Why formal charges are important in for organic chemistry?
Knowing formal charges can help us to understand the reactivity patterns in the molecule. For instance, negatively charged species tend to provide that density in reactions, while the positively charged species—accept electrons from other molecules.
Here is a couple of examples:
How do we calculate formal charges?
The “official” way is to subtract 1/2 bonding electrons and nonbonding electrons from possible valence electrons that an atom can have. In other words:
What are all those terms?
Bonding electrons are those that make up bonds. Each bond contains 2 electrons. So, 1/2 of bonding electrons equals to the number of bonds and atom has.
Nonbonding electrons are those that are not participating in any bonding, as the name suggests. In other words, nonbonding electrons are the spare electrons (usually electron pairs) on the atom. Using the examples from above we have:
You can also use a simple shortcut to this method. The shortcut is:
Formal charge = Valence Electrons – Bonds – Dots
This way you can quickly find the formal charge of any atom in a molecule.
The Difference Between the Formal and Actual Charge
So, what’s the difference between the formal and the actual charge of an atom? Formal charge is more of a bookkeeping tool that is important to help us keep track of the electron flow in the reaction.
Actual charge, however, is the actual electron density that is present on the atom. For instance, let’s take a look at borohydride anion:
However, the electronegativity of boron is 2.0 while electronegativity of hydrogen is 2.2. This means that hydrogen actually “pulls” the electron density towards itself. Thus:
While formal charges are merely a “formality,” they are very important for the reactions mechanisms understanding. Thus you need to make sure you master the skill of quickly finding the formal charge.
Check out the practice quiz below with more examples to practice the skill of finding the formal charges.