Spring 2009. Dr. Halligan CHM 236. Chapter 17. Benzene and Aromatic Compounds. 1. 1. 1. 1. Background. Benzene (C 6 H 6 ) is the simplest aromatic hydrocarbon (or arene). Benzene has four degrees of unsaturation ( U = 4 ).
Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
Benzene and Aromatic Compounds
The Kekulé structures satisfy the first two criteria but not the third, because having three alternating bonds means that benzene should have three short double bonds alternating with three longer single bonds.
Figure 17.4 Helicene and twistoflex—Two synthetic polycyclic aromatic hydrocarbons
Figure 17.5: Selected drugs that contain a benzene ring
Four structural criteria must be satisfied for a compound to be aromatic.
 A molecule must be cyclic.
To be aromatic, each p orbital must overlap with p orbitals on adjacent atoms.
 A molecule must be planar.
All adjacent p orbitals must be aligned so that the electron density can be delocalized.
Since cyclooctatetraene is non-planar, it is not aromatic, and it undergoes addition reactions just like those of other alkenes.
 A molecule must be completely conjugated.
Aromatic compounds must have a p orbital on every atom.
 A molecule must satisfy Hückel’s rule, and contain
a particular number of electrons.
Benzene is aromatic and especially stable because it contains 6 electrons. Cyclobutadiene is antiaromatic and especially unstable because it contains 4 electrons.
Note that Hückel’s rule refers to the number of electrons, not the number of atoms in a particular ring.
Considering aromaticity, a compound can be classified in one of three ways:
Both negatively and positively charged ions can be aromatic if they possess all the necessary elements.
We can draw five equivalent resonance structures for the cyclopentadienyl anion.
Combination of two p orbitals to form πand π* molecular orbitals
Consider benzene. Since each of the six carbon atoms in benzene has a p orbital, six atomic p orbitals combine to form six MOs.
The six molecular orbitals