Curved Arrows The Tools for Drawing Resonance Structures

The image of a peach is not adequate to describe a nectarine. Neither is the image of a plum. But by imagining both together at the same time, you can get a sense of what a nectarine looks like. 

The problem with drawing molecules is similar to the problem above with the nectarine. No single drawing adequately describes the nature of the electron density spread out over the molecule. To solve this problem, we draw several drawings and then meld them together in our mind into one image. Just like the nectarine. 

The compound above has two important resonance structures. Notice that we separate resonance structures with a straight, two-headed arrow, and we place brackets around the structures. The arrow and brackets indicate that they are resonance structures of one molecule. The molecule is not flipping back and forth between the different resonance structures. The electrons in the molecule are not actually moving at all. 

In the beginning of the course, you might encounter problems like this here is a drawing now draw the other resonance structures. But later on in the course, it will be assumed and expected that you can draw all of the resonance structures of a compound. If you cannot actually do this, you will be in big trouble later on in the course. So how do you draw all of the resonance structures of a compound To do this, you need to learn the tools that help you curved arrows. 

Here is where it can be confusing as to what is exactly going on. These arrows do not represent an actual process (such as electrons moving). This is an important point, because you will learn later about curved arrows used in drawing reaction mechanisms. Those arrows look exactly the same, but they actually do refer to the flow of electron density. In contrast, curved arrows here are used only as tools to help 

2 CURVED ARROWS THE TOOLS FOR DRAWING RESONANCE STRUCTURES  

Every cnrved arrow has a head and a tail. It is essential that the head and tail of every arrow be drawn in precisely the proper place. The tail shows where the electrons are coming from, and the head shows where the electrons are going (remember that the electrons aren t really going anywhere, bnt we treat them as if they were so we can make sure to draw all resonance strnctnres)  

Therefore, there are only two things that yon have to get right when drawing an arrow the tail needs to be in the right place and the head needs to be in the right place. So we need to see rules about where you can and where you cannot draw arrows. But first we need to talk a little bit abont electrons, since the arrows are describing the electrons. 

---

Curved arrows used for drawing resonance structures do not represent the motion of electrons— they are simply tools that allow us to draw resonance structures with ease. These tools treat the electrons as i/ they were moving, even though the electrons are actually not moving at all. In Chapter 3, we will encounter curved arrows that actually do represent the flow of electrons. For now, keep in mind that all curved arrows in this chapter are just tools and do not represent a flow of electrons. 

Now we have all the tools we need. We know why we need resonance structures and what they represent. We know what curved arrows represent. We know how to recognize bad arrows that violate the two commandments. We know how to draw arrows that get you from one structure to another, and we know how to draw formal charges. We are now ready for the final challenge using curved arrows to draw resonance structures. 

---