Common Mistakes: Formal Charges Can Mislead

by James

in Organic Chemistry 1, Understanding Electron Flow, Where Electrons Are

Formal charges have their plusses and minuses. Har har.

One one hand, they’re an indispensable accounting tool. If a molecule bears a charge, it would drive us nuts (for nomenclature reasons) if we didn’t adopt some kind of system where a charge was unambiguously assigned to one atom.

In many instances, the formal charge on an atom is an “honest” expression of its electron density. We’re all familiar with the ions Cl(-), HO(-), CH3O(-),Br(-), Li(+) and so on. The formal charge assigned to these atoms truly reflects that these molecules bear additional positive or negative charge.

However! then there are the outlier cases. And these cause problems. From someone who preaches “opposite charges attract, like charges repel“, it’s important to know when to pay attention to formal charge, and when to ignore it.

“Formal” charge is called “formal” because it’s ultimately an accounting issue. It doesn’t take into account the true electron densities of a molecule, which are based on a combination of electronegativity and resonance.

When trying to understand a new reaction, apply electronegativity to understand electron densities , not formal charge.

For instance in the bottom two examples, the curved arrows, as drawn, would be showing the formation of an oxygen-oxygen bond. This doesn’t make sense.

When you apply electronegativities, however, you get a much better picture of the true electron density of a molecule. And this can help you figure out how a reaction might proceed.

Here are some other common molecules where formal charge can be a misleading indicator of electron density.

Keep this in mind, and you’ll have a much easier time of properly understanding how reactions work.

Next Post: Seven Factors that Stabilize Negative Charge in Organic Chemistry

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{ 4 comments… read them below or add one }

Patrick

I think you’re missing a proton on each side of your first example. (Should be H3O+, not H2O+!)

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james

Thanks for the spot. Fixed!!!

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mevans

Kind of an esoteric application, but some organic chemistry researchers have built entire research programs on the idea of misleading formal charges. See “Lewis Base Activation of Lewis Acids” here:

http://www.scs.illinois.edu/denmark/research/index.html

The idea is that for certain acid-base pairs, coordination increases the partial charge on the coordinating atoms, in contrast to the intuitive “acid + base = neutralization” idea. Frustrated Lewis acid/base pairs are kind of the same idea, taken to an extreme:

http://en.wikipedia.org/wiki/Frustrated_Lewis_pair

Great post! Using electronegativity to think about formal charge is GREAT advice.

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james

You’re right, he has based an entire research program out of that! Scott Denmark gives great talks. I love it when you can come away from a lecture not only having a good idea of what the researcher does, but also having learned something: he really dug into the physical organic chemistry aspect of it. I remember vividly him asking the audience (via a story about Albert Eschenmoser) “where are the electrons on the borohydride ion”, and when you think about it, the boron actually has a partial positive charge. Then he extended it to silicon. Great stuff.

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