Resonance contributors curved arrows

Chemists do not like to use dotted lines when drawing structures because, unlike a solid line that represents two electrons, the dotted lines do not specify the number of electrons they represent. Therefore, chemists use structures with localized electrons (indicated by solid lines) to approximate the resonance hybrid that has delocalized electrons (indicated by dotted lines). These approximate stmctures are called resonance contributors. Curved arrows are used to show the movement of electrons in going from one resonance contributor to the next. 

Curved-arrow notation is also a very useful device with which to generate resonance structures. In this application it is truly a bookkeeping system. Since individual canonical forms do not exist but are only thought of as resonance contributors to the description of a real molecule, the use of curved-arrow notation to convert one canonical form to another is without physical significance. Nevertheless it provides a useful tool to keep track of electrons and bonds in canonical structures. For example, the structures of carboxylate resonance contributors can be interconverted as follows ... 

Draw the electron-dot formulas that show all important contributors to a resonance hybrid and show their electronic relationship using curved arrows. 

All of the bonds in the carbonate ion (COs ) are between C and 0. Write Lewis structures for the major resonance contributors, and use curved arrows to show their relationship. Apply the resonance concept to explain why all of the C—0 bond distances in carbonate are equal. 

In Section 1.8 we introduced curved arrows as a tool for systematically converting one resonance contributor to another. Their more common use is to track electron flow in chemical reactions. Organic chemistry involves a vast number of reactions—far too many to memorize By learning to track electron flow, you will be able to see trends in reactivity and to predict the outcome of almost any given reaction. 

Refer to the structure of imidazolium ion in the preceding equation and write a second resonance contributor that obeys the octet rule and has its positive charge on the other nitrogen. Use curved arrows to show how you reorganized the electrons. 

Draw electron-dot formulas for the two contributors to the resonance hybrid structure of the nitrite ion, NO2 A (Each oxygen is connected to the nitrogen.) What is the charge on each oxygen in each contributor and in the hybrid structure Show by curved arrows how the electron pairs can relocate to interconvert the two structures. 

The carbocation shown next has delocalized electrons. To draw its resonance contributor, move the tt electrons toward an sp carbon. The curved arrow shows you how to draw the second contributor. Remember the tail of the curved arrow shows where the electrons start from, and the head shows where the electrons end up. The resonance hybrid shows that the tt electrons are shared by three carbons, and the positive charge is shared by two carbons. 

We have seen that chemists use curved arrows to show how electrons move when reactants are converted into products (see the Tutorial on page 225). Chemists also use curved arrows when they draw resonance contributors. 

PROBLEM 9 Draw curved arrows to show how one resonance contributor leads to the next one. 

PROBLEM 4 Draw curved arrows to show the movement of the electrons as one resonance contributor is converted to the next. 

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