Resonance structure drawing arrows

Consider the compounds and ions with curved arrows drawn below. When the curved arrows give a second valid resonance structure, draw the resonance structure. When the curved arrows generate an invalid Lewis structure, explain why the structure is unacceptable. 

Although these curved arrows look exactly like the curved arrows used for drawing resonance structures, there is an important difference. When drawing resonance structures, curved arrows... 

Practice Problem B Given the following two resonance structures, draw the curved arrows on the first structure that will give rise to the second structure. 

CURVED ARROWS THE TOOLS FOR DRAWING RESONANCE STRUCTURES ... 

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 leam the tools that help you curved arrows. 

Now we know what curved arrows are, but how do we know when to push them and where to push them First, we need to learn where we cannot push arrows. There are two important rules that you should never violate when pushing arrows. They are the two commandments of drawing resonance structures ... 

Now that we know how to identify good arrows and bad arrows, we need to get some practice drawing arrows. We know that the tail of an arrow must come either from a bond or a lone pair, and that the head of an arrow must go to form a bond or a lone pair. If we are given two resonance structures and are asked to show the arrow(s) that get us from one resonance structure to the other, it makes sense that we need to look for any bonds or lone pairs that are appearing or disappearing when going from one structure to another. For example, consider the following resonance structures ... 

In this example, we can see that one of the lone pairs on oxygen is coming down to form a bond, and the C=C double bond is being pushed to form a lone pair on a carbon atom. When both arrows are pushed at the same time, we are not violating either of the two commandments. So, let s focus on how to draw the resonance structure. Since we know what arrows mean, it is easy to follow the arrows. We just get rid of one lone pair on oxygen, place a double bond between carbon and oxygen, get rid of the carbon-carbon double bond, and place a lone pair on carbon ... 

EXERCISE 2.20 Draw the resonance structure that you get when you push the arrows shown below. Be sure to include formal charges. 

PROBLEMS For each of the structures below, draw the resonance structure that you get when you push the arrows shown. Be sure to include formal charges. (Hint In some cases the lone pairs are drawn and in other cases they are not drawn. Be sure to take them into account even if they are not drawn—you need to train yourself to see lone pairs when they are not drawn.)... 

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. 

Once you learn to recognize this pattern (a lone pair next to a pi bond), you will be able to save time in calculating formal charges and determining if the octet rule is being violated. You will be able to push the arrows and draw the new resonance structure without thinking about it. 

Sometimes when writing the Lewis structure of a species, we may draw more than one possible correct Lewis structure for a molecule. The nitrate ion, N03 , is a good example. The structures that we write for this polyatomic anion differ in which oxygen has a double bond to the nitrogen. None of these three truly represents the actual structure of the nitrate ion—it is an average of all three of these Lewis structures. We use resonance theory to describe this situation. Resonance occurs when more than one Lewis structure (without moving atoms) is possible for a molecule. The individual structures are called resonance structures (or forms) and are written with a two-headed arrow (<- ) between them. The three resonance forms of the nitrate ion are ... 

To communicate the bonding in SO2 more accurately, chemists draw two Lewis structures and insert a double-headed arrow between them. Each of these Lewis structures is called a resonance structure. Resonance structures are models that give the same relative position of atoms as in Lewis structures, but show different places for their bonding and lone pairs. 

These simple examples illustrate the basic rules for mechanism and the use of curly arrows. The concepts are no different from those we have elaborated for drawing resonance structures (see Section 2.10) ... 

Every curved 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 coining from, and the head shows where the electrons are going (remember that the electrons aren t really going anywhere, but we treat them as if they were so we can make sure to draw all resonance structures) ... 

EXERCISE 2.1 For the compound below, look at the arrow drawn on the structure and determine whether it violates either of the two commandments for drawing resonance structures ... 

Of course, we should push one arrow at a time so that we can draw all of the resonance structures. But it is nice to know how the formal charges will end up so that we don t have to calculate them every time we push an arrow. 

Draw the important resonance structures for aniline. Use the curved arrow convention to show how the electrons are moved to create each new resonance structure. Discuss the relative contribution of each to the resonance hybrid and the overall resonance stabilization of the compound. 

Delocalization or conjugation arrows showing two different ways to draw the same molecule. The two structures ( canonical forms or resonance structures ) must differ only in the position of electrons... 

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