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15. Benzene and Aromaticity Aromatic Compounds

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15. Benzene and Aromaticity Aromatic Compounds 15. Benzene and Aromaticity Aromatic Compounds Benzene is the parent of the family of aromatic compounds. Its six carbons lie in a plane in a hexagon, each carbon has one hydrogen attached. Benzene is a resonance hybrid of two Kekulé structures. Note: It’s okay to use circle representation if explicitly showing π electrons is not necessary, since circle doesn’t indicate number of π electrons in ring 2 Aromatic Compounds In orbital terms, each carbon is sp2-hybridized. These orbitals form σ bonds to 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 π bonds, which lie above and below the plane. The bond angles in benzene are 120. All CC bond distances are equal (1.39 Å). The compound is more stable than either of the contributing Kekulé structures and has a resonance or stabilization energy of about 36 kcal/mol. 3 Aromatic Compounds The origin of the term aromatic comes from the description of fragrant substances such as benzaldehyde present in cherries, peaches, and almonds. Benzaldehyde Today aromatic refers to benzene and its structural relatives. Many compounds synthetic and isolated from nature are aromatic in part, including estrone, morphine and Valium. 4 15.1 Sources of Aromatic Hydrocarbons Coal and petroleum are two main sources for simple aromatic compounds. Coal has an array of benzene like rings joined together and some compounds that coal tar yields are benzene, toluene, naphthalene, and xylene. Petroleum yields mainly alkanes and only a few aromatic compounds. 5 15.2 Naming Aromatic Compounds: Common names and structures to memorize include benzene toluene phenol aniline xylene (methylbenzene) (hydroxybenzene) (aminobenzene) (o,m,p) (pronounced feenol) benzyl phenyl benzoic acid benzaldehyde benzonitrile (pronounced fenyl) benzene sulfonic acid ortho meta para 15.2 Naming Aromatic Compounds Monosubstituted benzenes systematic names as hydrocarbons with –benzene C H Br 6 5 C6H5NO2 bromobenzene nitrobenzene 1-propylbenzene 7 The Phenyl and Benzyl Groups When a benzene ring is a substituent (when the alkyl group has more carbons), the term phenyl is used (for C6H5 ) You may also see “Ph” or “f” in place of “C6H5” ” “Benzyl” refers to “C6H5CH2 8 Disubstituted Benzenes Relative positions on a benzene ring ortho- (o) on adjacent carbons (1,2) meta- (m) separated by one carbon (1,3) para- (p) separated by two carbons (1,4) Describes reaction patterns (“occurs at the para position”) 9 Naming Benzenes With More Than Two Substituents Choose numbers to get lowest possible values List substituents alphabetically with hyphenated numbers 4-chloro-1,2-dimethylbenzene 1-bromo-3-ethyl-2-nitrobenzene Common names, such as “toluene” can serve as root name 2,4,6-trinitrotoluene 5-bromo-2-chlorophenol o-bromobenzoic acid TNT 10 Naming Benzenes A. Br B. NO2 C. F I Cl Br NO2 C2H5 Br H2N D. E. H F. Cl Cl C O A. B. C. D. E. F. 11 15.3 Structure and Stability of Benzene Benzene is very stable it undergoes a slow substitution reaction, where the ring itself does not react. The addition product is not formed Evidence of Stability Heat of hydration is 150 kJ/mol less than expected, so it is 150 kJ/mol more stable than expected. All carbon-carbon bonds (double and single) are the same length. Resonance 12 15.4 Molecular Orbital Description of Benzene The 6 p-orbitals combine to give Three bonding orbitals with 6 electrons, Two nonbonding and two antibonding orbitals Orbitals with the same energy are degenerate 13 Aromaticity and the 4n + 2 Rule Huckel’s rule, based on calculations – a planar cyclic molecule with alternating double and single bonds has aromatic stability if it has 4n+ 2 electrons (n is 0,1,2,3,4) Benzene Three double bonds Six π electrons For benzene = 6 4n+2 = 6, n = 1 and is aromatic 14 Compounds With 4n Electrons Are Not Aromatic (May be Anti-aromatic) Planar, cyclic molecules with 4 n electrons are much less stable than expected (anti-aromatic) They will distort out of plane and behave like ordinary alkenes 4- and 8-electron compounds are not delocalized (single and double bonds) cyclobutadiene Cyclobutadiene is so unstable that it dimerizes by a self-Diels-Alder reaction at low temperature Cyclooctatetraene has four double bonds, and will distort out of plane and behave like an ordinary alkene, it reacts with Br2, KMnO4, and HCl as if it were four alkenes cyclooctatetraene 15 15.6 Aromatic Ions The 4n + 2 rule applies to ions as well as neutral species Both the cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic The key feature of both is that they contain 6 electrons in a ring of continuous p orbitals 16 15.7 Aromatic Heterocycles: Pyridine and Pyrrole Heterocyclic compounds contain elements other than carbon in a ring, such as N,S,O,P Aromatic compounds can have elements other than carbon in the ring Pyridine: The nitrogen Pyrrole: Nitrogen atom is sp2- lone pair electrons are not hybridized, and lone pair of part of the aromatic system electrons occupies a p orbital (6 (perpendicular orbital) electrons) 17 15.8 Why 4n +2? When electrons fill the various molecular orbitals, it takes two electrons (one pair) to fill the lowest-lying orbital and four electrons (two pairs) to fill each of n succeeding energy level This is a total of 4n + 2 18 15.9 Polycyclic Aromatic Compounds: Naphthalene Aromatic compounds can have rings that share a set of carbon atoms (fused rings) Compounds from fused benzene or aromatic heterocycle rings are themselves aromatic, if the follow Huckel’s rule 19 Naphthalene Orbitals Three resonance forms and delocalized electrons 20 15.10 Spectroscopy of Aromatic Compounds IR: Aromatic ring C–H stretching at 3030 cm1 and peaks 1450 to 1600 cm1 UV: Peak near 205 nm and a less intense peak in 255-275 nm range 21 15.10 Spectroscopy of Aromatic Compounds 1H NMR: Aromatic H’s strongly deshielded by ring and absorb between 6.5 and 8.0 22 Ring Currents Ring Current: The circulation of electrons induced in aromatic rings by an external magnetic field. This effect accounts for the downfield shift of aromatic ring protons (6.5 to 8.0 ) compared to vinylic protons (4.5 to 6.5 ). The presence of a ring current is a characteristic of all Hückel aromatic molecules and is a good test for aromaticity. 23 13C NMR of Aromatic Compounds Carbons in aromatic ring absorb at 110 to 140 Shift is distinct from alkane carbons but in same range as alkene carbons 24 16. Chemistry of Benzene: Electrophilic Aromatic Substitution Substitution Reactions of Benzene and Its Derivatives Reactions of benzene lead to the retention of the aromatic core Electrophilic aromatic substitution replaces a hydrogen on benzene with another electrophile Typical reactions include chlorination, bromination, iodination, nitration, sulfonation, alkylation, and acylation ( the last two are Friedel- Crafts reactions) 26 Substitution Reactions of Benzene and Its Derivatives Electrophilic substitution reactions occur on aromatic compounds other than benzene and are a good test for aromaticity. 27 Substitution Reactions of Benzene and Its Derivatives 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. General Mechanism benzenonium ion 28 16.1 Bromination of Aromatic Rings Benzene’s electrons participate as a Lewis base in reactions with Lewis acids The product is formed by loss of a proton, which is replaced by bromine FeBr3 is added as a catalyst to polarize the bromine reagent 29 Mechanism: Formation of Product from Intermediate The addition of bromine occurs in two steps In the first step the electrons act as a nucleophile toward Br2 (in a complex with FeBr3) This forms a cationic addition intermediate from benzene and a bromine cation - The cationic addition intermediate transfers a proton to FeBr4 - (from Br and FeBr3) This restores aromaticity (in contrast with addition in alkenes) slow fast Aromatic rings are less reactive towards electrophiles than alkenes. Alkenes undergo addition reactions while aromatic compounds prefer to undergo substitution reactions. 30 16.2 Other Aromatic Substitutions The reaction with bromine involves a mechanism that is similar to many other reactions of benzene with electrophiles The cationic intermediate was first proposed by G. W. Wheland of the University of Chicago and is often called the Wheland intermediate George Willard Wheland 1907-1974 31 Aromatic Chlorination and Iodination Chlorination: Iodination: Iodine itself is unreactive towards an aromatic ring and requires an oxidizing agent (H2O2 or CuCl2). I2 is oxidized and it becomes a more powerful electrophile that reacts as I+. Fluorination: Poor yields of monofluoro-aromatic compounds are made since fluorine is so reactive. 32 Aromatic Nitration Nitration: + The combination of nitric acid and sulfuric acid produces NO2 (nitronium ion) 33 Aromatic Sulfonation Sulfonation: Reaction with a mixture of sulfuric acid and SO3 Reactive species is sulfur trioxide or its conjugate acid Reaction occurs via Wheland intermediate and is reversible 34 Alkali Fusion of Aromatic Sulfonic Acids Sulfonic acids are useful as intermediates Heating with NaOH at 300 ºC followed by neutralization with acid replaces the SO3H group with an OH Must be alkyl substituted (17.10) Example
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