Chemistry Tutorial Aromaticity Based on a Chemistry 14C Honors Project Six carbons once formed in a ring with sp2 hybridization The strain was relieved and all six achieved electron delocalization The stability itself is dramatic said a puzzled o chemist fanatic All these factors at work just add a new perk And thus was proclaimed aromatic Contents Section 1 Vocabulary Section 2 Determining Aromaticity Section 3 Conjugation and Aromaticity Section 4 Resonance and Aromaticity Section 5 Frequently Asked Questions Section 6 Common Errors Section 7 Readings and Sources Section 1 Vocabulary a Define each term b Explain how it relates to aromaticity Aromaticity Conjugation Cyclic delocalization Resonance Pi bonds Partial Pi bonds Ring Strain Section 1 Vocabulary Solutions Aromaticity a Extra stability possessed by a molecule that meets specific criteria pi bonds all must lie within a cyclic structure loop of p orbitals p orbitals must be planar and overlap must follow H ckel s Rule b n a Conjugation a The special stability provided by three or more adjacent parallel overlapping p orbitals b Aromatic molecules by default have conjugation As it takes a minimum of three adjacent overlapping p orbitals for planarity aromaticity requires a minimum of this for conjugation Aromaticity is like conjugation but extra stable Cyclic delocalization a Electron delocalization or distribution of electron density occurs as a result of overlapping p orbitals in a planar cyclic structure b A closed loop of overlapping p orbitals must be present for aromaticity to occur therefore cyclic delocalization occurs in aromatic molecules and is a contributor to the molecule s extra stability Resonance a A situation in which a molecule can be represented by two or more valid Lewis structures b By looking at resonance structures such as a benzene ring we can determine where p orbitals and partial pi bonds occur Partial pi bonds and planar p orbitals can contribute to aromaticity so resonance also contributes to the stability of aromatic molecules Pi bonds a Pi bonds are formed by the overlap of p orbitals between two adjacent atoms b In order for a molecule to have aromaticity it must first have pi bonds so that overlapping p orbitals and electron delocalization are present Partial Pi bonds a Partial pi bonds are formed as a result of close pi bonds lying in the same plane b Through resonance aromatic molecules achieve conjugation and a ring of overlapping p orbitals through partial pi bonds within a structure Ring Strain a Ring strain occurs as a combination of torsional and angle strain and from deviation from the ideal or preferred bond angles b Because aromatic molecules include a closed ring of p orbitals ring strain occurs however the benefits of aromaticity generally outweigh the ring strain in terms of molecular stability Check the Illustrated Glossary of Organic Chemistry available at the course web site for more definitions Section 2 Determining Aromaticity Overview Aromaticity is special stability provided to a molecule upon possessing four specific qualities mentioned below To determine if a molecule is aromatic investigate its structure for the qualifiers If all are present within the molecule then it is aromatic Criteria for Aromaticity Pi bonds must lie within cyclic structure Each atom in the cycle must have p orbital forming p orbital loop All p orbitals in the loop must overlap planarity H ckel s Rule orbital arrangement must result in a lowering of energy 4n 2 pi electrons n is an integer 0 1 2 3 etc in the loop Example Problem Determine if the following molecule is aromatic Step 1 Do pi bonds lie within a cyclic structure Three pi bonds are present each lying within the cyclic structure of benzene Yes Step 2 Does each atom in the cycle have a p orbital forming a p orbital loop Each atom has a p orbital forming a loop Yes Step 3 Do all p orbitals overlap and lie within the same plane By rotating the molecule we see that all orbitals lie within the same plane Yes Step 4 Does the molecule follow H ckel s Rule 6 total pi electrons 4n 2 6 n 1 Yes Conclusion Benzene is an aromatic molecule Practice Problems Determine whether or not the following molecules are aromatic Practice Problems Solutions 1 Yes meets all criteria 2 No does not include loop of overlapping p orbitals 3 No does not contain loop of overlapping p orbitals 4 Yes meets all criteria 5 No does not follow H ckel s Rule 6 Yes meets all criteria Section 3 Conjugation and Aromaticity Overview Conjugation requires at least three overlapping p orbitals in the same plane so that electrons can be delocalized for better stability Aromaticity cannot exist without conjugation because aromatic molecules require planarity and overlapping p orbitals However conjugation can exist in a molecule without being aromatic Example Problem Determine if the molecule below has conjugation aromaticity both or neither Step 1 Does the molecule have conjugation Yes the atoms are all planar with more than three overlapping p orbitals Step 2 Is the molecule aromatic Pi bonds are present within a cyclic structure Each atom has a p orbital forming a loop P orbitals overlap and lie in the same plane Violates H ckel s Rule 4n 2 8 n 6 4 Not aromatic Conclusion The molecule has conjugation but is not aromatic Practice Problems Conjugation and Aromaticity Determine if the molecules below have conjugation aromaticity both or neither 1 2 4 5 3 N 6 H Practice Problems Solutions 1 Not conjugated the molecule has a tub shape and the C C are perpendicular Not aromatic violates H ckel s rule 2 Not conjugated does not contain at least three atoms with planar p orbitals Not aromatic does not contain closed loop of planar overlapping p orbitals 3 Conjugated Aromatic meets all criteria 4 Conjugated Not aromatic does not contain closed loop of planar overlapping p orbitals 5 Conjugated Aromatic meets all criteria 6 Conjugated Aromatic meets all criteria Section 4 Resonance and Aromaticity Overview Resonance exists as a result of electron delocalization in a molecule Different patterns emerge as a result of structure and atom arrangement within a molecule Resonance provides an extra stability due to electron delocalization and consequently aromatic rings have resonance structures due to cycling double bonds Aromatic molecules must have resonance however not all molecules with resonant structures are aromatic Example Problem Determine if the molecule below has resonance aromaticity