Hydrocarbons aromatic

Aromatic compounds have special characteristics of aromatidty, which include a low hydrogen carbon atomic ratio C—C bonds that are quite strong and of intermediate length between such bonds in alkanes and those in alkenes tendency to undergo substitution reactions rather than the addition reactions characteristic of alkenes and delocalization of n electrons over several carbon atoms. The last phenomenon adds substantial stability to aromatic compounds, and is known as resonance stabilization. 

Benzene is a volatile, colorless, highly flammable liquid used to manufacture phenolic and polyester resins, polystyrene plastics, alkylbenzene surfactants, chlorobenzenes, insecticides, and dyes. It is hazardous both for its ignitability and for its toxicity (exposure to benzene causes blood abnormalities that may develop into leukemia). Naphthalene is the simplest member of a large number of polycyclic aromatic hydrocarbons having two or more fused rings. It is a volatile white crystalline solid with a characteristic odor and has been used to make mothballs. The most important of the many chemical derivatives made from naphthalene is phthalic anhydride, from which phthalate ester plasticizers are synthesized. 

Because there are so many partial combustion and pyrolysis processes that favor production of PAHs, these compounds are encountered abundantly in the atmosphere, soil, and elsewhere in the environment from sources that include engine exhausts, wood stove smoke, cigarette smoke, and charbroiled food. Coal tars and petroleum residues such as road and roofing asphalt have high levels of PAHs. Some PAH compounds, including benzo[a]pyrene, are of toxicological concern because they are precursors to cancer-causing metabolites. 

Aromatic Hydrocarbons. The rr-election cloud of the aromatic ring is much more susceptible to electron removal than a saturated hydrocarbon. Thus, benzene is oxidized at +2.45 V versus SCE electron-donating substituents reduce 

In acetonitrile 5 mM H20 is oxidized at +2.80 V versus SCE the presence of benzene would facilitate electron removal by stabilizing the resulting hydroxyl radical to give an ECEC process  

In this case the potential is determined by the acidity of H30+ in the solvent matrix (pATa, -8.8 in MeCN pKa, 0.00 in H20). 

Dithiols. Thiols and mercaptans (RSH) are similar to alcohols, as oxidizable reductants, but ate much stronger nucleophiles and usually couple to form disulfides  

When the substrate is a dithiol (e.g., propane dithiol, HSCH2CH2CH2SH), it undergoes an apparent reversible oxidation at a gold electrode 19 

Aromatic hydrocarbons are unusually stable compounds with ring structures in which electrons are shared by many atoms. 

Real-World Reading Link What do bright, colorfui fabrics and essential oils for perfumes have in common They both contain aromatic hydrocarbons. 

Although this structure has a molecular formula of CeHe, such a hydrocarbon would be unstable and extremely reactive because of its many double bonds. However, benzene was fairly unreactive, and it did not react in the ways that alkenes and alkynes usually react. For that reason, chemists reasoned that structures such as the one shown above must be incorrect. 

Kekule claimed that benzenes structure came to him in a dream while he dozed in front of a fireplace in Ghent, Belgium. He said that he had dreamed of the Ouroboros, an ancient Egyptian emblem of a snake devouring its own tail, and that had made him think of a ring-shaped structure. The flat, hexagonal structure Kekule proposed explained some of the properties of benzene, but it did not explain benzenes lack of reactivity. 

Originally the word aromatic was applied to pleasant-smelling substances. The word now describes benzene, its derivatives, and certain other compounds that exhibit similar chemical properties. Some have very foul odors because of substituents on the benzene ring. On the other hand, many fragrant compounds do not contain benzene rings. 

Steel production at one time required large amounts of coke. This was prepared by heating bituminous coal to high temperatures in the absence of air. This process also favors production of coal gas and coal tar. Because of the enormous amount of coal converted to coke, coal tar was produced in large quantities. It served as a source of aromatic compounds. For each ton of coal converted to coke, about 5 kg of aromatic compounds were obtained. Today petroleum refining is the major source of aromatic hydrocarbons. 

Early research on the reactions of the aromatic hydrocarbons led to methods for preparing a great variety of dyes, drugs, flavors, perfumes, and explosives. More recently, large numbers of polymeric materials, such as plastics and fabrics, have been prepared from these compounds. 

Benzene is the simplest aromatic hydrocarbon. By studying its reactions, we can learn a great deal about aromatic hydrocarbons. Benzene was discovered in 1825 by Michael Faraday when he fractionally distilled a by-product oil obtained in the manufacture of illuminating gas from whale oil. 

Elemental analysis and determination of its molecular weight show that the molecular formula for benzene is CsEE- The formula suggests that it is highly unsaturated. But its properties are quite different from those of alkenes and alkynes. 

A complete definition of aromatic compounds must wait until Chapter 16. For the present we will define them as benzene and its substituted derivatives. They are also called arenes. These aromatic compounds have a six-membered ring with three conjugated doable bonds. It is this cycle of conjugated double bonds that makes arenes special. Examples include the following  

If you have spent much time in an organic laboratory, you are well aware that many organic compounds have rather strong (and sometimes disagreeable) odors. Aromatic compounds, however, tend to have more fragrant odors than other compounds. If you have a chance, compare the odor of toluene, for example, with that of cyclohexene. In fact, the term aromatic was originally given to 

Look for this logo in the chapter and go to OrganicChemistryNow at http //now.brookscole.com/hornback2 for tutorials, simulations, problems, and molecular models. 

When several substituents are present on a benzene ring, the ring is numbered in the same manner as the rings of cycloalkanes—that is, so that the numbers for the substituents are as low as possible. In addition, some special terms are used with disuhsti-tuted benzenes only. Two substituents on adjacent carbons (positions I and 2) are said to be ortho, or o-. Two substituents on positions 1 and 3 are meta, or m-. And two substituents on positions 1 and 4 are para, or p-. Finally, if an alkyl group with six or more carbons is attached to a benzene ring, the compound is named as an alkane with a phenyl substituent. Some examples are as follows  

Click Coached Tutorial Problems for more practice Naming Aromatic Compounds. 

Chlorinated aromatic hydrocarbons Volatile methyl-silicon compounds Methylcyclopentadienyl manganese (CO)3 

chemists isolated other more complex aromatic compounds in which the benzene rings are condensed (two rings are condensed if they have a common bond). The most important examples are naphthalene, anthracene and phenanthrene  

Aromatic structures can also appear as substituents on the hydrocarbon chains. We then use special prefixes in their nomenclature. For instance, the C6H5 group is named phenyl and the C6H5CH2 group is called benzyl as is shown in the next scheme. 

The structure of the benzene molecule proposed by Kekule was not accepted without criticism. The basic concerns appeared related to the study of substituted derivates of benzene. For example, if the structure includes the alternating double bonds the dimethylbenzene derivative should exist as four isomers  

The isomers 1,2-dimethylbenzene, 1,3-dimethylbenzene and 1,4-dimethylbenzene also have traditional names 6 -dimethylbenzene, m-dimethylbenzene and p-dimethylbenzene (6 rf/z6 -dimethylbenzene, m fa-dimethylbenzene, and para-dimethylbenzene). If the positions of double bonds were localized, as proposed by Kekule, 1,2-dimethylbenzene and 1,6-dimethylbenzene would be different isomers because the bond between carbons Ci and C2 is double while the bond between Ci and Ce would be a single bond. However, all the experiments have demonstrated that only three isomers exist, hence 1,2-dimethylbenzene and 1,6-dimethylbenzene are identical. Consequently, the classical structure theory which implies that the double bonds are localized is unable to describe the structures of the benzene molecule and other aromatic compounds. 

The resolution of this conflict between the structure theory and experimental results was not possible during the Kekule s time, because the electronic structure of the chemical bond has not been discovered. 

The oxidation of aromatic hydrocarbons originating from coal is one of the first organic gas phase oxidation processes carried out on an industrial scale. The development of these processes was initiated by the discovery that the V2Os catalyst used for the oxidation of sulphur dioxide was also applicable to the partial oxidation of benzene to maleic anhydride and naphthalene to phthalic anhydride. Remarkably, V2Os-based catalysts are still used in these processes today as they appear superior to any other type of catalyst. 

There are a number of analogies between the oxidation of aromatic hydrocarbons and olefins. Two classes of aromatic oxidations are to be distinguished. 

Ammoxidation can be successfully applied to methyl aromatics (e.g. toluene and xylene) as it can to propene. However, the subject has not received much attention in the literature, mainly due to the fact that there are no important applications for aromatic nitriles at present. 

Catalysts based on V2Os resemble the binary oxide catalysts such as 

Bi—Mo—O and U—Sb—O in many aspects. The participation of oxygen from the V2Os lattice is proved by the selective oxidation capacity of the catalyst in the absence of gas phase oxygen, while ESR investigations have shown the reduction of Vs + to V4+ and demonstrated a correlation between the activity and the V4+ content of a working catalyst. These facts strongly support the assumption of a redox mechanism. 

The above discussions have concentrated on hydrocarbons, both saturated and unsaturated, with the unsaturated hydrocarbons containing only one multiple bond. The unsaturated hydrocarbons are the alkenes with one double bond and the alkynes with one triple bond. There are other straight-chain hydrocarbons that are unsaturated containing more than one multiple bond, some with more than one double bond, and some with a mixture of double bonds and triple bonds. The combinations and permutations are endless, but there are only a few of the highly unstable materials. 

From a commercial standpoint, there is a large body of hydrocarbons that is very important and hence these are of relevance to first responders to a hazardous-materials incidents. These hydrocarbons are different in that they are not straight-chain hydrocarbons but have a structural formula that can only be called cyclical. The most common and most important hydrocarbon in this group is benzene. It is the first and simplest of the six-carbon cyclical hydrocarbons referred to as aromatic hydrocarbons. 

Benzene s molecular formula is C6H6, but it does not behave like hexane, hexene, or any of their isomers. One would expect it to be similar to these other six-carbon hydrocarbons in its properties. Table 4 provides a comparison between benzene, hexane and 1-hexene. The table shows that there are major differences between benzene and the straight-chain hydrocarbons of die same carbon content. Hexene s ignition temperature is very near to hexane s. The flash point difference is not great, however, there are significant differences in melting points. The explanation for these differences is structure which in the case of benzene is a cyclical form with alternating double bonds. 

Compound Formula Melting Point (°F) Boiling Point (°F) Flash Point (°F) Ignition Temp. rtf Molecular weight 

Benzene s particular hexagonal structure is found throughout nature in many forms, almost always in a more complicated way and usually connected to many other benzene rings to form many exotic compounds. Benzene s derivatives include toluene and xylene. Some typical properties are given in Table 3.9, which illustrates the differences caused by molecular weight and structural formulas. There are other cyclical hydrocarbons, but they do not have the structural formulas of the aromatics unless they are benzene-based. These cyclical hydrocarbons may have three, four, five, or seven carbons in the cyclical structure in addition to the six-carbon ring of the aromatics. None of them have the stability or the chemical properties of the aromatics. 

The aromatic hydrocarbons are used mainly as solvents and as feedstock chemicals for chemical processes that produce other valuable chemicals. With regard to cyclical hydrocarbons, the aromatic hydrocarbons are the only compounds discussed. These compounds all have the six-carbon benzene ring as a base, but there are also three-, four-, five-, and seven-carbon rings. These materials will be considered as we examine their occurrences as hazardous materials. After the alkanes, the aromatics are the next most commonly 

Some attempts have been made to correlate total hydrocarbons or alkanes with the biological activity of marine waters. 

Zsolnay (1973b) reported the existence of a significant linear correlation (r = 0.63 P 0.001) between the non-aromatic hydrocarbons and the chlorophyll a content in the euphotic zone of the water off West Africa during a short period (six days) of high biological activity in March 1972. It was suggested, therefore, that the non-aromatic hydrocarbons present resulted essentially from phytoplankton activity. 

More recently, from the analyses of 23 water samples collected over a short period (two days) from the euphotic zone between the Gulf Stream and Nova Scotia, Zsolnay (1977b) developed a simple model relating hydrocarbon to chlorophyll contents. The equation proposed, (H = 0.6 + 36.10 Chi — 55.327 Chl ), only explained 19% of the variance in the hydrocarbon distribution, with a correlation coefficient of 0.434. 

From observations of the annual variations of dissolved, particulate and phytoplankton hydrocarbons and chlorophyll a content in Mediterranean clean coastal waters, Goutx and Saliot (1980) found a significant correlation (r = 0.63) between total hydrocarbons and chlorophyll a, and this supports the conclusions of Zsolnay for unpolluted areas (upwelling regions). 

In order to establish certain terms used in defining aromatic reactions, consider the following, where the structure of benzene is represented by the symbol 

Based on the early work of Norris and Taylor [55] and Bernard and lbberson [56], who confirmed the theory of multiple hydroxylation, a general low-temperature oxidation scheme was proposed [57, 58] namely, 

There are two dihydroxy benzenes that can result from reaction (3.131)— hydroquinone and pyrocatechol. It has been suggested that they react with oxygen in the following manner [55]  

Thus maleic acid forms from the hydroquinone and oxalic acid forms from pyrocatechol. However, the intermediate compounds are triplets, so the intermediate steps are spin-resistant and may not proceed in the manner indicated. The intermediate maleic acid and oxalic acid are experimentally detected in this low-temperature oxidation process. Although many of the intermediates were detected in low-temperature oxidation studies, Benson [59] determined that the ceiling temperature for bridging peroxide molecules formed from aromatics was of the order of 300°C that is, the reverse of reaction (3.128) was favored at higher temperatures. 

It is interesting to note that maleic acid dissociates to two carboxyl radicals and acetylene 

The photochemistry of aromatic hydrocarbons has recently been reviewed,196 and there has been an unusual amount of exemplary structural work on the lowest-lying triplets of aromatic molecules. 

Hutchison reported the first ESR spectrum of a metastable phosphorescent state by study of naphthalene oriented in durene crystals.4 Since then, similar spectra have been recorded for several other polynuclear aromatics both oriented in host crystals and randomly suspended in glassy matrices. D values for all these ir,n excited states are quite low, indicating little interaction between the unpaired electrons. Interestingly, D for the quinolines equals 0.10cm 1 just as in naphthalene,197 indicating that the presence of a heteroatom does not necessarily change the ir,w nature of the lowest triplet state very much. A similar conclusion has been reached from a comparison of the ESR spectra of fluorene, carbazole, dibenzofuran, and dibenzothiophene.198 

Very efficient delocalization of singlet excitation between non-conjugated benzene rings has been observed in isotactic polystyrene, 

Most of the fairly extensive photochemistry of aromatic compounds has not been studied in sufficient detail to permit disentanglement of singlet and triplet mechanisms. Theoretical calculations indicate that the pattern of substituent effects on side-chain reactions of excited disubstituted benzenes should be quite different from that observed in the ground states of the molecules. One problem associated with these predictions is the question of whether or not they are appropriate for triplets as well as for corresponding singlet excited states. Consider the following system  

Model molecular orbital calculations suggest that in the lowest-lying excited states the methoxy group will selectively increase the electron density at meta positions.205,208 The fact that compounds such as that shown above undergo photosolvolysis,205,207,208 whereas para isomers 

One last group of hydrocarbons found in crude petroleum is the aromatic hydrocarbons, of which benzene is the simplest example. Benzene is a cyclic molecule with the formula CgHg. In the language of Chapter 3, benzene is represented as a resonance hybrid of two Lewis diagrams  

Benzene is sometimes represented by its chemical formula CgHg and sometimes (to show structure) by a hexagon with a circle inside it  

The six points of the hexagon represent the six carbon atoms, with the hydrogen atoms omitted for simplicity. The circle represents the de-localized tt electrons, which are spread out evenly over the ring. The molecules of other aromatic compounds contain benzene rings with various side groups or two (or more) benzene rings linked by alkyl chains or fused side by side, as in naphthalene (CioHg)  

Besides benzene, the most prevalent aromatic compounds in petroleum are toluene, in which one hydrogen atom on the benzene ring is replaced by a methyl group, and the xylenes, in which two such replacements are made  

This set of compounds is referred to as BTX (for fcenzene-roluene-xylene). The BTX in petroleum is important to polymer synthesis (see Section 23.1). These components also significantly increase octane number and are used to make high-performance fuels with octane numbers above 100, as are required in modern aviation. 

The lowest energy electronic transitions in homocyclic aromatic hydrocarbons occur at near-ultraviolet, visible, and infrared wavelengths from 2500 A out to beyond 7000 A. They involve excitations of electrons in delocalized 7t-type MOs, which are composed principally of carbon 2p orbitals oriented perpendicular to the aromatic plane. The remaining minimal-basis carbon valence orbitals (the 2s orbitals and the 2p orbitals oriented in the molecular plane) are utilized to form in-plane ff-type MOs directed along the chemical bonds. Excitations of electrons in ff-type MOs to unoccupied MOs require far higher photon energies (in the vacuum ultraviolet), and are not considered in this Section. 

The singlet ground state Sq is therefore totally symmetric, while the lowest excited singlet Sj is either a Aj state or a 82 state. Both types of excited states are El-accessible from the ground state the A A and 82 - A  

To determine the symmetry of the Si state, Christofferson et al. [7] analyzed the rotational structure of several of the bands. The appearance of the Og absorption band under high resolution is shown in the top portion of Fig. 7.7. In a molecule as large as aniline, such a contour comprises some 30,000 rotational transitions, and so there is little hope for resolving individual lines. Instead, the contour is compared with a computer simulation that calculates the asymmetric rotor energy level differences, weights the intensities of allowed rotational 

The lowest few excited singlet states are then expected to be 

The Si So transition to the lowest excited singlet state in napthalene is the - Ag transition. Since the Ag state is totally symmetric and the vector x transforms as 83 in D2h, this transition should presumably be El symmetry-allowed and polarized along the long axis. The next higher spin-allowed transition (S2 Sq) is a symmetry-allowed 82 - Ag transition. The vector y transforms as 82 j so this transition should be polarized along the short axis. The S2 So transition is in fact responsible for an intense electronic band system in the near ultraviolet. 

When mixtures of hydrocarbons from natural sources, such as petroleum or coal, are separated, certain of the compounds that emerge have pleasant odors and are thus known as aromatic hydrocarbons. When these substances, which include wintergreen, cinnamon, and vanillin, are examined, they are all found to contain a common feature a six-membered ring of carbon atoms called the benzene ring. Benzene has the formula and a planar (flat) structure in which all of the bond angles are 120°. H 

When we examine the bonding in the benzene ring, we find that more than one Lewis structure can be drawn. That is, the double bonds can be located in different positions, as shown below. 

Because the actual bonding is a combination of the structures represented above, the benzene ring is usually shown with a circle. 

Cycloaliphatic hydrocarbons are also referred as cycloparaffins or naphthenes. Most of their properties such as solvency, odor, and specific gravity are intermediate between those of aliphatic and aromatic hydrocarbon solvents. The use of cycloaliphatic solvents is less common in paint and coatings than that of paraffin hydrocarbons. 

CIO aromatics. Styrene and vinyl toluene are also aromatic hydrocarbons that act simultaneously as solvents and reactive diluents for chemical crosslinking with unsaturated polyester resins and in UV-cured coatings. 

Ionization of benzene undoubtedly also involves loss of an electron from the highest occupied v-molecular orbital. The relatively high energy required (PI I = 9.245 e.v.) (Table VIII) compared to that for hexatriene (UV / = 8.26 e.v.) (Table VII) demonstrates clearly the additional v-electronic stability of the aromatic ring. The more extensive x-network in naphthalene results in a lower ionization potential, 8.12 e.v. 

The effect of multiple substitution is of unusual interest because of the curious identity of o-xylene and m-xylene /-values, 8.56 e.v., and the substantially lower /-value of p-xylene, 8.445 e.v. This phenomenon, which seems general for many other substituents, is difficult to interpret with resonance structures alone, but Bralsford et al. (48) have given an elegant and satisfying explanation in terms of simple MO theory. The introduction of methyl substituents is treated as a perturbation of benzene. The three occupied x-molec-ular orbitals of benzene are given schematically in Fig. 6. The extent of interaction of a substituent with a molecular orbital de- 

The mass spectra of the xylene isomers (Figs. 4.22 and 4.23 for example) show a medium peak at m/z = 105, which is due to the loss of a hydrogen atom and the formation of the methyltropylium ion. More importantly, xylene loses one methyl group to form the tropylium (m/z = 91). The mass spectra of ortho-, meta-, and para-disubstituted aromatic rings are essentially identical. As a result, the substitution pattern of polyalkylated benzenes cannot be determined by mass spectrometry. 

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The formation of a substituted tropylium ion is typical for alkyl-substituted benzenes, hi the mass spectrum of isopropylbenzene (Fig. 4.24), a strong peak appears at miz = 105. This peak corresponds to loss of a methyl group to form a methyl-substituted tropyhum ion. The tropylium ion has characteristic fragmentations of its own. The tropylium ion can fragment to form the aromatic cyclopentadienyl cation (m/z = 65) plus ethyne (acetylene). The cyclopentadienyl cation in turn can fragment to form another equivalent of ethyne and the aromatic cyclopropenyl cation (m/z = 39) (Fig. 4.25). 

Benzene and all substances with structures and chemical properties resembling benzene are classified as aromatic compounds. The word aromatic originally referred to the rather pleasant odor many of these substances possess, but this meaning has been dropped. Benzene, the parent substance of the aromatic hydrocarbons, was first isolated by Michael Faraday in 1825. Its correct molecular formula, CsHg, was established a few years later, but finding a reasonable structural formula that would account for the properties of benzene was difficult. 

Finally, in 1865, August Kekule proposed that the carbon atoms in a benzene molecule are arranged in a six-membered ring with one hydrogen atom bonded to each carbon atom and with three carbon-carbon double bonds  

Modem theory suggests that the benzene molecule is a hybrid of the two Kekule structures shown earlier. 

For convenience, chemists usually write the structure of benzene as one or the other of these abbreviated forms  

In both representations, it is understood that there is a carbon atom and a hydrogen atom at each comer of the hexagon. The classical Kekule structure is represented by A the modem molecular orbital structure is represented by B. These hexagonal structures are also used to represent the structural formulas of benzene derivatives— that is, substances in which one or more hydrogen atoms in the ring have been replaced by other atoms or groups. Chlorobenzene (CsHsCl), for example, is written in this fashion  

Benzene (1) is the simplest aromatic hydrocarbon upon which our knowledge of aromatic chemistry is based. This hydrocarbon, the alkylbenzenes (2), the arylmethanes [e.g. diphenylmethane (3)], the biphenyls [e.g. biphenyl (4)] and the condensed polycyclic systems [e.g. naphthalene (5) and anthracene (6)] all exhibit chemical reactivity and spectroscopic features which are markedly different from their aliphatic and alicyclic hydrocarbon counterparts. Indeed the term aromatic character was introduced to specify the chemistry of this group of hydrocarbons and their substituted functional derivatives, and it was soon used to summarise the properties of certain groups of heterocyclic compounds having five- and six-membered ring systems and the associated condensed polycyclic analogues (Chapter 8). 

The crucial structural feature which underlies the aromatic character of benzenoid compounds is of course the cyclic delocalised system of six n-electrons. Other carbocyclic systems similarly possessing this aromatic sextet of electrons include, for example, the ion C5Hf formed from cyclopentadiene under basic conditions. The cyclopentadienide anion is centrosymmetrical and strongly resonance stabilised, and is usually represented as in (7). The analogous cycloheptatrienylium (tropylium) cation (8), with an aromatic sextet delocalised over a symmetrical seven-membered ring, is also demonstrably aromatic in character. The stable, condensed, bicyclic hydrocarbon azulene (Ci0H8) possesses marked aromatic character it is usually represented by the covalent structure (9). The fact that the molecule has a finite dipole moment, however, suggests that the ionic form (10) [a combination of (7) and (8)] must contribute to the overall hybrid structure. 

Bridie et al. [27] have studied the solvent extraction of hydrocarbons and their oxidative products from oxidised and non-oxidised kerosine-water mixtures, using pentane, chloroform and carbon tetrachloride. Extracts are treated with Florasil to remove non-hydrocarbons before analysis by temperature programmed gas chromatography. From the results reported it is concluded that, although each of the solvents extracts the same amount of hydrocarbons, pentane extracts the smallest amount of non hydrocarbons. Florasil effectively removes non hydrocarbons from pentane extracts, but also removes 10-25% of aromatic hydrocarbons. However, as the other solvents are less susceptible than pentane to treatment with Florasil, pentane is considered by those workers to be the most suitable solvent for use in determining oil in water. 

Other applications of gas chromatography to the determination of aliphatic hydrocarbons in non saline waters are discussed in Table 15.9. 

Wasik and Tsang [28,29] have described a method for the determination of traces of arene contaminants using isotope dilution gas chromatography. They used perdeuterated benzene as the isotope source and anadysed solutions containing 10-20mg L of benzene and toluene. This method is most effective when the isotope can be completely separated from the 

Gas chromatography has foimd some applications in the determination of simple aromatics in water. Mel kanovitskaya [31] has described a method for determining Q-Cg aromatics in subterranean waters. In this method the sample (25-50mL) is adjusted to pH8-9 and extracted for 3min with 0.5 or l.OmL of nitrobenzene the extract is washed with 0.3mL of 5% hydrochloric acid or 5% sodium hydroxide solution and with 0.3mL of water adjusted to pH7. The purified extract is subjected to gas chromatography at 85°C on a column (Im x 4mm) packed with 15% of polyoxyethylene glycol 2000 on Celite 545 (60-80 mesh) and operated with nitrogen (lOmL min ) as carrier gas, decane as internal standard and flame ionisation detection. 

Orito and Imai have shown that the hydrogenation of benzene over nickel and cobalt catalysts is inhibited by alcoholic solvents and some ethers.5 As seen from the results shown in Table 11.2, benzene is hydrogenated extremely slowly or not at all in primary alcohols but very rapidly without solvent or in hydrocarbons. Benzene is hydrogenated at a considerable rate at 110°C even over Urushibara Ni A, which is known to be a poor catalyst toward the hydrogenation of aromatic nucleus,10 when used without solvent or in hydrocarbons after the water or alcohol on the catalyst has been carefully removed. 

Adkins and Kramer hydrogenated benzene and alkylbenzenes to the corresponding cyclohexanes quantitatively over Ni-kieselguhr without solvent at 135-175°C and initial hydrogen pressures of 15.5-18 MPa.11 Examples are shown in eqs. 11.1 and 

2 for hydrogenations of benzene and toluene, respectively. Mesitylene was hydrogenated to 1,3,5-trimethylcyclohexane at 200°C (eq. 11.3).12 Triphenylmethane was hydrogenated to tricyclohexylmethane in methylcyclohexane at 175°C. However, hydrogenation in ethanol or in the presence of water gave dicyclohexylphenylmethane 414 

TABLE 11.1 Relative Rates of Hydrogenation of Methyl-Substituted Benzenes over Platinum and Nickel Catalysts 

TABLE 11.2 The Effect of Solvents on the Rate of Hydrogenation of Benzene over Nickel and Cobalt Catalystsai  

Benzene- a total of six electrons can be found In the donut-shaped clouds. 

At a quite different level of approximation, this class of compounds was investigated by means of STO-3G calculations involving a detailed optimization of all the geometric and exponent parameters [42]. The Mulhken net atomic charges and the 

TABLE 6.2. Carbon Net Charges and NMR Shifts of Selected Aromatic Hydrocarbons (me) 

The behavior of the para carbon atoms of substituted benzenes is similar. Using STO-3G Mulliken cr + tt net charges, the correlation with NMR shifts resembles that shown in Fig. 6.1, with a 384ppm/e [127]. 

A similar study on meta carbons [127], while giving results of the same type, is perhaps somewhat less conclusive because of the very limited range of variation of the meta-carbon NMR shifts ( 1.5 ppm). It remains, however, that the major conclusions drawn for the para carbons apply to the meta carbons as well. 

As one would anticipate from the similarity in the chemical nature of these sub-stimted benzenes and the compounds indicated in Table 6.2, the gross features are quite similar, namely, in terms of the increase in electron population at carbon resulting in a high-held shift. 

More recently, two papers have described higher resolution RAIRS results for benzene on Pt(lll) (311, 312) and on Cu(110) (312). The latter spectra, obtained by Haq and King, were particularly informative in relation to VEELS results characterizing the same system obtained earlier by Leh-wald et al. (284). Furthermore, Raman spectra have been obtained for C6D6 on Ag(lll) and Ag(110) (313). 

There is general agreement, based on measurements of vibrational spectra taken at room temperature or below, that benzene adsorbs nondissocia-tively on single-crystal metal surfaces with the C6 ring oriented parallel or near-parallel to the surface. Furthermore, there is a strong general resem- 

The principal uncertainty in the literature concerns the assignment of the strong shoulder some 100-120 (105-115) cm 1 higher in wavenumber 

We summarize in the following the principal VEEL evidence for or against these alternatives. 

Other questions can be raised with respect to the v2 assignment. This assignment of the many C6D6 spectra implies that the mode is of very limited variability, whereas v4, as we have seen, varies gradually but substantially from metal to metal. If v2 is to move downward from the value of 992 (943) cm-1 for benzene itself, it might have been expected that this too would be a gradual process. Fujisawa et al. (291) also raised the question 

Doubly-charged molecular ions (C6H6)2+ have been formed from 

Isomerisation of the molecular ions of 2-benzylindanes has been studied by field ionization [501]. 

Benzene has been identified as a carcinogen. (CAUTION All procedures involving benzene must be carried out in a well-ventilated fume cupboard, and protective gloves should be worn.) The analytical reagent grade benzene is satisfactory for most purposes if required dry, it is first treated with anhydrous calcium chloride, filtered and then placed over sodium wire (for experimental details, see under 15. Diethyl ether) or a Type 5A molecular sieve. Phosphorus pentoxide, lithium aluminium hydride or calcium hydride may be used as alternatives to sodium wire. 

Toluene free from sulphur compounds may be purchased. Commercial toluene may contain methyl thiophenes (thiotolenes), b.p. 112-113 °C, which cannot be removed by distillation. It may be purified with concentrated sulphuric acid in a similar manner to the purification of benzene, but care must be taken that the 

For solvent purposes various grades of xylenes (the mixture of isomers and ethylbenzene) are available purification and drying procedures are similar to those used for benzene and toluene. For chemical purposes the commercially available pure isomeric xylenes are usually available in at least 99 per cent purity. 

De-aluminated mordenites were claimedto give more active and stable catalysts for toluene disproportionation than conventional H-mordenite. Becker, Karge, and StreubeP studied the alkylation of benzene with ethene and propene over an H-mordenite catalyst. Shape-selective catalysis was found because only ethylbenzene, w-diethylbenzene, p-diethylbenzene, cumene, p-di-isopropylbenzene, and m-di-isopropylbenzene were detected in the products neither o-diethylbenzenes nor higher alkylated products were found. The results are in agreement with earlier transalkylations over H-mordenite. Catalyst aging was caused by olefin polymerization. The selectivity of Be-mordenite 

Earlier process studies on toluene disproportionation and liquid-phase xylene isomerization with ZSM-4 catalyst showed advantages over previous catalysts, but the performance was not as good as that achieved later with ZSM-5 catalysts. 

HL zeolite was shown to have a higher activity than CaL for cumene cracking. It was also about twice as active as HY in the isomerization of o-xylene, but less active than HY in the hydrocracking of cumene. 

Methanol.—Strong homogenous acids, e.g. phosphoric acid, catalyse the dehydration of methanol to a mixture of hydrocarbons. Nominal Lewis acids. 

Grandio, F. H. Schneider, A. B. Schwartz, and J. J. Wise, American Chemical Society, Div. Petroleum Chem., Preprints, 1971, 16, B70, B78 Mobil Oil Corp., U.S.P. 3 907 914. 235 Y. Nishimura and H. Takahashi, Bull. Japan Petroleum Inst., 1971, 13,201. 

A number of decompositions of n-butylbenzene and n-pentylbenzene following FI have been elucidated through D- and C-labelling [90, 902]. Methyl and ethyl radicals were lost from the ends of the alkyl chains, but there was evidence of interaction between the chains and the phenyl rings. Hydrogen transfers from the side chains via 5-, 6- emd 

7-membered cyclic transition states were involved in the formation of (C7Hg)f. Formation of (C7H7) was also investigated. Hydrogen exchange between side chains and the ortho positions of the ring did occur but not at times less than 100 ps. 

The rates of loss of ethylene from 1,4- C2-labelled, 2,3- C2-labelled and various D-labelled tetralins following FI have been measured over the range from tens of picoseconds to microseconds [524, 770]. Interpretation of results was complicated by loss at short times of two other neutrals, viz. ethyl and ethane. Ethylene was lost by both a retro Diels—Alder process and another process effecting loss of C-1 and C-2 (or C-3 and C-4) [524]. Hydrogen exchanges between the benzenoid ring and the alicyclic ring did occur, but only after 10 ns [770]. 

Usually only one or very few crystal structures are observed for a given molecule. These are dictated by the molecular potential field, through mutual polarization, dispersion forces, electrostatic interactions, and hydrogen bonds. We shall show that, for non-hydrogen bonded crystals, some features of the molecular field can be described in terms of the molecular size and shape parameters so far discussed, and hence that some crystal properties can be inferred from molecular properties. 

Planar condensed aromatic molecules are a particularly simple example, because their shapes are easy to describe, and the intermolecular potential is similar in different crystals [22]. Two basic interaction types emerge [23] a core-edge interaction. 

The broken bonds (boldface=dissociated atom), AfH°(R), kcal/mol (kj/mol) BDEs (boldface = recommended data reference in parentheses) Methods  

BDEs (boldface = recommended data reference in parentheses) 

A series of n-alkylbenzenes, cooled by supersonic expansion and excited to what are initially well localized ring distortion vibrations within the first excited singlet states, show fluorescence spectral behaviour that is dependent upon the alkyl chain length. For the first three members of the series (toluene to n-propylbenzene) resonance fluorescence from the initially pumped mode in 5, was observed, but for 

One-photon excitation studies on the fluorescence decay properties of isolated aniline,24 deuteriated26 and difluoro-benzenes,27 and other simply substituted benzenes28 have been carried out. In the case of aniline, the results quoted by different groups of workers differ markedly. As an example, results for excitation of four transitions populating two levels are given in Table 1. As can be seen from Table 1, the vast difference in results makes discussion of the trends difficult. 

As stated above, similar but smaller differences exist between values of r and measured by Rice et al. and other workers for difluorobenzenes,27 as evidenced in Table 2. Again, the more serious difficulty concerns the self-consistency, 

An elegant molecular-beam study of the photofragmentation of aryl halides and methyl iodide has permitted extraction of excited-state lifetimes from a measured anisotropy parameter which depends upon the lifetime of excited state, the rotational correlation time of the molecule, and the orientation of the electronic transition dipole with respect to the —X bond.38 The lifetimes obtained were methyl iodide 0.07 ps, iodobenzene 0.5 ps, a-iodonaphthalene 0.9 ps, and 4-iodobiphenyl 0.6 ps, from which it was concluded that, whereas methyl iodide dissociates directly, the aryl halides predissociate. A crossed-beam experiment using electron-beam excitation has yielded the results for the Si Tt intersystem-crossing relaxation time in benzene, [sHe]benzene, fluorobenzene, and 

Sporborg, and . E. Williams, Chem. Phys. Letters, 1974,26, 541. 

Emission measurement from the excited states is also a powerful method to investigate the ion beam radiation chemistry because a very sensitive time resolved photon-counting technique can be applied. In 1970s, temporal behavior of the emission from benzene excited states in 40 mM benzene in cyclohexane irradiated with pulsed proton and He ion particles was measured and compared with UV pulse irradiation. It was found that immediately after the irradiation there is a short decay ( 10 ns) followed by a longer decay corresponding to the life-time of the benzene excited states (26-28 ns). The fraction of the shorter decay component increases with increasing LET of the particle. This was explained by a quenching mechanism that radical species formed in the track core attack and quench the benzene excited states, which would take place only shorter period less than 10 ns after irradiation [69]. 

The formation of a substituted tropylium ion is typical for alkyl-substituted benzenes. In the mass spectrum of isopropylbenzene (Fig. 8.19), a strong peak appears at m/e - 105. This peak corresponds to loss of a methyl group to form a methyl-substituted tropylium ion. 

Again in the mass spectrum of propylbenzene (Fig. 8.20), a strong peak due to the tropylium ion appears at m/e =91. 

The intensity of the molecular ion peak in the mass spectrum of a primary or secondary alcohol is usually rather low. The molecular ion peak may be entirely absent in the mass spectrum of a tertiary alcohol. Fragmentation involves the loss of an alkyl group or the loss of a molecule of water. 

The mass spectrum of 1-butanol (Fig. 8.21) shows a very weak molecular ion peak at mie = lA, while the mass spectrum of 2-butanol (Fig. 8.22) has a molecular ion peak (rnie = 74) that is too weak to be detected. The molecular ion peak for tertiary alcohol, 2-methyl-2-propanol (Fig. 8.23), is entirely absent. The most important fragmentation reaction for alcohols is the loss of an alkyl group  

(Farbwerke Hoechst A.G.), German Patent 1010958, Chem. ZW., 2569 (1958). 

Benzene (CeHe) is the parent compound of this large farrrily of organic substances. As we saw in Section 9.8, the properties of benzene are best represented by both of the fohowing resonance structures (p. 297)  

An electron micrograph of benzene molecule, which shows clearly the ring stracture. 

In the ethylene molecule, the overlap of the two Ip orbitals gives rise to a bonding and an antibonding molecular orbital, which are locahzed over the two C atoms. The interaction of the Ip orbitals in benzene, however, leads to the formation of delocalized molecular orbitals, which are not confined between two adjacent bonding atoms, but actually extend over three or more atoms. Therefore, electrorrs residing in atty of these orbitals are free to move aroimd the benzene ring. For this reasorr, the structure of benzene is sometimes represented as 

The sigma bond jmmework in the benzene molecule. Eadi C atom is sp -hybridized and forms sigma bonds with two adjacent C atoms and another sigpia bond with an H atom. 

We can now state that each carbon-to-carbon linkage in benzene contains a sigma bond and a partial pi bond. The bond order between any two adjacent carbon atoms 

This structure shows alternating single and double bonds. When we examine the bond lengths in benzene, however, we find that all of the bonds are of the same length. 

The structure of benzene is better represented by the following resonance structures. 

I The concept of resonance structures was first introduced in Section 10.6. 

Recall that the resonance structures indicate that the true structure of benzene is an average between the two structures. In other words, all carbon-carbon bonds in benzene are equivalent and are midway between a single and double bond. The space-filling model of benzene is  

Benzene is often represented with the shorthand notations  

12 Given the name (or structural diagram) of an alkyl- or halogen-substituted benzene compound, write the stmctural diagram (or name). 

Initially, aliphatic compounds were associated with oils and fats, which contain long carbon chains. By contrast, the term aromatic was associated with a series of compounds found in such pleasant-smelling substances as oil of cloves, vanilla, wintergreen, and cinnamon. Ultimately, chemists found that the key structure in aromatic hydrocarbons is the benzene ring. Now any hydrocarbon that does not contain a benzene ring— a hydrocarbon that is not an aromatic hydrocarbon— is an aliphatic hydrocarbon. 

The simplest aromatic hydrocarbon is benzene, CgHg. Chemists have struggled for decades to find a structural diagram for benzene that is consistent with its physical and chemical properties, but without success. Common forms and its space-filling model are 

Consumer products containing compounds that contain a benzene ring. The ibuprofen in Advil, the propoxur in Raid, the diphenhydramine hydrochloride in Benadryl, the sodium benzoate in Sprite, and the benzoyl peroxide in Oxy-10 all have at least one benzene ring in their molecular structures. 

An alkyl group, halogen, or other species may replace a hydrogen on a benzene 

Chlorine atoms bonded to the benzene ring (as in chlorobenzene CgHfC) are much less reactive than those bonded to a side-chain (as in benzylchloride C6H5CH2CI). 

Chlorine derivatives of benzene such as chlorobenzene CgHfCl, dichlorobenzene C6H4CI2, trichlorobenzene C6H3CI3, and hexachlorobenzene CeClg are solvents, intermediates for synthesis or insecticides. 

In general, for a given number of chlorine atoms, chlorinated benzene derivatives are much less reactive towards aluminium than chlorinated aliphatic derivatives. 

They have no action on aluminium, even at intermediate temperatures for example, aluminium resists dichlorobenzene very well, even at 80 °C. 

Tests performed over 1 month at 20 °C have shown that 1050A and 3(X)3, immersed in trichlorobenzene, suffer only very superficial pickling, corresponding to a decrease in thickness of less than 1 j,m per year. 

Alkenes are compounds containing carbon-carbon double bonds. The double bond consists of one a- and one t-bond. The a-bond is formed from two sp hybrid orbitals, and the t-bond is formed from two p orbitals. Alkenes are planar. The more substituted an alkene, the more stable it is. 

In naming alkenes, we select the longest carbon chain that contains the double bond and number it so that the double bond has the lowest possible number. Substituents are then located and specified. 

The stereochemistry of alkenes is specified by ranking the substituents at each end of the double bond according to the Cahn-Ingold-Prelog system. When the higherranking substituents are on the same side of the double bond, the stereochemistry isZ. When they are on opposite sides of the double bond, the stereochemistry is E. 

Alkenes with alternating double and single bonds are described as conjugated conjugation significantly lowers energy and increases stability. 

The formula of benzene as CgHg was established early in the nineteenth century, but its structure was long problematic. Structures such as 3.18 could be written, but the reactions of benzene are quite unlike those of alkenes or alkynes. Benzene is extremely stable, and the characteristic reaction is substitution (3.1) rather than the addition typical of alkenes (3.2)  

Effect of Substituents on l c Chemical Shifts of Monosubstituted Benzenes (6 in ppm) 

Estimation Chemical Shifts of Multiply Substituted Benzenes 

The chemical shifts of multiply substituted benzenes and naphthal es (see next pages) can be estimated using the substitnent effects in the corresponding monosubstituted hydrocarbons. 

Larger discrepancies between estimated and experimental values are to be expected if the substituents are ortho to each other or if strongly electron-donating and electron-accepting groups occur simultaneously. 

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Edeleanu process An extraction process utilizing liquid sulphur dioxide for the removal of aromatic hydrocarbons and polar molecules from petroleum fractions. 

Gattermann-Koch reaction Formylation of an aromatic hydrocarbon to yield the corresponding aldehyde by treatment with CO, HCl and AICI3 at atmospheric pressure CuCl is also required. The reaction resembles a Friedel-Crafts acylation since methanoyl chloride, HCOCl, is probably involved. 

Methods of producing B —C bonds include hydroboration, nucleophilic displacement at a boron atom in BX., (X = halogens or B(0R>3) by e.g. a Grignard reagent, and a psewiio-Friedel-Crafts reaction with an aromatic hydrocarbon, BX3, and AICI3. 

Its charge transfer complexes with aromatic hydrocarbons have characteristic melting points and may be used for the identification and purification of the hydrocarbons. 

Table 1.3 summarizes data for these aromatic hydrocarbons. 

Outside of carbon monoxide for which the toxicity is already well-known, five types of organic chemical compounds capable of being emitted by vehicles will be the focus of our particular attention these are benzene, 1-3 butadiene, formaldehyde, acetaldehyde and polynuclear aromatic hydrocarbons, PNA, taken as a whole. Among the latter, two, like benzo [a] pyrene, are viewed as carcinogens. Benzene is considered here not as a motor fuel component emitted by evaporation, but because of its presence in exhaust gas (see Figure 5.25). 

More precisely, the rate of ozone formation depends closely on the chemical nature of the hydrocarbons present in the atmosphere. A reactivity scale has been proposed by Lowi and Carter (1990) and is largely utilized today in ozone prediction models. Thus the values indicated in Table 5.26 express the potential ozone formation as O3 formed per gram of organic material initially present. The most reactive compounds are light olefins, cycloparaffins, substituted aromatic hydrocarbons notably the xylenes, formaldehyde and acetaldehyde. Inversely, normal or substituted paraffins. 

Nevertheless, this type of analysis, usually done by chromatography, is not always justified when taking into account the operator s time. Other quicker analyses are used such as FIA (Fluorescent Indicator Analysis) (see paragraph 3.3.5), which give approximate but usually acceptable proportions of saturated, olefinic, and aromatic hydrocarbons. Another way to characterize the aromatic content is to use the solvent s aniline point the lowest temperature at which equal volumes of the solvent and pure aniline are miscible. 

Burdett, R.A., L.W. Taylor and L.C. Jones Jr (1955), Determination of aromatic hydrocarbons in lubricating oil fractions by far UV absorption spectroscopy , p. 30. In Molecular Spectroscopy Report Conf. Institute of Petroleum, London. 

Wiliams D E 1965 Non-bonded potential parameters derived from crystalline aromatic hydrocarbons J. Chem. Phys. 45 3770... 

Since aliphatic hydrocarbons (unlike aromatic hydrocarbons, p. 155) can be directly nitrated only under very special conditions, indirect methods are usually employed for the preparation of compounds such as nitroethane, CjHsNO. When ethyl iodide is heated with silver nitrite, two isomeric compounds are formed, and can be easily separated by fractional distillation. The first is the true ester, ethyl nitrite, C,HiONO, of b.p. 17° its identity is shown by the action of hot sodium hydroxide solution, which hydrolyses it, giving ethanol and... 

One of the chief differences between aliphatic and aromatic hydrocarbons... 

Identification of Aromatic Hydrocarbons. Picric acid combines with many aromatic hydrocarbons, giving addition products of definite m.p. Thus with naphthalene it gives yellow naphthalene picrate, C oHg,(N08)jCeHiOH, m.p. 152°, and with anthracene it gives red anthracene picrate, C 4Hio,(NOj)jCeHjOH, m.p. 138 . For practical details, see p. 394. 

A further difference between aliphatic and aromatic hydrocarbons is that only the latter are capable of direct sulphonation. Thus benzene when heated with concentrated sulphuric acid gives benzenesulphonic acid, a reaction which proceeds more readily, however, if chlorosulphonic acid is used instead of sulphuric acid an excess of chlorosulphonic acid however may convert the sul phonic acid into the sulphonyl chloride (c/. p. 181). 

When an aqueous solution of a diazonium salt is added to an alkaline solution of a phenol, coupling occurs with formation of an azo-compound (p. 188). If ho vc cr the ntiueous solution of the diazonium salt, t. . ., />-bromohenzene diazonium chloride, is mixed with an excess of an aromatic hydrocarbon, and aqueous sodium hydroxide then added to the vigorously stirred mixture, the diazotate which is formed, e.g., BrC,H N OH, dissolves in the hydrocarbon and there undergoes decomposition with the formation of nitrogen and two free radicals. The aryl free radical then reacts with the hydrocarbon to give a... 

The free radical mechanism is confirmed by the fact that if a substituted aromatic hydrocarbon is used in this reaction, the incoming group (derived from the diazotate) may not necessarily occupy the position in the benzene ring normally determined by the substituent present—a characteristic of free radical reactions. 

TTie true ketones, in which the >CO group is in the side chain, the most common examples being acetophenone or methyl phenyl ketone, C HjCOCH, and benzophenone or diphenyl ketone, C HjCOC(Hj. These ketones are usually prepared by a modification of the Friedel-Crafts reaction, an aromatic hydrocarbon being treated with an acyl chloride (either aliphatic or aromatic) in the presence of aluminium chloride. Thus benzene reacts with acetyl chloride... 

Two methods may conveniently be used to ascend the homologous series of aromatic hydrocarbons ... 

The Friedel-Crafts Reaction, in which an aromatic hydrocarbon reacts with an alkyl halide under the influence of aluminium chloride ... 

Benzene, toluene, anthracene, phenanthrene, biphenyl. Aromatic hydrocarbons with unsaturated side-chains. Styrene, stilbene. 

Aliphatic mono-halides, and aromatic hydrocarbons with halogen in side-chain, precipitate silver hdide on treatment with cold aqueous silver nitrate solution. 

The aliphatic hydrocarbons are extremely unreactive and do not respond to any of the following tests for aromatic hydrocarbons. 

Aromatic hydrocarbons with unsaturated side-chains. 

Iodine solutions. Dissolve i crystal of iodine in diethyl ether and note the brown colour. Aromatic hydrocarbons e.g. benzene) give purple solutions. 

The purpose of this eornpuLer project is Lo examine several polynuclear aromatic hydrocarbons and to relate their electron density patterns to their carcinogenic activity. If nucleophilic binding to DN.A is a significant step in blocking the normal transcription process of DN.A, electron density in the hydrocarbon should be positively correlated to its carcinogenic potency. To begin with, we shall rely on clinical evidence that benzene, naphthalene, and phenanthrene... 

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