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Organic Chemistry
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Why 4n+2 π Electrons?
According to Hückel's http://chemwiki.ucdavis.edu/skins/common/icons/icon-external.gif); background-attachment: initial; background-origin: initial; background-clip: initial; background-color: initial; padding-right: 13px; background-position: 100% 50%; background-repeat: no-repeat no-repeat; " href="http://en.wikipedia.org/wiki/Molecular_orbital_theory">Molecular Orbital Theory, a compound is particularly stable if all of its bonding molecular orbitals are filled with paired electrons. This is true of aromatic compounds, meaning they are quite stable. With aromatic compounds, 2 electrons fill the lowest energy molecular orbital, and 4 electrons fill each subsequent energy level (the number of subsequent energy levels is denoted by n), leaving all bonding orbitals filled and no anti-bonding orbitals occupied. This gives a total of 4n+2 π electrons. You can see how this works with the molecular orbital diagram for the aromatic compound, benzene, below. Benzene has 6 π electrons. Its first 2 π electrons fill the lowest energy orbital, and it has 4 π electrons remaining. These 4 fill in the orbitals of the succeeding energy level. Notice how all of its bonding orbitals are filled, but none of the anti-bonding orbitals have any electrons.
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How does the 4n+2 Rule Work?
To apply the 4n+2 rule, first count the number of π electrons in the molecule. Then, set this number equal to 4n+2 and solve for n. If n turns out to be 0 or any positive integer (1, 2, 3,...), the rule has been met. For example, benzene has six π electrons:
4n + 2 = 6
4n = 4
n = 1
For benzene, we find that n=1, which is a positive integer, so the rule is met.
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if a compound has 2,6,10,14,.... 4n+2 pie electrons the molecule is aromatic.
If compound has 0,4,8,12.. 4n pie electrons the molecule is anti aromatic..
if anything else the compound is non aromatic.
General stability of organic compounds : strain relief > aromatic> mesomeric > hyperconjugation > inductive effect.
aromatic > non aromatic > anti aromatic,
for calculating pie electrons:
Every double bond has 2 pie electrons.
benzene has 3 double bonds so 6 pie electrons. hence benzene is aromatic.
1,3 cyclobutadiene has 2 double bonds so 4 pie electrons hence it is antiaromatic.