Key Mechanism Electrophilic Addition to Alkenes

Most addition reactions involve a second step in which a nucleophile attacks the carbocation (as in the second step of the S l reaction), forming a stable addition product. In the product, both the electrophile and the nucleophile are bonded to the carbon atoms that were connected by the double bond. This reaction is outlined in Key Mechanism Box 8-1, identifying the electrophile as E and the nucleophile as Nuc . This type of reaction requires a strong electrophile to attract the electrons of the pi bond and generate a carbocation in the rate-limiting step. Most alkene reactions fall into this large class of electrophilic additions to alkenes. 

The p i bond as a nucleophile. A strong electrophile attracts the electrons out of the pi bond to form a new sigma bond, generating a carbocation. The (red) curved arrow shows the movement of electrons, from the electron-rich p i bond to the electron-poor electrophile. 

A wide variety of electrophilic additions involve similar mechanisms. First, a strong electrophile attracts the loosely held electrons from the pi bond of an alkene. The electrophile forms a sigma bond to one of the carbons of the (former) double bond, while the other carbon becomes a carbocation. The carbocation (a strong electrophile) reacts with a nucleophile (often a weak nucleophile) to form another sigma bond. 

Step I Attack of the pi bond on the electrophile forms a carbocation. 

Step 2 Attack by a nucleophile gives the addition product. 

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