On average, SN2 reactions take less than 30 seconds and SN1 reactions take more than 30 seconds. This is because SN1 is a two-step mechanism and SN2 is an one-step mechanism. In the experiment, bromobenzene should perform SN1 reaction but it does not. In contrast, 1-chlorobutane should not perform SN1 reaction but it does. Furthermore, 1-chlorobutane and 2-chlorobutane should perform SN2 reaction but they do not. To explain for these observations, the tubes using for the three chemical may contain other compounds from the previous experiment. Therefore, some tubes have precipitations due to the reactions between unknown substrates and the reagents - AgNO3 and NaI.
