University:
Augusta University
Course:
CHEM 3412 | Organic Chemistry II
Academic year:

2024
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288

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1
Author:

kellimitchell27

Aromaticity Guide Sheet Definitions: Aromatic Compounds: Compounds that are unusually stable. Anti-Aromatic Compounds: Unstable, highly reactive compounds. Non-Aromatic Compounds: Do not have aromatic properties. More Resonance Structures = More Stable the Compound Examples of Aromatic Compounds: Benzene Toluene Ortho-Xylene Rules for Resonance Structures: All resonance structures must be correct Lewis structures. Find electron sources, which are either π electrons or lone pair electrons or both. Move your electron sources towards a bond, but the other end of the bond MUST be a sp² or sp hybridized atom. Example: How many π electrons? Answer: 2π electrons. The double bond does NOT affect the π electrons of the cyclic compound. In the example, you will remove that bond. Example (Bottom Left): The carbon CANNOT be part of the bond because it’s an sp³ hybridized carbon. IR Spectroscopy (Top Center): IR Spectroscopy ranges for aromatic bonds: 3100-3000 cm⁻¹ - C-H stretching. 1700-2000 cm⁻¹ - Combination bands. 1450-1600 cm⁻¹ - Stretching of carbon-carbon bonds as well as ring vibrations. Example: Aromatic or Anti-Aromatic? Answer: Anti-aromatic. No electron movement to an sp² carbon, not in resonance. Hybridization (Top Right): Key Information: Sp³: Tetrahedral. Sp²: Trigonal Planar. Sp: Linear / Bent. Aromaticity (Middle Right): Aromatic compounds: Cyclic structure. Conjugated (Sp²). Fully conjugated. Cyclic Structure: All contributing structures must have the same placement. Continuously conjugated, so π electrons can move in a complete circle. Delocalization: Must have delocalized π electrons. Delocalized π electrons help in stabilizing the compound. Aromaticity Rules: A cyclic structure with π electrons must follow Huckel’s rule to be aromatic. Anti-Aromaticity: Fails Huckel’s rule, but conjugated cyclic. Huckel’s Rule: Aromatic when: 4n+2 π electrons (Where n = 0,1,2,...). Aromatic or Anti-Aromatic? Answer: Aromatic (10π electrons). Follows Huckel’s rule. Conjugated π system. 4(2)+2 = 10 π electrons. Heteroatoms: (Lone atoms more than C-H). Follow aromaticity rules. Note: π electrons must move to a double bond. Rule: Each atom, count one pair π electrons. Example (Bottom Right): Aromatic, anti-aromatic, or non-aromatic? Answer: Aromatic. Only count 1 double bond passing through the nitrogen. Anti-aromatic: Follows Huckel's rule, does not have conjugated cyclic structure. Conjugated π System Example: 4n+2 = 10π electrons.

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