Aromatic- compound

Aromatic Compounds.—A number of 2,3-dihydroxyoestra-l,3,5(10)-trienes have been prepared from the corresponding 2-amino-3-hydroxy-compounds using a novel inverse oxidation procedure followed by reduction with KI. Addition of the substrate to sodium metaperiodate in high dilution ensures no coupling with the intermediate quinonimines. 2-Bromo-oestradiol was readily converted into 2-methoxyoestradiol by treatment with NaOMe-MeOH-DMF-CuI. Novel preparations of the biologically interesting 11/3-methyl- and 11/3-ethyl-oestradiol have been reported in full. The key intermediates were the 11-oxo-oestradiol 3-benzyl ether (82) and its 9/3-epimer (83). The latter was derived from the 9,H-epoxides (81) by treatment with KOH followed by benzylation. The thermodynamically unstable 9a-epimer (82) was prepared from the 9j8-epimer (83) by 

Yamada, K, Hosaka, T. Sawahata, Y. Watanabe, and K. Iguchi, Tetrahedron Letters, 1977, 2675. 

Improved selectivity has been reported for the metal-ammonia reductions of androstan-ll-one and -17-one by the incorporation of ethanol in the reaction 

LiAlH4 as this avoids protonation of the enolate and the production of any over-reduction products. Cholest-4-en-3-one may be reduced to cholestanone (5a 5/8,1 19) with alkali-metal carbonyl chromates. The studies on intramolecular hydride shifts on hydroxy-ketones and -aldehydes have been extended. The hydride shifts were examined in a number of y- and 5-hydroxy-carbonyI compounds by heating the substrates with alkaline alumina containing D2O. Exchange of protons on the carbon a to both oxygen functions signals the intramolecular hydride shift typically, the hemiacetals (95) and (96) each incorporate up to six deuterium atoms. The general conclusion, in common with literature precedent, is that, whereas 1,5-shifts are common, 1,4-shifts are rare. 

Kasai and A. Trka, Coll. Czech. Chem. Comm., 1977,42, 1389. 

Aromatic Compounds.—Regioselective mercuration at position 2 was reported for oestradiyl 3-methyl ether 17-acetate with Hg(OAc)2-CH3CN and allowed the preparation of the 2-chloro-, -bromo-, and -iodo-derivatives.54 The major product (41 %) of the reaction between oestrone and Ph5Bi was reported to be the 2,4- 

Aromatic compounds are compounds which have benzene rings in them. Aromatic hydrocarbons are also called arenes. Some compounds have structures which look like fused benzene rings. Such compounds are called polycyclic aromatic compounds. 

Aromatic compounds have two characteristic types of hydrogens aromatic ring hydrogens (benzene ring hydrogens) and benzylic hydrogens (those attached to an adjacent carbon atom). 

Hydrogens attached to an aromatic (benzenoid) ring have a large chemical shift, usually near 7.0 ppm. They are deshielded by the large anisotropic field generated by the electrons in the ring s n system. 

Benzylic hydrogens are also deshielded by the anisotropic field of the ring, but they are more distant from the ring, and the effect is smaller. 

Splitting patterns for the protons on a benzene ring are discussed in Section 7.10. It is often possible to determine the positions of the substituents on the ring from these splitting patterns and the magnitudes of the coupling constants. 

Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. 

Fused polycyclic aromatic hydrocarbons that contain four or more rings with an angular region are carcinogenic. Their structures resemble phenanthrene. Three of die most potent carcinogens are 1,2-benzanthracene, 1,2,5,6-dibenzanthracene, and 3,4-benzpyrene. The angular region is outlined 

Small amounts of these angular fused-ring aromatic hydrocarbons cause cancer in about a month when applied to the skin of a mouse. These compounds are present in the effluent from coal-burning power plants and in automobile exhaust. They are also present in tobacco smoke and in meat cooked over charcoal. The incidence of lung cancer among smokers and inhabitants of large urban areas may partly result from inhaling these airborne compounds in minute amounts over time. 

1 Determine whether each of the following is an aromatic compound 

3 Explain the observation that borazole is an aromatic compound. 

146 Nuclear Magnetic Resonance Spectroscopy Part One Basie Concepts 

For simple aromatic compounds the ring is orientated such that the 1 position is at the top or top right of the ring and numbered in a clockwise direction, again keeping the general horizontal layout. 

No interactions of silylenes with aromatic rings are reported except for those of Sip2 shown in Table 5 and those of SiCl2 and SiClR as shown in Table 7. For the reactions of Sip2 with C5H5, it is the polymeric species which add across the ring at the 1,4 positions. For C5H5F and C Pg, the apparent 

Sip2 insertions into C-F bonds, as shown in equations (123) and (124), are likely to involve an initial attack on the aromatic 7r-bond systems followed by isomerization. 

Very recently, Ando, Ikeno, and Sekiguchi established that SiMcj may add to the carbonyl groups of ketones to produce oxasilacyclopropane as intermediates  

Thermal cleavage of the Si-C ring bond in this intermediate gives a 1,3-diradical which may undergo either intramolecular H abstraction or addition to 71 bonds of the aromatic rings. 

This work is partially supported by U.S. Department of Energy, Contract No. EY-76-05-3898. The kind help provided by E.E. Siefert, E.B.M. Siefert, E.-C. Wu, B.J. Ezzell, and E.C.G. Tang in the preparation of this article is also appreciated. 

Bitter almonds are the source of the aromatic compound benzaldehyde. 

11 Ortho, Para-D rect ngan6 Meta-Directing Groups 

A WORD ABOUT... Polycyclic Aromatic Hydrocarbons and Cancer A CLOSER LOOK AT... Polycyclic Aromatic H ydrocarbons A WORD ABOUT... 

It turned out that many of these aromatic substances have rather simple structures. Many contain a six-carbon unit that passes unscathed through various chemical reactions that alter only the rest of the structure. This group, CgHs—, is common to many substances, including benzaldehyde (isolated from the oil of bitter almonds), benzyl alcohol (isolated from gum benzoin, a balsam resin obtained from certain Southeast Asian trees), and toluene (a hydrocarbon isolated from tolu balsam). When any of these three compounds is oxidized, the QH5 group remains intact the product is benzoic acid (another constituent of gum benzoin). The calcium salt of this acid, when heated, yields the parent hydrocarbon QHg (eq. 4.1). 

Online homework for this chapter can be assigned in OWL, an online homework assessment tool. 

Benzene is the parent of the family of aromatic hydrocarbons. Its six carbons lie in a plane at the corners of a regular hexagon and each carbon has one hydrogen attached. Benzene is a resonance hybrid of two contributing Kekule structures  

In orbital terms, each carbon is sp -hybridized. These orbitals form a bonds to the hydrogen and the two neighboring carbons are all in the ring plane. A p orbital at each carbon is perpendicular to this plane, and the six electrons, one from each carbon, form an electron cloud of % bonds which lie above and below the ring plane. 

The bond angles in benzene are 120°. All C-C bond distances are equal (1.39 A). The compound is more stable than either of the contributing Kekule structures and has a resonance or stabilization energy of about 36 kcal/mol. 

The nomenclature of benzene derivatives is described in Sec. 4.6. Common names and structures to be memorized include those of toluene, styrene, phenol, aniline, and xylene. Monosubstituted benzenes are named as benzene derivatives (bromobenzene, nitrobenzene, and so on). Disubstituted benzenes are named as ortho- (1,2-), meta- (1,3-), or para- (1,4-), depending on the relative positions of the substituents on the ring. Two important groups are phenyl (C6H5-) and benzyl (C6H5CH2-). 

Aromatic compounds react mainly by electrophilic aromatic substitution, in which one or more ring hydrogens are replaced by various electrophiles. Typical reactions are chlorination, bromination, nitration, sulfonation, alkylation, and acylation (the last two are Friedel-Crafts reactions). The mechanism involves two steps addition of the electrophile to a ring carbon, to produce an intermediate benzenonium ion, followed by proton loss to again achieve the (now substituted) aromatic system. 

In recent years further novel classes of compounds were added to cyclophane chemistry. The multilayeredphanesiS derived from [2.2]paracyclophane contain coaxially stacked benzene rings connected by ethano bridges para to one another. The first members of this series were described in 196416. Quadruple layered phane hydrocarbons 3 and 4 reveal in their UV-spectra long range electronic effects penetrating several arene units. 

The synthesis and properties of the highly symmetrical hydrocarbon phanes 2 and 6 have been reported in 197018 X-ray data of 619 indicate a molecular strain exceeding that of [2.2]paracyclophane-diene (7). The latter was estimated by Gantzel and Trueblood to be about 39 kcal/mole (163.0 kJ/mole)20). 

Rehybridization would partially displace the intraanular tr-electron density to the outside of the benzene rings, thus reducing destabilizing interaction in the interior of the molecule. In addition, rehybridization rationalizes a striking feature of the triene 6, also found in [2.2]paracyclophane 112 its diene 72V and in [3.3]paracyclo-phane20 the aromatic C—H-bonds of all phane hydrocarbons mentioned are inclined towards the inside of the molecule, e.g. in triene 6 by an angle of 13°. 

High field shift is a characteristic feature of face-to-face phanes27 and was attributed to two factors pushing into the same direction27 5  

However, already Longone and Chow pointed out that rehybridization should play a minor role in diamagnetic shift This in indicated by the 1H—NMR-spectra of the more strained members of the [mjparacyclophane series 8. Despite even a drastic ring distortion their phenylene protons resonate in the usual range (see Table 1). Triene 6 displays two singlet signals of equal intensity (5 = 6.24 and 7.37 ppm). By 

For a long time aromatic compounds were believed to be stable when exposed to ultraviolet irradiation. The interest in the photochemistry of arenes only started in the late 1950s when several groups observed both isomerization and addition reactions of benzene and its derivatives. Among the pioneers who were active at that time are the groups of Bryce-Smith and Schenck. Since then the photochemistry of aromatic compounds has become the subject of innumerable papers dealing with their conversion to other aromatic systems (by substitution or isomerization) or even to nonaromatics. 

In addition, there is another type of reaction which does not change the aromatic character involving either a substitution at the ring or a substitution at the side group. As in the case of acyclic addition an electron transfer mechanism operates. 

The free enthalpy of electron transfer can easily be calculated from the electrochemically measured redox potentials of the substrates and the excitation energy of the arene using 

The simultaneous formation of two or more a-bonds has always attracted synthetic chemists since complex molecules can be built up in one single step. Therefore it is not surprising that highly developed syntheses of natural and artificial products often made use of cycloaddition procedures. The meta photocycloaddition of aromatic compounds to alkenes certainly belongs to this category and has reached its summit of application in the admirable work of Wender s group. Hence this chapter describes photocycloadditions of various types covering mainly meta [3+2], ortho [2+2] and para [4+2] ones. Even an unusual example of an [6+6] cycloaddition is presented. The fact that only one substitution reaction is described may indicate that synthetic studies of electron transfer activation only started recently. 

Although the ortho cycloaddition was discovered more than three decades ago its application to organic synthesis is scarce. Two recent examples are described 

A diverse and important group of organic compounds is derived from benzene (CgHg), which has the special feature of being a six-carbon ring with three double bonds between 

Benzene, and compounds derived from it, are among the most commonly used solvents in industry. 

Our knowledge of rate coefficients for these compounds is due almost entirely to Warhurst and co-workers [24, 25, 39, 74] with the exception of data from the comparative method study for a small number of compounds [37]. The results [39] for a number of substituted bromo-benzenes are summarized in Table 2. The values are all standardized to a 

Rate coefficients relative to bromobenzene for substituted bromobenzenes (Y = Br) C6H4XY + Na - C6H4X + Y Na  

The values for substituted chlorobenzenes are by no means as extensive. Table 3 summarizes the available data obtained at 520°K at which temperature the rate coefficient for chlorobenzene is 2.6 x 10 cm mole sec .  

The authors point out that the substituted bromo- and chlorobenzenes fall into two classes based upon the magnitude of the accelerating effect of the substituent upon the reaction rate and the relationship between the rates of the m- and p-substituted halides. Class I comprises the substituents CH3O, HO, Cl and F for which the rate coefficient sequence is o m p u(u = unsubstituted) and Class II comprises the substituents CN and CH3 OOC for which the sequence iso p m u and for which the substituent effects are more powerful. These variations are understood in terms of the negative group effect, it being expected that the extra stabilization of the transition state decreases as the X and Y substituents are further separated from one another. The Class II substituents exhibit a more powerful accelerating effect because of the character of the multiple 

This results in a characteristic pattern of four lines in the aromatic region. 

Hemoglobin, the oxygencarrying protein in blood, contains a large aromatic cofactor called heme. 

In the early days of organic chemistry, the word aromatic was used to describe such fragrant substances as benzene (from coal distillate),benzaldehyde (from cherries, peaches, and almonds), and toluene (from Tolu balsam). It was soon realized, however, that substances classed as aromatic differed from most other organic compounds in their chemical behavior. 

the association of aromaticity with fragrance has long been lost, and we now nse the word aromatic to refer to the class of compounds that contain six-membered benzene-like rings with three double bonds. Many naturally occurring compounds are aromatic in part, such as the steroidal hormone estrone and the analgesic morphine. In addition, many synthetic drugs are aromatic in part, such as the antidepressant fluoxetine (Prozac). Benzene itself has been found to cause bone marrow depression and consequent leukopenia, or lowered white blood cell count, on prolonged 

7 Alkylation and Acylation of Aromatic Rings The Fried el-Crafts Reaction 

In contrast to proton shifts, carbon-13 shifts cannot be used as criteria for aromaticity (Section 3.1.3.10). No difference exists between aromatic (128.5 ppm for benzene) and comparable alkene carbon nuclei (127.5 ppm for cyclohexene). Aromatic ring carbon nuclei are practically not influenced by the ring current (Section 3.1.3.4), which makes up a deshielding of about 2 ppm and thus is small compared with other (e.g. steric) effects on carbon-13 shifts. 

A linear correlation between 13C chemical shifts and local n electron densities has been reported for monocyclic (4n + 2) n electron systems such as benzene and nonbenzenoid aromatic ions [76] (Section 3.1.3, Fig. 3.2). In contrast to theoretical predictions (86.7 ppm per n electron [75]), the experimental slope is 160 ppm per it electron (Fig. 3.2), so that additional parameters such as o electron density and bond order have to be taken into account [381]. Another semiempirical approach based on perturbational MO theory predicts alkyl-induced 13C chemical shifts in aromatic hydrocarbons by means of a two-parameter equation parameters are the atom-atom polarizability nijt obtained from HMO calculations, and an empirically determined substituent constant [382]. 

Chemical shifts of aromatic compounds occur between 120 and 150 ppm. On inclusion of electron releasing and electron withdrawing substituents as well as multiple substitution, this shift range may expand considerably (90-185 ppm). 

In aromatic compounds carbon-13 shifts are largely determined by mesomeric (resonance) and inductive effects. Field effects arising from through-space polarization of the n system by the electric field of a substituent, and the influences of steric (y) effects on the ortho carbon nuclei should also be considered. Substituted carbon (C-l) shifts are further influenced by the anisotropy effect of triple bonds (alkynyl and cyano groups) and by heavy atom shielding. 

Carbon skeletons of some polycyclic aromatic compounds of the (a) benzenoid type and (b) non-benzenoid type. 

Many aromatic compounds have considerable resonance stabilization but do not possess a benzene nucleus, or in the case of a fused polycyclic system, the molecular skeleton contains at least one ring that is not a benzene ring. The cyclopentadienyl anion C5HJ, the cycloheptatrienyl cation C7H+, the aromatic annulenes (except for [6]annulene, which is benzene), azulene, biphenylene and acenaphthylene (see Fig. 14.2.2(b)) are common examples of non-benzenoid aromatic hydrocarbons. The cyclic oxocarbon dianions C Of (n = 3,4,5,6) constitute a class of non-benzenoid aromatic compounds stabilized by two delocalized n electrons. Further details are given in Section 20.4.4. 

Chapter 4. Pu rification of Organic Chemicals — Aromatic Compounds 

Acenaphthene [83-32-9] M 154.2, m 94.0 . Ciystallise acenaphthene from EtOH. It has also been purified by chromatography from CCl on alumina with benzene as eluent [McLaughlin Zainal J Chem Soc 2485 I960]. [Beilstein 5IV 1834.] 

Acenaphthenequinone [82-86-0] M 182.2, m 260-261 . Extract it with, then lecrystallise it twice from C6H6- Dry it in vacuo. [LeFevie et 3I.J Chem Soc 974 1963, Beilstein 7IV 2498.] 

Acenaphthylene [208-96-8] M 152.2, m 92-93 , h 280 / 760mm. Dissolve acerraphtl lene in warm redistilled MeOH, filter through a sirttered glass frrrmel and cool to -78 to precipitate the material as yellow 

4-Acetamidobenzaldehyde [122-85-0] M 163.2, m 155 , 156 , 160 . Recrystallise it from water. The 4-nitrophenylhydrazone, m 264-265°, crystallises as orange needles from EtOH [Hodgson Beard J Chem Soc 21 1927, Beilstein 14 H 38,14 II25,14 III 75,14IV 71.] 

Fused tetra- and pentacyclic aromatic alkaloids are a new, emerging group of compounds from marine organisms. Amphimedine (187) was isolated from a Pacific sponge (Amphimedon sp.) as a cytotoxic compound in 1983 and was the first example of a polycyclic alkaloid (158). A pigment from the sea anemone Calliactis parasitica, named calliactine, has been known for many years, but the structure elucidation of calliactine was a difficult problem (159). In 1987 the structure of calliactine was proposed to be 188 on the basis of modern spectroscopic methods as well as chemical 

Kuanoniamines A-D (213-216) were isolated from an unidentified Mi-cronesian purple colonial tunicate and its predator, Chelynotus semperi, and exhibits cytotoxicity (177). Judging from the more than 20 examples mentioned above, it seems likely that these metabolites embracing the common tetracyclic unit (189) should be classed together biosynthetically. The diversity of source organisms would suggest that the metabolites are produced by symbionts. 

Two pentacyclic aromatic alkaloids, plakinidines A (217) and B (218) were recently isolated from a Vanuatuan red sponge of the genus Plakortis 

The Okinawan yellow sponge Aaptos aaptos contained a new type of alkaloid, aaptamine (220), having a 17/-benzo[ fe][l,6] naphthyridine skeleton (185). Demethylaaptamine (221) and demethyl(oxy)aaptamine (222) 

Benzene carbon atoms absorb at 128.5 ppm, neat or as a solution in CDC13. Substituents shift the attached aromatic carbon atom as much as 35 ppm. Fused-ring absorptions are as follows  

Shifts of the aromatic carbon atom directly attached to the substituent have been correlated with substituent electronegativity after correcting for magnetic anisotropy effects shifts at the para aromatic carbon have been correlated with the Hammett a constant. Ortho shifts are not readily predictable and range over about 15 ppm. Meta shifts are generally small-up to several parts per million for a single substituent. 

The substituted aromatic carbon atoms can be distinguished from the unsubstituted aromatic carbon atom by its decreased peak height that is, it lacks a proton and thus suffers from a longer 7) and a diminished NOE. 

Incremental shifts from the carbon atoms of benzene for the aromatic carbon atoms of representative monosubstituted benzene rings (and shifts from TMS 

Shifts of the aromatic carbon atom directly attached to the substituent have been correlated with substituent 

C Atom Calculated Observed C Atom Observed C Atom Observed 

The electronic structure of the lowest excited states of planar aromatic compounds is well described by MO theory (Sections 4.5 and 4.7). Benzenoid aromatic hydrocarbons exhibit three or four7t,7t absorption bands in the near-UV region, which are labelled Lb,1 La, 1 Bb 

The fluorescence rate constants of aromatic compounds predicted by Equation 2.11 differ by about two orders of magnitude depending on the nature of the lowest singlet state the smaller representatives with S i 1 Lb have iffs2x 106 s 1 and the larger ones with 

The first absorption band of nonaltemant hydrocarbons and the band shifts induced by substitution are generally well described by HMO theory (Section 4.7). Absorption to Si corresponds to the HOMO LUMO transition. Nonradiative decay often dominates the photophysical properties of nonaltemant hydrocarbons and also alternant hydrocarbons with a 4 -membered ring (biphenylene), so that they generally have short singlet lifetimes and low triplet yields and are less prone to undergo photoreactions upon direct irradiation. 

The use of liquid chromatography for the analysis of chlorinated aromatic compounds is rarely reported. An exception exists for LC methods used for sample cleanup. The complexity of environmental samples often necessitates the fractionation of sample extracts, which are then analyzed separately by GC-based methods. These LC fractionation methods have been developed based on size-exclusion chromatography 

FIGURE 13.4 Separation of PCB congeners by UHPLC with three sub-2-pm particle diameter columns. Peak identifications are PCB designations. Source From Reference [62] reproduced by permission of Elsevier. 

The use of pesticides is highly regulated in most countries [65]. The World Health Organization includes numerous pesticides and 

Whereas most multiresidue methods employ SPE for sample enrichment, Grulick and Alder [80] utilized direct sample injection for the determination of 300 pesticides in water samples. A conventional reversed-phase LC separation was developed with gradient elution using a Cl 8 column with a run time of approximately 23 min. The LC was coupled to a triple quadrupole mass spectrometer with electrospray ionization in the positive ion mode. Two selected reaction monitoring transitions were collected for each analyte by means of repeated analyses (i.e., two sample injections). Because interfering signals were noted for many of these transitions, both the LC separation and confirmatory transitions were essential for correct analyte identification. 

FIGURE 13.5 UHPLC—MS/MS separation of an apple extract, fortified with 17 polar pesticides. Peak identification (1) carbendazim, (2) thiabendazole, (3) carbofuran, (4) carbaryl, (5) linuron, (6) methiocarb, (7) epoxiconazole, (8) flusilazole, (9) diflubenzuron, (10) tebuconazole, (11) imazalil, (12) propiconazole, (13) trUlumuron, (14) bitertanol, (15) prochloraz, (16) teflubenzuron, (17) flufenoxuron. Source From Reference 181], reproduced by permission of Elsevier. 

Q Use IR, NMR, UV, and mass spectra to determine the structures of aromatic compounds. Given an aromatic compound, predict the distinguishing features of its spectra. 

Many other compounds discovered in the nineteenth century seemed to be related to benzene. These compounds also had low hydrogen-to-carbon ratios as well as pleasant aromas, and they could be converted to benzene or related compounds. This group of compounds was called aromatic because of their pleasant odors. Other organic compounds without these properties were called aliphatic, meaning Tatlike. As the unusual stability of aromatic compounds was investigated, the term aromatic came to be applied to compounds with this stability, regardless of their odors. 

The Kekule Structure In 1866, Friedrich Kekule proposed a cyclic structure for benzene with three double bonds. Considering that multiple bonds had been proposed only recently (1859), the cyclic structure with alternating single and double bonds was considered somewhat bizarre. 

The Kekule structure has its shortcomings, however. For example, it predicts two different 1,2-dichlorobenzenes, but only one is known to exist. Kekule suggested (incorrectly) that a fast equilibrium interconverts the two isomers of 1,2-dichlorobenzene. 

The Resonance Representation The resonance picture of benzene is a natural extension of Kekule s hypothesis. In a Kekule structure, the C — C single bonds would be longer than the double bonds. Spectroscopic methods have shown that the benzene ring is planar and all the bonds are the same length (1.397 A). Because the ring is planar and the carbon nuclei are positioned at equal distances, the two Kekule structures must differ only in the positioning of the pi electrons. 

This section will encompass the reactions of carbocyclic and heterocyclic aromatic compounds in which oxidation affects the aromatic rings and the attached side chains. 

Smith and coworkers have screened the solid catalysts for aromatic nitration, and found that zeolite (3 gives the best result. Simple aromatic compounds such as benzene, alkylbenzenes, halogenobenzenes, and certain disubstituted benzenes are nitrated in excellent yields with high regioselectivity under mild conditions using zeolite (3 as a catalyst and a stoichiometric quantity of nitric acid and acetic anhydride.11 For example, nitration of toluene gives a quantitative yield of mononitrotoluenes, of which 79% is 4-nitrotoluene. Nitration of fluorobenzene under the same conditions gives p-fluoronitrobenzene exclusively (Eqs. 2.1 and 2.2) 

To avoid excessive acid waste, lanthanide(III) triflates are used as recyclable catalysts for economic aromatic nitration. Among a range of lanthanide(III) triflates examined, the ytterbium salt is the most effective. A catalytic quantity (1-10 mol%) of ytterbium(III) triflate catalyzes the nitration of simple aromatics with excellent conversions using an equivalent of 69% nitric acid in refluxing 1,2-dichloromethane for 12 h. The only by-product of the reaction is water, and the catalyst can be recovered by simple evaporation of the separated aqueous phase and reused repeatedly for further nitration.12 

However, this catalyst is not effective for less reactive aromatics such as o-nitrotoluene. In such cases, hafnium(IV) and zirconium(IV) triflates are excellent catalysts (10 mol%) for mononitration of less reactive aromatics. The catalysts are readily recycled from the aqueous phase and reused (Eqs. 2.3 and 2.4).12 

Phenols are easily mononitrated by sodium nitrate in a two-phase system (water-ether) in the presence of HC1 and a catalytic amount of La(N03)3.13 Various lanthanide nitrates have been used in the nitration of 3-substituted phenols to give regioselectively the 3-substituted 5-nitrophenols.14 

Vanadium oxytri nitrate is an easy to handle reagent that can be used to nitrate a range of substituted aromatic compounds in dichloromethane at room temperature, leading to 99% yields of nitration products (Eq. 2.5).16 

When an aromatic compound adds MA, generally, the resonance stabilization of such a system is destroyed. When loss of resonance energies are significant, high activation energies are anticipated for these reactions. Thus, simple aromatic compounds tend to be less reactive as dienes. In the series benzene, naphthalene, and anthracene, examination of canonical structure suggests that the middle ring of anthracene has the most dienelike character. The normal order of reactivity observed, therefore, is benzene naphthalene anthracene. 

Benzene reacts very sluggishly, and no 1 1 adduct with MA has been isolated.However, the benzene ring in cyclophanes may be sufficiently strained to be more trienic than aromatic. A 1 1 DA adduct of MA with a cyclophane has been reported.Hydroquinone 108 also is sufficiently activated to produce low yields of the adduct 110. Note that product 110 is the ketonic form of the initially produced adduct 109. 

Under ordinary conditions, naphthalene 111 gives very low yields of a 

Nitro substitution, as in 1-nitronaphthalene, makes forcing conditions necessary, and addition takes place in the unsubstituted ring across the 5,8 positions. 

The ring in 2-naphthol 113 is sufficiently activated to act as a diene. On reaction with MA two stereoisomers of adduct 114 are formed, which is the 

In ordinary conversation, the word aromatic conjures pleasant associations—the odor of freshly prepared coffee, or of a cinnamon bun. Similar associations occurred early in the history of organic chemistry, when pleasantly aromatic compounds were isolated from natural oils produced by plants. As the structures of these compounds were elucidated, a number of them were found to contain a highly unsaturated six-carbon structural unit that is also found in benzene. This special ring structure became known as a benzene ring, and the aromatic compounds containing a benzene ring became part of a larger family of compounds now classified as aromatic on the basis of their electronic structure rather than their odor. 

The following are a few examples of aromatic compounds including benzene itself. In these formulas we foreshadow our discussion of the special properties of the benzene ring by using a circle in a hexagon to depict the six TT electrons and six-membered ring of these compounds, whereas heretofore we have shown benzene rings only as indicated in the left-hand formula for benzene below. 

Eugenol (in oil of cloves) Anethole (in oil of anise) Cinnamaldehyde (in oil of cinnamon) Vanillin (in oil of vanilla) 

As time passed, chemists found or synthesized many compounds with benzene rings that had no odor, such as benzoic acid and acetylsalicylic acid (aspirin). 

In this chapter we shall discuss in detail the structural principles that underlie how the term aromatic is used today. We will also see how the stracture of benzene proved so elusive. Even though benzene was discovered in 1825, it was not until the development of quantum mechanics in the 1920s that a reasonably clear understanding of its structure emerged. 

In the experiments, 2,4,6-trichlorophenoxyacetic acid, pentachlorophenoxyace-tic acid and pentachlorophenoxyisobutyric acid revealed a systemic action against Botrytis fabea and Alternaria solani. Fungicides of more favourable action were obtained with such derivatives of phenoxi compounds in which the carbonyl group has also been changed. 4-Chloro-3,5-dimethylphenoxy ethanol (26), effective 

Nitrothal-isopropyl, diisopropyl-5-nitroisophthalate, (27) is not systemic, but is a fungicide of specific activity against apple powdery mildew. It is used in combination (sulfur, zineb). This fungicide is of low toxicity to mammals, bees and earthworms. The acute oral is 6400 mg/kg for rats for the formulated product (50% a. i.) (Phillips et al., 1973 Archer, 1979). 

Fenaminosulf (28) is a diazo compound, the sodium salt of p-dimethylamino-benzenediazo sulfonic acid. It is prepared by the reaction of diazotized p-dimethyl-aminoaniline with sodium sulfite (Urbschat, 1960). 

Fenaminosulf is relatively toxic to mammals, its acute oral LD50 being 60 mg/kg for rats. Its dermal toxicity is low. Treated seeds are toxic to birds and other wildlife. 

Fenaminosulf reduces respiration in sensitive fungi. Thus, it inhibits the mitochondrial oxidation of nicotinamide adenine dinucleotide (NADH) in Pythium (Tolmsoff, 1962). On the other hand, in nonsensitive fungi (Rhizoctonia solani) a reductase has been identified which decomposes the active substance. But 

The Resonance Representation The resonance picture of benzene is a natural extension of Kekuld s hypothesis. In a Kekuld structure, the C—C single bonds would be longer than the double bonds. Spectroscopic methods have shown that the benzene 

Benzene is actually a resonance hybrid of the two Kekule structures. This representation implies that the pi electrons are delocalized, with a bond order of 11 between adjacent carbon atoms. The carbon-carbon bond lengths in benzene are shorter than typical single-bond lengths, yet longer than typical double-bond lengths. 

Back before the time of Michael Faraday (who discovered benzene in 1825 it was known that a ubiquitous structural unit was fonnd in natural compounds and seemed to be associated with the pleasant smells of many of those substances. The stmcture of the unit was not known at first, but its relative lack of reactivity was recognized early on. The structural unit could be seen to proceed unchanged through many series of reactions. Compounds that contained this unit were called aromatic because of their often fragrant odors, but this designation later became correctly and more nsefiilly associated with their lack of reactivity. Subsequently, the structure of benzene was unraveled, and, indeed, its chemistry proved to be quite different from the nsual 

While the thermochemical definition of an aromatic compound is commonly used and understood, there are two other definitions that have also been introduced. Sometimes it may be desired to decide whether or not a compound is aromatic, but it may not be that easy to obtain and interpret the desired thermochemical information. Hence, these subsidiary definitions. But these subsidiary definitions do not always lead to the same conclusion regarding the presence, absence, or degree of aromaticity as does the thermochemical definition. These two definitions will be briefly discussed in turn. 

We will examine benzene with different bases and also discuss some of the ideas that consideration of this molecule has led to, such as resonance and resonance energy. 

We show again the traditional five covalent Rumer diagrams for six electrons and six orbitals in a singlet coupling and emphasize that the similarity between the ring of orbitals and the shape of the molecule considerably simplifies the understanding of the S5mimetry for benzene. 

Most of the discussion we give here on the nature of the wave function will focus on HLSP functions. An early ab initio study by Norbeck and the present author 

In this ease all of the terms in a symmetry function have the same sign as well as magnitude for the eoeffieient. 

The Weyl dimension formula (Eq. (5.115)) tells us that six electrons in six orbitals in a singlet state yield 175 basis functions. These may be combined into 22 Aig symmetry functions. Table 15.1 shows the important HLSP functions for a rr-only calculation of benzene for the SCF optimum geometry in the same basis. The a orbitals are all treated in the core , as described in Chapter 9, and the tt electrons are subjected to its SEP. We discuss the nature of this potential farther in the next section. The functions numbered in the first row of Table 15.1 have the following characteristics. 

In benzo[b]thiophene itself, both rings are coplanar, but the introduction of a substituent on the thiophene ring usually causes the two rings to be inclined to each other at about 1.0° (74AX(B)2058, 84AX(A)C277 . 

Early work on the experimentally established conformational preferences in solution for a variety of other 2-substituted heterocycles is summarized in Table 30. Most of these conclusions have been deduced either from dipole moment measurements in benzene or by the use of lanthanide induced shifts for chloroform solutions. The aforementioned MO studies correctly predict the preferred conformations, (63, R = H) or (64, R = H), of pyrrole-2-carbaldehyde, thiophene-2-carbaldehyde and furfural in the gas phase. 

Substituent Pyrrole Furan Thiophene Selenophene Tellurophene Phase 

Although theoretical studies indicate very small gas-phase energy differences between the syn and anti conformers of the 3-carbaldehydes of furan, thiophene and pyrrole with a slight preference for the syn conformer (65a, R = H), in chloroform solution the furan- and thiophene-3-carbaldehydes adopt the anti conformers (65b, R = H) to the extent of 100 and 80%, respectively (82T3245). However, N-substituted 3-(trifluoroacetyl)pyrroles (65, R = CF3) exist in solution as mixtures of rotational isomers (80JCR(S)42). 

Structure of Five-membered Rings with One Hetematom 

The study of the reactivity of towards an extensive series of mono- 

The specific reaction rates of substituted benzenes with have been related to that of benzene and expressed in terms of 77 values, where 77 = log ( c Hsx/ c hsh)- Comparable values of 77 are obtained for mono-substituted toluenes and phenols. 

A series of para-derivatives of benzoic acid was examined for their rates of reaction with e, and their 77 values, relative to benzoic acid, were calculated. These 77 values are proportional to the a values of the monosubstituted benzene series, and p = 0-74 was found for the benzoate series. The behaviour of the benzoic acid series shows that in contrast to 

Surprisingly similar results are obtained when monosubstituted benzenes react with solvated electrons in methanol (p = 4-7) (Sherman, 1966). This result, which is in accord with other cases examined both in H20 and MeOH (Anbar and Hart, 1964b), suggests that the rates of eaq reactions are independent, at first approximation, of the dielectric constant of the solvating medium and of its viscosity. The implication of this finding will be discussed in Section III. 

The e j rate constants correlate better with normal o- values derived from electrophilic substitution than with a para values obtained from data of nucleophilic reactions. This is not surprising in view of the fact that the e]fq reactions constitute an interaction of an electron with the 77-orbitals of the ring, as in electrophilic substitution, rather than with effects on electron distribution and polarizability of a certain substituent. 

Note The representation of benzene with a circle to represent the n system is fine for questions of nomenclature, properties, isomers, and reactions. For questions of mechanism or reactivity, however, the representation with three alternating double bonds (the Kekule picture) is more informative. For clarity and consistency, this Solutions Manual will use the Kekule form exclusively. 

Models show that the angles between p orbitals on adjacent n bonds approach 90°. 

16-8 Azulene satisfies all the criteria for aromaticity, and it has a Huckel number of n electrons 10. Both heptalene (12 7i electrons) and pentalene (8 n electrons) are antiaromatic. 

The crystalline material soluble in polar organic solvents is cyclopropenium tetrafluoroborate. 

16-15 Draw resonance forms showing the carbonyl polarization, leaving a positive charge on the carbonyl 

These are TOP views. Nodes are shown by dashed lines. 

The presence of a benzene ring in a molecule gives rise to absorptions in six regions of the spectrum and these are summarised in Table 4.3d. 

Previous sections have dealt with electrophilic additions to olefins and to acetylenes. It is also true that addition can be a minor mode of reaction during electrophilic substitution. De la Mare et al. have demonstrated that adducts, resulting from reactions such as (7.1) and (7.2), are isolated during 

Comprehensive Organic Reactions in Aqueous Media, Second Edition, by Chao-Jun 

Using BF3 OEt2 as a catalyst, MBH acetate 106 reacts with readily available a-EWG ketene-(5,iS)-acetals 107 to afford effectively 5 N2 -type product 108 in 

Manganese(iii)- and Ce(iv)-induced intramolecular homolytic malonylation of malonate derivatives with an aromatic ring at the 5-position has been utilized to construct tetrahydro- or dihydro-naphthalene frameworks.5 N2 -type adducts 134 of MBH acetates were used to investigate the Mn(iii)-mediated 

If necessary, review the suggested sections to prepare for this chapter  

Dissolving Metal Reduction (Section 10.5) MO Description of Conjugated Dienes (Section 17.3) 

PLUS Visit www.wileyplus.com to check your understanding and for valuable practice. 

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LEARNING GOAL Write the lUPAC names and formulas for alkenes and alkynes draw the condensed structural formulas for the monomers that form a polymer. 

11 Identify each of the foUowing as an alkane, alkene, or alkyne  

15 I raw the condensed structural formula for each of the following compounds  

LEARNING GOAL Draw the condensed structural formulas and give the names for the organic products of addition reactions of hydrogenation and hydration of alkenes. 

33 Draw the condensed structural formula, or skeletal formula if cyclic, for the product in each of the following reactions  

In 1825, Michael Faraday isolated a hydrocarbon called benzene, which had the molecular formula C6H5. A molecule of benzene consists of a ring of six carbon atoms with one hydrogen atom attached to each carbon. Because many compounds containing benzene have fragrant odors, the family of benzene compounds became known as aromatic compounds. 

In benzene, each carbon atom uses three valence electrons to bond to the hydrogen atom and two adjacent carbons. That leaves one valence electron, which scientists first thought was shared in a double bond with an adjacent carbon. In 1865, August Kekule proposed that the carbon atoms in benzene were arranged in a flat ring with alternating single and double bonds between the carbon atoms. There are two possible structural representations of benzene in which the double bonds can form between two different carbon atoms. 

Fill in the blanks below. To verify that your answers are correct, look in your textbook at the end of Chapter 18. Each of the sentences below appears verbatim in the section entitled of Concepts and 

Benzene is comprised of a ring of six identical C-C bonds, each of which has a bond order of 

The stability of benzene can be explained with MO theory. The six % electrons all occupy MOs. 

The presence of a fully conjugated ring of % electrons is not the sole requirement for aromaticity. 

The requirement for an odd number of electron pairs is called rule. 

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Azulene is an aromatic compound and undergoes substitution reactions in the 1-position. At 270 C it is transformed into naphthalene. 

The composition of coal tar varies with the carbonization method but consists, largely, of mononuclear and polynuclear aromatic compounds and their derivatives. Coke oven tars are relatively low in aliphatic and phenolic content while low-temperature tars have much higher contents of both. 

The nitration, sulphonation and Friedel-Crafts acylation of aromatic compounds (e.g. benzene) are typical examples of electrophilic aromatic substitution. 

Hammen equation A correlation between the structure and reactivity in the side chain derivatives of aromatic compounds. Its derivation follows from many comparisons between rate constants for various reactions and the equilibrium constants for other reactions, or other functions of molecules which can be measured (e g. the i.r. carbonyl group stretching frequency). For example the dissociation constants of a series of para substituted (O2N —, MeO —, Cl —, etc.) benzoic acids correlate with the rate constant k for the alkaline hydrolysis of para substituted benzyl chlorides. If log Kq is plotted against log k, the data fall on a straight line. Similar results are obtained for meta substituted derivatives but not for orthosubstituted derivatives. 

It is a typically aromatic compound and gives addition and substitution reactions more readily than benzene. Can be reduced to a series of compounds containing 2-10 additional hydrogen atoms (e.g. tetralin, decalin), which are liquids of value as solvents. Exhaustive chlorination gives rise to wax-like compounds. It gives rise to two series of monosubstitution products depending upon... 

Kovats, E. and A. Wehrli (1959), Gas-chromatography characterization of organic compounds. III. Calculation of the retention indexes of aliphatic, alicyclic and aromatic compounds . Helv. Chim. Acta, Vol. 42, p. 2709. 

We will show here the classification procedure with a specific dataset [28]. A reaction center, the addition of a C-H bond to a C=C double bond, was chosen that comprised a variety of different reaction types such as Michael additions, Friedel-Crafts alkylation of aromatic compounds by alkenes, or photochemical reactions. We wanted to see whether these different reaction types can be discerned by this... 

HMO theory is named after its developer, Erich Huckel (1896-1980), who published his theory in 1930 [9] partly in order to explain the unusual stability of benzene and other aromatic compounds. Given that digital computers had not yet been invented and that all Hiickel s calculations had to be done by hand, HMO theory necessarily includes many approximations. The first is that only the jr-molecular orbitals of the molecule are considered. This implies that the entire molecular structure is planar (because then a plane of symmetry separates the r-orbitals, which are antisymmetric with respect to this plane, from all others). It also means that only one atomic orbital must be considered for each atom in the r-system (the p-orbital that is antisymmetric with respect to the plane of the molecule) and none at all for atoms (such as hydrogen) that are not involved in the r-system. Huckel then used the technique known as linear combination of atomic orbitals (LCAO) to build these atomic orbitals up into molecular orbitals. This is illustrated in Figure 7-18 for ethylene. 

Figure 8-7, Comparison of tlie RDF code for aromatic compounds with different subslilulic patterns (hydrogen atoms are not considered).

It should be noted that aliphatic compounds (except the paraffins) are usually oxidised by concentrated nitric acid, whereas aromatic compounds (including the hydrocarbons) are usually nitrated by the concentrated acid (in the presence of sulphuric acid) and oxidised by the dilute acid. As an example of the latter, benzaldehyde, CjHsCHO, when treated with concentrated nitric acid gives ffi-nitrobenzaldehyde, N02CgH4CH0, but with dilute nitric acid gives benzoic acid, CgHgCOOH. 

When an aromatic compound having an aliphatic side chain is subjected to oxidation, fission of the side chain occurs between the first and second carbon atoms from the benzene ring, the first carbon atom thus becoming part of a carboxyl ( -COOH) group. For example ... 

Since Grignard reagents can easily be obtained from aryl halides, they are of special value in the s nthesis of many aromatic compounds, particularly as, for reasons already stated (pp. 270, 276), aromatic compounds cannot generally be prepared by means of ethyl acetoacetate and ethyl malonate. 

In practice superheated steam is generally employed for substances with a low vapour pressure (< 5-1 mm.) at 100°. Thus in the recovery of the products of nitration or aromatic compounds, the ortho derivative e.g., o-nitrophenol) can be removed by ordinary steam distillation the... 

Table 111,42 deals with a number of aliphatic halogen compounds together with their crystalline derivatives. Some aromatic compounds, which simulate the properties of aliphatic haUdes in some respects, are included. 

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