In the following pairs of halogen compounds, which compound undergoes faster S_N1 reaction?
         and        
 

Greater the stability of the carbocation, faster is the rate of S_N1 reaction. Since tertiary carbocation 2-chloro-2-methylpropane is more stable than secondary carbocation 3-chloropentane. Hence S_N1 reaction proceed via tertiary cation such as 2-chloro-2-methylpropane.