© E.V. Blackburn, 2011 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 chlorobenzenenitrobenzene

© E.V. Blackburn, 2011 Nomenclature toluene benzoic acid aniline phenol benzenesulfonic acid anisole

© E.V. Blackburn, 2011 Nomenclature o-dibromobenzene 1,2-dibromobenzene m-chloronitrobenzene 1-chloro-3-nitrobenzene p-dinitrobenzene 1,4-dinitrobenzene

© E.V. Blackburn, 2011 Nomenclature p-iodoaniline 4-iodoaniline 1,3,5-trichlorobenzene 2,4,6-trinitrotoluene

© E.V. Blackburn, 2011 oil of wintergreen methyl anthanylate - grape taste and odor safrole - root beer smellmescaline - euphoric A few aromatic compounds

© E.V. Blackburn, 2011 Benzene The molecular formula of benzene is C 6 H 6. 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 C 6 H 5 Y - all benzene hydrogens must therefore be equivalent. 2. There are three disubstituted isomers:-

© E.V. Blackburn, 2011 Benzene However single bond double bond What is the structure of benzene? What do we learn in the lab?

© E.V. Blackburn, 2011 Reactions of benzene Therefore benzene cannot be a simple triene as it does not react with bromine in carbon tetrachloride. The benzene ring is very stable - it undergoes substitution reactions rather than addition reactions. However:

© 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 Benzene is a planar, cyclic molecule containing six atoms of carbon. All carbon - carbon distances are 1.397Å and all angles are 120 o. 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.” The structure of benzene

© 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 A planar structure

© E.V. Blackburn, 2011 Orbital description of benzene

© E.V. Blackburn, 2011 Aromatic character Compounds do not readily undergo addition reactions. Compounds undergo electrophilic substitution reactions. Compounds whose molecules are cyclic and planar. Compounds whose molecular formulae indicate a high degree of unsaturation.

© 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 5 aromatic  electrons: 4 antiaromatic antiaromaticity: R. Breslow, D.R. Murayama, S. Murahashi, and R. Grubbs, J. Amer. Chem. Soc., 95, 6688 (1973). 6

© E.V. Blackburn, 2011 Dicyclopentadienyliron Ferrocene

© E.V. Blackburn, 2011 The tropylium cation Tropylium bromide, C 7 H 7 Br, mp > 200C. It is soluble in water but insoluble in non-polar solvents. It forms a precipitate of silver bromide on addition of AgNO 3.

© E.V. Blackburn, 2011 Aromatic character?

© E.V. Blackburn, 2011 Heme 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:

© 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. 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.