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Aromaticity © E.V. Blackburn, 2011 Aromatic hydrocarbons Originally called aromatic due to fragrant odors, today this seems strange as many possess distinctly non-fragrant smells! Their properties differ markedly from those of aliphatic hydrocarbons. Aromatic hydrocarbons undergo ionic substitution whereas aliphatic hydrocarbons undergo free radical substitution coupled with ionic addition to double and triple bonds. © E.V. Blackburn, 2011 Nomenclature Cl chlorobenzene NO2 nitrobenzene © E.V. Blackburn, 2011 Nomenclature CH3 toluene CO2H benzoic acid NH2 aniline SO3H benzenesulfonic acid OH phenol OCH3 anisole © E.V. Blackburn, 2011 Nomenclature NO2 Br Br Cl o-dibromobenzene 1,2-dibromobenzene m-chloronitrobenzene 1-chloro-3-nitrobenzene NO2 O2N p-dinitrobenzene 1,4-dinitrobenzene © E.V. Blackburn, 2011 Nomenclature I Cl NH2 Cl Cl 1,3,5-trichlorobenzene p-iodoaniline 4-iodoaniline CH 3 O2 N NO2 NO2 2,4,6-trinitrotoluene © E.V. Blackburn, 2011 A few aromatic compounds CO2CH 3 OH CO2CH 3 NH 2 oil of wintergreen methyl anthanylate grape taste and odor O CH2CH2NH2 O CH2CH=CH2 safrole - root beer smell H3CO OCH3 OCH3 mescaline - euphoric © E.V. Blackburn, 2011 Benzene The molecular formula of benzene is C6H6. How are the atoms arranged? In 1865 Kekulé proposed that benzene has a “cyclohexatriene” structure:- © E.V. Blackburn, 2011 Benzene However there are other structures having this molecular formula:- Evidence points to the “cyclohexatriene” structure. © E.V. Blackburn, 2011 Benzene 1. There is only one monosubstituted benzene of formula C6H5Y - all benzene hydrogens must therefore be equivalent. 2. There are three disubstituted isomers:Br Br Br Br Br Br © E.V. Blackburn, 2011 Benzene However...... double single bond bond Br Br Br Br What is the structure of benzene? What do we learn in the lab? © E.V. Blackburn, 2011 Reactions of benzene The benzene ring is very stable - it undergoes substitution reactions rather than addition reactions. Br2/CCl4 X However: Br2/CCl4 Br Br Therefore benzene cannot be a simple triene as it does not react with bromine in carbon tetrachloride. © E.V. Blackburn, 2011 Heats of hydrogenation The heats of hydrogenation and combustion are lower than predicted for a cyclohexatriene structure. © E.V. Blackburn, 2011 Heats of hydrogenation © E.V. Blackburn, 2011 Heats of hydrogenation © E.V. Blackburn, 2011 Heats of hydrogenation © E.V. Blackburn, 2011 Heats of hydrogenation © E.V. Blackburn, 2011 Heats of hydrogenation The heats of hydrogenation and combustion are lower than predicted for a cyclohexatriene structure. The heat of hydrogenation of one mole of benzene is 152 kJ less than that of three moles of cyclohexene. Benzene is therefore 152 kJ more stable than expected for “cyclohexatriene.” © E.V. Blackburn, 2011 The structure of benzene Benzene is a planar, cyclic molecule containing six atoms of carbon. All carbon - carbon distances are 1.397Å and all angles are 120o. The Kekulé structure cannot explain the physical and chemical properties of benzene. Remember CHEM 261 and the concept of resonance...... “Whenever a molecule can be represented by 2 or more structures which differ only in the arrangement of their electrons, there is resonance.” © E.V. Blackburn, 2011 Resonance The structure of benzene is a resonance hybrid of the two Kekulé structures: The resonance hybrid is more stable than any one contributing canonical form (resonance-contributing structure). This energy, 150 kJ, is called the resonance energy. © E.V. Blackburn, 2011 Orbital description of benzene H H sp bond H o 2 120 H H H A planar structure © E.V. Blackburn, 2011 Orbital description of benzene © E.V. Blackburn, 2011 Aromatic character • Compounds whose molecular formulae indicate a high degree of unsaturation. • Compounds do not readily undergo addition reactions. • Compounds undergo electrophilic substitution reactions. • Compounds whose molecules are cyclic and planar. © E.V. Blackburn, 2011 Hückel’s Rule Hückel proposed the hypothesis that aromatic compounds possess molecules containing cyclic clouds of electrons delocalised above and below the plane of the molecule and that the electron clouds must contain a total of (4n+2) electrons. Therefore, in order to possess aromatic character, the number of electrons must be 2 or 6 or 10 etc. © E.V. Blackburn, 2011 Cyclopentadiene + - cyclopentadienyl cation . cyclopentadienyl anion cyclopentadienyl radical electrons: 4 5 6 antiaromatic aromatic antiaromaticity: R. Breslow, D.R. Murayama, S. Murahashi, and R. Grubbs, J. Amer. Chem. Soc., 95, 6688 (1973). © E.V. Blackburn, 2011 Dicyclopentadienyliron Ferrocene Fe © E.V. Blackburn, 2011 The tropylium cation • Tropylium bromide, C7H7Br, mp > 200C. • It is soluble in water but insoluble in non-polar solvents. • It forms a precipitate of silver bromide on addition of AgNO3. © E.V. Blackburn, 2011 Aromatic character? + + + + H H N H © E.V. Blackburn, 2011 Heme H2C=HC CH3 H3C CH=CH2 N N Fe N H3C HO2CH2CH2C N CH3 CH2CH2CO2H Heme is the prosthetic group (non-peptide portion) of hemoglobin. © E.V. Blackburn, 2011 Aromatic compounds in biochemistry Three amino acids necessary for protein synthesis contain a benzene ring: HO CO2- CO2- H NH3+ phenylalanine H NH3+ tyrosine H N CO2H + NH3 tryptophan © E.V. Blackburn, 2011 Aromatic compounds in biochemistry Humans do not have the biochemical ability to synthesize the benzene ring. Thus phenylalanine and tryptophan derivatives are essential in our diet. CO2H NH3+ H N CO2H NH3+ Tyrosine can be synthesized from phenylalanine in a reaction catalyzed by phenylalanine hydroxylase. © E.V. Blackburn, 2011 Aromatic compounds in biochemistry Heterocyclic aromatics are present in many biochemical systems. Thus derivatives of purine and pyrimidine are essential parts of DNA and RNA. N N N H purine N N N pyrimidine © E.V. Blackburn, 2011
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