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Chemistry 2131:
Organic Chemistry for the Life Sciences (3)

Introduction to Aromatics

• benzene was first isolated by Michael Faraday in 1825. He found it in the illuminating gas-lines of London
• its structure was established early on to be C₆H₆. This suggested a gigh degree of unsaturation and/or a cyclic nature. But there was a problem, when the presence of unsaturation was looked for, evidence of electrophilic addition reactions typical of alkenes and alkynes, was missing. Benzene is very resistant to these reactions.
• in 1872 a fellow named August Kekule got it right, with the ring of alternating "double" bonds.
• with a ring of alternating double and single bonds, all of the carbon atoms are sp² hybridized, all bond angles are exactly 120 degrees.
• the ring is planar
• to understand the chemistry of benzene and all of the other aromatics, you ust appreciate that each carbon of the ring has a p orbital perpendicular to the plane of the ring with a pair of electrons.
• instead of forming conventional pi bonds, this ring of p orbitals forms a cloud or ring of electron density above and below the plane of the ring. The electrons are equally shared by all of the carbon atoms.
• the carbon-carbon bonds are intermediate in length between single and double bonds (benzene bonds are 0.139 nm, single bonds are 0.147 nm and double bonds are 0.133 nm)
• to draw a proper structure of benzene with Lewis structures you must draw the two contributing resonance structure. It is important to remember that neither of these structures actually exists, the average of them is more true.
• you will sometimes see benzene depicted as a hexagon with a circle in the middle to depict the sharing of electrons.

2. Resonance Energy of Benzene:

• the fact that the p electrons of benzene are delocalized leads to a phenomenon known as the resonance energy.
• we'll approach this simplistically. If you compare the heat of formation of cyclohexane and cyclohexene, the difference is -28.6 kcal/mol (this means that the cyclohexane is that much more stable than cyclohexene). If we use this value to calculate the theoretical heat of formation for 1,3,5-cyclohexatriene, we get a value of -85.8 kcal/mol. The actual value for benzene is -49.8 kcal/mol.
• why is benzene so much more stable than predicted? This is attributed to the resonance stabilization, this value of 36 kcal/mol.