Aromatic hydrocarbons

Aromatic hydrocarbons Originally called aromatic due to fragrant odors, today this seems strange as many posses distinctly non-fragrant smells! Their properties differ markedly from those of aliphatic hydrocarbons. Aromatic hydrocarbons undergo ionic substitution whereas aliphatic hydrocarbons undergo ionic addition to double and triple bonds and free radical substitution.

Nomenclature chlorobenzene nitrobenzene

Nomenclature toluene aniline phenol benzoic acid benzenesulfonic acid anisole

Nomenclature o-dibromobenzene 1,2-dibromobenzene m-chloronitrobenzene p-dinitrobenzene 1,4-dinitrobenzene

Nomenclature p-iodoaniline 4-iodoaniline 1,3,5-trichlorobenzene 2,4,6-trinitrotoluene

A few aromatic compounds methyl anthanylate - grape taste and odor oil of wintergreen safrole - root beer smell mescaline - euphoric

Benzene The molecular formula of benzene is C6H6. How are the atoms arranged? In 1865 Kekulé proposed that benzene has a “cyclohexatriene” structure:-

Benzene However there are other structures having this molecular formula:- Evidence points to the “cyclohexatriene” structure.

Benzene 1. There is only one monosubstituted benzene of formula C6H5Y - all benzene hydrogens must therefore be equivalent. 2. There are three disubstituted isomers:-

Benzene However...... double bond single bond What is the structure of benzene? What do we learn in the lab?

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

Heats of hydrogenation The heats of hydrogenation and combustion are lower than predicted for a cyclohexatriene structure.

Heats of hydrogenation

Heats of hydrogenation

Heats of hydrogenation

Heats of hydrogenation

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.”

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.”

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.

Orbital description of benzene A planar structure

Orbital description of benzene

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.

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, 6, 10 etc.

Cyclopentadiene cyclopentadienyl cation cyclopentadienyl anion cyclopentadienyl radical p electrons: 4 5 6 antiaromatic aromatic antiaromaticity: R. Breslow, D.R. Murayama, S. Murahashi, and R. Grubbs, J. Amer. Chem. Soc., 95, 6688 (1973).

Dicyclopentadienyliron Ferrocene

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.

Aromatic character?

Heme Heme is the prosthetic group (non-peptide portion) of hemoglobin.

Aromatic compounds in biochemistry Three amino acids necessary for protein synthesis contain a benzene ring:

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.

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.

IR spectroscopy Frequency Vibration (cm-1) benzene ring 3000 - 3100 C-H stretching 1500 and 1600 C=C stretching 690 - 710, 730 - 770 C-H bending, mono 735 - 770 C-H bending, ortho 690 - 710, 750-810 C-H bending, meta 810 - 840 C-H bending, para

Problems Try problems 14.18 – 14.23 and 14.28 – 14.30 in Solomons.