huckel rule of aromaticity

Hückel's Rule (4n+2 rule): In order to be aromatic, a molecule must have a certain number of pi electrons (electrons with pi bonds, or lone pairs within p orbitals) within a closed loop of parallel, adjacent porbitals. The pi electron count is defined by the series 4n+2 where n = zero or a positive integer (0, 1, 2, etc.). The most common case in six pi electrons (n = 1) which is found for example in benzene, pyrrole,furan, and pyridine.

For  monocyclic planar compounds in which each atom has a p orbital (as in benzene) Hückel showed that compounds with (4n + 2) p electrons, where n = 0, 1, 2, 3 etc, would have closed shells of delocalised p electrons and should show exceptional stability (high resonance energy º “aromatic”).

i.e. planar monocycles with 2, 6, 10, 14….delocalised p electrons should be “aromatic”.

i.e. p electrons are delocalised over the entire ring and the compound is thereby stabilised by the delocalisation.

Based on the properties of aromatic compounds, there are FOUR criteria about the π system that need to be met inorder for the "special" aromatic stabilisation to be observed:

1. Conjugated : there needs to one "p" orbital from each atom in the ring, so each atom must be either sp² or sp hybridised.
2. Cyclic : linear systems are not aromatic, all atoms in the ring must be involved in the π system (i.e. no sp³ atoms)
3. Planar : if the ring is planar flat then this means there is good overlap / interaction between the "p" orbitals....not always easy to consider.
4. The Huckel Rule..... 4n+2 π electrons in the cyclic conjugated π system (n = 0, 1, 2, 3 etc.) This is equivalent to an odd number of π-electrons pairs).

In order for a compound to be aromatic, all FOUR of these criteria must be met.

Aromatic Molecules

Cyclopropenyl cation Two pi electrons n = 0		Benzene Six pi electrons n = 1		Pyrrole Six pi electrons n = 1
Furan Six pi electrons n = 1		Pyridine Six pi electrons n = 1		Imidazole Six pi electrons n = 1
		Naphthalene Ten pi electrons n = 2 some  questions Which of the following is nonaromatic? A) B) C) D) Which of the following cations is aromatic? A) b, d B) d C) a, b, d D) b, c, d Which of the following molecules is aromatic according to Hückel's rule? A) B) C) D)