Chapter 21: Benzene and the Concept of Aromaticity
Table of Contents:
The Structure of Benzene
The Concept of Aromaticity
Nomenclature
Phenols
Reactions at a Benzylic Position
The Structure of Benzene
Benzene was first isolated by Michael Faraday in 1825. It is a a carbon ring with 3 double bonds. Benzene is usually stable. Originally benzene and its derivates were called aromatic because of their distance odors. However, now aromaticity refers to these molecules being unusually stable and would not undergo alkene reactions we learned about before.
The Concept of Aromaticity
How To Determine If A Molecule Is Aromatic?
Mayya's Trick
To be aromatic, molecule must
Be a ring
Be planar (usually can be assumed that it is)
Be conjugated (have one 2p orbital on each atom of the ring)
4.Huckel's Rule: Have 4n+2 pi electrons in the ring, where n=0,1,2,3,4,5...
PS: if the molecule has 4n pi electrons it is antiaromatic.
How to use Huckel's rule? Count the number of pi electrons and write out 4n+2 = number of electrons you counted.
Solve for n. If n=0,1,2,3,4...., Huckel's rule is satisfied
Frost Circle
Frost circle is a method to construct pi molecular orbital diagrams for planar and conjugated rings.
How to draw Frost circles?
Draw a circle
Inscribe the shape of the molecule into the circle making sure that one of the vertices hits the bottom of the circle.
Put a horizontal line through the center of the circle.
Put a dash in each place where the vertices hit the circle. Dashes represent molecular orbitals. Those below the horizontal lines are bonding. Orbital on the horizontal lines are nonbonding and orbitals above the horizontal line are antibonding.
Distribute the pi electrons from the bottom up.
Frost Circle For Cyclobutadiene
We can see that cyclobutadiene has two electrons in nonbonding orbitals which makes it unstable and not aromatic.
For a molecule to be aromatic, all of its electrons will be in the bonding orbitals.
HNMR of aromatic molecules
Hydrogens on the outside of an aromatic ring such as benzene have the HNMR shift around 7-8ppm.
Hydrogens inside aromatic rings will have the shift around -3ppm.
Nomenclature
For benzene rings that contains two groups we use: ORTHO (two groups are next to one another), META (two groups are one carbon away from one another), PARA (two groups are opposite one another).
Polysubstituted Benzenes
If there is a substituent that gives benzene a special name, then the molecule is named as a derivative of that name. If there is no special name, then substituents are numbered to get the smallest numbers.
Phenols
Kolbe Carboxylation: Synthesis of Salicylic Acid
Reactions at a Benzylic Position
Oxidation
Halogenation using NBS
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