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8.4 Aromatic Compounds: BenzeneNext we consider a class of hydrocarbons with molecular formulas like those ofunsaturated hydrocarbons, but which, unlike the alkenes, do not readily undergoaddition reactions. These compounds comprise a distinct class, called aromatichydrocarbons. Aromatic hydrocarbons are compounds that contain a benzene ringstructure.The simplest aromatic compound is benzene (C6H6) and it is of greatcommercial importance, but it also has noteworthy deleterious health effects (see “ToYour Health: Benzene and Us”).The formula C6H6 seems to indicate that benzene has a high degree of unsaturation.(Hexane, the saturated hydrocarbon with six carbon atoms has the formula C6H14—eight more hydrogen atoms than benzene.) However, despite the seeming low level ofsaturation, benzene is rather unreactive. This is due to the resonance structure formedfrom the alternating double bond structure of the aromatic ring.NoteBenzene is a liquid that smells like gasoline, boils at 80 C, and freezes at 5.5 C. It is thearomatic hydrocarbon produced in the largest volume. It was formerly used todecaffeinate coffee and was a significant component of many consumer products, suchas paint strippers, rubber cements, and home dry-cleaning spot removers. It wasremoved from many product formulations in the 1950s, but others continued to usebenzene in products until the 1970s when it was associated with leukemia deaths.Benzene is still important in industry as a precursor in the production of plastics (suchas Styrofoam and nylon), drugs, detergents, synthetic rubber, pesticides, and dyes. It isused as a solvent for such things as cleaning and maintaining printing equipment andfor adhesives such as those used to attach soles to shoes. Benzene is a naturalconstituent of petroleum products, but because it is a known carcinogen, its use as anadditive in gasoline is now limited.To Your Health: Benzene and UsMost of the benzene used commercially comes from petroleum. It is employed as astarting material for the production of detergents, drugs, dyes, insecticides, and plastics.Once widely used as an organic solvent, benzene is now known to have both short- andlong-term toxic effects. The inhalation of large concentrations can cause nausea andeven death due to respiratory or heart failure, while repeated exposure leads to a12

progressive disease in which the ability of the bone marrow to make new blood cells iseventually destroyed. This results in a condition called aplastic anemia, in which there isa decrease in the numbers of both the red and white blood cells.Concept Review Exercises1. How do the typical reactions of benzene differ from those of the alkenes?2. Briefly describe the bonding in benzene.3. What does the circle mean in the chemist’s representation of benzene?Answers1. Benzene is rather unreactive toward addition reactions compared to an alkene.2. Valence electrons are shared equally by all six carbon atoms (that is, theelectrons are delocalized).3. The six electrons are shared equally by all six carbon atoms.Recognizing Aromatic CompoundsWhich compounds are aromatic?1.2.3.4.Solution5. The compound has a benzene ring (with a chlorine atom substituted forone of the hydrogen atoms); it is aromatic.13

6. The compound is cyclic, but it does not have a benzene ring; it is notaromatic.7. The compound has a benzene ring (with a propyl group substituted forone of the hydrogen atoms); it is aromatic.8. The compound is cyclic, but it does not have a benzene ring; it is notaromatic.Skill-Building ExerciseWhich compounds are aromatic?9.10.11.In the International Union of Pure and Applied Chemistry (IUPAC) system,aromatic hydrocarbons are named as derivatives of benzene. Five examples areshown below. In these structures, it is immaterial whether the single substituentis written at the top, side, or bottom of the ring: a hexagon is symmetrical, andtherefore all positions are equivalent.14

These compounds are named in the usual way with the group that replaces ahydrogen atom named as a substituent group: Cl as chloro, Br as bromo, I asiodo, NO2 as nitro, and CH3CH2 as ethyl.Although some compounds are referred to exclusively by IUPAC names, someare more frequently denoted by common names, as is indicated below.Key Takeaway Aromatic hydrocarbons appear to be unsaturated, but they have a special type ofbonding and do not undergo addition reactions.(Back to the Top)Polycyclic Aromatic HydrocarbonsSome common aromatic hydrocarbons consist of fused benzene rings—rings that sharea common side. These compounds are called polycyclic aromatic hydrocarbons(PAHs)An aromatic hydrocarbon consisting of fused benzene rings sharing a commonside.The three examples shown here are colorless, crystalline solids generally obtained fromcoal tar. Naphthalene has a pungent odor and is used in mothballs. Anthracene is used15

in the manufacture of certain dyes. Steroids, including cholesterol and the hormones,estrogen and testosterone, contain the phenanthrene structure.To Your Health: Polycyclic Aromatic Hydrocarbons and CancerThe intense heating required for distilling coal tar results in the formation of PAHs. Formany years, it has been known that workers in coal-tar refineries are susceptible to atype of skin cancer known as tar cancer. Investigations have shown that a number ofPAHs are carcinogens. One of the most active carcinogenic compounds, benzopyrene,occurs in coal tar and has also been isolated from cigarette smoke, marijuana smoke,automobile exhaust gases, and charcoal-broiled steaks. It is estimated that more than1,000 t of benzopyrene are emitted into the air over the United States each year. Only afew milligrams of benzopyrene per kilogram of body weight are required to inducecancer in experimental animals.Figure 8.6 Benzo[a]pyrene is a polycyclic aromatic hydrocarbon produced as abyproduct in coal tar, cigarette and marijuana smoke, and in charbroiled steaks.Benzo[a]pyrene is metabolized to produce biologically active compounds that can formphysical adducts on DNA molecules. These adducts can cause genetic mutations thatcause cancer.Photo of cigarette smoke16

Biologically Important Compounds with Benzene RingsSubstances containing the benzene ring are common in both animals and plants,although they are more abundant in the latter. Plants can synthesize the benzene ringfrom carbon dioxide, water, and inorganic materials. Animals cannot synthesize it, butthey are dependent on certain aromatic compounds for survival and therefore mustobtain them from food. Phenylalanine, tyrosine, and tryptophan (essential amino acids)and vitamins K, B2 (riboflavin), and B9 (folic acid) all contain the benzene ring. Manyimportant drugs, a few of which are shown in Table 8.2 also feature a benzene ring.NoteSo far we have studied only aromatic compounds with carbon-containing rings.However, many cyclic compounds have an element other than carbon atoms in the ring.Organic ring structures that contain an atom other than carbon are called heterocycliccompounds., Heterocyclic aromatic compounds also have unique and medicallyrelevant properties.Table 8.2 Some Drugs That Contain a Benzene etaminesulfanilamide17

(Back to the Top)8.5 Geometric IsomersWithin alkane structure there is free rotation about the carbon-to-carbon single bonds(C–C). In contrast, the structure of alkenes requires that the carbon atoms form adouble bond. Double bonds between elements are created using p-orbital shells (alsocalled pi orbitals). These orbital shells are shaped like dumbbells rather than thecircular orbitals used in single bonds. This prevents the free rotation of the carbonatoms around the double bond, as it would cause the double bond to break during therotation (Figure 8.7). Thus, a single bond is analogous to two boards nailed togetherwith one nail. The boards are free to spin around the single nail. A double bond, on theother hand, is analogous to two boards nailed together with two nails. In the first caseyou can twist the boards, while in the second case you cannot twist them.Figure 8.7 The formation of double bonds requires the use of pi-bonds. Formolecules to create double bonds, electrons must share overlapping pi-orbitals betweenthe two atoms. This requires the dumbbell-shaped pi-orbitals (show on the left) toremain in a fixed conformation during the double bond formation. This allows for theformation of electron orbitals that can be shared by both atoms (shown on theright). Rotation around the double bond would cause the pi orbitals to be misaligned,breaking the double bond.Diagram provided from: JoJanderivative work - Vladsinger (talk)18

The fixed and rigid nature of the double bond creates the possibility of an additionalchiral center, and thus, the potential for stereoisomers. New stereoisomers form if eachof the carbons involved in the double bond has two different atoms or groups attachedto it. For example, look at the two chlorinated hydrocarbons in Figure 8.8. In the upperfigure, the halogenated alkane is shown. Rotation around this carbon-carbon bond ispossible and does not result in different isomer conformations. In the lower diagram, thehalogenated alkene has restricted rotation around the double bond. Note also that eachcarbon involved in the double bond is also attached to two different atoms (a hydrogenand a chlorine). Thus, this molecules can form two stereoisomers: one that has the twochlorine atoms on the same side of the double bond, and the other where the chlorinesreside on opposite sides of the double bond.19

Figure 8.8 Alkene Double Bonds Can Form Geometric Isomers. (a) Shows the freerotation around a carbon-carbon single bond in the alkane structure. (b) Shows the fixedposition of the carbon-carbon double bond that leads to geometic (spatial) isomers.Click Here for a Kahn Academy Video Tutorial on Alkene Structure.For this section, we are not concerned with the naming that is also included in this videotutorial.(Note: All Khan Academy content is available for free using CC-BY-NC-SAlicensing at www.khanacademy.org )Cis-Trans NomenclatureThe cis-trans naming system can be used to distinguish simple isomers, where eachcarbon of the double bond has a set of identical groups attached to it. For example, inFigure 8.8b, each carbon involved in the double bond, has a chlorine attached to it, andalso has hydrogen attached to it. The cis and trans system, identifies whether identicalgroups are on the same side (cis) of the double bond or if they are on the opposite side(trans) of the double bond. For example, if the hydrogen atoms are on the opposite sideof the double bond, the bond is said to be in the trans conformation. Whenthe hydrogen groups are on the same side of the double bond, the bond is said to be inthe cis conformation. Notice that you could also say that if both of the chlorine groupsare on the opposite side of the double bond, that the molecule is in the transconformation or if they are on the same side of the double bond, that the molecule is inthe cis conformation.To determine whether a molecule is cis or trans, it is helpful to draw a dashed linedown the center of the double bond and then circle the identical groups, as shown infigure 8.9. Both of the molecules shown in Figure 8.9, are named 1,2dichloroethene. Thus, the cis and trans designation, only defines the stereochemistryaround the double bond, it does not change the overall identity of themolecule. However, cis and trans isomers often have different physical and chemicalproperties, due to the fixed nature of the bonds in space.20

Figure 8.9 A Guide for Determining Cis or Trans Conformations.Click Here for a Kahn Academy Video Tutorial on Cis/Trans Isomerization(Note: All Khan Academy content is available for free using CC-BY-NC-SA licensing atwww.khanacademy.org )Cis-trans isomerism also occurs in cyclic compounds. In ring structures, groups areunable to rotate about any of the ring carbon–carbon bonds. Therefore, groups can beeither on the same side of the ring (cis) or on opposite sides of the ring (trans). For ourpurposes here, we represent all cycloalkanes as planar structures, and we indicate thepositions of the groups, either above or below the plane of the ring.21

To Your HealthPossibly the most common place that you will hear reference to cis-trans conformationsin everyday life is at the supermarket or your doctor's office. It relates to ourconsumption of dietary fats. Inappropriate or excessive consumption of dietary fats hasbeen linked to many health disorders, such as diabetes and atherosclerosis, andcoronary heart disease. So what are the differences between saturated and unsaturatedfats and what are trans fats and why are they such a health concern?Figure 8.10 Common Sources of Dietary Fats.Photo from: TyMaHeThe most common form of dietary fats and the main constituent of body fat in humansand other animals are the triglycerides (TAGs). TAGs, as shown in figure 8.10, are builtfrom one molecule of glycerol and three molecules of fatty acids that are linked togetherby an ester bond. In this section, we will focus on the structure of the long fatty acidtails, which can be composed of alkane or alkene structures. Chapter 10 will focus moreon the formation of the ester bonds.22

Figure 8.11. Example of a Triglyceride (TAG) Structure. Notice that each triglyceridehas three long chain fatty acids extending from the glycerol backbone. Each fatty acidcan have different degrees of saturation and unsaturation.Structure adapted from: Wolfgang SchaeferFats that are fully saturated will only have fatty acids with long chain alkane tails. Theyare said to be 'saturated' with hydrogen atoms. Saturated fats are common in theAmerican diet and are found in red meat, dairy products like milk, cheese and butter,coconut oil, and are found in many baked goods. Saturated fats are typically solids atroom temperature. This is because the long chain alkanes can stack together havingmore intermolecular London dispersion forces. This gives saturated fats higher meltingpoints and boiling points than the unsaturated fats found in many vegetable oils.Most of the unsaturated fats found in nature are in the cis-conformation, as shown inFigure 8.11. Note that the fatty acids shown in Figure 8.11 are drawn for convenience,so that they are easy to look at and do not take up too much space on the paper, butthe bond angles written do not adequately portray the true spatial orientation of themolecules. When the fatty acids from the TAG shown in Figure 8.11 are drawn withcorrect bond angles, it is easy to see that cis-double bonds cause bends in the alkenechain (Fig. 8.12).23

Figure 8.12 Cis-Double Bonds Cause Bends in Fatty Acid StructureThus, monounsaturated and polyunsaturated fats cannot stack together as easily anddo not have as many intermolecular attractive forces when compared with saturatedfats. As a result, they have lower melting points and boiling points and tend to beliquids at room temperature. It has been shown that the reduction or replacement ofsaturated fats with mono- and polyunsaturated fats in the diet, helps to reduce levels ofthe low-density-lipoprotein (LDL) form of cholesterol, which is a risk factor for coronaryheart disease.Trans-fats, on the other hand, contain double bonds that are in the trans conformation.Thus, the shape of the fatty acids is linear, similar to saturated fats. Trans fats also havesimilar melting and boiling points when compared with saturated fats. However, unlikesaturated fats, trans-fats are not commonly found in nature and have negative healthimpacts. Trans-fats occur mainly as a by-product in food processing (mainly thehydrogenation process to create margarines and shortening) or during cooking,especially deep fat frying. In fact, many fast food establishments use trans fats in theirdeep fat frying process, as trans fats can be used many times before needing to bereplaced. Consumption of trans fats raise LDL cholesterol levels in the body (the badcholesterol that is associated with coronary heart disease) and tend to lower highdensity lipoprotein (HDL) cholesterol (the good cholesterol within the body). Trans fatconsumption increases the risk for heart disease and stroke, and for the development oftype II diabetes. The risk has been so highly correlated that many countries havebanned the use of trans fats, including Norway, Sweden, Austria and Switzerland.Within the United States, the Food and Drug Administration (FDA) has recently passeda measure to phase out the use of trans fats in foods by 2018. This measure isestimated to prevent 20,000 heart attacks and 7,000 deaths per year.24

Figure 8.13 Structural differences in saturated, polyunsaturated and trans fats.Click Here for a Kahn Academy Video Tutorial on Saturated-, Unsaturated-, andTrans-Fats(Note: All Khan Academy content is available for free using CC-BY-NC-SA licensing atwww.khanacademy.org )Key Factors for Determining Cis/Trans Isomerization1. The compound needs to contain a double or triple bond, or have a ring structurethat will not allow free rotation around the carbon-carbon bond.2. The compound needs to have two non-identical groups attached to each carboninvolved in the carbon-carbon double or triple bond.Worked ExampleWhich compounds can exist as cis-trans (geometric) isomers? Draw them.1.2.3.4.CHCl CHBrCH2 CBrCH3(CH3)2C CHCH2CH3CH3CH CHCH2CH3SolutionAll four structures have a double bond and thus meet rule 1 for cis-trans isomerism.25

1. This compound meets rule 2; it has two nonidentical groups on each carbonatom (H and Cl on one and H and Br on the other). It exists as both cis and transisomers:2. This compound has two hydrogen atoms on one of its doubly bonded carbonatoms; it fails rule 2 and does not exist as cis and trans isomers.3. This compound has two methyl (CH3) groups on one of its doubly bonded carbonatoms. It fails rule 2 and does not exist as cis and trans isomers.4. This compound meets rule 2; it has two nonidentical groups on each carbonatom and exists as both cis and trans isomers:Skill-Building Exercise1. Which compounds can exist as cis-trans isomers? Draw them.1. CH2 CHCH2CH2CH32. CH3CH CHCH2CH33. CH3CH2CH CHCH2CH326

4.5.Concept Review Exercises1. What are cis-trans (geometric) isomers? What two types of compounds canexhibit cis-trans isomerism?2. Classify each compound as a cis isomer, a trans isomer, or neither.1.2.3.4.27

Answers1. Cis-trans isomers are compounds that have different configurations (groupspermanently in different places in space) because of the presence of a rigidstructure in their molecule. Alkenes and cyclic compounds can exhibit cis-transisomerism.2.1. trans2. cis3. cis4. neitherKey Takeaway Cis-trans (geometric) isomerism exists when there is restricted rotation in amolecule and there are two different groups on each carbon atom involved in thechemical bond.(Back to the Top)E-Z NomenclatureThe situation becomes more complex when there are 4 different groups attached to thecarbo

3. Classify each compound as saturated or unsaturated. Identify each as an alkane, an alkene, or an alkyne. 1. 2. CH. 3. CH. 2. C CCH. 3. 3. Answers. 1. Unsaturated hydrocarbons have double or triple bonds and are quite reactive; saturated hydrocarbons have only single bonds and are rather