CHAPTER 15 Benzene and Aromaticity Electrophilic Aromatic Substitution 15-1 Naming the Benzenes he于 0wegLco0reataercm B.a 4= ted benzenes are named using the prefixes en6grtwre。benenesareonannamedbyaddngaprafikto 1
1 CHAPTER 15 Benzene and Aromaticity: Electrophilic Aromatic Substitution Benzene has the empirical formula CH. Its molecular formula has been shown to be C6H6, which implies four degrees of unsaturation in the molecule. Several incorrect structures of benzene have been proposed in the past: Of these, only Claus benzene has not been synthesized. All of the other structures represent unstable substances which isomerize to benzene in very exothermic reactions. The correct molecular structure of benzene is: Benzene is relatively inert. Among the reactions it does undergo is the reaction with bromine in the presence of catalytic amounts of FeBr3: The addition product is not formed, as might be expected. Further reaction with bromine leads to a mixture of isomeric di-substituted products: 15-1 Naming the Benzenes Benzene and its derivatives were originally called “aromatic compounds” because of their strong aromas. Benzene is considered the “parent” aromatic molecule. The structure of benzene is written as a pair of resonance structures or as a regular hexagon containing an inscribed circle: Monosubstituted benzenes are often named by adding a prefix to the word benzene: Disubstituted benzenes are named using the prefixes: •1,2- (ortho-, or o-) •1,3- (meta-, or m-) •1,4- (para-, or p-) The substituents are then listed in alphabetical order:
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2 For tri- and more highly substituted benzenes, the ring carbons are numbered to give the substituents the lowest set of numbers, as in cyclohexane nomenclature: The following benzene derivatives will be used often in this course: The naming of three substituted benzene systems do not follow IUPAC nomenclature in the Chemical Abstracts indexing preferences: phenol, benzaldehyde and benzoic acid. Ring substituted compounds of these substances are named by numbering the ring positions or using the prefixes o-, p- and m-. The carbon carrying the substituent giving the compound its base name is given the number 1. Several other common names have been accepted by IUPAC, including: A substituted benzene is called an arene. An arene, when used as a substituent, is called an aryl group (Ar). The parent aryl substituent is phenyl, C6H5-. The group C6H5CH2- is called phenylmethyl (benzyl). Structure and Resonance Energy of Benzene: a First Look at Aromaticity 15-2 At room temperature, benzene is inert to acids, H2, Br2 and KMnO4, reagents that usually add to conjugated alkenes. The cyclic 6-electron arrangement of double bonds imparts a special stability in the form of a large resonance energy
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3 The benzene ring contains six equally overlapping p orbitals. Experimentally, benzene can be shown to be a completely symmetrical hexagon; there are no alternating single (long) and double (short) bonds. The electronic structure of the benzene ring consists of 6 sp2 carbon atoms, with each p-orbital overlapping the p-orbitals of its nearest neighbors. Benzene is especially stable: heats of hydrogenation. It is useful to compare the heats of hydrogenation of benzene, 1,3-cyclohexadiene and cyclohexene. In each case, cyclohexane is the final product. The heat of hydrogenation of benzene can be experimentally measured using special catalysts and is measured to be 29.6 kcal mol-1 less than the calculated heat for the hypothetical molecule, 1,3,5-cyclohexatriene. Benzene is much more stable than a molecule containing alternating double and single bonds. The difference in stability is called the resonance energy of benzene, about 30 kcal mol-1. 15-3 π Molecular Orbitals of Benzene Cyclic overlap modifies the energy of benzene’s molecular orbitals. The molecular π orbitals of benzene can be compared to those of 1,3,5-hexatriene: The cyclic π system is stabilized relative to the acyclic one. Two of the orbitals have been lowered in energy in benzene (π1 and π3) while one has been raised (π2). The drop in energy of π1 and π3 more than offsets the rise in energy of π2. Some reactions have aromatic transitions states. The transition states in the Diels-Alder reaction, the addition of osmium tetroxide to alkenes and the first step in ozonolysis all exhibit a cyclic overlap of six electrons in π orbitals or orbitals having π character: These concerted aromatic transition states have lower energies than the alternative sequential bond-breaking and bond-making mechanism
15-4 Spectral Characteristics of the Benzene Ring eahgb8enestvmofbenzaneroveatt ves have char 1,3-dmethylbenzone(m-) 690 and 765 cm 650-100 C-plane bending ass spectrum of benz ne indicates stability heancdRea2emreshowhe icimghndrogens 655p ne hydroge .27 ppm(single peak) 4
4 15-4 Spectral Characteristics of the Benzene Ring The UV-visible spectrum of benzene reveals its electronic structure. The energy gap between bonding and antibonding orbitals is greater in benzene than for acyclic trienes. The UV spectra of benzene shows absorbances at smaller wavelengths (higher energy) than does the spectra of 1,3,5- hexatriene: The electronic spectra of aromatic compounds varies with the introduction of substituents (useful in designing dye molecules). Simple substituted benzenes absorb between 250 and 290 nm. 4-Aminobenzoic acid (PABA) has a λmax of 289 nm and a high extinction coefficient of 18,600. It is used in sunscreen lotions to filter out harmful UV light in this wavelength region. The infrared spectrum reveals substitution patterns in benzene derivatives. The IR spectra of benzene and its derivatives have characteristic bands in three regions: •3030 cm-1 phenyl-hydrogen stretching •1500-2000 cm-1 aromatic ring C-C stretching •650-1000 cm-1 C-H out-of-plane bending The specific substitution pattern determines the precise location of the C-H out-of-plane bending absorptions. For the dimethylbenzenes: •1,2-dimethylbenzene (o-) 738 cm-1 •1,4-dimethylbenzene (p-) 793 cm-1 •1,3-dimethylbenzene (m-) 690 and 765 cm-1 The mass spectrum of benzene indicates stability. The mass spectrum of benzene shows little fragmentation due to its unusual stability. The (M+1)+. peak shows the correct peak height (6.8%) for the relative abundance of 13C in a six-carbon molecule. The NMR spectra of benzene derivatives show the effects of an electronic ring current. The cyclic delocalization of the electrons in the aromatic ring gives rise to unusual deshielding: Aromatic ring hydrogens: 6.5-8.5 ppm Alkenyl hydrogens: 4.6-5.7 ppm Benzene hydrogens: 7.27 ppm (single peak) This aromatic deshielding is due to ring currents produced by the π electrons moving in the external magnet field, H0
C have more complicate NMR apsdaya04o8pmmonh (Benzene:7.27 ppm 6tmeao2482e0e2hnteso6rpetn Ortho hydroo 。 ” 5Rn2a 15-5 Polycyclic Aromatic Hydrocarbons e sicommaoawhe2oenaoon 男 aronreaa 位女拉 v 5
5 The magnetic field from the ring current opposes H0 inside the loop but opposes it outside the loop where the hydrogens are located, resulting in deshielding. The effect is strongest closest to the ring and diminishes rapidly with distance. Benzylic nuclei are deshielded only about 0.4-0.8 ppm more than their allylic counterparts. Hydrogens farther away from π system have chemical shifts similar to those in the alkanes. Substituted benzenes may have more complicated NMR patterns. The presence of a substituent renders the ortho, meta and para hydrogens non-equivalent and subject to mutual coupling. (Benzene: 7.27 ppm) 4-(N,N-dimethylamino)benzaldehyde shows a large chemical shift difference between the two sets of ring hydrogens and a near first-order pattern of two doublets. The 9 Hz coupling constant is typical of splitting between ortho protons. All three types of coupling can be seen in the first order spectrum of 1-methoxy-2,4-dinitrobenzene (2,4-dinitroanisole). Ortho hydrogen (to methoxy) Doublet, δ=7.23 ppm, 9 Hz coupling Hydrogen flanked by nitro groups Doublet, δ=8.76 ppm, 3 Hz coupling Remaining ring hydrogen Doublet of doublets, δ=8.45 ppm, Para coupling between C3 and C6 is too small to be resolved. The 13C NMR spectra of benzene derivatives is not greatly affected by ring current shifts, since the induced ring current flows directly above and below the ring carbons. Benzene carbons exhibit chemical shifts similar to those in alkenes (120-135 ppm when unsubstituted). Benzene exhibits a single line at δ=128.7 ppm. 15-5 Polycyclic Aromatic Hydrocarbons Molecules containing several fused benzene rings are called polycyclic benzenoid or polycyclic aromatic hydrocarbons (PAHs). Common names are used for these systems, since there is no simple naming system for them. The series of linearly fused benzene rings is called the acenes. Angular fusion (“annulation”) results in phenanthrene. Quaternary carbons are numbered using the preceding carbon in the sequence followed by a letter indicating its distance to the preceding carbon
Naphthalene is aromatic:a look at spectra. eaeeaaaoagtaeaceereeamehaa -9-c0 te8rCe.cthe1opcrtbtalsresusnafewevenastrtl 我郭@ XaYee2aehgehspta。etem Ra9”786p shows two sym glen an dou onn s with chemical shifts in Mostusedbenzenoldhydrocabonsare romatic phenanthre C0-00-00-Q0 o0-o9-o0-c0-o9 6
6 Naphthalene is aromatic: a look at spectra. Naphthalene is a colorless crystalline material with a melting point of 80o C. The UV spectrum of naphthalene indicates an extended, conjugated system with peaks at wavelengths as long as 320 nm. The electrons in naphthalene are more delocalized than in benzene and several resonance structures can be drawn: The overlap of the 10 p orbitals results in a fairly even distribution of electron density. X-ray crystallographic measurements have determined the exact bond distances and angles in naphthalene. The C-C bond distances deviate slightly from those in benzene, 1.39 Å, C-C single bonds, 1.54Å and C=C double bonds, 1.33 Å. The 1H NMR spectrum of naphthalene shows two symmetric multiples at δ = 7.49 and 786 ppm. Coupling constants in the naphthalene nucleus are similar to those in substituted benzenes: •Jortho = 7.5 Hz •Jmeta = 1.4 Hz •Jpara = 0.7 Hz These peak positions are characteristic of ringcurrent deshielded aromatic hydrogens. The 13C NMR spectrum shows three lines with chemical shifts in the range of other benzene derivatives: Most fused benzenoid hydrocarbons are aromatic. Linear and angular fusion of a third benzene ring onto naphthalene result in anthracene and phenanthrene. Anthracene is about 6 kcal mol-1 less stable than phenanthrene due to differences in resonance stabilization
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7 15-6 Other Cyclic Polyenes: Hückel’s Rule Cyclic conjugated polyenes can be aromatic as long as they contain 4n+2 π electrons (n=0,1,2,…). Cyclic polyenes containing 4n π electrons may be destabilized by conjugation, or are antiaromtic. These observations are known as Hückel’s rule. Nonplanar cyclic systems in which p-orbital overlap is disrupted sufficiently to impart alkenelike properties are classified as nonaromatic. 1,3-Cyclobutadiene, the smallest cyclic polyene, is antiaromatic. 1,3-Cyclobutadiene is a 4n π system and is antiaromatic. It is air-sensitive and extremely reactive (compared to 1,3- butadiene or cyclobutene). It is destabilized through π overlap by more than 35 kcal mol-1. Its structure is rectangular and it exists as two isomers, equilibrating through a symmetrical transition state, rather than resonance forms. Free 1,3-cyclobutadiene can be prepared and observed only at very low temperatures. Cyclobutadiene can act as either a diene or a dienophile in its rapid Diels-Alder reactions: Substituted cyclobutadienes are less reactive and have been used to study the spectroscopic features of the 4 π electron cyclic system. In 1,2,3-tris(1,1-dimethylethyl)cyclobutadiene, the ring hydrogen resonates at δ = 5.38 ppm, much higher than expected for an aromatic system. 1,3,5,7-Cyclooctatetraene is non-planar and nonaromatic. 1,3,5,7-cyclooctatetraene is a 4n π system and is antiaromatic. It can be made by a nickel-catalyzed cyclotetramerization of ethyne. It is a yellow liquid (b.p. 152oC), is stable while cold, but polymerizes when heated. It is oxidized by air, catalytically hydrogenated to cyclooctane, and subject to electrophilic additions and to cycloadditions. The 1H NMR spectrum shows a sharp singlet at δ = 5.68 ppm, typical of an alkene. The molecular structure of cyclooctatetraene is non-planar and tub shaped. The double bonds are nearly orthogonal and are not conjugated. Only cyclic conjugated polyenes containing 4n+2 π electrons are aromatic. An alternative naming system for completely conjugated monocyclic hydrocarbons, (CH)n, is [N]annulene, in which N is the ring size. The system 1,3,5,7,9,11,13,15,17-cyclooctadecanonaene, or [18]annulene, contains 18 π electrons and is aromatic (4n+2, n=4). [18]annulene is fairly planar with little alternation of single and double bonds. It is relatively stable and undergoes electrophilic aromatic substitution. It can be described by a set of 2 resonance forms
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8 Hückel’s rule (2n+2) in cyclical conjugated polyenes can be explained by the following: The p orbitals mix to give an equal number of π molecular orbitals. All levels are composed of degenerate pairs of orbitals except for the lowest bonding and highest antibonding orbitals. A closed shell system (aromatic) is possible only if all of the bonding orbitals are fully occupied, 4n+2 π electrons. In the case of 4n π electrons there will always be a pair of singly occupied orbitals, an unfavorable electronic arrangement. 15-7 Hückel’s Rule and Charged Molecules The cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic. 1,3-Cyclopentadiene is unusually acidic because the anion resulting from deprotonation contains a delocalized, aromatic system of six π electrons: The cyclopentadienyl cation (four π electrons) can only be produced at low temperatures and is highly reactive. If 1,3,5-cycloheptatriene is treated with bromine, a stable salt is formed containing the cycloheptatrienyl cation, an aromatic, 6 π electron system. The cycloheptatrienyl cation is remarkably unreactive for a carbocation, as expected for an aromatic system. The cycloheptatrienyl anion, on the other hand, is antiaromatic (8 π electrons) and has a much lower acidity (pKa = 39) than cyclopentadiene (pKa ~ 16). Non-aromatic cyclic polyenes can form aromatic dianions and dications. Cyclic 4n π systems can be converted into their aromatic counterparts by two-electron oxidations and reductions. Cyclooctatetraene is reduced by alkali metals to a planar, aromatic dianion: The dianion exhibits an aromatic ring current in 1H NMR. [16]Annulene can be either reduced to its dianion or oxidized to its dication; both products are aromatic. On formation of the dication, the configuration of the molecule changes. Synthesis of Benzene Derivatives: Electrophilic Aromatic Substitution 15-8 Benzene undergoes substitution reactions with electrophiles. Electrophiles attack benzene by substituting for a hydrogen atom, not addition to the ring. Under the conditions of this type of reaction, ordinary non-aromatic conjugated polyenes would polymerize rapidly
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9 Electrophilic aromatic substitution in benzene proceeds by addition of the electrophile followed by proton loss. Electrophylic aromatic substitution is a two-step process. The cationic intermediate is resonance-stabilized: The initial attack of the electrophile is endothermic because the sp3 carbon generated interrupts the cyclic conjugation. The transition state is not aromatic. The loss of the proton regenerates the sp2 carbon atom and aromaticity is restored. This process is more favored than the nucleophilic trapping by the anion accompanying E+. The overall reaction is exothermic because the bonds formed are stronger than the bonds broken. Halogenation of Benzene: the Need for a Catalyst 15-9 Benzene in normally unreactive to halogens because they are not electronegative enough to disrupt its aromaticity. Halogens can be activated by Lewis acid catalysts, however, such as ferric halides (FeX3) or aluminum halides (AlX3), to become much more powerful electrophiles. This activated bromine complex can attack the benzene molecule, allowing the other bromine atom to depart with the good leaving group FeBr4 - The FeBr4 - next abstracts a proton from the cyclohexadienyl cation intermediate, and in the process regenerates the original FeBr3 catalyst. Bond energy calculations show that the electrophilic bromination of benzene is exothermic: Phenyl-H +112 kcal mol-1 Br-Br +46 kcal mol-1 Phenyl-Br -81 kcal mol-1 H-Br -87.5 kcal mol-1 --------------------------------------------------------- Reaction -10.5 kcal mol-1 Fluorination of benzene is very exothermic (explosive). Chlorination and bromination require an activating catalyst. Iodination is endothermic and does not occur. 15-10 Nitration and Sulfonation of Benzene Benzene is subject to electrophilic attack by the nitronium ion. Benzene can be attacked by concentrated nitric acid in the presence of concentrated sulfuric acid
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10 The purpose of the sulfuric acid is to generate the more electrophilic nitronium ion: The nitronium ion subsequently attacks the benzene molecule: Aromatic nitration is the best way to introduce nitrogencontaining substituents into the benzene ring. The aromatic nitro group serves as a directing group in further substitutions and as a masked amino function. Sulfonation is reversible. Fuming sulfuric acid (8% SO3 in concentrated H2SO4) reacts with benzene to form benzenesulfonic acid. Because the reaction of SO3 with water is so exothermic, the sulfonation of benzene can be reversed by heating benzenesulfonic acid in dilute aqueous acid. Because sulfonation is reversible, it can be used as a blocking group to control further aromatic substitution and then later removed. Benzenesulfonic acids have important uses. Long chain-branched alkylbenzenes can be sulfonated to the corresponding sulfonic acids. After conversion to their sodium salts, these compounds can be used as synthetic detergents. Sulfonate detergents have now been replaced by more biologically friendly, biodegradable detergents. Sulfonation is often used to impart water solubility to organic compounds, as in the manufacture of certain dyes. Sulfonyl chlorides can be prepared by reaction of the sodium salt of the acid with PCl5 or SOCl2. Sulfonyl chlorides are frequently used in synthesis, for example to convert the hydroxy group of an alcohol into a good leaving group
