Typical Characteristics of Polar Mechanisms

The reaction of isobutylene (2-methylpropene) with HCl to give t-butyl chloride is a typical example of a polar reaction. 

HCl is an acid it dissociates into H and Cl. A curved arrow is drawn to show the movement of electrons from the bond to the more electronegative element. 

Three species are in the reaction mixture now. How would you characterize their reactivity One (H+) is an electrophile, and two (Cl , isobutylene) are nucleophiles. Nucleophiles react with electrophiles Cl can combine with H , but this reaction is simply the reverse of the first reaction. More productively, the electrons in the tt bond in isobutylene can be used to form a new bond to H to give a carbocation. 

(Remember Markovnikov s rule ) Fourth, the number of electrons around the nucleophilic carbon (on the right in both starting material and product) does not change in the course of the reaction, but a pair of electrons is taken away from the nonnucleophilic carbon, so it is electron-deficient in the product. 

The carbocation is an electrophile, and there is still a nucleophile (Cl ) in the reaction mixture. In the final step of the reaction, a lone pair on Cl is used to form a 7 bond to the electrophilic C of the carbocation. The product is t-BuCl. 

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