Aromatic Compounds

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 C 6 H 6. Other related compounds with low C:H ratios had a pleasant smell, so they were classified as aromatic.

Kekulé Structure Proposed in 1866 by Friedrich Kekulé, shortly after multiple bonds were suggested. Failed to explain existence of only one isomer of 1,2-dichlorobenzene.

Resonance Structure Each sp 2 hybridized C in the ring has an unhybridized p orbital perpendicular to the ring which overlaps around the ring. =>

Unusual Reactions Alkene + KMnO 4  diol (addition) Benzene + KMnO 4  no reaction. Alkene + Br 2 /CCl 4  dibromide (addition) Benzene + Br 2 /CCl 4  no reaction. With FeCl 3 catalyst, Br 2 reacts with benzene to form bromobenzene + HBr (substitution!). Double bonds remain.

Unusual Stability

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 Br 2 readily.

What Does It Take to Be Aromatic? Alternating double and single bonds A magic number of pi electrons Resonance structures must be able to move the pi electrons in a circular manar. Non bonding electrons can also participate in the resonance structures.

Hückel’s Rule Hückel’s Rule is used to generate the magic number of pi electrons. If the compound has a continuous ring of overlapping p orbitals and has 4N + 2 pi electrons, it is aromatic. If the compound has a continuous ring of overlapping p orbitals and has 4N electrons, it is antiaromatic. N is any integer, starting at zero

Generation of Magic Pi Electrons Value of N 4N

[N]Annulenes [4]Annulene is antiaromatic (4N e - ’s) [8]Annulene would be antiaromatic, but it’s not planar, so it’s nonaromatic. [10]Annulene is aromatic except for the isomers that are not planar. Larger 4N annulenes are not antiaromatic because they are flexible enough to become nonplanar.

Tropylium Ion The cycloheptatrienyl cation has 6 p electrons and an empty p orbital. Aromatic: more stable than open chain ion.

Which of the Following are Aromatic?

Disubstituted Benzenes The prefixes ortho-, meta-, and para- are commonly used for the 1,2-, 1,3-, and 1,4- positions, respectively. =>

3 or More Substituents Use the smallest possible numbers, but the carbon with a functional group is #1.

Common Names

Phenyl and Benzyl Phenyl indicates the benzene ring attachment. The benzyl group has an additional carbon.

Fused Ring Hydrocarbons Naphthalene Anthracene Phenanthrene

Polynuclear Aromatic Hydrocarbons Formed in combustion (tobacco smoke). Many are carcinogenic. Epoxides form, combine with DNA base. pyrene =>

Physical Properties Melting points: More symmetrical than corresponding alkane, pack better into crystals, so higher melting points. Boiling points: Dependent on dipole moment, so ortho > meta > para, for disubstituted benzenes. Density: More dense than nonaromatics, less dense than water. Solubility: Generally insoluble in water.

Chapter 1721 Nitration of Toluene Toluene reacts 25 times faster than benzene. The methyl group is an activating group. The product mix contains mostly ortho and para substituted molecules.

Chapter 1722 Catalytic Hydrogenation Elevated heat and pressure are required. Possible catalysts: Pt, Pd, Ni, Ru, Rh. Reduction cannot be stopped at an intermediate stage.