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

Aromaticity

• so far we have approached the aromatics by looking at the structure of benzene, which can be thought of as the parent aromatic compound. We saw that benzene is a planar molecule where all of the carbon atoms are sp² hybridized. There is an evenly shared cloud of electrons above and below the plane of the ring.
• other molecules can be said to be aromatic as well. Like benzene they have a high degree of unsaturation but are resistant to characteristic alkene addition and oxidation reactions.
• in the 1930's the german chemist Erich Huckel worked out a number of criteria for aromatic rings (mainly the 5 and 6 membered rings). They are:
  1. the molecule must be planar
  2. the molecule must have one 2p orbital on each member of the ring
  3. the molecule must have 6 pi electrons (what is called an aromatic sextet) in the cyclic arrangement of 2p orbitals
• we'll look at a few heterocyclic aromatic compounds. Heterocyclics contain more than one kind of atom in a ring structure.
• pyridine is very similar to benzene, with one of the carbons replaced with N:. It is important to remember that the lone pair is not part of the aromatic sextet, but the p orbital on nitrogen is. The resonance energy is 32 kcal/mol.
• pyrimidine, which is like pyridine with a nitrogen at the 3 position.
• there are also 5-membered aromatic rinds. Furan has an oxygen atom (and two double bonds. For this molecule, one of the unshared electron pairs on oxygen contributes to the aromatic sextet and the other doesn't.
• other examples of 5-membered rings are thiophene (sulfur analog of furan), pyrrole (N:-H analog of furan) and imidazole (like pyrrole with an extra N: at position 3)

2. Nomenclature:

• aromatics can be named both systematically and with common names. In general common names prevail. Some frequently encountered examples are listed below.
• toluene is methylbenzene
• cumene is isopropylbenzene
• styrene is vinylbenzene, or ethenylbenzene
• phenol is hydroxybenzene
• aniline is aminobenzene
• benzoic acid is benzene with a carboxyl group on it.
• when benzene itself is a group on another larger molecule it is referred to as a phenyl group. When the group has a CH₂-then a benzene ring it is called a benzyl group
• that covers many of the monosubstituted benzenes, but there can be more than one group hanging off of the ring. One must tell where the groups are in relation to each other. For this there are two systems. The old system uses the prefixes ortho-, meta- and para- to indicate the spacing.
• 1,2 is ortho; 1,3 is meta; and 1,4 is para.
• this naming system is only very useful for 2 substituents, beyond that numbering is the only unambiguous method.