Resonance structure recognizing patterns

There are five patterns that you should learn to recognize to become prohcient at drawing resonance structures. First we list them, and then we will go through each pattern in detail, with examples and exercises. Here they are ... 

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. 

As previously recognized (e.g., Table 4.52), this corresponds to the ideal pattern of three cu bonds, three lone pairs, and sd2 (90°) hybridization, locking in a rigid octahedral structure. On the basis of the formally duodectet-rule-consistent parent species FeH3, the overall co-bonding is compactly described by the two resonance structures... 

Now we are ready to go back to some problems. Let s try to draw resonance structures again. Only this time, let s try to focus on recognizing some patterns. Look at the examples below, and see if you can recognize any of the patterns we just discussed ... 

The ability to draw and manipulate resonance structures is an important skill that will be needed throughout your study of organic chemistry. With practice, you will begin to recognize certain common bonding patterns for which more than one Lewis structure can be drawn. For now, notice that two different re.sonance structures can be drawn in the following situations ... 

Notice that each pattern used in this example involves only one curved arrov/. If we had started by recognizing a lone pair next to a Tt bond (which utilizes two curved arrows), then we might have missed the middle resonance structure above ... 

This mesomerism (or resonance )59 between equivalent Kekule structures was recognized as the quintessential feature underlying the aromaticity of benzene, conferring highly distinctive symmetry, stability, and reactivity patterns. 

---