Aromatic Compounds 1 1 Discovery of Benzene Isolated in 1825 by Michael Faraday who .determined C:H ratio to be 1:1 Synthesized in 1834 by Eilhard Mitscherlich who determined molecular .formula to be C6H6
Other related compounds with low C:H ratios had a pleasant smell, so they . were classified as aromatic => 2 Kekul Structure Proposed in 1866 by Friedrich Kekul, shortly .after multiple bonds were suggested Failed to explain existence of only one isomer
.of 1,2-dichlorobenzene H H C C H C C C H
C => H H 3 Resonance Structure
Each sp2 hybridized C in the ring has an unhybridized p orbital perpendicular to .the ring which overlaps around the ring => 4 Unusual Reactions Alkene + KMnO4 diol (addition) .Benzene + KMnO4 no reaction Alkene + Br2/CCl4 dibromide (addition)
.Benzene + Br2/CCl4 no reaction With FeCl3 catalyst, Br2 reacts with benzene to form bromobenzene + HBr .(substitution!). Double bonds remain => 5 Unusual Stability Hydrogenation of just one double ! bond in benzene is endothermic
=> 6 Annulenes All cyclic conjugated hydrocarbons were .proposed to be aromatic However, cyclobutadiene is so reactive that it dimerizes before it can .be isolated
And cyclooctatetraene .adds Br2 readily Look at MOs to explain . =>aromaticity 7 MO Rules for Benzene Six overlapping p orbitals must form six .molecular orbitals .Three will be bonding, three antibonding
Lowest energy MO will have all bonding .interactions, no nodes As energy of MO increases, the number of . => nodes increases 8 MOs for Benzene => 9
Energy Diagram for Benzene .The six electrons fill three bonding pi orbitals All bonding orbitals are filled (closed shell), an . extremely stable arrangement => 10
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