Nucleophilic substitution reactions occur when a nucleophilic molecule attacks a positive or partially positive electrophilic center by breaking and creating a new bond. S1 and S2 are two different mechanisms for nucleophilic substitution, and S1 involves a carbocation intermediate. In S1, a leaving group is broken off to create a carbocation reaction intermediate. Then, a nucleophile attacks and forms a new bond with the carbocation intermediate to form the final, substituted product, as shown in the reaction of 2-bromo-2-methylpropane to form 2-methyl-2-propanol.
In this reaction,  is the formed carbocation intermediate to form the alcohol product.