INTERCHAPTER H Aromatic Hydrocarbons Scanning tunneling micrograph of benzene molecules adsorbed on the surface of a rhodium crystal. The hexagonal shape of the benzene molecules is revealed in this remarkable micrograph. University Science Books, ©2011. All rights reserved. www.uscibooks.com Title General Chemistry - 4th ed H. AROMATIC HYDROCARBONS H1 Author McQuarrie/Gallogy At one time, the term aromatic was used to describe a Artist George Kelvin number of substances isolated from coal tar that had Figure # 007-012 characteristic strong fragrances. Many of these sub- stances were subsequently shown to contain benzene Date 08/05 /09 where a hydrogen atom is understood to be bonded to rings. Nowadays, we use the term aromatic to refer Check if revision x Approved the carbon atom at each vertex in the benzene ring. to the class of compounds that contain benzene-like Although each resonance form of benzene shows rings. Many important compounds such as the hor- localized double bonds, the resonance hybrid does mone estrone and the analgesic ibuprofen are aro- not (Section 9-13). matic in part of their structure. O CH CH3 3 CH C OH H 3 H C C H H C H C O H 3 H HO 2 estrone ibuprofen H-1. Benzene Belongs to a Class of Hydrocarbons Called Aromatic Hydrocarbons Molecular model of benzene, C6H6 Benzene, which was first identified by Michael Faraday (Chapter 25 Frontispiece) in 1825, is the quintessen- tial example of an aromatic compound. Benzene is a colorless, carcinogenic, flammable liquid with a characteristic odor. A benzene ring is relatively stable H-2. Most Reactions of Benzene Are because of p-electron delocalization (Section 9-13). Substitution Reactions Benzene and many other aromatic hydrocarbons are The benzene molecule does not have localized dou- obtained from petroleum and coal tar. About eight ble bonds and does not react as an unsaturated hydro- million metric tons of benzene are produced annu- carbon. In fact, the p-electron delocalization causes ally in the United States. the ring to be so stable that many of the reactions We noted in Sections 7-6 and 9-13 that benzene is that unsaturated hydrocarbons undergo do not occur a planar, perfectly hexagonal molecule and that the with benzene. For example, two principal resonance forms are acid (aq) H H KMnO (aq) 4 no reaction H C H H C H C C C C HCl (aq) C C C C H C H H C H Most of the reactions that benzene undergoes are substitution reactions in which the hydrogen atoms H H on the ring are replaced by other atoms or groups. The resonance hybrid is For example, the usual reaction of benzene with bro- mine is a substitution reaction (not an addition reac- H tion), as given by H C H C C H Br C C H C H H C H C C C C H C H FeBr3 + Br2(l ) + HBr(g) H C C C C H C H H C H or, more compactly, H H Title General Chemistry - 4th ed Author McQuarrie/Gallogy Artist George Kelvin Figure # bromobenzine Date 07 /28/10 Check if revision Approved H2 GENERAL CHEMISTRY, FOURTH EDITION | McQuarrie, Rock, and Gallogly Sulfonation H2SO4 + SO3 SO3H benzenesulfonic acid Alkylation AlCl3 + CH3Cl CH3 + HCl toluene (methylbenzene) In each case above, the primary reactant also requires a facilitator such as H2SO4, which reacts with bromobenzene, C6H5Br + the primary reactant to produce NO2 in the case of nitration and HSO+ in the case of sulfonation. or more succinctly, 3 Title General Chemistry - 4th ed FeBr3 C6H6(l ) + Br2(l ) → C6H5Br(l ) + HBr(g) H-3. SubstitutedAuthor McQuarrie/Gallogy Benzene Rings Are Named AccordingArtist to IUGeorgePAC KelvinRules The fact that only one kind of monobromoben- Many aromaticFigure # hydrocarbonstoluene have widely used com- zene has ever been isolated indicates that all the hydro- mon names. Although the aromatic product in the Date 07/28/10 gen atoms in benzene are equivalent, as you should alkylation reaction above should, strictly speaking, be expect from the hexagonal symmetry of a benzene called methylbenzene,Check if revision its commonApproved name toluene is ring. The hexagonal geometry of the benzene mol- an IUPAC accepted name. Toluene is a widely used ecule has been confirmed by X-ray and spectroscopic industrial solvent often used in paints and thinners data. This reaction requires FeBr . The FeBr acts as a 3 3 (Figure H.1). Toluene, like benzene, is a carcinogen, facilitator by reacting with Br (l ) according to 2 but it is significantly less toxic than benzene because the methyl group allows it to be more readily broken FeBr + Br → Br+ + FeBr– 3 2 4 down by the body. Even though the benzene ring is stable to most re- actants, the positively charged Br+ ions attack the electron-rich ring of the benzene molecule to form an adduct, such as H Br + that can then eliminate an H+ ion to form bromo- benzene. Some other types of substitution reactions that benzene undergoes are given by the equations below. toluene, C6H5CH3 Chlorination FeCl3 + Cl2 Cl + HCl When a benzene ring is a substituent in a mol- ecule, it is called a phenyl group. For example, the chlorobenzene compound Nitration H2CC CH2 CH3 H2SO4 + HNO3 NO2 + H2O nitrobenzene University Science Books, ©2011. All rights reserved. www.uscibooks.com H. AROMATIC HYDROCARBONS H3 Br Br Br Br Br Br 1,2-dibromobenzene 1,3-dibromobenzene 1,4-dibromobenzene These names result from numbering the six positions around the benzene ring. One of the substituents on the ring is always placed at the number 1 position and the numbers assigned so as to give the lowest num- bers to each of the other substituents. Some other examples of substituted benzenes are H3C CH3 Figure H.1 Many brands of nail polish use toluene as a solvent. Because toluene is a known carcinogen, some Cl Cl manufacturers now offer toluene-free products. Nail CH3 Br polishes containing toluene or other volatile solvents 1,3,5-trimethylbenzene 2-bromo-1,3-dichlorobenzene should always be applied outdoors or in a well-ventilated room. Br CH3 2-bromotoluene is systematically named 2-phenyl-1-butene. Because the main carbon chain must include the double bond, There is a less systematic but more commonly used the benzene ring is a substituent in this case. way to designate the positions of the bromine atoms The hexagonal symmetry of a benzene ring allows in the three disubstituted benzene molecules shown for three different disubstituted benzenes. For exam- above. Substituents at the 1,2 positions are desig- ple, the formulas for the three isomers of dibromo- nated ortho- (o-), those at the 1,3 positions are des- benzene are ignated meta- (m-), and those at the 1,4 positions 1,2-dibromobenzene 1,3-dibromobenzene 1,4-dibromobenzene H4 GENERAL CHEMISTRY, FOURTH EDITION | McQuarrie, Rock, and Gallogly are designated para- (p-); the letter designations are shorthand for the full prefix. Thus, we write Br Br Br Br Br Br Br Br o-dibromobenzene (or) m-dibromobenzene (or) oor-dibromobenzenetho-dibromobenzene (or) mmeta-dibromobenzene-dibromobenzene (or) ortho-dibromobenzene meta-dibromobenzene Br Br Br Br p-dibromobenzene (or) ppar-dibromobenzenea-dibromobenzene (or) para-dibromobenzene Benzene is used in the manufacture of medicinal chemicals, dyes, plastics, varnishes, lacquers, linoleum, and many other products. Some important derivatives of benzene and their uses are shown below. Figure H.2 The explosive TNT (2,4,6-trinitrotoluene) is a substituted benzene molecule. HC CH2 NH2 styrene aniline (used in the manufacture (used in the manufacture of polystyrene) of polyurethane plastics) CH3 O O C CH3 C OH toluene (or methylbenzene) O (used as a solvent for paints, nail polish, 2-(acetyloxy)benzoic acid and in paint thinners) (acetylsalicylic acid or aspirin) OH CH3 O CH3 O2N NO2 O C NO 2 H Figure H.3 Vanillin is the chemical responsible for the 2,4,6-trinitrotoluene 4-hydroxy-3-methoxybenzaldehyde smell of vanilla and is used as a flavoring, as a scent in (TNT, Figure H.2) (vanillin, Figure H.3) perfumes, and as a precursor to the synthesis of various pharmaceuticals. Natural vanillin is extracted from the We discuss the formation of polymers such as poly- seed pods of the Mexican orchid Vanilla planifola. The styrene and polyurethane in Interchapter S. Aztecs used vanillin as a flavoring for chocolate. University Science Books, ©2011. All rights reserved. www.uscibooks.com H. AROMATIC HYDROCARBONS H5 H-4. There Are a Number of Polycyclic balls. Anthracine is a solid with a melting point of Aromatic Compounds 218°C. Anthracine is a primary constituent of some coal tars. One of the principal uses of anthracene is Polycyclic aromatic molecules contain regions in in the preparation of alizarin dyes (Figure H.4). which two or more benzene rings are fused together. These compounds, as well as other polycyclic The two simplest polycyclic aromatic hydrocarbons are compounds, show many of the characteristics that we associate with aromaticity. Both, for example, react with bromine to yield substituted products instead of and addition products, as in naphthalene anthracene Br which we could also write even more succinctly as FeBr3 + Br2 (l ) + HBr (g) and 1-bromonaphthalene How many isomers of bromonaphthalene do you think Naphthalene is a solid with a melting point of exist? 80.2°C, but it is very volatile at room temperature. In addition to naphthalene and anthracene, there Naphthalene confers the characteristic odor of moth- are a great many more complex polycyclic aromatic Figure H.4 Alizarin is a substituted anthracene molecule: O OH OH O alizarin Alizarin was first extracted from the root of the madder plant and used as a dye to produce a deep crimson color.