Loading [MathJax]/jax/output/HTML-CSS/jax.js
Skip to main content
Library homepage
 

Text Color

Text Size

 

Margin Size

 

Font Type

Enable Dyslexic Font
Chemistry LibreTexts

14.2: Frost's Circle

( \newcommand{\kernel}{\mathrm{null}\,}\)

Huckel's rule is easy to remember, but what is the reason behind the aromaticity phenomenon? For that, we need to look at the molecular orbital description. Let’s take a look at benzene first.

Frost's Circle

The best way to visualize the molecular orbitals for anything aromatic (or antiaromatic) is to inscribe a polygon inside a circle (Frost’s circle). Draw a circle and then inscribe the polygon so that one point of the polygon touches the bottom of the circle. Then, fill n the rest of the polygon. Each point that touches the circle corresponds to a molecular orbital. The midpoint of the circle is the nonbonding orbital energy level.

Screen Shot 2022-12-29 at 10.21.05 AM.png

Notice that π2 and π3 are degenerate, as are π4 and π5. The aromatic stabilization energy is 35.5 kcal/mol.

What do the orbitals look like? A bird’s eye view is shown below (with a view from the side provided for π1 only). Notice the increase in the number of nodes as you increase energy levels.

Screen Shot 2022-12-29 at 10.32.47 AM.png

So, with this method, can we see why molecules are antiaromatic? Let’s take cyclobutadiene.

Screen Shot 2022-12-29 at 10.32.53 AM.png

Two orbitals in cyclobutadiene are degenerate and lie at the nonbonding energy level, so each orbital gets one electron (hence, non-bonding). This suggests that cyclobutadiene should really be drawn like a diradical. Since paired electrons are lower in energy, this makes cyclobutadiene extremely reactive. In this case, it blows itself up (decomposes) by making two molecules of acetylene, or it dimerizes.

Screen Shot 2022-12-29 at 10.33.10 AM.png

When something is antiaromatic, it will do everything in its power to break conjugation, including becoming a diradical or puckering the ring so it is no longer planar. Try these other examples and see if you can show why they are aromatic or antiaromatic.

Screen Shot 2022-12-29 at 10.37.29 AM.png


14.2: Frost's Circle is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

  • Was this article helpful?

Support Center

How can we help?