Resonance structures are multiple electronic representations of the same molecule. The only difference between resonance structures is the location of electrons. A molecule is a hybrid of all of its resonance structures. What this means is that a molecule does NOT switch from one resonance form to another. Rather, it is a combination of all of the resonance structures at the same time.

Many organic chemistry students find drawing resonance structures difficult because of the lack of clear guidelines. Resonance is definitely learned through practice. In this resonance guide, I will teach you the foundation skills needed to draw correct resonance structures along with major resonance patterns and practice.

Curved Arrows

When drawing curved arrows, we need to remember a very important RULE: second row elements cannot have more than 8 electrons (octet). Atoms, such as nitrogen, carbon and oxygen can not have more than an octet. How does this apply to curved arrows? If we draw an arrow towards an atom that already has 8 electrons and cannot lose any of its electrons, this arrow is WRONG. This is a very common mistake among organic chemistry students.

Example of wrong curved arrows:

All resonance structures must have the same total charge. For example, if one resonate structure has a +1 and -1 (1-1=0) and another resonance has no charges, both are valid since their total charge is the same. However, if one resonance has a charge of +1 and another is neutral, you did something wrong!

Double way arrows are used between resonance forms. These arrows are used exclusively for resonance.

Resonance Patterns

There are three major patterns in resonance that we can recognize and implement to make resonance easier. Please note that these are not the only patterns. These are encountered most often and will help you with drawing resonance correctly.

1. Double bond next to a positive charge.
2. Double bond next to a lone pair of electrons.
3. A lone pair next to a positive charge

DOUBLE BOND NEXT TO A POSITIVE CHARGE

This is one of the most common and easiest patterns. We only need to show one arrow coming from the double bond to create a new bond next to the carbocation.

Note: In organic chemistry, we usually use line-angle structure to draw molecules easily. If you are not yet familiar with line-angle structure, I strongly suggest drawing the molecules out fully including carbons, hydrogens and lone pairs.

PRACTICE : DRAW ALL RESONANCE STRUCTURES FOR THE FOLLOWING MOLECULES:

DOUBLE BOND NEXT TO A LONE PAIR OF ELECTRONS

We need to show two arrows. One moving from the lone pair to the neighboring bond and another moving from the double bond to the carbon to create a lone pair.

PRACTICE : DRAW ALL RESONANCE STRUCTURES FOR THE FOLLOWING MOLECULES:

Example 1

Example 2

A LONE PAIR NEXT TO A POSITIVE CHARGE

Draw one arrow from the lone pair to the neighboring bond with the positively charge atom.

Which resonance structures are most important?

Rule 1: The more bonds, the better the resonance

Rule 2: The less charge, the better resonance is.

Rule 3: The more electronegative an atom is, the easier it is for it to bear a negative charge, and not positive charge.

Practice: Which resonance is the major contributor?

How to identify INCORRECT resonance forms?

1. Check if there are any row two elements with more than octet.
2. Check if any of the atoms moved. In resonance structures, the only electrons can move. If you see an atom such as hydrogen move, that is not a correct resonance structure.
3. Check if the sum of all the formal charges is equal for all resonance forms.

Practice: Which of the following is not a correct resonance structure?

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Resonance Structures Practice Worksheet

LINKS

Organic Chemistry Course

Online Organic Chemistry Tutoring

References:

Organic Chemistry as a Second Language by David Klein