This post discusses the following topics:
1. What is Formal Charge and how to calculate it
2. What are Nucleophiles and Electrophiles, how to determine which compounds fall under each category, and how to do NU/E reactions
3. What are Resonance Structures, rules to make them, and how to determine the best resonance structure.
Please see this video for examples relating to everything we discuss below!
Formal Charge
What is it? Formal charge is the actual charge on an atom.
Why do we need it? It can be used in helping to determine the most correct shape (eg. Lewis structure) of a compound.
How do we find it? It can be calculated per atom or per compound. The formal charge per atom is found by the equation below. To find the formal charge per compound, find the formal charge per each atom in the compound, then sum them all up for the entire compound.
# valence electrons of that element from the periodic table – # bonds the element is making to other elements – # lone electrons on the element
Nucleophile/Electrophile Reactions
Nucleophiles (NU) are commonly characterized as negatively charged, electron rich bases. Conversely, Electrophiles (E) are commonly characterized as positively charged, electron poor acids. In a NU/E reaction, the nucleophile always uses its electrons to attack the electrophile when the two are reacting with one another. There are three main spots on an E where a NU can attack, depending on what is given in the shape:
(a) The element with the positive charge
(b) The central atom
(c) An end hydrogen, in which case only the hydrogen is taken, and homolytic bond cleavage occurs between hydrogen and the rest of the compound
Resonance Structures
What are they? Resonance structures are chemical structures that have the same chemical bonds between atoms in the compound, but the electrons in the shape (either lone electrons that were sitting on an atom, or electrons that are in a double or triple bond) are moved around. These electrons are not associated with one specific bond or atom, and are called “delocalised.”
Why do we need them? A chemical compound may not look exactly like its Lewis Structure. In reality, the electrons resonate around the shape, but there is one most stable structure, otherwise known as the most “correct” resonance structure. They give an approximation of the true electronic structure of a compound.
How do we find a resonance structure of a compound?
Given a chemical compound:
(a) First identify where the delocalised electrons are.
These include the extra bonds in double or triple bonds that can be broken to move electrons around (can’t break single bonds though), and lone electron pairs sitting on top of an atom. We always move an electron pair together, usually not one electron on its own. We can move these electron pairs between bonds and atoms.
(b) Resonance structures also need to have the same overall formal charges of their shape.
Sum up the formal charge of each atom in both compounds. If they are the same overall formal charge, you have resonance structures between those two shapes.
(c) Also make sure Octet Rule is fulfilled as much as possible!
How do we pick the most correct/stable resonance structure?
One shape can have multiple resonance structures. To pick the most correct shape, use the following rules:
(a) Negative charges should try to be placed on the most electronegative atoms.
Positive charges on some of these atoms are okay as long as you fulfill the Octet Rule (always try to follow this rule, especially for F, O, N! Otherwise it is very unstable for these elements)
(b) Carbon should avoid having any charges on it.
However the more attachments (eg. primary, secondary, tertiary) a carbon has on it, the more stable it can be with a POSITIVE charge, otherwise known as a carbocation
(c) The less charge distribution (meaning the less charges you have on your atoms), the better.
You could have two resonance structures with overall charges of zero, with one shape showing no charges (neutral) and the other shape having three charges that add up to zero. So the one that looks neutral would be more stable.