Chapter 2, Klein: Resonance

What is Resonance?

Imaginary set of Lewis Structures used to represent a molecule or ion that cannot be accurately described by a single Lewis diagram

Shows delocalization of electrons and provide more realistic depictions of electron distribution, contributing to overall stability of molecule.

Why does delocalization of electrons ensure stability

*Electrons repel each other, so delocalizing them over a greater number of bonds will reduce the repulsive forces between them to result in a lower energy state

To know how/when molecules will interact with each other, we must know…

regions of high and low electron density

Curved Arrows CURRENTLY represent…

Not electron movement, but rather, that electrons were in one place in one drawing, and in another in another drawing

Explain the curved arrows in resonance…

They consist of two parts, the tail and the head

The head shows where the electrons are going, and the tail shows where the electrons came from

There are 3 options regarding atomic orbitals… what are they?

Since electrons can hold a max of two orbitals, there are only three options:

0 electrons, uninteresting

1 electron in the orbital, so it is able to overlap with another orbital with only 1 electron to form a bond

2 electrons in the orbital, so it is filled and referred to as a lone pair

What are the 3 combinations that the curved arrow can move?

From lone pair to bond, from double or triple bond to lone pair, from double or triple bond to single or double bond

When can we push electrons and where? What are the first 2 commandments?

Two main rules to follow:

1. Never start a curved arrow from a single bond, as resonance structures must have same atoms connected to same atoms

2. For second row elements, never exceed the octet rule, even though you can have less. There should never be more than 8 electrons around a second row element nor more than 4 orbitals at any given time, as they only have 4.

What should you look for when drawing good arrows?

Potential disappearable lone pairs or bonds

In resonance structures, what typically is and is not there?

Lone pairs are typically not shown, only formal charges

How should you approach drawing resonance structures?

Locate where resonance is prominent: find places with pi bonds or lone pairs

Find ways to push the electrons without violating the 2 commandments: exceeding octet or moving from a sigma bond

3 ways we can convert arrows is from pi bond to pi bond or sigma bond, lone pair to single or double bond, or pi bond to lone pair.

One thing to note, you never have to worry about lone pair to lone pair because electrons cannot jump from one atom to another

Another thing to note, never assume that a first arrow will always violate the 2 commandments. Not true, as there might be another electron transfer (example: if there is a positive and negative formal charge but it is a few atoms away, you may have to keep moving the arrows to neutralize it)

What are the 5 main patterns/guidelines to follow when making resonance structures?

Lone pair next to a pi bond

Lone pair next to a C+

pi bond next to a C+

pi bond between 2 atoms, where one of the atoms is electronegative

pi bonds going all the way around the ring

Rule #1 is, a lone pair next to a pi bond. What does this look like?

Means the lone pair and the pi bond are separated by exactly one bond, often referred to as being “next to each other”

Rule #1, is a lone pair next to a pi bond. What can you do with this in terms of resonance?

You can bring down the lone pair to form a pi bond, or vice versa, turning the pi bond into a lone pair.

When a molecule has a lone pair and a negative charge, it will turn the pi bond into a lone pair, and the lone pair into a pi bond, transferring the charge to the side with the lone pair

Do the rule of pushing two arrows. One from the lone pair to the pi bond, and one from the pi bond to form the lone pair.

Often, just move the arrows to most electronegative atom if not sure.

Rule #2, is a lone pair next to a C+. What does this look like?

A lone pair is one sigma bond away from the C+ Carbon, which has potential to turn into a pi bond and make it so that the Carbon atom is neutralized

Rule #2, is a lone pair next to a C+. What can you do with this in terms of resonance?

Often times, the lone pair from the one next to the C+ is converted into a bond between the two, causing the neutralization of the C+ atom. Just one arrow.

When an atom with a lone pair has a negative charge, it ends up being neutral. When an atom with a neutral charge has a lone pair, it ends up becoming a positive charge, as it loses an electron.

Often times, we can cancel out the + and - charges to become a double bond, but one situation where it is not possible is… and why?

Nitrogroup, which is O double bonded to N single bonded to O and vice versa. This is because if you completely neutralize this atom, it might LOOK good, but it violates the octet rule as now Nitrogen will have five bonds, when it only has four orbitals. While 5 valence electrons MIGHT BE accurate, the maximum number of bonds it can have is 5 so this is wrong.

Rule #3, is a pi bond next to C+. What does it look like?

This is basically just a pi bond next to a single bond/sigma bond that is bonded to a C+.

Rule #3, is a lone pair next to a C+. What can you do with this in terms of resonance?

This is an easier solution, as we just move the pi bond one over so that it can neutralize the C+ charge, but consequently, this leads to a C+ charge on the other end.

Conjugation

This means that double bonds are only separated by a single bond between each, so it is double bond every other, and a single bond every other.

It is common to find these next to a positive charge, and once you push over the bonds one at a time, eventually the positive charge will transfer over to the opposite side.

Rule #4 is a pi bond between 2 atoms, where one of them is electronegative (NOF). What does it look like?

This looks like an electronegative atom pi bonded (triple or double bonded) to a carbon.

Rule #4 is a pi bond between 2 atoms, where one of them is electronegative (NOF). What can you do with this in terms of resonance?

You should move the pi bond up to the electronegative atom to become a lone pair, as it is relatively stable being negatively charged. A neutral molecule will diverge into a positive and negative charge.

Rule #5 is pi bonds going all the way around the ring. What does it look like?

Conjugated double bonds, where there are alternating double and single bonds going around the ring.

Rule #5 is pi bonds going all the way around the ring. What can you do with this in terms of resonance?

You can move the electrons around one in the same direction in a circle.

Explain Resonance Significance the concept

Not all resonance structures are equally significant, having varying levels of importance. However, even those that contribute only a bit are significant in adding something to predict/explain reactivity, etc.

Why is this significant?

Understanding the true nature of a compound

What are the four rules, listed in order of importance, regarding determining resonance significance?

The most important resonance structures contribute the greatest number of filled octets (meaning no formal charges) or covalent bonds

Fewer the formal charges a molecule has, the more significant it is

If other things are equal, the molecule that has the negative charge on the most electronegative atom is the most significant

Resonance structures that have equally good Lewis Diagrams are equivalent and contribute equally to the hybrid

What are some cases that are insignificant, and you should never draw?

Regarding Rule #1, Oxygen lacking an octet / having a positive charge

Regarding Rule #2, when the structure already has a charge (not neutral) but performing resonance causes there to be more formal charges than the already non-neutral charge