The S_N¹ reaction is a substitution reaction in organic chemistry. "SN" place for nucleophilic substitution and the "1" shows  the fact that the rate-determining step is unimolecular.Therefore, the rate equation is often shown as having first-order dependence on electrophile and zero-order dependence on nucleophile. This relationship holds for conditions where the amount of nucleophile is much larger than that of the carbocation intermediate. Instead, the rate equation may be more accurately explain using steady-state kinetics. The reaction involves a carbocation intermediate and is commonly seen in reactions of secondary or tertiary alkyl halides under strongly basic conditions or, under strongly acidic conditions, with secondary or tertiary alcohols. With primary alkyl halides, the alternative S_N² reaction happens. In inorganic chemistry, the S_N¹ reaction is often called as the dissociative mechanism