While the specific chemical and physical reasons as to what determines “aromaticity” or if a compound is aromatic is difficult to understand without having taken a year of college chemistry, the basic structure can be understood easily enough. Molecules must be cyclic, which is any polygon shape. There can't be a break in the shape, thus each side must touch two other sides. Next the compound must be completely conjugated which means that there is no more than one bond between each double bond. Bonds are represented by a line in a line-angle structure(see picture), and double bonds are represented by a double line. These compounds actually share those double bonds so each bond has a bond order of 1.5, thus a more accurate depiction would have a double bond on each carbon (corners of the compound), but for visual aid they are only on every other carbon. A classic example that depicts these explanations is the benzene ring (figure 1). Notice the double lines(double bonds) as well as how they are actually shared by all of the carbons(corners of the internal structure) illustrated by the bottom image.
(Figure 1- Benzene Resonance Structures)
Finally, they must have a multiple of 4n +2 pi electrons. Each double bond contains 2 pi bonded electrons so by counting the double bonds one can determine the amount of pi electrons. For example the two dimensional structure of a benzene ring has alternating double bonds thus it has six pi electrons(see figure 2), which is a multiple of 4(n) +2=6, where n=1. In this compound, each carbon is sharing the electrons that belong to the Hydrogen (H) and each Carbon to which it is connected. This type of electron "sharing" is called covalent bonding. Pi bonds are covalent chemical bonds where two lobes of one involved electron orbital overlap two lobes of the other involved electron orbital.
(Figure 2- Benzene Orbitals)
Resource:
1. Janice Smith,Organic chemistry. 3rd. New York City: McGraw-Hill Companies, 2008. pg 616-618
2. Benzene Resonance Structures. February 24, 2011. http://en.wikipedia.org/wiki/File:Benzene_resonance_structures.png
3. Benzene Orbitals. February 24, 2011. http://en.wikipedia.org/wiki/File:Benzene_Orbitals.svg
I noticed you only stated three of the four rules, a molecule must also be planar to be considered aromatic.
ReplyDeleteIt is difficult to explain every aspect of this concept to a person who has not had at minimum a year of college chemistry.
I am amazed at your ability to not only understand this, but also your choice of words. You are very intelligent, as shown by your writing capabilities.
I visited my grandmother about a week before this assignment, she asked me what my major was and I told her Chemistry. She then asked what Chemistry was, I said science grandma. =)