Aromatics 260

Aromatic Chemistry Aromatic compounds can be generated from coal and petroleum processing. They include: benzene
toluene or
methylbenzene
naphthalene
phenyl group
They are a class of hydrocarbons that have special properties. Nomenclature: You can use numbering of substituents or for disubstituted benzenes, use ortho (1,2), meta (1,3), and para (1,4): ortho-xylene
or o-xylene or
1,2-diemthylbenzene
meta-xylene
or m-xylene
para-xylene
or p-xylene Benzene is very stable and has different reactivity to alkenes: HBr
Br2
H2/Pd/C
No Reaction
Hydrogenation requires a catalyst and releases far less energy than one would expect from three alkene π-­‐bonds: H2/Pd/C
ΔG° = –29 kcal/mol
H2
ΔG° = –49 kcal/mol
special catalyst
high pressure
Note that –49 kcal is far less than 3×(–29). The differential is 36 kcal/mol: the π-­‐
bonds of benzene are 36 kcal/mol stronger (in total) than three normal alkene bonds. Why? The enhanced stability is due to aromaticity, which is an interaction of the π-­‐bonds as demonstrated by resonance structures: =
The bonds are all the same, neither single nor double, but 1.5 bonds. The six electrons are delocalized around the ring, shared equally among all six carbons. Huckel Rule: For an integer n, (4n+2) electrons in a planar cyclic arrangement of p-­‐
orbitals leads to aromatic stabilization. There must be no break in the sp2 carbons. Note the compound below, buckyball, is aromatic; it is a spherical version of graphite. Electrophilic Aromatic Substituition: To react with Br2, aromatic rings require a catalyst. Moreover, they do not undergo addition; instead they undergo substitution. Both differences from the reactivity of alkenes result from the stability of the aromatic ring. They require more energy to react and if they underwent addition, they would lose their aromaticity. For bromination of benzene, the catalyst is irontribromide, which acts as a Lewis Acid: it coordinates (bonds reversibly) to the Br2 and thus both weakens the Br—Br bond and polarizes it. The result is a more highly electrophilic form of Br2, electrophilic enough to react with the modestly nucleophilic benzene. Alkylation: Aromatic rings can be alkylated via Freidel Crafts Alkylation. This requires an alkyl chloride and the catalyst AlCl3. The AlCl3 acts as a Lewis Acid: it coordinates to the chloride, weakens the C—Cl bond and polarizes it. The mechanism can be written in the form of a carbocation if it is sufficiently stable (e.g., secondary). There are issues with overalkylation and potential carbocation rearrangements, but we will not be concerned about these. Acylation: Benzene rings can undergo Freidel Crafts Acylation, which entail use of AlCl3 and an acylchloride (acid chloride). The AlCl3 forms a full bond with the chloride as the ensuing acylium ion (acyl cation) is sufficiently stable to exist on its own as it is resonance stabilized. There are other common electrophilic aromatic substitution reactions (e.g., nitration), but we will not be concerned with them. Likewise, there are other issues such as substitution patterns and reactivity of substituted benzenes and other aromatic rings, but we will not be concerned with these issues either.