Sometimes The “Formal Charge” Does Not Accurately Represent The Electron Density Around An Atom!
Formal charges have their plusses and minuses. Har har.
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. If a molecule bears a charge, it makes things a lot easier (for nomenclature reasons) if we adopt some kind of system where a charge was unambiguously assigned to one atom.
Just like the rules in baseball sometimes assign a “Win” or “Loss” to a pitcher who didn’t contribute much to the team’s overall performance, formal charge 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.
Where Formal Charge Can Lead One Astray
For instance in the two examples below-left, the curved arrows, as drawn, would be showing the formation of an oxygen-oxygen bond. This doesn’t make sense given the weakness of the oxygen-oxygen bond (about 35 kcal/mol).
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.
Several Examples Of Species Where The Formal Charge Does Not Accurately Represent Electron-Density (And Therefore, Reactivity)
Here are some other common species where formal charge can be a misleading indicator of electron density.
AlH4 (–) and BH4 (–) are hydride donors (sources of H-). The nucleophilic atom is actually hydrogen, not Al or B.
NH4 (+) is a weak acid (source of H+). Bases react with NH4 at H, not N.
The species on the right is called an “iminium ion”. Nucleophiles react with the iminium ion at carbon, not at nitrogen.
Keep this in mind, and you’ll have a much easier time of properly understanding how reactions work.
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