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A Conceptually Simple Approach to the Analysis of Aromaticity in Pericyclic Transition States

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Richard Francis Langler

Department of Chemistry, Mount Allison University, Sackwille, New Brunswick, Canada EOA 3CO

J. Chem. Educ., 1996, 73 (9), p 899

DOI: 10.1021/ed073p899

Publication Date (Web): September 1, 1996

Abstract

One approach (Zimmerman-Dewar) to anticipating outcomes for concerted reactions examines possible transition states for aromatic/antiaromatic character. Some transition states are isoconjugate with well-known annulenes e.g. cyclobutadiene or benzene and can be readily recognized as aromatic or antiaromatic. Other transition states are said to be analogous to aromatic or antiaromatic Möbius annulenes - which have each p-orbital in the "pi-system" tipped by an angle Î with respect to each of its neighboring p-orbitals. "Aromatic" Möbius structures constitute a significant pedagogical obstacle for students of this method.

Aromatic Möbius cyclobutadiene is shown to be an artifact of Hückel theory. It is then shown that transition states may be classified as aromatic / antiaromatic / non-aromatic by exploiting both the extent of orbital overlap which develops in competing transition states and Hückel's rule. Thus some transition states are analogous to familiar annulenes while others are analogous to acyclic polyenes. This new approach to aromatic character in transition states is successfully applied to a representative assortment of known concerted reactions. Möbius annulenes have no role in the new approach.

Keywords (Audience):

Upper-Division Undergraduate

Keywords (Domain):

Organic Chemistry

Keywords (Subject):

Molecular Modeling

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This article has been cited by 1 ACS Journal articles (1 most recent appear below).

A Century of Chemical Dynamics Traced through the Nobel Prizes. 1981: Fukui and Hoffmann
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Journal of Chemical Education2002 79 (6), 667
- A Century of Chemical Dynamics Traced through the Nobel Prizes. 1981: Fukui and Hoffmann
  J. Van Houten
  Journal of Chemical Education2002 79 (6), 667
  The 1981 Nobel Prize was awarded to Kenichi Fukui and Roald Hoffmann "for their theories, developed independently, concerning the course of chemical reactions." Those theories, which have come to be known as "frontier orbital theory" and the "Woodward-...
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