a well-definedmechanism.The steps can often be identified,the rates and equilibrium constants

35、can bemeasured,and the mechanisms can be studieduntil the reaction is well understood.Explosions,however,are different.Theirrates are so fast and their mechanisms are socomplex that the detailsby which explosionsoccur defy a complete understanding.Chemical explosions are characterized by thespontane

36、ous breakdown of molecules into fragments that then recombine togive the final products-usually stable gases such as N2,H2O,and CO2.Theresult is a nearly instantaneous release of large quantities of hot gases,which set up a devastating shock wave as they expand.The shock wave cantravel at speeds of

37、up to 9000 m/s(approximately 20,000 mi/h)and generatea pressure of up to 700,000 atm,causing enormous physical devastationto the surroundings.Explosives are categorized as eitherprimary or secondary,depending ontheir sensitivity to shock.Primary explosives,such as lead azide,Pb(N3)2,are the most sen

38、sitive.They are used in detonators,blasting caps,andmilitary fuses to initiate the explosion of a less sensitive,secondaryexplosive.Secondary explosives,orhigh explosives,are less sensitiveto heat and shock than primary explosives and are therefore safer tomanufacture and transport.Most secondary ex

39、plosives simply burn ratherthan explode when ignited in air,and most can be detonated only by thenearby explosion of a primary initiator.The first commercially important high explosive was nitroglycerin,prepared in 1847 by reaction of glycerin with nitric acid in the presenceof sulfuric acid:8Chemis

40、tryWorkChemistryWork Organic Chemistry Organic Chemistry 6e 6e McMurryMcMurry9As you might expect,the reaction is extremely hazardous to carry out,and it was not until 1865 that the Swedish chemist Alfred Nobel succeededin finding a reliable method of producing nitroglycerin and incorporatingit into

41、 the commercial blasting product calleddynamite.(The fortuneNobel accumulated from his discovery was subsequently used to fund theNobel Prizes.)Modern industrial dynamite used for quarrying stone andblasting roadbeds is a mixture of ammonium nitrate and nitroglycerinabsorbed onto diatomaceous earth.

42、The military explosives used as fillings for bombs or shells must havea low sensitivity to impactshock on firing and must havegood stabilityfor long-term storage.TNT(trinitrotoluene),PETN(pentaerythritoltetranitrate),and RDX(royal demolition explosive)are the most commonlyused military high explosiv

43、es.PETN and RDX are also compounded with waxesor synthetic polymers to make so-called plastic explosives.6.Natural RubberNatural Rubber9ChemistryWorkChemistryWork Organic Chemistry Organic Chemistry 6e 6e McMurryMcMurry10Rubber an unusual name for an unusual substance is a naturally occurring alkene

44、polymer produced by more than 400 different plants.The major source is the so-calledrubber tree,Hevea brasiliensis,from which the crude material is harvested as it drips froma slice made through the bark.The namerubber was coined by Joseph Priestley,thediscoverer of oxygen and early researcher of ru

45、bberchemistry,for the simple reason that one of rubbers earlyuses was to rub out pencil marks on paper.Unlike polyethylene and other simple alkene polymers,natural rubber is a polymer of adiene,isoprene(2-methyl-1,3-butadiene).The polymerization takes placeby addition of isoprene monomer units to th

46、e growingchain,leading to formation of a polymer that still containsdouble bonds spaced regularly at four-carbon intervals.As the following structure shows,these double bondshaveZ stereochemistry:Crude rubber,calledlatex,is collected from the tree as an aqueous dispersion that iswashed,dried,and coa

47、gulated by warming in air.The resultant polymer has chains thataverage about 5000 monomer units in length and have molecular weights of 200,000 to500,000 amu.This crude coagulate is too soft and tacky to be useful until it is hardened byheating with elemental sulfur,a process calledvulcanization.By

48、mechanisms that are stillnot fully understood,vulcanization cross-links the rubber chains together by formingcarbon-sulfur bonds between them,thereby hardening and stiffening the polymer.Theexact degree of hardening can be varied,yielding material soft enough for automobile tiresor hard enough for b

49、owling balls(ebonite).The remarkable ability of rubber to stretch and then contract to its original shape is due tothe irregular shapes of the polymer chains caused by the double bonds.These doublebonds introduce bends and kinks into the polymer chains,thereby preventing neighboringchains from nestl

50、ing together.When stretched,the randomly coiled chains straighten outand orient along the direction of the pull but are kept from sliding over one another by thecross-links.When the stretch is released,the polymer reverts to its original random state.10ChemistryWorkChemistryWork Organic Chemistry Or