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I 1.9 Aromotic Compounds

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I 1.9 Aromotic Compounds 5t8 CHAPTERI I CarbonChains and Rings PRACTICE EXERCISEII.S \ Draw structural formulas for the following alkenes.If a compound has geometric isomers, draw both the cls and the transforms. (a) l-pentene (b) 2-hexene (c) 2-methyl-2-hexene (d) 2,3-dimethyl-2-butene Alkynes Organic compounds containing carbon-carbon triple bonds are called alkynes. Like alkenes,alkynes are unsaturated compounds. This is the car- bon-carbon triple bond found in alkynes: -C:C- Allqmes are quite rare in nature. The simplest alkyne is the gasethlme, C2H2. The common name for ethyne is acetylene-the fuel burned in oxyacety- lene torches used in welding. The single bonds that extend from the car- bons involved in the carbon-carbon Figurel l .l0 triple bond of ethpre are separatedby (Fig. Ball-and-stickmodel of ethyne the maximum bond angle, IB0 degrees f f .10). Ethyne is a linear (commonname acetylene). molecule. The development of an orbital picture for ethyne requires us to go through the same steps that we used for methane and ethene: hybridiza- tion of carbon atomic orbitals and orbital overlap. As with methane and ethene,remember that each orbital representsone electron.The descrip- tion of ethyne that best flts the experimental data is obtained if the 2s atomic orbital of carbon is mixed with only one of the three available 2p atomic orbitals (Fig. f 1.11).The result of this mixing is two sp atomic orbitals; two 2p orbitals remain unused. Overlap of one of the sp hybrid orbitals of one carbon with that of another carbon producesa carbon-car- bon sigma bond. The remaining sp orbital on each carbon overlapswith the ls orbital of a hydrogen to form two more sigma-bonding orbitals. The remaining pair of p atomic orbitals on each carbon now overlaps side by side to form two pi bonds. The bonding in ethyne consists of three sigma bonds and two pi bonds.Each pi bond has two regionsin which the proba- bility of finding the bonding electrons is high, so the pi bonds in ethyne resemble four sausagestightly packed around the carbon-carbon sigma bond. I 1.9Aromotic compounds AIM: To describethe bonding, structure,ond chemicolproperties of henzeneond other simple oromotic ond fused-ring oromotic compounds. The organic compounds we have seen thus far are all aliphatic. Aliphatic compounds contain only carbon-carbon single, double, or triple bonds. Benzene is the simplest arene. Now we will turn to the group of carbon compounds knor.tm as arenes. Arenes are a classof carbon compounds that haue structures based on the parent molecule benzene,C6I16. All arenes contain a benzene ring or a ,40 CHAPTERI I CarbonChains and Rings "Connecting the dots" to complete the structural formula of benlene, we seethat there are two dffirent waysto form three double bonds: I One ertreme Another extreme (doublebond) These two structural formulas only describe the extremes of electron sharing between any two adjacent carbons in benzene. One extreme is a normal single bond; the other is a normal double bond. Thesetwo extremes of bonding-single and double bonds-are called resonance structures. When resonancestructures can be drar,rmfor a certain molecule or ion, that molecule or ion is more stable than others forwhich only one structural for- mula can be drar,rm.As a result, benzene resistschemical reactions to which any ordinary alkene easily succumbs. A more accuratedescription of bonding in benzeneis an aueragepicture or resonance hybrid of theseforms. (The offspring of a jackass and a mare, two extreme forms of the horse family, is a hybrid, a neuter mule. The hybrid resembleseach of its parents but also has its ornmunique character- istics.)\Mhen organic chemists talk about benzene and related arenes,they usually call this hybrid bonding aromatic characteror aromaticity.The ben- zene ring is a perfectly flat molecule; ring puckering or bending would make it less stable. The orbital description of benzene is helpful in furthering our under- standing of the low chemical reactivity of benzene and related arenescom- pared with alkenes such as ethene. As mentioned previously, all orbitals representone electron.The carbonsofbenzene aresp2 hybridized, and the formation of the carbon-hydrogenand carbon-carbonsigma bonds of ben- zene is similar to their formation in ethene. Once again, one 2p atomic orbital is available on each carbon. In benzene, however, each of these p orbitals overlapsside by side with both of its neighbors (FiB. 11.12).The Electron clouds for 2p orbitals, each describing one electron. HH \,,/, t----r--, ,' --tt , n-'\_ ^/'-" \" ^^, \t' i o bonds Figurell.l2 Benzene Side-by-sideoverlap of 2p atomic orbitalsresults in an extensive pi-bondingorbital in the benzenemolecule. 11.9 Aromatic Compounds 541 result is a pi bond in which there is a high probability of findin$ the elec- trons in doughnut-shapedregions above and below the carborJring. The orbital picture shows that the pi electrons in benzene are no longer identi- fiable with any particular carbon. Thus the electrons in the pi-bonding orbitals of benzene are said to be delocalized.This bonding is quite differ- ent from that in the carbon-carbon double bond of an alkene.In the double bond of an alkene, the pi electrons are localized above and below the two carbons in the bonding orbitals. The benzene ring is found in many The low chemical reactivity of benzene and related compounds com- compounds used medicinally: the pared with that of alkenes results from their extensivepi-bonding systems. analgesic aspirin, the deconges- The more the electrons in pi bonds are smeared over a molecule, the lower tant ephedrine, the painkiller and the energy of the bonding electrons and the more stable the molecule. cough suppressantcodeine, and Therefore, the pi bonds of benzene are less chemically reactive than those the antibiotic sulfa drugs. of ordinary alkenes. When drawing an abbreviated structure of benzene, chemists often inscribe either a complete or dashed circle in the ring to indicate the delo- calized pi-bonding electrons. The inscribed circle is useful for showing the aromatic character of benzene and related compounds, but it does not give anyindicationof how manyelectrons are involved. For this reason, we will stickwith the time-honored benzene structure shor,rmat the right. A r,/\ <---\ t(ll li-\rz )l ttl v \-/ Alterna representations ofbenzene Naming arenes Most compounds containing alkyl substituents attached to the benzene ring are named as derivatives of benzene. Here are some examples of ben- zene derivativeswith one substituent: f', 00CH. CH, Methylbenzene Ethylbenzerte (toluene) Another name for methylbenzene is toluene. Sometimes the benzene ring is named as a substituent on an alkane or cycloalkane. In such instances, the benzene ring is called a phenyl group, as in 3-phenylhexane or phenyl- cyclohexane. cH3-cH2-cH-cHz-cH2-cH3 3-Phenylhexane Phenvlcvclohexane t42 CHAPTERI I CarbonChains and Rings PRACTICEEXERCISE II.9 Name the following compounds: (a) cH2cH2cH3 b) *^ CH. \-" CH. CH. tl (c) CH.-CH-CHz-CH-CH-CH3 \2r> Structural isomers of benzene derivatives There are derivatives of benzene with two substituents of the same kind. Such derivatives are called disubstituted benzenes.There are three different structural isomers of the liquid aromatic compound dimethylbenzene, C6H4(CH3)2.The dimethylbenzenes are also called xylenes.The difference between the structural isomers of dimethylbenzene is the relative positions of the methyl groups on the benzene ring. We can distinguish among the possible positions of the methyl groups by assigning the methyl groups the numbers I,2;1,3; or 1,4.It is alsopermissible in naming disubstitutedben- zenesto use the prefixes ortho-, nlett-, and para- (abbreviated o, m, and p) in place of numbers. Ortho- means that the substituentsare on adjacent carbons on the benzene ring. Meta- substituents are separatedby one ring carbon, and para- substituents are separatedby two ring carbons. The physical properties of the structural isomers of dimethylbenzene are differ- ent, as indicated by their boiling points. CFI. CHs CHg I CH. ('\l ttl2-^ .U \-A UI 13 Y CH, 1,2-Dimethylbenzene 1,3-Dimethylbenzene 1,4-Dimethylbenzene (o-dimethylbenzene, (ru-dimethylbenzene, (p-dimethylbenzene, o-xylene) rn-xylene) p-xylene) bP laa "C) (bp l3s "C) (bp 138"C) Fused-ring aromatics Fused-ring (polycyclic) aromatic compounds are deriuatiuesof benzenein which carbons are shared between benzene rlngs. Naphthalene, used in mothballs, is the simplest fused-ring aromatic. Anthracene is found in anthracite coal, and the carbon skeleton of phenanthrene forms the basic structure of the steroids, amongwhich are the sexhormones. Benzpyreneis t 1.9 AromaticCompounds 545 I I 300 carcinogens have been identified in cigarette smoke. One such compound is the hydrocarbon Hydro carb ons end Health benzpyrene. This carcinogen is converted by the Some hydrocarbons body into highly reactive compounds that react ':, with DNA molecules in have medical ap- cell nuclei to cause the : plications. Cyclopro- transformation of normal cells into cancerous ', pane is an anesthetic. ones.Benzene is implicated as a causativeagent in Mineral oil is a leukopenia (low white cell count) and in certain : mixture of hydrocar- types of leukemia. Toluene poses less of ahazard l bons of high molar than benzene, although prolonged exposure to its , mass.Petroleum jelly vapors can cause headaches,nausea, and vomit- :l (Vaseline is one ing. Breathing toluene vapors also can produce a brand name) is a narcoticeffect.ThepermissibleexpoSureofchem- semisolid mixture of ical workers to benzene and other aromatic hydro- . solid and liquid carbons is regulated by law. , Every year hospitals admit patients who have aliphatic hydrocar- Petroleumjelly, gasoline, gasoline. l Lrons of high molar paintthinners, and mineral swallowed The saturated aliphatic mass.Mineral oil and oil are all examplesof hydrocarbons in gasoline are not particularly petroleum jetly (see hydrocarbonsfound in the toxic, since they are chemically unreactive in our figure) are used to home.
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