Lecture notes Aromaticity Rules PDF

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CHEM 273

10/1/18

Huckel’s Rule: a compound can be 1. Aromatic: cyclic, planar, completely conjugated, 4n+2 electrons 2. Anti-aromatic cyclic, planar, completely conjugated, 4n electrons 3. Non-aromatic: lacks one or more of the requirements for aromaticity +

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aromatic

anti-aromatic

non-aromatic

non-aromatic

Examples of Aromatic Rings: -completely conjugated rings larger than benzene with 4n+2 electrons -annulenes: hydrocarbons containing a single ring with alternating double/single bonds -name an annulene: number of atoms in the ring in brackets and add the word annulene (ex. [14]-annulene) -six-membered rings with alternating double/single bonds can be put together to form polycyclic aromatic hydrocarbons (PAHs) -as # of rings increases, # of resonance structures increases Examples of Non-Aromatic: not planar -[10]-Annulene -cyclooctatetraene

Types of orbitals: -bonding: p-orbitals of similar phase overlap side-by-side -lower in energy -antibonding: p-orbitals of opposite phase overlap side-by-side -higher in energy To Draw Molecular Orbitals (MOs): Inscribed Polygon Method -draw structure so a point is facing down -draw a circle intersecting all atoms in ring -each intersection = MO -split it in half horizontally -top = antibonding -bottom = bonding -add in electrons starting with filling the bonding MOs -aromatic compounds are very stable, so when treated with base they are much more acidic...