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Introduction

Chapter 7 is all about delocalized electrons. Delocalized electrons are electrons that are shared by more than two atoms.

The chapter starts by looking at the structure of benzene, a compound whose structure puzzled early chemists because they were not aware that electrons could be shared by more than two atoms (delocalized). Then you are shown how to recognize when a compound has delocalized electrons, and you are taught how to draw the structures of compounds with delocalized electrons. Having delocalized electrons causes a compound to be more stable than if all its electrons were localized. You need to know only a few rules to be able to determine to what extent electron delocalization affects the stability of a compound.

Delocalized electrons play a very important role in organic chemistry. A few examples are given that show how delocalized electrons can affect the reactions a compound undergoes, and how they can affect the pKa of a compound.

This chapter also revisits the reactions of alkenes. This time you see how compounds that have two double bonds (a diene) react. If the two double bonds are separated by more than one single bond, the reactions of compounds with two double bonds are exactly the same as the reactions of compounds with one double bond. If, however, the two double bonds are separated by only one single bond, the carbocation intermediate has delocalized electrons. Now you must combine what you learned about the reactions of alkenes with what you learned about delocalized electrons. As a consequence of these delocalized electrons, the positive charge on the carbocation intermediate is shared by two different carbons. That means there are two different carbons that a nucleophile is attracted to. As a result, two products can be formed. You then learn how to predict which of these two possible products will be the predominant product of the reaction.

The chapter ends with a discussion of ultraviolet/visible (UV/Vis) spectroscopy, an instrumental technique that gives information about compounds with conjugated double bonds.




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