Addition Reactions to Polarized Multiple Bonds

An addition reaction is formally the reverse of an elimination reaction. We will see both electrophilic and nucleophilic additions in this section. There are few new generic classes of reactants to consider, and there are three new routes, but these are just the reverse of the elimination routes just covered. The new generic classes are discussed in much more detail in Chapter 5 and 6, but need to be introduced here. A carbon-carbon double bond can range from electrophilic to nucleophilic depending on what is attached to it (Fig. 4.33). Another way to make a double bond electrophilic is to replace one of its carbon atoms with an electronegative heteroatom like oxygen, C=0, a carbonyl. 

Electron donor group attached More electron rich Nucleophilic pi bond HOMO more accessible 

Electron-withdrawing group attached More electron deficient Electrophilic pi bond LUMO more accessible 

The AdE2 is the conceptual reverse of the El elimination. The first step, Ag, (Association, Electrophilic) is a new electron flow path and has the following general form. The arrow starts from the pi bond, breaking it and forming a new sigma bond to the electrophile, creating a carbocation next to the carbon-electrophile bond. 

Additions to Alkenes from a HOMO-LUMO Perspective 

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