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Aromatic vs. Antiaromatic Systems

In simplest terms, both aromatic and antiaromatic molecules are fully-conjugated, cyclic polyenes.   Aromatic compounds exhibit a special stability not seen in acyclic polyenes, whereas antiaromatic compounds are especially unstable.   This module reviews the reasons accounting for the contrasting behavior of aromatic and antiaromatic systems.   Drill exercises are also provided to help students distinguish at a glance whether a given compound is aromatic, nonaromatic, or antiaromatic.

Related Topics

Strongly Related

Bonding / Bond Types
Resonance and Conjugated Systems
EAS:  Substituent Effects

Somewhat Related

Hybridization and
Molecular Geometry

NMR Spectroscopy
IR Spectroscopy
UV Spectroscopy

What are Carbocations?

Carbocations are species bearing a formal "+" charge on carbon. They have sp2 hybridization and trigonal planar geometry, with an empty p orbital on carbon, perpendicular to the plane containing the substituents (see diagrams shown to the right). Carbocations are "hypovalent" species which have only three shared pairs of electrons around the carbon, instead of the usual four. This incomplete octet around carbon makes carbocations very unstable and very reactive. Nevertheless, carbocations are known to be formed as intermediates in many types of organic reactions. The relatively high energy of a carbocation, however, means that it will usually be formed in the rate-determining step for a reaction, and so it is important to understand how the substituents -- shown as R1, R2, and R3 in the diagrams -- can act to stabilize a carbocation, thus making it easier to form from a neutral molecule in the first place.

A Point to Remember

There are basically two types of carbocations:

  1. those that are not stabilized by resonance effects, and

  2. those that are stabilized by resonance (either through lone pair electrons on adjacent atoms or through conjugated pi bonding electrons).

For more information on this topic, please consult the following:

  • Related Computer-based Materials
    • Related Chem TV Files
      • Vol. 1, Topic 13: The SN1 Reaction
      • Vol. 1, Topic 16: The E1 Reaction
    • Related Beaker Menu Functions
    • Related Internet Resources

  • Related reading in textbook (McMurry, Organic Chemistry, 4th ed.)
    • Pg. 156: Carbocations
    • Pgs. 211, 581-582: Alkyl Shifts and Carbocations
    • Pg. 581: Friedel-Crafts Reaction and Carbocations
    • Pgs. 207-210: Hammond Postulate and Carbocations
    • Pg. 204: Hybridization of Carbocations
    • Pgs. 211, 581-582: Hydride Shifts and Carbocations
    • Pgs. 205-206: Hyperconjugation in Carbocations
    • Pg. 205: Inductive Effects and Carbocations
    • Pgs. 204-206: Markovnikov's Rule and Carbocations
    • Pgs. 210-212, 581-582: Rearrangements of Carbocations
    • Pgs. 204-206: Stability of Carbocations
    • Pgs. 203-204: Structure of Carbocations
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Aromatic vs. Antiaromatic Systems

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Dr. Thomas H. Eberlein

Copyright © 1998 Thomas H. Eberlein




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