Structure aromaticity and Huckels rule

organic chemistry

1. STRUCTURE AROMATICITY 1

2. CONTENTS Aromaticity Rules for aromaticity Huckels rule Anti aromaticity Example References 2

3. Aromaticity Aromaticity is a characteristic in which any planar system in which there is complete delocalization of pi electron in the ring, and the system contain 4n+2 pi electrons in the ring is called aromatic . EXAMPLES : 1.Benzene 2.Naphthalene 3.Anthracene 3

4. RULES FOR AROMATICITY First, it must be cyclic. Second, every atom in the ring must be conjugated. Third, the molecule must have [4n+2] pi electrons (Huckels rule) Fourth, the molecule must be flat/planar (2d structure). Benzene 4

5. HUCKELS RULE It was discovered by ERICH HUCKEL in 1931. It states that: “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 p orbitals .” 4n+2∏ electrons Where n is any positive integer. If n=0 than pie electrons are 2. If n=1 than pie electrons are 6. Similarly pie electrons would be 2,6,10,14……. 5

6. ANTI-AROMATICITY Anti aromaticity is a characteristic of a cyclic molecule with a π electron system that has higher energy due to the presence of 4n delocalized (π or lone pair) electrons in it. 4n∏ electrons 6

7. EXAMPLES Planar 4n+2 pie electrons i.e. 10 electrons Conjugated Ring 7

8. REFERENCES Dr. Dietmar Kennepohl FCIC (Professor of Chemistry, Athabasca University) Prof . Steven Farmer (Sonoma State University) William Reusch, Professor Emeritus (Michigan State U.), Virtual Textbook of Organic Chemistry bon and Geoff Hutchison from Chemistry StackExchange 8

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